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 Post subject: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Feb Sat 24, 2018 11:10 pm 
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So, picked up what was advertised as a "Radiola 60" from Goodwill. Got it home, and started cleaning it. Removed the plates and such to polish, and noted it's NOT a Radiola 60, but a 1928 GE Graybar 310, which uses the Radiola 18 chassis (I'm guessing GE licensed the design from RCA).

Anyway, it had all 6 tubes, and after replacing a bad power cord, I gave it an initial test. All tube filaments are glowing, which is a real plus! However, odd readings on voltages and such.

So, on to getting actual measurements and replacing the old paper caps. Here's what I have right now:

Image

Image

Image

First of all, does the reading of 367 Ohms across the plates of UX-280 rectifier tube mean the windings are going bad? If so, I should probably stop there! I expected a very low resistance, as with the other windings.

Next, I'm confused by the use of "MF" on the schematics. Do they mean "milli-Farads" or "micro-Farads"? Either way, some of those caps are way off!

Also, when it comes to caps, would you recommend 630v film caps or another type and voltage? A quick search showed this voltage/type in other Radiola 18 threads. As for the out-of-spec resistor, what's the wattage? Would 1W work, or do I need a higher wattage? As with my other restorations, I'll be sure to upload a schematic with as many values/ratings as possible once all is done so that folks won't have to guess in the future!

At the moment, I'm sticking with the original wirewound resistors (R1-R6), as they're all within about 10% of spec. If you suggest differently, what wattage for those? I've looked at prices for high-watt (10+) wirewounds, and I'd prefer to not spend that kind of money on this radio, seeing as how I'm trying to sell my entire collection (still for sale, BTW).

One other issue is the pot between antenna and ground. Windings were open, and finding a replacement is highly unlikely. The design is not typical; there's a 2nd set of windings to provide a slower increase in the first 1/3 of the sweep , then faster increase in latter 2/3 of the sweep. I found an article about another restoration of a Radiola 18, and the author used a modern 2K pot in its place. That's what I'm choosing to do, as I found one with a stem that is nearly identical to the old one. That will maintain the external look of the knobs without having to do a lot of custom work on some other vintage pot.

The cabinet will need minor work, and it was a bit of a surprise to find that asbestos pad under the Power Chassis! I guess those wirewounds really put out some heat! Not worried about the asbestos; it's intact and not obviously friable. I figure I'll be fine if I'm careful moving the power chassis back in, and vacuum up afterward. Also, mesothelioma takes 20-30 years given a heavy exposure, and since I'm just shy of 57, I doubt I'll live long enough to worry about it!

Any other suggestions would be appreciated, as always.


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 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Feb Sat 24, 2018 11:31 pm 
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Two relatively easy answers:

1. Something in the 100s of ohms is quite normal for a high voltage secondary. For a complete test of the transformer, remove all tubes and disconnect any dial lights. Apply power to the primary and verify that the transformer does not get hot. With no loads, it should stay cold or maybe warm up very slightly. If there is a significant increase in temperature, it might have a shorted winding,

2. If the old capacitors are leaky or otherwise defective, you cannot get a meaningful capacitance reading----just replace them.

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 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Feb Sun 25, 2018 12:25 am 
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If the capacitors were measured in-circuit, the reading could be affected. You need to disconnect at least one side of a capacitor to get a proper reading.

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Last edited by processhead on Feb Sun 25, 2018 1:27 pm, edited 1 time in total.

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 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Feb Sun 25, 2018 12:54 am 
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processhead wrote:
If the capacitors were measured in-circuit, the reading would be affected. You need to disconnect at least one side of a capacitor to get a proper reading.


Yeah, measured in-circuit, but some are close to stated value while others are way off. Guess it just depends on where in the circuit the cap sits.

Still, any idea on the "milli" vs. "micro" issue? I plan to redo the schematics to show them all in "uF" values, since that seems to be the standard today.


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 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Feb Sun 25, 2018 12:58 am 
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pixellany wrote:
Something in the 100s of ohms is quite normal for a high voltage secondary. For a complete test of the transformer, remove all tubes and disconnect any dial lights. Apply power to the primary and verify that the transformer does not get hot. With no loads, it should stay cold or maybe warm up very slightly. If there is a significant increase in temperature, it might have a shorted winding.


As best as I could tell, the individual shielded cases didn't get hot. The problem with the Radiola 18 is that the rectifier tube puts out a fair amount of heat, as do the wirewound resistors (thus the need for the asbestos pad). On a short run, everything seems cool enough.

I'll assume it's good to go and continue on.


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 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Feb Sun 25, 2018 2:38 am 
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In regards to the "MF" markings, yes, they indicate microfarads. The other commonly seen designation is mmF, which is micro-microfarads or picofarads.

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 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Feb Sun 25, 2018 4:02 am 
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ForkInSocket wrote:
In regards to the "MF" markings, yes, they indicate microfarads. The other commonly seen designation is mmF, which is micro-microfarads or picofarads.


I'm not saying I doubt you, but here's the thing: I removed the 2 paper caps (physical size is ~ 2" x 2" x 3/4") from the Tuning section, and they both measure 5 microfarads by my meter. Does that sound correct for a paper cap of that physical size? Would you expect a paper cap to gain 10x its original rated capacitance over the past 90 years?

OTOH, if "MF" is millifarads, then an 0.5 millifarad cap is equivalent to a 500 microfard cap, and I could see that cap losing 95% of its capacitance over the same period.

Anyone have similar paper caps in their collection that measure 5 uF? Any schematics that show the use of a 5 uF cap?


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 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Feb Sun 25, 2018 4:31 am 
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Once again, I would not trust those measurements. If you want to get to the bottom of this, then get them on an impedance bridge and get the measured C and DF (dissapation factor). Regardless of the result, you still should replace them with the values specified on the schematic.

To the best of my knowledge, NO radio builders specified capacitance in milli-farads.

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 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Feb Sun 25, 2018 4:36 am 
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indy_kid wrote:
ForkInSocket wrote:
In regards to the "MF" markings, yes, they indicate microfarads. The other commonly seen designation is mmF, which is micro-microfarads or picofarads.


I'm not saying I doubt you, but here's the thing: I removed the 2 paper caps (physical size is ~ 2" x 2" x 3/4") from the Tuning section, and they both measure 5 microfarads by my meter. Does that sound correct for a paper cap of that physical size? Would you expect a paper cap to gain 10x its original rated capacitance over the past 90 years?

OTOH, if "MF" is millifarads, then an 0.5 millifarad cap is equivalent to a 500 microfard cap, and I could see that cap losing 95% of its capacitance over the same period.

Anyone have similar paper caps in their collection that measure 5 uF? Any schematics that show the use of a 5 uF cap?



Not surprising to find 2 uF paper capacitors of that physical size in a 1920's radio. The technology and materials back then was just not that good and the capacitors of that time used thicker paper and foil plates with large surface area resulting in much larger caps than we are used to seeing today. A few years later, electrolytic caps came on the scene with much smaller packages.

Everything you find in a consumer radio power supply from that era is going to be rated in microfarads.
Some of the confusion results from the nomenclature of the time. Abbreviations used for various measurement units can lead to some confusion for those not familiar with them.

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Last edited by processhead on Feb Sun 25, 2018 1:26 pm, edited 1 time in total.

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 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Feb Sun 25, 2018 5:34 am 
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pixellany wrote:
Regardless of the result, you still should replace them with the values specified on the schematic.


Okay, so what type and voltage would you recommend? The values would be 0.5uF and 1uF. I noted above a thread that recommended 630V film caps, but whatever is cheapest works for me.

BTW, I went back and isolated the various caps in the power chassis:

Image





Values are lower, but still 2x or more higher than the schematic.

Also, I need to replace one resistor in the Tuning section, so can I get away with 1W? The original look pretty beefy.


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 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Feb Sun 25, 2018 11:59 am 
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The second & very important Non Polarised cap check is their electrical leakage; & that can give a false capacity reading. My normal tester for them, in that set would be an Insulation Tester at 250VDC.

I would be surprised if any Wax Paper type did not leak. As for a previous post (elsewhere) ... If they leak here, they're duds. Test with one end out of circuit.

Marc


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 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Feb Mon 26, 2018 7:38 am 
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indy_kid wrote:
pixellany wrote:
Regardless of the result, you still should replace them with the values specified on the schematic.


Okay, so what type and voltage would you recommend? The values would be 0.5uF and 1uF. I noted above a thread that recommended 630V film caps, but whatever is cheapest works for me.

.

For B+ filters, just about any film cap is fine. Many of us stock only the 630-volt parts---no money to be saved by using a lower voltage rating.

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 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Feb Mon 26, 2018 9:56 am 
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I note the transformer is shielded; I normally fit a supply cable with an earth / ground wire to them. +1 on cap voltage 630V covers most tube radios; but you need line caps where there is utility power. Also keeps the inventory down if you fix lots.

Personally I would not panic about the value of a Wax Paper type cap now. The probability of good is so low, just replace all of them with circuit value.

Marc


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 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Mar Sat 03, 2018 5:50 pm 
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So, I recapped the unit as suggested. I get a small hum from the unit, but the voltages are still way off.

0.35VAC instead of 1.5VAC
2.6VAC instead of 2.25VAC
0.5VAC instead of 5VAC

Nothing for 145VDC or 165VDC (using terminal 9 as the GND).

So, I'm stopping at this point. I've spent $30+ on parts. I intend to sell it anyway, so I'll let the buyer figure it out. There is a working supply on eBay for $50 + S/H, so that might be the easiest way to go. I also thought that a custom ARBE to be mounted in place of the supply could be an option. No more worries about 90-year-old parts with the ARBE.

AF1 and AF2 both appear good. I got good resistance readings on those. They're potted in a metal case, which would make replacement a nightmare. There were some other caps for the pentode, along with a 2M grid leak resistor (that one was good).

Anyone have a schematic with ALL values listed?


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 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Mar Sat 03, 2018 11:24 pm 
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Double "B" voltage is consistent with no loading and is the reason why one uses filter caps twice the "B" voltage.

Where are the LV voltages being measured?
5V: 5 to 6
2.25V: 4 to 3
1.5V 1 to 2

What are the R4, R6, Voltages (probably to ground), 's 4, 5,6, are humdingers, with rivets, I have had issues with them & corrosion. Double check the LV wiring, for a phase inversion & a faulty tapped resistor can cause that: Seen it in a Lyric. AC can do strange things.

Marc


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 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Mar Sun 04, 2018 1:35 am 
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Kid,

IMHO suspect what the meter is saying...

Try an analog VOM known to be reliable...


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 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Mar Sun 04, 2018 4:27 am 
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Chas wrote:
Kid,

IMHO suspect what the meter is saying...

Try an analog VOM known to be reliable...


I've reassembled it, but I can get access to the terminal strip with an analog VOM. I'll give it a try. Another member wondered about voltages at the resisters, but since it's back together, I don't want to take it apart again to find out. I had the resistors isolated and got values within 10% of spec, so I'm confident they're still good. No obvious signs of damage to the underside, so no excessive heat or arcing.

If the analog readings show promise, I might take out the power unit, just to satisfy my curiosity of the voltages at the resisters. I haven't given up on a set yet once I've been inside, but I'm really worn down from repairing 9 different models in about 3 months. Some of those had wiring that you wouldn't believe! I know my knowledge is limited, but whomever worked on these before me was doing "hit & miss"! Things like putting fixed and variable caps in serieswith a B voltage input and other components. They probably had no idea why their ARBE was reading 45V or 67.5V, yet there was no voltage on the other side of the cap!!!

I tested the tubes (excluding the rectifier), and the four 26 tubes measured weak/bad, (between 30-40) on my Supreme 589A tester (an early-mid 1940s model). The 71A and 27 pentode both measured good at 60 or above. I haven't calibrated the 589A, so can't judge the accuracy of the readings; I do know that the tubes work, with no shorts indicated. At least I got my money's worth with the tubes! Even if they are weak, they can be used for testing purposes, then swap in known good tubes for normal radio use.


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 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Mar Sun 04, 2018 6:01 am 
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At this point and with double the B+ I would say that if all the plates have that voltage there is the possibility of no negative, perhaps, neutral return, or the heater / filaments are cold (no volts).

Generally if there are no heaters / filaments operating the tubes won't & B+ doubles.

Marc


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 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Mar Sun 04, 2018 10:18 am 
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When I restored mine the capacitors were all good with little leakage which many folks will tell you is true for the 18.

I had read that the capacitors were made with very high quality paper and wax along with excellent sealing and seldom needed replacement. This turned out to be the case, as with the supplies suitably loaded, and later with the radio working, I measured a DC current of less than 200 micro A in the earth lead of the filters C2 and C3. That through the speaker output capacitor C1 was less than 40 micro A. Had the components been bad then the only sensible thing to do would have been to disconnect them and use modern small electrolytics under the chassis. Removing the metal box has been made very difficult as they are attached to the chassis by tabs passing through it and pressed over. Apparently the tabs are likely to break off if straightened.

The only faulty components were wire wound resistors R1 and R2 which were open circuit. Unfortunately, most of the circuit values were not on the Rider’s schematic I had obtained, including these. However, there was another diagram showing values. This gave 1K8 Ohm which could be confirmed by measuring the lengths of R1, R2 and R3 wound on the same former, with the same wire, and doing a quick sum. The faulty sections were easily bridged with 3W metal film items.

I checked the capacitors C6 – C9 which were not so good being less than 1M Ohm leakage at 10V. As it was easy to remove this box I ‘re-stuffed’ it using new poly types to avoid any problems in the future.

This is from an article I wrote for a Mag: if I dig back in note books I can find what values I measured for the caps. Obvioulsy nothing odd like you have found as they would have been changed. But you have already done it so I wont bother.

All electronic cap meters measure odd if there is inductance in circuit: you have to disconnect one end to get a proper result as others have said.

Gary


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 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Mar Sun 04, 2018 11:08 am 
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The fact that there is double the B+ suggests that its capacitors have little, or more likely naught to do with the problem. The problem is conduction, or phase. If you removed connexions and did not mark where they came from, which is crucially important with transformers this may be part of the problem.

What would perhaps be interesting would be to remove the rectifier & the heater filament wires & quickly check the open circuit voltages of the three low voltage windings. That may give a clue as to it being the PSU or something wired wrongly in the set? Its got to be obvious in such a simple circuit. Normally if I disconnect the windings of a transformer I will check them for earth leakage & winding to winding shorts.

There will be no DC with the rectifier out.

Marc


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