Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Sep Sat 22, 2018 12:16 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 34 posts ]  Go to page Previous  1, 2
Author Message
 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Mar Sun 04, 2018 4:30 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 12600
Location: S. Dartmouth MA 02748-1225 USA
Hi again Kid,

Established, you have an analog ohmmeter that appears to be accurate enough for your garage :D

Let's use that to make any readings in posts referencing this response.

I'm gonna guess what is going on..

See this schema which is here, certainly seen many times before:

http://www.nostalgiaair.org/pagesbymode ... 040317.pdf

This is a very simple power supply, no regulation, minimal series resistors and minimal caps... RCA (et al) calls it an SPU, Socket Power Unit. One or two interesting bits. It HAS a form of regulation :shock: Whaaaa? Yep, the first and only "passive" device after the rectifier is a choke. No one knows if it is a "swinging" choke but none the less choke input filter systems have somewhat better regulation and a lower output voltage. Both chokes have a relatively high inductance because they are connected to low mfd capacitors. That is a primary reason to NOT use replacement caps greater than say 10% or so...

Factoid #1 RCA's potted caps "generally" have a good life expectancy.

Look at the filament resistors (tapped?) if one or both are open of have poor connections the B+ cannot return to the grounded center tap of the transformer.

If the tubes are weak the B+ will rise, this is a sort of self preserving feature of the radio. As the emission falls off on the tubes, the rising plate voltage will encourage greater emission. There is an end point.

Factoid #2 The 26 tube has a high filament current demand, 1.4 amps. Tubes of that era have a high tin content in the pin solder. The tin becomes crystalline over the last 90 years. The pins will have high resistance. Since there are two filament pins, there could be enough resistance to impair emission :shock: Even enough to show the tube open... Re-solder the pins by adding solder first and then extracting all the old solder. Scrape the interior of the pin and the wire with the point of an Exacto knife, careful :wink:

I often file the very end of the tube pin so the end of the pin becomes more open and removes most of the tin. This leaves a large opening for scraping the interior and the wire and clear evidence that the pin has been re-soldered

Use liquid rosin flux sparingly if you have it especially the "active" type, remove excess flux.

Use a heat sink on the pins, Bakelite will blister if overheated and the pin will become loose in the base. Simple sink is rubber bands around the tips of long nose pliers to hold them closed around the pin. Use a large hemostat or a "regulation" heat sink if you have them.

Tube tester internal line voltage adjustments are not accurate. Tubes of different filament loads will not yield the same corrected filament voltage as shown on the tube tester meter. The worse errors are tubes with high filament current.

Construct an adapter that can plug into the tube tester, use heavy wire for the filaments. Such adapter will allow the connection of the analog voltmeter with a low voltage AC scale.

Factoid #3 26 tubes especially the "S" bulb, tend go sleepy and show a gas content.

Use the filament voltage adapter and measure the filament voltage from the tester under the load of the 26 re-adjust the filament control to give 1.4 volts (ac)...

Burn the 26 for at least half an hour. No NEVER, raise filament voltage, test at elevated filament voltage or make any attempt to rejuvenate. What happens is residual gas will get re-absorbed by some of the getter. even though the getter is not being flashed at high temperature. Test the 26 normally after the 1/2 hour cook. Most will recover emission. Those that don't are bad.

Why the blather on the tubes?

There are no shunting load resistors in the chassis, without tubes present the SPU will have high unrealistic voltages, period!!!

See the table of current vs voltage on the Riders schema 8)

If you want to run the SPU disconnected from the radio for B+ measurement, the find resistors to take the load, using Ohms' Law...

A 10ma load from terms 8 to 9. A 18.5ma from terms 7 to 9. Since the voltage is given, the wattage can be determined. This would be the load of the radio with GOOD tubes.

Therefore, if the voltage is higher at the SPU terminals, it may be because of poor emission of the tubes, OR issues in grid circuits, OR poor connections in the radio, OR SPU, OR filament resistors.

There are two .5mf bypass caps on the B+, if they are leaking badly they will draw down the B supply.

Obviously, there could be unforeseen mysteries, like leakage of the chokes to the core, but that would reduce the voltage. A dramatic change in value of the filament resistors or the series resistor for the radio. But, you have found those values.

IMHO reading of no other feedback in your posts. I would look at poor connections, Bad lugs on wires, tube issues, possibly a bad audio transformer.

Play around with those concepts and see what smokes.

As always, YMMV

Chas


Top
 Profile  
 
 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Mar Sun 04, 2018 5:13 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2862
Location: Nr London, England, SS1 3PT
Well, couldn't resist a look at my Notebook from 2010!

Here is a pic of the page: maybe readable?

Attachment:
Notebook.jpg
Notebook.jpg [ 127.23 KiB | Viewed 333 times ]


Yep! reads OK. I wont dive in on the technical discussion of your problems as there is enough experts doing that and I'm busy at the Mo!

See how the power supply (psu) was tested in isolation with dummy loads as others have told you that you need. "o/c" is Brit speak for open circuit.

Gary

Another bit of info picked up off of the Forum before I started on mine:

Attachment:
Radiola PSU caps etc.jpg
Radiola PSU caps etc.jpg [ 19.85 KiB | Viewed 332 times ]


Top
 Profile  
 
 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Mar Sun 04, 2018 11:05 pm 
Moderator
User avatar

Joined: Feb Sun 01, 2009 2:56 pm
Posts: 9712
Location: Victoria, Australia
I am still, at this point, not convinced that the capacitors & tubes are the problem. As long as a few of the tubes work they will drag down the B+ (Even at 40%): Not happening.

I am still of the opinion (as are others) that the rectifier needs removing, whilst a total overhaul & assessment is made of the three filament windings. As before I have seen this sort of thing in a Lyric 70 series.

Marc


Top
 Profile  
 
 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Mar Mon 05, 2018 4:40 am 
Member

Joined: Sep Mon 28, 2009 1:53 am
Posts: 192
Location: Bloomington, IN
I checked the voltages with all tubes in, PSU set to 120V.

1.5VAC - reads 11VAC
2.25VAC - reads 313VAC (not a typo!)
5VAC - reads 10VAC

145VDC - reads 10VDC
165VDC - reads 0

The tubes don't connect to the sockets via tabs on the bottom (like many tabs, the tube held in place by pressure and the pin on the tube base). The tubes slip between 2 contact plates, so inserting and removing tends to scrap the pins clean naturally.

Given the really odd voltages, I'm not going to mess with it. Beyond my skills at this point, and lifting the 60lb+ cabinet is absolute murder on my back! Besides, if a winding is failing, or the choke is bad, there's little I can do with it. That $50 supply on eBay is probably the best option for the next owner to pursue.


Top
 Profile  
 
 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Mar Mon 05, 2018 12:05 pm 
Moderator
User avatar

Joined: Feb Sun 01, 2009 2:56 pm
Posts: 9712
Location: Victoria, Australia
My idea was not to mess with the chassis: My idea was to mess with the power supply Low voltage, initially without any DC. One fixes the PSU first and I am far from convinced that it is wired correctly or faultless. Once it has been established that it is correct, we then move to the chassis to find if there are miswires & problems?

I would however, now be suspicious of C1 & be interested to see what the red & black wires hook to. I think a different approach is required than what you are using. At the moment L1, L2, & L3 are not in my sights. However, had I have disconnected the windings to check resistances and had actually isolated the LV windings, they would have been checked for earth leakage to the transformer frame & between all other windings. That would also apply to L1, L2, & L3 if I was suspicious of them. Earth leakage is done here @ 500VDC with an insulation tester. One of the testers here has a range of voltages.

The insulation tester can also test the HV Non polarised caps for leakage. Caps should not pass DC and therefore, have resistance. There is only one policy here with them: If it leaks its a dud. The Only way to test High Voltage caps is with a voltage at their rating, or at the voltage that will be applied to them.

I suggest a stop, a rethink & appraisal of the methodology, rather than quitting as a better option. You learn from swines of sets like this, not the ones where you replace the known duds, & it works.

Marc


Top
 Profile  
 
 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Mar Mon 05, 2018 6:27 pm 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 36880
Location: Livermore, CA
Filament voltages seem like they are being measured to chassis? Measurements seem to include bias voltages? For 26 tubes filaments read between two large pins. You should find 1.5 volts AC there. Same with the 71A, read between large pins. There should be 5 volts AC.

DC voltages may be missing but that's another issue after filaments check out.

_________________
Norm


Top
 Profile  
 
 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Mar Mon 05, 2018 11:01 pm 
Moderator
User avatar

Joined: Feb Sun 01, 2009 2:56 pm
Posts: 9712
Location: Victoria, Australia
I have already asked for voltages measured across the pins LV pins, not to chassis. There will be a voltage between windings to ground on any transformer winding, even that it is not physically connected to it.

Transformers with a shield will always bleed a charge to the chassis, that is why I connect that sort of transformer to a cable with a utility ground.

As said I think, it needs to stop, for a review of the whole methodology here.

It would be an idea to have a link to the radio part, not just the PSU, to see how it actually connects.

Marc


Top
 Profile  
 
 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Mar Tue 06, 2018 9:29 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2862
Location: Nr London, England, SS1 3PT
I think the best thing is to quit Marc. The OP obviously wants to leave it now and sell it on.

Flogging a dead horse :D

Gary


Top
 Profile  
 
 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Mar Tue 06, 2018 12:26 pm 
Moderator
User avatar

Joined: Feb Sun 01, 2009 2:56 pm
Posts: 9712
Location: Victoria, Australia
Radio Fixer wrote:
I think the best thing is to quit Marc. The OP obviously wants to leave it now and sell it on.

Flogging a dead horse :D

Gary


I can see where this is going, but just have the uneasy feeling that it just might be something simple. I have had the odd one hand balled onto me only to find that it was, just a joint that someone did not solder at all in the factory, or it was a dry joint.

It is very hard to find ones own mistakes & I often advise the walking away from it, for a while, top overhaul a diesel tractor engine, or something not radio, to get the mind out of its rut, & reappraise the thing a few days later. Often the mistake is then obvious. Just refitting the tractor cylinder head & everything around it is the best part of a day, on a medium sized tractor, that's an ideal sort of thing to get the mind off radio.

Marc


Top
 Profile  
 
 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Mar Mon 12, 2018 1:30 am 
Member

Joined: Sep Mon 28, 2009 1:53 am
Posts: 192
Location: Bloomington, IN
Would 10H chokes work for the filter & output chokes? I'm tempting to gut the 2 cans and install new components. Hammond has a model that appears would fit inside the cans.

Wirewound resistors seem good. I did take readings with the leads unsoldered.

If anyone has original values for those chokes, it would be appreciated!


Top
 Profile  
 
 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Mar Mon 12, 2018 4:08 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3195
Location: Waukegan, IL
voltages measured with a digital meter vs an analog meter will vary. since they used an analog meter then, id use one anyway. the voltages can be off by a little, but if more than 10-20 volts then id say something is wrong

_________________
Tony Pignaloni


Top
 Profile  
 
 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Mar Mon 12, 2018 5:42 am 
Member

Joined: Sep Mon 28, 2009 1:53 am
Posts: 192
Location: Bloomington, IN
trainman wrote:
voltages measured with a digital meter vs an analog meter will vary. since they used an analog meter then, id use one anyway. the voltages can be off by a little, but if more than 10-20 volts then id say something is wrong


Analog meter was used.


Top
 Profile  
 
 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Mar Mon 12, 2018 6:30 am 
Moderator
User avatar

Joined: Feb Sun 01, 2009 2:56 pm
Posts: 9712
Location: Victoria, Australia
An open choke should have been detected long before this. It is consistent with double B+ one side & nothing the other, so if the plates relative to chassis have double voltage the choke is not open. Besides, choke continuity can be checked with an ohmmeter.

Marc


Top
 Profile  
 
 Post subject: Re: Repairs on a Radiola 18 - Questions of Schematic Values
PostPosted: Mar Mon 12, 2018 10:11 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2862
Location: Nr London, England, SS1 3PT
I cant see why you are still bothering with it and messing it up? Chokes are probably fine (values in my notes) and removing them means the bent over tabs will break off anyway on reinstatement.

You are going to sell it on, so you say, and it wont be worth anymore working, in fact less to most of us.

So best thing is to remove all the components you have soldered on and put it back as close as you can to how it was. Then sell it to someone who knows how to sort it.

Gary


Top
 Profile  
 
Post New Topic Post Reply  [ 34 posts ]  Moderators: Marcc, Norm Leal Go to page Previous  1, 2

All times are UTC [ DST ]


Who is online

Users browsing this forum: addyboy, Google Adsense [Bot], Lou deGonzague, mjeansonne and 12 guests



Search for:
Jump to:  
























Privacy Policy :: Powered by phpBB