Forums :: NEW! Web Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Jan Mon 24, 2022 8:58 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 17 posts ] 
Author Message
 Post subject: Restoration of Monitor, Model TC56M radio
PostPosted: Sep Wed 15, 2021 10:24 pm 
Member

Joined: Sep Thu 02, 2021 9:13 pm
Posts: 30
I have owned this table top radio for over 40 years. I have only played it a few time over the years.
After retiring a few years ago from a job in the electronics field I have become interested in vacuum tube and transistor radio repair.
Recently I decided to remove the radio from the cabinet and inspect the condition of the radio. I found some brittle wires running to the antenna that I replaced recently. When I inspected the radio it looks like most of the paper capacitors were replaced at one time, except one.
The dual Sprague electrolytic was still there. There was also one I believe mica capacitor ( looks like a domino) that was never replaced.
I have a Rider schematic and a Sam’s Photofact. The Sam’s is for the model TA56M & TW56M.
I was hoping for advice on what to replace from my findings.

1). The Sprague capacitor is a 30 mfd & 50mfd 150v electrolytic that is working. I am not getting any noticeable hum. When I check the capacitance in circuit with my Fluke meter I get 75 mfd & 90 mfd. Should I replace this capacitor(s)?

2) The paper capacitor is a .05 mfd 200v. The Sams states that it is a Line Isolation capacitor.
From what I have learned so far about paper capacitors I am thinking that this capacitor should be replaced.

3) The 220 mmfd 500v ( domino) capacitor. The Sams states it is a Audio Plate Bypass capacitor.
From what I have learned these types of capacitors seldom go bad. I was thinking of not replacing it.

4) I checked all of the resistors in circuit and found 3 resistors that were questionable.

3.3 Meg AVC network resistor read 2.873 Meg

470,000 Grid Output tube resistor read 408,000 ( Sams states that the resistor is suppose to
be a 510,000, Rider states the resistor is suppose to be 470,000.

22,000 Oscillator Grid resistor reads 26,000.

While checking the operation of the radio I discovered the Volume control/ on-off switch stopped working. I played with the on-off switch numerous time and couldn’t get the radio to turn on.The next day I tried it again and it started working again. I think I should replace the volume control on-off switch as well?

I found myself going back and forth between the Sams and Rider information. I was wondering if the differences was because of the different model numbers?

Any parts that need to be replaced I want to replace them with good quality replacements. I know this is not an expensive radio but I want it to work as long as possible without any problems.
I also wanted to know what type and values you would recommend as well as where best to purchase them.
Any information will be greatly appreciated.
Thanks,
Mark


Top
 Profile  
 
 Post subject: Re: Restoration of Monitor, Model TC56M radio
PostPosted: Sep Thu 16, 2021 3:52 am 
Member
User avatar

Joined: Jun Tue 17, 2008 4:11 pm
Posts: 247
Location: Mt. Rest, SC, USA
Sounds like a nice one. Some people don't care for them, but I always liked sets that use loctal tubes. :)

1. In order to get an accurate measurement of capacitance you need to remove it from the circuit. The readings will be affected by resistors and other capacitors in parallel. But this is a poor way to gauge the health of a capacitor, particularly an electrolytic. To get an accurate test of its health, you also need to measure its ESR (equivalent series resistance, i.e. its lossiness) under rated voltage. (The Jackson 650A is a good example of a vintage capacitor tester designed to make such tests.) An electrolytic that still tests good as far as capacitance may be excessively lossy.

If the Sprague capacitor is the original I would definitely recommend replacing it for the health of the 35Y4 rectifier even if it appears to still be good. The first section I would replace with a 47µF, 200V capacitor. A 200V rating provides needed margin to cope with today's higher AC line voltage and the surges it will experience on turn-on. (Even when new, 150V was marginal.) The second section sees a lower voltage, and so a 33µF 160V replacement could be used, but I would still go with a 33µF 200V replacement as there is little difference in price.

2. Yes, it should be replaced. The 0.05 µF (also written 50 nF or nanofarad) capacitor is special due to its safety significance, and should be replaced with a modern class-Y capacitor. It is designed to isolate the chassis from the AC line to prevent shocks due to a hot chassis. Class-Y capacitors are specially designed to "fail-safe" or open-circuit, preventing a potential shock even if the capacitor is damaged by a transient. Here is a suitable replacement https://www.digikey.com/en/products/detail/kemet/PME271YC5470MR30/5730903.

Likewise, capacitor 18 (the noise-suppression 50 nF capacitor) should be replaced by a modern class-X1 capacitor, like https://www.digikey.com/en/products/detail/kemet/R49AI247000B1M/8565806 Across-the-line capacitors need a higher rating now than they did back in the 1940s, as there is much more noise on the power line due to the increase in power-electronic loads like switching power supplies, light dimmers, LED bulbs, etc. For more on class-X and class-Y capacitors there is a good write-up on them on Just Radios https://www.justradios.com/safetytips.html.

3. Mica and ceramic capacitors are highly reliable and seldom go bad. The 220 pF capacitor does not need replacing.

4. Any resistors that have drifted outside their tolerance range I always replace. The AVC and oscillator resistors for instance are outside even a ±20% tolerance and are bad. Assuming the 470k resistor also has a 20% tolerance, 408kΩ would still be within that and would be good. In my experience resistors always go bad by drifting up in value, seldom do they drift down. A lower than nominal reading is usually the result of shunting other resistances in parallel or normal production variance. The only resistors I have seen that fail out-of-tolerance low are metal-housed wirewound resistors (which fail by shorting).

5. I would try to save the potentiometer and power switch by cleaning it with De-Oxit or a similar product. You don't say if whether pot or switch is giving you trouble but they make separate cleaners for both. Ex., De-Oxit Fader-series cleaner for pots and D-series for cleaning switch contacts. A good squirt of contact cleaner can often bring a dirty switch back to life.

Digi-Key https://www.digikey.com/is an excellent source for top-grade parts. Their website makes it very easy to do complex filters and searches. For instance, you can specify whether you want a ultra-high grade 47 µF, 200V capacitor able to survive 12,000 hours at 105°C or a cheap one that will die after 2000 hours at 85°C with just a few clicks. (Ironically the difference in cost between best and worst is only 28 cents. Maybe significant to a cell-phone manufacturer ordering 5,000 but not to restorers like us ordering one or two. :lol: )

As far as 470k versus 510k either would work fine. After WWII the RMA standardized all electronic component values to the system we use today. Pre-WWII, components had slightly different standard values (which could vary between manufacturers). For instance the nearest standard replacement for a 50 µF electrolytic capacitor is now 47 µF, or a 500 kΩ ±20% pre-war dog-bone could be replaced by either a 470 kΩ or 510 kΩ modern replacement. The 470k resistor was available in 20%, 10% and 5% tolerances, whereas the 510k was only available in 5% tolerance. Nowadays even the cheapest resistors are at least 5%, but back in the 1940s these would have been considered precision and were more expensive. However, being as this was made in 1946, there were tons and tons of war-surplus parts available, so many radios shipped with a mix of commercial and military-grade components. I have seen models that had war-surplus 2% precision paper capacitors and ultra-low-noise metal-film resistors in place of carbon-composition dog-bones, just because the manufacturer had so many of them on hand.

As far as the models at least according to the Riders the only difference between them is the cabinet.

Good luck, with a couple new parts, that Monitor ought to go on working at least another 80 years! :D

_________________
"We are all refugees of a future that never happened."--Lee Weiner


Top
 Profile  
 
 Post subject: Re: Restoration of Monitor, Model TC56M radio
PostPosted: Sep Fri 17, 2021 6:09 pm 
Member

Joined: Sep Thu 02, 2021 9:13 pm
Posts: 30
Thanks for all of the information. I will have to look into the Jackson 650A capacitor checker.

I plan on ordering the line and isolation capacitors that you recommend replacing.

CAPACITORS
I also will order the two electrolytic capacitors that you recommend as well. The 47mfd 200v and the 33mfd 200 v. I want to order high quality, long lasting capacitors. It’s a little confusing to figure out the best capacitors to buy. Can you tell me which capacitors you would recommend?

RESISTORS
MOST OF the resistors in the radio are .5 watt carbon resistors. There are however three brownish/reddish metal resistors that values are numerically marked and have 2% tolerance.
I have never seen his type of resistor before. They look like the military grade resistors that you mentioned. They are as follows:

Rider No. 33 resistor, (Power Supply). The resistor used is a 50 ohm resistor and reads 49.8.
The Sams doesn’t show this resistor

Sams No. 21 resistor, (Hum Feedback). The resistor used is a 5000 ohm resistor and reads 4980.
The Sams value is 4700 ohms.

Sams No. 19 resistor, (1st AF grid) The resistor used is a 400k ohm resistor and it reads 393k.
The Sams shows a value of that resistor to be 5 Meg?? I couldn’t find the resistor on the Rider schematic.
I was wondering if it is common to see such differences in Sams and Rider schematics.
All three resistors read good.

If I find any of the carbon resistors out of tolerance, should I replace them with another carbon resistor or is there a better type of resistor that you would recommend. Also, would it be good to use a 1 watt resistor instead?

The problem that I mentioned with the Volume control on-off switch in my first post had the problem with the on-off switch. It was the first time that the switch stopped working. I checked it with an ohm meter and it showed an open. The next day I tried the switch and it’s started working without a problem. The volume control is good. No static or erratic behavior. I am debating if I should replace it?

I just wanted to thank you again for your help. It’s greatly appreciated.

Mark


Top
 Profile  
 
 Post subject: Re: Restoration of Monitor, Model TC56M radio
PostPosted: Sep Sat 18, 2021 1:26 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11057
Location: Beaver Falls, PA. USA
Try squirting the switch with WD-40 and operate it a dozen times or so; that may make it work again. There were parts and tube shortages in the late 1940s and early 1950s, and it's not uncommon to see imported or even prewar parts used. Those resistors might be from Germany; I have seen them before, and they seem to hold their value well.

_________________
Tim KA3JRT


Top
 Profile  
 
 Post subject: Re: Restoration of Monitor, Model TC56M radio
PostPosted: Sep Sat 18, 2021 7:40 pm 
Member
User avatar

Joined: Jun Tue 17, 2008 4:11 pm
Posts: 247
Location: Mt. Rest, SC, USA
The catalog of a major distributor like Digi-Key or Allied may seem overwhelming at first. The trick is using filters things down to just a few results. First thing, starting on their capacitor page, https://www.digikey.com/en/products/category/capacitors/3, is to select the type of capacitor. Paper types are replaced by plastic "Film Capacitors" (ceramics may also work depending on the application, but let's keep things simple for starters.). The Electrolytic capacitors are Aluminum Electrolytics.

The main thing to look for in a good electrolytic capacitor is temperature rating. For instance, putting in the filters In Stock, Capacitance 47 µF, Voltage-Rated 200, Mounting Type Through-Hole, you will get 31 results. Be sure to select Through-Hole, as you don't want some microscopic surface-mount part! :lol:
Second, look for a part with a reasonable lifespan at 105°C. Tube equipment runs hot, so more margin here equates to a longer lifespan. The best grade are 10,000 and 12,000 hours @ 105°C. Adding that filter reduces the results to 7. Any of those would be a good option. In general, Japanese manufacturers like Nichicon and United Chemi-Con make the best grades of electrolytic capacitors.

Film capacitors have a lot more options. Start with the filters of In Stock, Capacitance, and Mounting Type: Through-Hole. After that select the voltage rating. For a safety capacitor, just select the option in "Ratings," X1 for across-the-line types like 18, or Y1 and Y2 for capacitor #10. For any other type, the rating given on the schematic is the DC voltage rating. For 200V parts also look for 250V as 200V is not common anymore. Also select the tolerance desired. None of the paper capacitors in this radio are critical so look for 10% and 20% tolerances.

Once you filter on all that, look for a film capacitor will as long of leads as you can possibly find to minimize the need for jumpers.

Old carbon composition resistors can be replaced by carbon-film, metal-film, or metal-oxide-film types. Any would work equally well, but modern small resistors (1/2W and below) would usually be carbon-film and power resistors metal-oxide. Power rating, value, and tolerance are the main criteria for a resistor. Higher powers are ok; you just don't want it too low.

The simplest approach is simply to replace the resistor with the same rating as it originally had. The scientific way, assuming you have a schematic with accurate voltage readings for everything, is to calculate the dissipation of the resistor using
    2 * (V1 - V2) ^2 / R
, (the times 2 adds a safety margin) then rounding up to the next bigger size. A spreadsheet makes this easy. For instance resistor 29 has 5V across it so 2 * 5^2/150 = 0.33, so it should be 1/2W. Resistor 33 seems to have been undersized by Monitor as it has 2*(117-110)^2 / 47=2.09W, so it should be replaced by either a 2 or 3W. The 3.3M and 15M resistors carry only microwatts, so modern 1/6W types could be used.

When things get complicated is that resistors also have voltage ratings. A resistor too physically small might arc over even though it is not being overheated. The attached data-sheet is typical of carbon-film resistors. "Maximum Working Voltage" is the largest voltage that can be placed across the resistor. "Voltage Proof on Insulation" is the highest voltage between either side of the resistor and earth before the insulation fails. Your radio uses such low voltages that this isn't an issue; even a 1/6W resistor can handle 100V assuming its power rating isn't exceeded. Only on radios with voltages of 200V and above do resistor voltage ratings become an issue. For example I worked on a McIntosh MC225 amplifier for a client that uses a 390V supply. All the 2.2M plate resistors had to be replaced by special high-voltage types. Many engineers of the day were largely ignorant of resistor voltage ratings, and used ordinary 1/2W resistors in places where we today would use a specialty type. (Although to be fair, the technology to make a HV resistor did not exist back in the 1940s.)

I agree with Tim Tess. If you don't have any DeOxit on hand, WD-40 will work as a substitute. WD-40 even makes their own brand of contact cleaner that is sold at Home Depots and other hardware stores. Just spray the cleaner thru an open hole in the switch and operate it a couple of times.


Attachments:
Yageo CFR Resistors.pdf [113.01 KiB]
Downloaded 26 times

_________________
"We are all refugees of a future that never happened."--Lee Weiner
Top
 Profile  
 
 Post subject: Re: Restoration of Monitor, Model TC56M radio
PostPosted: Sep Sun 19, 2021 6:50 pm 
Member

Joined: Sep Thu 02, 2021 9:13 pm
Posts: 30
Last night I did a search on the Digikey website and came up with these choices to replace the
30 & 50 mfd , 150v electrolytic capacitors in my radio. ALL 4 are Nichicon brand. The size of the capacitors are about the same and the price for each is under $1.52.

UCA2D470MHD 47 mfd , 200v , 12,000 hrs, 105C
UCA2E470MHD6 47 mfd , 250v , 12,000 hrs, 105C

UCA2D330MHD. 33 mfd, 200v , 12,000 hrs, 105C
UCY2E330MHD3TO 33 mfd, 250v , 12,000 hrs, 105C

Is there any advantage in going with the 200v vs 250v?
If you think that there is a better reliable, quality capacitor to use please let me know.

RESISTORS

The Sams has tube pin voltages that I can use to calculate the wattage needed for some of the resistors. Can I also take voltage readings WITH a DVM across the resistors to determine the voltage drop?

You mentioned R33 should be around a 2-3 watt resistor. The resistor in the radio is a brownish/ reddish resistor. It has the numerical value of 50 ohms, 2%, 2. I am guessing that the last 2 stands for 2watts. It reads 49.8 ohms in circuit.

There is a resistor. R21 in the Sams ( Hum Feedback) is a 4.7k resistor, In the Rider I believe it’s R34. The resistor in the radio is a brownish/ reddish resistor, 5k , 2%, 2. It reads 4980 ohms in circuit. I am also guessing that it is a 2watt also

There is a third resistor that is also brownish/ reddish that in the Sams that I think is R19 ( 1st AF Grid) It is a 5 Meg resistor , In Rider it is a 4.7 Meg. The value of the resistor in the radio is a
400k 2%, 2 resistor. What I don’t understand is the big difference in the values. This resistor test good as well, 398 k.

I will take your advice in replacing the resistors out of tolerance and wattage.

Thanks again for your help. It is greatly appreciated.


Top
 Profile  
 
 Post subject: Re: Restoration of Monitor, Model TC56M radio
PostPosted: Sep Sun 19, 2021 9:20 pm 
Member
User avatar

Joined: Jun Tue 17, 2008 4:11 pm
Posts: 247
Location: Mt. Rest, SC, USA
No difference in 200 versus 250V. For whatever reason many companies have stopped making 200V film types in favor of 250V whereas it still remains a common size for electrolytics. So long as the rating is well above the voltage put across it that's all that matters.

I don't know what the Sams schematic says--all I have is the Riders. (Note when I said 33, I mean component 33 on the Riders.) So long as the voltages were taken with a VTVM or 20kΩ/V meter they are accurate and can be used for judging ratings. For instance the Riders manual says "voltages measured with an electronic voltmeter" aka a VTVM. Just look out for voltages that were taken with a 1000Ω/volt meter. Such a low-impedance meter loads the circuit too much, resulting in misleading readings.

Yes a DVM will substitute for a VTVM for taking readings. (Of course you usually can't take a voltage reading to find the correct replacement part to repair a radio as the radio is usually nonworking when you find it, so you have to trust the voltages on the schematic or the original engineers' judgment in sizing them. :lol: )

As far as 47Ω versus 50Ω and 4.7kΩ versus 5kΩ, when WWII ended, companies were scrambling to get civilian production going again, so they made due with what they had in stock. Even through the design engineers revised their schematic to use the recently standardized RMA values (e.g. 47Ω), they still had pre-war 50Ω resistors (made using the old numbering convention) in stock so they used them. There is only a 6% difference between 47 versus 50.

Are you sure about the 400kΩ 1st AF grid-leak resistor? That is way too small for a grid-leak. :shock: Double-check that it is not the plate resistor instead (27 on Riders). The plate resistor is on pin 2, the grid resistor between pin 3 and pin 4, 5, or 7. 400k would substitute for 470k, but not 4.7M!

_________________
"We are all refugees of a future that never happened."--Lee Weiner


Top
 Profile  
 
 Post subject: Re: Restoration of Monitor, Model TC56M radio
PostPosted: Sep Tue 21, 2021 1:44 am 
Member

Joined: Sep Thu 02, 2021 9:13 pm
Posts: 30
I tried to research the concern with the 400k resistor. I attached a diagram that I drew of where the 400 k resistor is located in the chassis. There is no 5 Meg or near equivalent resistor in the chassis. I have found a number of differences between the Rider and Sams schematics besides the 400k resistor. This radio looks like it was worked on by someone in the past because most of paper capacitors were replaced with newer capacitors. Please see attached photo.


Attachments:
File comment: Diagram of wiring of the 400k resistor.
FB4B136D-7012-4293-A8E9-784A6F8B57E1.jpeg
FB4B136D-7012-4293-A8E9-784A6F8B57E1.jpeg [ 536.93 KiB | Viewed 747 times ]
Top
 Profile  
 
 Post subject: Re: Restoration of Monitor, Model TC56M radio
PostPosted: Sep Tue 21, 2021 2:10 am 
Member

Joined: Sep Thu 02, 2021 9:13 pm
Posts: 30
I tried to research the concern with the 400k resistor. I attached a diagram that I drew of where the 400 k resistor is located in the chassis. There is no 5 Meg or near equivalent resistor in the chassis. I have found a number of differences between the Rider and Sams schematics besides the 400k resistor. This radio looks like it was worked on by someone in the past because most of paper capacitors were replaced with newer capacitors. Please see attached photo.


Attachments:
File comment: Diagram of wiring of the 400k resistor.
FB4B136D-7012-4293-A8E9-784A6F8B57E1.jpeg
FB4B136D-7012-4293-A8E9-784A6F8B57E1.jpeg [ 536.93 KiB | Viewed 746 times ]
Top
 Profile  
 
 Post subject: Re: Restoration of Monitor, Model TC56M radio
PostPosted: Sep Tue 21, 2021 3:07 pm 
Member

Joined: Sep Thu 02, 2021 9:13 pm
Posts: 30
This is a continuation of the previous post.
This is the Sams schematic diagram which appears different from me to the Riders diagram.

R19 5 Meg resistor 1st AF Grid
R21 4700 ohms resistor Hum Feedback

Once again, any help is solving this mystery will be greatly appreciated.


Attachments:
File comment: Sams schematic
B579416C-027B-487F-81EF-8E31A0EF4E47.jpeg
B579416C-027B-487F-81EF-8E31A0EF4E47.jpeg [ 1.82 MiB | Viewed 729 times ]
File comment: Sams Schematic
3A3CC6B5-78C6-4EB2-9627-58E4B3141FA1.jpeg
3A3CC6B5-78C6-4EB2-9627-58E4B3141FA1.jpeg [ 1.82 MiB | Viewed 729 times ]
Top
 Profile  
 
 Post subject: Re: Restoration of Monitor, Model TC56M radio
PostPosted: Sep Tue 21, 2021 11:12 pm 
Member
User avatar

Joined: Jun Tue 17, 2008 4:11 pm
Posts: 247
Location: Mt. Rest, SC, USA
Thanks for posting the scan of the Sam's schematic. That makes it much easier for others reading your post to compare to the Riders http://www.nostalgiaair.org/pagesbymodel/972/M0010972.pdf. Maybe there was a change to the design mid-production?

If you analyze the circuit on the Sam's schematic, you'll notice the 14B6 is still getting its bias from the 5MΩ grid-leak resistor, same as shown on the Riders schematic. So the design will still work. The difference is a negative feedback loop as been added coupling B+ back to the input of the 14B6. The 5MΩ and 5kΩ (or 4.7) resistors form a voltage divider which sets the amount of feedback. I'm guessing they're using the feedback network to reduce the hum level. It sounds like your radio was built using this revised circuit shown on the Sam's schematic.

So, you should also have an extra 5kΩ resistor and 22nF capacitor, in addition to a 5MΩ/4.7MΩ resistor. Can you verify the rest of the wiring for your radio matches the Sam's schematic? Its possible whoever recapped the radio in the past replaced 19 with the wrong resistance. (Maybe he was colorblind and mistook yellow for green on the color code? :lol: ) To avoid confusion, note that the ground symbol shown on the Sams' means counterpoise ground, and not chassis-ground as on the Riders'. (Counterpoise ground is isolated from chassis by the 50nF class-Y capacitor.)

_________________
"We are all refugees of a future that never happened."--Lee Weiner


Top
 Profile  
 
 Post subject: Re: Restoration of Monitor, Model TC56M radio
PostPosted: Sep Wed 22, 2021 11:22 pm 
Member

Joined: Sep Thu 02, 2021 9:13 pm
Posts: 30
The person who put the 400k resistor in place of the 5 meg resistor wasn’t color blind. The 400k resistor has the numerical value on the side of the resistor. I attached a photo of the resistor. The reddish/brownish resistor on the right is the 400k resistor. The reddish/ brown resistor next to it is the 5k resistor from the Sams schematic.

I decided to do a few voltage checks on the 14B6 tube. I turned the volume up all the way and tuned off of a station according to Sam’s directions. These are the voltage reading that I got:

14B6.
Pin 2 should be 57 vdc DVM reading was 31.54 vdc
Pin 3 should be -.5 vdc. DVM reading was -.77 vdc

I don’t know what I should do at this time. I don’t know enough about vacuum tube radios to understand why the person who worked on this radio before me would of used the 400k resistor instead of the 5 Meg resistor that is indicated in the Sams schematic??
The radio is working fine. I am concerned what could happen to the radio if I put a 5 Meg resistor back into the radio.

You asked me if there were any other difference between the Rider and the Sams schematic. I have listed the ones that I have found. They are as follows:

1) Rider shows R33 to be 47 ohms, Sams does not show this resistor, radio has 50 ohm resistor.
2) Rider shows R25 to be 1.2k ohms, Sams does not show this resistor, radio has 1.2k resistor.
3) Rider shows R24 to be 150 ohms, Sams does not show this resistor, radio has 200 ohm resistor.
4) Rider shows R26 to be 4.7 Meg, Sams shows 5 Meg, radio has 400k resistor ??
5 Sams shows R21 to be 4.7k ohms , Rider does not show it. Radio has 5k resistor.

I have attached a photo of the Sams schematic. There may be more.

At this time I plan on replacing the 2 old paper capacitors that you recommend along with the old 30&50 mfd electrolytic capacitors as well. All of the other capacitors are newer.I will also check all of the resistors to see if they are out of tolerance and replace them as needed.

Any insight as what to do with the 400k resistor would be greatly appreciated.


Attachments:
File comment: Resistor on right is 400k. Resistor to the left is 5k
48535819-E609-448C-9956-7AE4360C9527.jpeg
48535819-E609-448C-9956-7AE4360C9527.jpeg [ 2.03 MiB | Viewed 690 times ]
File comment: Sams Schematic
4683580A-F446-4F8B-947B-8BBF41386039.jpeg
4683580A-F446-4F8B-947B-8BBF41386039.jpeg [ 2.15 MiB | Viewed 690 times ]
Top
 Profile  
 
 Post subject: Re: Restoration of Monitor, Model TC56M radio
PostPosted: Sep Thu 23, 2021 2:04 am 
Member
User avatar

Joined: Jun Tue 17, 2008 4:11 pm
Posts: 247
Location: Mt. Rest, SC, USA
I would replace the 400k resistor with a 5.1M or 4.7M resistor per the Sam's schematic. That is what the radio's designer intended it to have.

When I said the person misread the multiplier stripe I meant the person who recapped the radio in the past. Or even some old time repairman. He may have mistook a 4.7M resistor for 470k, measured the resistance as 4.7M, thought it was bad, and replaced it with a 400k he happened to have. Who knows?

The circuit is just a simple triode audio amplifier. I attached a copy of Sylvania's application notes for a 7B6/14B6 audio amplifier. Sylvania shows there are two ways to design an AF stage using it: "zero-bias" and self-bias. Zero-bias uses the natural grid leakage of the tube to bias the grid. Because this grid current is very small, less than a microamp, the grid resistor must be between 4.7M and 10M to generate sufficient bias. I circled the operating conditions specific to your radio: Ebb=100V, 470k plate resistor, 470k grid resistor for 50A5. The radio may work with an incorrect grid-leak resistor, but this would reduce gain and increase distortion in the 14B6 stage. This is a standard circuit which was used in millions of other AA5 radios.

Self-bias uses a cathode resistor to generate the bias. This resistor is typically bypassed by a capacitor to prevent degeneration, which would decrease gain. Self-bias uses a smaller grid resistor, on the order of 270k as shown in Sylvania Figure 4. Both Sam's and Rider's schematics show the stage was intended to use grid-leak bias with a 4.7M resistor, and there is no evidence of a cathode resistor.

The other changes all look fine to me and should be left as is:
1. Resistor 33 limits inrush current thru the 35Y4 and is not critical.
2. Your radio apparently uses a speaker with an electromagnet instead of a permanent magnet. The electromagnet substitutes for resistor 25.
3. Resistor 24 is a shunt resistor to limit the current thru the pilot light.
5. Resistor 21 is part of a feedback network to suppress hum.

There is a bad solder joint where the 3 nF capacitor was clipped to the 400k resistor.


Attachments:
7B6 Amplifier Data.jpg
7B6 Amplifier Data.jpg [ 455.19 KiB | Viewed 684 times ]
Amplifier Circuit.jpg
Amplifier Circuit.jpg [ 284.39 KiB | Viewed 684 times ]
File comment: Exert from Radiotron Designer's Handbook, 4th ed.
Grid-Leak Bias.jpg
Grid-Leak Bias.jpg [ 189.92 KiB | Viewed 684 times ]

_________________
"We are all refugees of a future that never happened."--Lee Weiner
Top
 Profile  
 
 Post subject: Re: Restoration of Monitor, Model TC56M radio
PostPosted: Sep Fri 24, 2021 3:10 am 
Member

Joined: Sep Thu 02, 2021 9:13 pm
Posts: 30
Thanks for the information.

I will replace the 400k resistor with a 5 Meg , .5 watt resistor.

In one of your previous response you mentioned that the manufacturer may of under valued the wattage of the Rider No. 33 resistor, which was a 47 ohm, 1 watt resistor. You said that according to your calculations it should be a 2 to 3 watt resistor. In my radio there is a 50 ohm resistor used, that is reddish/brownish and is marked numerically 50 ohm, 2%, 2. It is the same type and size as the 400k resistor in the photo in my previous post. I was wondering if you would have any idea what the wattage would be of that 50 ohm resistor?

When I physically look at all of the resistor there is one resistor, the 150 ohm that is connected to the cathode of the 50A5 that looks like it is slightly discolored. Possibly due to heat ?? I am getting a good reading of 150 ohms..

I started to look at resistors on the Digikey website. Are there any manufacturers that you could recommend using in my radio. I am thinking that I need to find resistors that have voltage ratings of 200 v or higher and that are heat tolerant . Is there a temperature value that I should use when searching? Does the type of resistor make a difference. I remember you saying that there are three types that can be used, carbon film, metal film and metal oxide. Are wire wound resistors not good for radios?

Picking resistor should be easy. There are so many types and sizes that I just want to make good choices and not overlook anything when choosing.

I really appreciate all of you help and insight. I am learning a lot from your posts.


Top
 Profile  
 
 Post subject: Re: Restoration of Monitor, Model TC56M radio
PostPosted: Sep Fri 24, 2021 3:34 pm 
Member
User avatar

Joined: Jun Tue 17, 2008 4:11 pm
Posts: 247
Location: Mt. Rest, SC, USA
I've never seen resistors like those red 2% ones, so I'm not sure. They don't look like carbon-composition resistors, so maybe they're wirewound resistors. The tight tolerance would suggest wirewound. Hard to say without breaking one open. Maybe someone else can offer a suggestion. If they are wirewound resistors, then they can safely run hotter than a carbon resistor.

Resistors are easy to choose, as there really isn't much to a resistor. Unlike a capacitor, resistors are designed to make heat. Some modern metal-oxide power resistors use exotic high-temperature ceramics that allow them to be made very small and yet still operate reliably. The standard factor-of-two safety factor I gave above is usually all that is needed to compensate for the fact you won't have perfect cooling under a chassis. This radio uses low enough voltages you don't have to worry about resistor voltage ratings. (Only with voltages 200 and above does resistor voltage rating become a concern.) The only criteria you need to focus on is sufficient power rating. Adequate power rating ensures the resistor will have a long life.

Wirewound resistors are good but have limited application compared to the other types. They are limited to lower resistances, and unless specially wound, are inductive (being essentially a coil of wire). They work well at DC and low frequencies, especially when large powers much be handled, i.e. in the power supply section. However, inductance means they will behave poorly at higher frequencies.

Nothing in this radio requires highly-precise resistor values, so you can stick to 5% tolerance for everything. Just go to the Through-Hole Resistor section and filter on in stock, 5% tolerance, power rating, and resistance needed. You can safely ignore criteria like temperature coefficient, pulse-withstanding, and automotive grade.

For example, most resistors will probably be made in Taiwan by either YAGEO or Vishay, as they make 90% of the world's quality resistors. I would avoid mainland Chinese companies like Stackpole. Resistors made in the People's Republic of China are flagged by Digi-Key with the warning "Import tariff will apply to this part if shipping to the United States," when you click on the part, like this one https://www.digikey.com/en/products/detail/stackpole-electronics-inc/CFM12JT470K/1741966. Unfortunately due to the present tensions between China and Taiwan :evil: , resistors like integrated circuits can be in short supply.

_________________
"We are all refugees of a future that never happened."--Lee Weiner


Top
 Profile  
 
 Post subject: Re: Restoration of Monitor, Model TC56M radio
PostPosted: Sep Sat 25, 2021 12:50 am 
Member

Joined: Sep Thu 02, 2021 9:13 pm
Posts: 30
I hope to place my order to Digikey this weekend. When I get the parts order, I will replace the
400k resistor with a 470k and the 5k with a 4.7k and then test the radio.
If everything is good after that I will replace all of the other capacitors and resistors that I order.

I will try to give you an update as soon as I can. Thanks again for all of you help. Having someone to answer all of my questions has really helped me.


Top
 Profile  
 
 Post subject: Re: Restoration of Monitor, Model TC56M radio
PostPosted: Sep Sun 26, 2021 2:37 am 
Member

Joined: Sep Thu 02, 2021 9:13 pm
Posts: 30
I ment to say that I will replace the 400k resistor with a 4.8 Meg resistor.
Thanks again.


Top
 Profile  
 
Post New Topic Post Reply  [ 17 posts ]  Moderators: Norm Leal, Marcc

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 30 guests



Search for:
Jump to:  






































Privacy Policy :: Powered by phpBB