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 Post subject: Royal 500 Fades - (Not Electrolytics)
PostPosted: Jan Fri 17, 2020 2:49 am 
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Joined: Jan Thu 09, 2020 6:46 am
Posts: 29
After my wife gave me a Trans-Oceanic D7000Y for Christmas (which I've physically cleaned up and will recap), I thought I would also restore a Royal 500 that my godmother gave to me years ago. It worked, but faded after a few minutes. I bought a "re-cap kit" from a guy on eBay (I wanted his notes, etc., I know it was only a few dollars of capacitors, support the hobby), and recapped it. The radio sounded better and a loud volume problem cleared up, but the radio would still fade after a few minutes of operation. I didn't have the "it worked until I recapped it" problem, so I can't be a COMPLETE idiot :)

I was thinking that it was a weak/failing transistor, so I bought some on eBay BEFORE learning that my particular radio did not have the usual transistors:

Royal 500 (b?)- 7ZT40Z1
Osc = 121-15 or 16, illegible
Mixer 121-13
IF's = 121-17
Driver=121-18
Outputs = 121-19

I bought replacement transistors for the outputs (unfortunately AFTER I found about the transistor differences). They are "equivalent" to the NTE102, not the 102A, so they have less power capacity. Nonetheless, I substituted the -18 and -19's and still have the same problem. So, it's probably not the amplifier.*

What should my next course of action be? TIA

* I was wondering about the .1uf capacitor across the outputs. What is its purpose. Can it go bad? I could remove it and measure it but it's pretty delicate in there and i don't want to just start desoldering parts. It is yellow and has a stripe on it as though it is polarized but the schematics don't seem to indicate that this is true.


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 Post subject: Re: Royal 500 Fades - (Not Electrolytics)
PostPosted: Jan Fri 17, 2020 5:12 am 
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Joined: Oct Mon 02, 2017 8:53 am
Posts: 408
Location: Panama City, FL 32401
I just looked at a 500B that has the same chassis. 7ZT40Z1
Osc=121-15
Mixer=121-16
1st,2nd IF's=121-17
Driver=121-18
Outputs=121-19

I've had that happen too after a radio has been playing to slowly fade. Usually is a weak transistor.


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 Post subject: Re: Royal 500 Fades - (Not Electrolytics)
PostPosted: Jan Fri 17, 2020 5:20 am 
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Joined: Jan Tue 31, 2012 1:55 am
Posts: 12054
Location: Mountains of Mourne. Ireland.
You have tried using fresh batteries?

The guy on eBay - - was his name Fast Eddie. lol
Greg.

Edit: some bedtime reading...
https://www.americanradiohistory.com/Ar ... Lemons.pdf


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 Post subject: Re: Royal 500 Fades - (Not Electrolytics)
PostPosted: Jan Fri 17, 2020 5:45 am 
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Joined: Oct Mon 02, 2017 8:53 am
Posts: 408
Location: Panama City, FL 32401
Attachment:
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I don't know what that .1uf cap does,but they don't seem to be an issue, so I leave them as is. They're in many of the Zeniths I have. Being a .1uf, it wouldn't be an electrolytic. At least I've never seen one below 1uf.


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 Post subject: Re: Royal 500 Fades - (Not Electrolytics)
PostPosted: Jan Fri 17, 2020 8:31 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 34506
Location: SoCal, 91387
Zenith 500 front end Transistors;

Converter, 2N94 (NTE 103A), GE 2N169 (NTE 101)

IF's, 2N94 (NTE 103A), GE 2N169A (NTE 101)

_________________
\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\He Who Dies With The Most Radios Wins/////////////////////////


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 Post subject: Re: Royal 500 Fades - (Not Electrolytics)
PostPosted: Jan Sun 19, 2020 12:28 am 
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Joined: Aug Wed 24, 2011 4:35 am
Posts: 4797
Location: Sunnyvale CA
OldHack wrote:
After my wife gave me a Trans-Oceanic D7000Y for Christmas


You married well.

Quote:
I bought replacement transistors for the outputs (unfortunately AFTER I found about the transistor differences). They are "equivalent" to the NTE102, not the 102A, so they have less power capacity. Nonetheless, I substituted the -18 and -19's and still have the same problem. So, it's probably not the amplifier.*

What should my next course of action be? TIA

* I was wondering about the .1uf capacitor across the outputs. What is its purpose. Can it go bad? I could remove it and measure it but it's pretty delicate in there and i don't want to just start desoldering parts. It is yellow and has a stripe on it as though it is polarized but the schematics don't seem to indicate that this is true.


Do any of the transistors get hot to the touch after a while? Fading suggests thermal runaway. Perhaps a failed trace going to or from the 4.7 ohm transistor between the two output transistors.

The usual caveats apply. The transistors are generally still good, if not shorted outright. But cracked traces, failing earphone jack, etc, are always the first guess after it has been re-capped. I would suspect a backwards capacitor, but it would have had to be backwards before...


Brett


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 Post subject: Re: Royal 500 Fades - (Not Electrolytics)
PostPosted: Jan Sun 19, 2020 2:18 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 312
I restored a Royal 500 a while ago. I documented my experiences with it here:

http://worldphaco.com/uploads/THE_ZENIT ... SISTOR.pdf

More than one transistor was defective and an electrolytic. In many (not all) transistor radios (but this generally goes unnoticed by most) there is a "classic mistake" and I have a number of radios with this problem. The filter electrolytic cap on the AGC line spends most of its life in a reversed biased condition. This is why it is usually the first electrolytic in the vintage transistor radio to go. The AGC capacitor should really be a bipolar type. It is explained on page 3 of the pdf, but it may not relate to your problem at all.

The transistors in these sets have very high base-collector feedback capacitances and the IF stages have heavy neutralization to compensate for it and stabilize the IF's. It is quite hard to find a replacement for the IF stages that is stable without having to adjust the neutralization components, so its better to find a very similar transistor. I found 2N94's were satisfactory without have to make modifications. There is some info in this in the article.

As a general fault finding method, it is worthwhile checking the DC conditions of the transistors first by checking their emitter voltages. Ideally though the radio is presented with a test modulated RF signal and monitored at the detector with a scope or milli-volt meter and the audio stages checked with a signal generator (1kHz sine wave) for normal output and symmetry. Its not uncommon for one or other of the output transistor pair to become unwell, lowering the output and increasing the distortion. If one has thermal runaway it tends to take the other out of conduction as they share the same emitter resistor. So look for the voltage across that resistor increasing with time after turn on. Also check the emitter voltage of the driver transistor, that it is normal & stable.

The 0.1uF poly cap across the output transformer is to improve the tone of the radio, highly unlikeley to be defective, as I recall it had quite a high voltage rating for the application.


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 Post subject: Re: Royal 500 Fades - (Not Electrolytics)
PostPosted: Jan Sun 19, 2020 3:31 pm 
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Joined: Jan Thu 09, 2020 6:46 am
Posts: 29
Thank you everyone for your suggestions. I am thinking it is not an audio output problem because I can still hear static and crackling from the speaker after the fade; I've substituted the driver and output transistors also. They don't seem to get hot. So, it's probably one of the RF transistors. I have NTE/ECG101 replacements coming. Hopefully, they will be close enough to work. If not, I have some pretty good test equipment (digital and analog oscilloscopes, waveform generatiors, etc.) that will help me. I would have actually -used- them if the easy temptation of just swapping socketed transistors would not have been so strong. I'm glad I got the other transistors because I suspect that this will not be the only germanium-based transistor radio I will be working on in the future.

I did check the batteries, and I'm very certain that I got the polarity of the capacitors right.


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 Post subject: Re: Royal 500 Fades - (Not Electrolytics)
PostPosted: Jan Sun 19, 2020 7:57 pm 
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Joined: Aug Wed 24, 2011 4:35 am
Posts: 4797
Location: Sunnyvale CA
OldHack wrote:
Thank you everyone for your suggestions. I am thinking it is not an audio output problem because I can still hear static and crackling from the speaker after the fade; I've substituted the driver and output transistors also. They don't seem to get hot. So, it's probably one of the RF transistors. I have NTE/ECG101 replacements coming. Hopefully, they will be close enough to work. If not, I have some pretty good test equipment (digital and analog oscilloscopes, waveform generatiors, etc.) that will help me. I would have actually -used- them if the easy temptation of just swapping socketed transistors would not have been so strong. I'm glad I got the other transistors because I suspect that this will not be the only germanium-based transistor radio I will be working on in the future.

I did check the batteries, and I'm very certain that I got the polarity of the capacitors right.


A signal tracer would likely help here.

With the radio on and malfunctioning, touch your soldering iron to the wiper terminal on the volume control pot. That should give you a loud hum. If so, the audio section of the radio is working.

If that works, do the second radio LO test.


Brett


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 Post subject: Re: Royal 500 Fades - (Not Electrolytics)
PostPosted: Jan Wed 22, 2020 3:41 am 
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Joined: Jan Thu 09, 2020 6:46 am
Posts: 29
Thanks again, everyone. It was the 121-16 transistor, which I replaced with an ECG101. It was literally like putting a round peg in a rectangular hole, but there was enough room; the radio plays great in spite of having a slightly separating speaker, which I am at this time afraid to try to repair anyway.

This radio is kind of an heirloom for me; my godmother was a fairly well off single "geek" of her time, working at Western Electric's Cicero IL plant in electronic/mechanical repair. She bought this as a present for herself! At the height of her career, she was the plant expert on Model 28 Teletypes. My grandmother, her aunt, actually WORKED at Zenith in Chicago during the war (THAT war) wiring up radar sets, the only years that she ever worked outside her home - possibly at the plant in which this radio was built. 35 or 40 years later, she still knew the resistor codes on my projects and would call them off to me.

Anyway...

To confirm that the transistor was bad, I put in in this new toy that grew from an open-source project and some Chinese company is producing it on eB** for $17 shipped. At first, it showed a paltry hFE of 20; the next time I tested it showed two diodes (which I assume means zero gain). So, that matched the "fading" symptoms.

This thing tests transistors, resistors, capacitors and inductors (sort of, within reason), diodes, and Zeners (within reason). This is NOT a professional tool with extreme accuracy or durability, but for $17 chargeable with a mini-USB adapter, not bad and the results were comparable to my better test equipment. The tool in operation on a bad (no gain) 121-16 and good ECG101 are shown. So, very useful for quickie tests, sanity checking, that sort of thing.


Attachments:
File comment: Bad Zenith 121-16 in M-Tester
t1.jpg
t1.jpg [ 723.39 KiB | Viewed 570 times ]
File comment: Good ECG101 in M-Tester
t2.jpg
t2.jpg [ 692.7 KiB | Viewed 570 times ]
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 Post subject: Re: Royal 500 Fades - (Not Electrolytics)
PostPosted: Jan Wed 22, 2020 2:15 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 312
..........hmmm...oops...maybe;


Unfortunately a lot of modern component testers are unable to successfully test early Germanium transistors.

The Vbe and Vbc voltage drops are very low compared to silicon transistors (confusing the meter) and the Vcb leakage is well over an order of magnitude higher than for a Silicon type.

Modern transistor testers were designed to test Silicon transistors. So unless the tester design explicitly says they are ok for germanium transistors, the readouts you will get will have little if any meaning. Maybe your meter is ok, but check that from its specs.

To test the Germanium types properly you need to be aware of the expected leakage properties and the frequency range over which the transistor is expected to operate.If the device passes initial simple tests on a meter, if there is any doubt about it, set it up in a test jig and measure its current gain and cut off frequency.

The many "open source" cheap transistor testers now available are actually inappropriate to the task of testing vintage Germanium transistors and the designers of them, often, were not aware of the vagaries of germanium types.


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 Post subject: Re: Royal 500 Fades - (Not Electrolytics)
PostPosted: Jan Wed 22, 2020 4:37 pm 
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Joined: Jan Tue 31, 2012 1:55 am
Posts: 12054
Location: Mountains of Mourne. Ireland.
I didn't read all of this...

Joe wrote: Jon Cusack — the pedal builder, not the actor recommended the TC1 ($41.50), which apparently is only available via eBay in the US. And it’s more useful than I could have imagined.

First off, it gives accurate hFE readings within a hundredth of an hFE unit. It works with silicon and germanium BJTs, FETS, JFETS, and MOSFETS. And dig this: It doesn’t matter which way you orient the pins — it knows which leg is which, so no more  jumping online to verify the pinout of a particular part.
https://tonefiend.com/diy/indispensable-diy-tool/

Greg.


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 Post subject: Re: Royal 500 Fades - (Not Electrolytics)
PostPosted: Jan Wed 22, 2020 10:13 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 312
egg wrote:
I didn't read all of this...

Joe wrote: Jon Cusack — the pedal builder, not the actor recommended the TC1 ($41.50), which apparently is only available via eBay in the US. And it’s more useful than I could have imagined.

First off, it gives accurate hFE readings within a hundredth of an hFE unit. It works with silicon and germanium BJTs, FETS, JFETS, and MOSFETS. And dig this: It doesn’t matter which way you orient the pins — it knows which leg is which, so no more  jumping online to verify the pinout of a particular part.
https://tonefiend.com/diy/indispensable-diy-tool/

Greg.


That looks very good because they explicitly say Germanium types are tested. Not all are like this. It might still pay to verify that by testing a few other known good germanium types. With the very low gain of some germanium types, a tester might for example report the part as a double diode.

Also, there is one other thing to look out for with some transistor testers. They exceed the base-emitter max rated current for Germanium RF transistors (such as the AF11X and AF12x series), which is a very low value compared to a silicon signal transistor, as of course do some Ohm meters depending on the design and the range setting. Historically it was recommended not to test these transistors on standard Ohm meters too. In the picture you posted, of the tester testing the Germanium transistor, the junction fwd voltage drops are very high for a germanium, at low currents the junctions are rated for it should be in the order of 250 to 300mV. It suggests that the applied currents are far too high for a Germanium signal transistor junction, on the other hand, if the transistor is defective that remark might mean nothing. But test a known good germanium and check that the tester identifies it as a germanium and that the junction forward voltage drops, say if you just check the B-E junction as a diode is correct.It looks correct in the 2nd photo checking the T0-5 case transistor.


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