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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Thu 06, 2020 2:30 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 422
Location: moreno valley
As per the drain, with my meter between the battery supply it reads around 10 - 12 mA. Cleaned the traces with brush and alcohol, no noticeable change. Will try the linking the two radios together thru the vol control as suggested, might show something. Need to get some of those little spring loaded probes so I don't have to take the H1 apart again just for this, I don't want to risk shorting anything in that tight vol/power switch area. Should have some of those anyway.


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Thu 06, 2020 3:31 am 
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Joined: Dec Wed 25, 2013 7:57 am
Posts: 4187
Location: USA
Backing up a bit...

In these transistor portable radios, most frequent issues is corrosion on battery contacts; almost as often are these thin wires barely hanging on with all but one or two strands broken. So it's worth the time to hunt down all these. Definitely check the pair that goes to the ferrite rod antenna.

Since the radio is receiving, I wouldn't spend too much time randomly guessing what component might be bad. Rule out all simple causes first.

No way to know if alignment is off just by looking. If radio alignment is new to you, I wouldn't touch any of the screws for now.

Once I couldn't figure out why a radio wouldn't play, a new set of batteries cured it. I thought the ones I was using were good :)


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Thu 06, 2020 3:54 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 422
Location: moreno valley
I already dumbly pulled one antenna wire off with the toothbrush cleaning and my fuzzy eyes, had to resolder it on. And yes no way would I mess with any alignment until everything else is positively ruled in or out, and even then I don't have the equipment for that. Not yet anyway... :)
The few really super strong stations I get sound pretty good, if you didn't know vol was maxed you might think, hey pretty good. But all the weaker stations, forget it, useless, can barely hear them.


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Thu 06, 2020 5:27 am 
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Posts: 34465
Location: SoCal, 91387
frenchmarky wrote:
Here's the best shot I could come up with, I thought it sure looked like ' - 37 V ' And the collector looks like ' - 44 V ' and the base maybe -5 or -5.1 volts (can't quite make that one out.) All emitters on the schems are negative voltages, most of them are less than -1 volts and are marked that way. Does - 37 make sense given it's hooked to a 45v cap on that end? I am measuring very close to what is on the schems on all the other emitters.

There is no way that a six volt battery supply will yield those types of voltages. 3.7 and 4.4 make more sense. AFA the capacitor working voltage rating, it really has no bearing in a Transistor set. The factory most likely simply used what was available at a good lot price.

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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Thu 06, 2020 6:11 am 
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Joined: Aug Wed 24, 2011 4:35 am
Posts: 4795
Location: Sunnyvale CA
frenchmarky wrote:
Here's the best shot I could come up with, I thought it sure looked like ' - 37 V ' And the collector looks like ' - 44 V ' and the base maybe -5 or -5.1 volts (can't quite make that one out.) All emitters on the schems are negative voltages, most of them are less than -1 volts and are marked that way. Does - 37 make sense given it's hooked to a 45v cap on that end? I am measuring very close to what is on the schems on all the other emitters.


There is no voltage that exceeds 6V anywhere in the radio, since that is the supply voltage.

On my properly-working 500H with a battery voltage of 5.11V:

2nd IF:
C = -4.4v
B= -0.63v
E= -0.40v

Driver:

C= -3.9v
B= -0.44v
E= -0.31v

Output (both are the same):

C= -5.11v (Vcc)
B= -0.146v
E= -0.02v

Which seems to correspond very well to the schematic I have, scaled to 5.11 volts instead of 6V.

I just checked the current draw on a properly-operating 500H, and with the volume control set to get an uncomfortably loud output, the current draw was 20 mA. With the volume all the way down, silent, the draw was 6.2 mA. This with a battery voltage of 5.11 volts. A normal running current at my normal listening volume is about 10 mA.

You problem is likely to be either a cracked trace, or a low-performing or failed 2nd IF or driver. Since I have many appropriate transistors at hand, I would quickly sub for those from known-good radios, or, failing that, an NTE102A (or even an NTE160 as long as you don't leave it too long). But the change in performance as the board is flexed is absolutely classic sign of a failed trace, which is far and away the most common problem for 500Hs. Other possible failures are poorly-seated transistors, or, unfortunately, an intermittent interstage/phase setting transformer, which has been known to be a weak point in the radio.

Brett


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Thu 06, 2020 7:07 am 
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Location: Lexington, KY
There is a little better schematic here in Beitman's 1962 p.155:

https://www.americanradiohistory.com/Ar ... n-1962.pdf

But if you want to make your life even easier order the SAMS download for $6.99. It has 5 pages of data and a crystal clear schematic, it was the best $6.99 I spent when working on my 500H. It is in set 592 folder 13 for chassis 8HT40Z2:

https://theschematicman.com/instantdown ... rt=nameAsc

I'd offer to email you a copy but the collector that runs the site, Steve Johnson, is a member here and he works hard to provide a quality product.

_________________
John KK4ZLF


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Thu 06, 2020 9:02 am 
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Posts: 34465
Location: SoCal, 91387
Brett_Buck wrote:

On my properly-working 500H with a battery voltage of 5.11V:

2nd IF:
C = -4.4v
B= -0.63v
E= -0.40v

Driver:

C= -3.9v
B= -0.44v
E= -0.31v

Output (both are the same):

C= -5.11v (Vcc)
B= -0.146v
E= -0.02v

frenchmarky, notice on a properly operating Transistor circuit, the Base/Emitter difference is one or two tenths of a volt, while the Collector approaches the supply voltage.

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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Thu 06, 2020 1:37 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 292
egg wrote:

Greg.
oh... and as Rich says... it's best to use a fresh set of alkaline batteries when servicing a transistor radio.



Really ? I totally disagree.

Firstly Alkaline cells are made for high current devices, those with high power output, apparatus with DC motors etc.

Alkaline batteries are unsuitable for low current drain devices like transistor radios. There are multiple reasons, I'll go through it:

The main risk, especially while working (servicing) a vintage transistor radio, is an inadvertent short between some terminals. The problem is with alkaline cells is they have an extremely low internal resistance. The short circuit currents are enormous, they can open circuit coils like IF transformers and even fuse fine pcb tracks, heat up wires and melt insulation & destroy transistors with a slip up.

Zinc carbon cells have a much higher internal resistance and cause less destruction with errors, as the short circuit currents are lower, due to their higher internal resistance.

In addition, consider what is inside the Zinc carbon cell vs the alkaline cell (remember, batteries sooner or later always leak).

Inside the alkaline cell, the chemistry is extremely corrosive with a very high pH. It will destroy rapidly most things it comes in contact with. On the other hand zinc carbon cells (which are a dry version of the classic LeClanche cell), have a relatively benign ammonium chloride based electrolyte with a closer to neutral pH. Sure when they leak, they can cause corrosion, but it is nowhere near as destructive as the fluid leaked from an alkaline cell and is easier to clean up.

For this reason I don't let alkaline cells anywhere near my precious vintage transistor radios, especially not during servicing.

There is no argument that an alkaline cell has a higher higher ampere-hour capacity than a zinc carbon cell of the same size, and will "last longer" , like it shows on the Duracell TV adverts, but I am afraid for a vintage transistor radio application, that is not the point at all.

Also nickel metal hydride or nickel cadmium rechargeable batteries represent the same potential hazards for vintage transistor radios as alkaline cells for the same reasons.

Obviously if you have a working transistor radio (not undergoing servicing) there is little risk in putting alkaline cells in it, that is if you don't leave them in the radio too long and forget about them.


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Thu 06, 2020 5:25 pm 
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Joined: Aug Wed 24, 2011 4:35 am
Posts: 4795
Location: Sunnyvale CA
ACORNVALVE wrote:
egg wrote:

Greg.
oh... and as Rich says... it's best to use a fresh set of alkaline batteries when servicing a transistor radio.



Really ? I totally disagree.

Firstly Alkaline cells are made for high current devices, those with high power output, apparatus with DC motors etc.

Alkaline batteries are unsuitable for low current drain devices like transistor radios. There are multiple reasons, I'll go through it:



I believe you overthinking the situation. Any battery you put in there and gives the right voltage will work, the issues with the dangers of shorts is overblown. If something shorts, even Zinc-carbon will throw enough current to burn up the #55 wires in a interstage transformer, pretty quick.

Another point is that with alkaline batteries, they will leak, but cause only a mess that is relatively easy to clean up. Zinc-carbon have an acid electrolyte, and cause the classic leaky-battery damage and severe corrosion when they leak.

Standard Ni-MH cells generally have too much self-discharge to make sense in a 10 mA radio, but low-self discharge types are fine, and leak only in extremely rare, almost unheard-of rates. That makes them a very good choice - they will run down faster than ideal, because the while self-discharge is still significant, at least you can recharge them.

From a practical standpoint, you can get alkaline batteries nearly anywhere, and you have to dig through bins at Dollar General or similar to even find zinc-carbon, they are very uncommon. Yes, you can get them, but they aren't at any retail outlet. And, even not being optimized for low-current operation, they will still run the average transistor radio 50% to twice as long.

Just use whatever batteries will go in the holder, it's not a significant issue no matter how you look at it.

I use low-self-discharge Ni-MH batteries in all my transistor radios, primarily because they are not at all prone to leaking the electrolyte. They last plenty long enough for any practical purpose, even just sitting. In my highest-use application use (which is about 8-12 hours a day, every day), they require a recharge about once a month. That's a lot of radio usage. For emergency use (like in the emergency kit), I leave them in the radio, charge them about once every 6 months, and keep a pack of alkaline batteries separate in the original package, so if they leak, they won't be in the radio. If an extended power outage occurs, I would run the rechargable batteries until they were dead (again, on the order of 3-4-500 hours) then use the brand new alkalines, which would go for about 2-6 months or more with careful use.

Brett


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Thu 06, 2020 5:37 pm 
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Joined: Aug Wed 24, 2011 4:35 am
Posts: 4795
Location: Sunnyvale CA
fifties wrote:
Brett_Buck wrote:

On my properly-working 500H with a battery voltage of 5.11V:

2nd IF:
C = -4.4v
B= -0.63v
E= -0.40v



frenchmarky, notice on a properly operating Transistor circuit, the Base/Emitter difference is one or two tenths of a volt, while the Collector approaches the supply voltage.


Exactly, you can use that the same principle to just eyeball whether just about any transistor is working, without a schematic or any references to specific voltages. It's why a transistor can amplify a signal - small currents from base/emitter control are replicated in larger currents from collector to emitter.

In any case, without some sort of boost convertor, you can't have any DC voltage in any device that exceeds the supply voltage. You *might* see spikes or transient noise above the supply voltage, if you were to put it on a scope, to surprising voltages, but those are only possible in transient conditions.

Brett


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Thu 06, 2020 10:23 pm 
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Location: SoCal, 91387
ACORNVALVE wrote:

Alkaline batteries are unsuitable for low current drain devices like transistor radios.

For this reason I don't let alkaline cells anywhere near my precious vintage transistor radios, especially not during servicing.

Obviously if you have a working transistor radio (not undergoing servicing) there is little risk in putting alkaline cells in it, that is if you don't leave them in the radio too long and forget about them.

Are you serious? Oh please. As long as the power source approaches what the radio needs to operate, it doesn't matter what kinda battery is used, or left in the set. Of course, I've only serviced a few hundred Transistor sets, so maybe you know better...

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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Fri 07, 2020 12:49 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 292
Brett_Buck wrote:

Another point is that with alkaline batteries, they will leak, but cause only a mess that is relatively easy to clean up. Zinc-carbon have an acid electrolyte, and cause the classic leaky-battery damage and severe corrosion when they leak.



From a practical standpoint, you can get alkaline batteries nearly anywhere, and you have to dig through bins at Dollar General or similar to even find zinc-carbon, they are very uncommon. Yes, you can get them, but they aren't at any retail outlet. And, even not being optimized for low-current operation, they will still run the average transistor radio 50% to twice as long.

Brett


Well I guess we will have to disagree on the corrosive properties of the contents of Zinc carbon vs alkaline cells. I have always found the destructive effects of leaked alkaline batteries much worse than zinc carbon cells.

Interesting though on the second point, Zinc carbon cells of all sizes AA, C, D, 9V types are very easy to get here in AU, I had no idea they were so difficult to get elsewhere. Also they have less of a propensity to actually leak than many of the alkaline types made in the far East that have flooded the market.

As an experiment, get a fresh zinc-carbon and fresh AA Alkaline cell, put the meter on the 10A DC current range and connect it across the battery terminals, you may be surprised at the difference in the short circuit currents.

One other thing which is interesting, I'm not sure if other have noticed this, once an alkaline cell is flat, it leaks fairly promptly after that and the leaked alkaline material has a penchant for travelling down the insides of wires under the insulation (say those connected to battery clips) along the wire strands, corroding the wire and finds its way all the way to pcb's or where the wires lead. I have seen a few meters destroyed this way.


Last edited by ACORNVALVE on Feb Fri 07, 2020 2:19 am, edited 1 time in total.

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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Fri 07, 2020 12:51 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 292
fifties wrote:
ACORNVALVE wrote:

Alkaline batteries are unsuitable for low current drain devices like transistor radios.

For this reason I don't let alkaline cells anywhere near my precious vintage transistor radios, especially not during servicing.

Obviously if you have a working transistor radio (not undergoing servicing) there is little risk in putting alkaline cells in it, that is if you don't leave them in the radio too long and forget about them.

Are you serious? Oh please. As long as the power source approaches what the radio needs to operate, it doesn't matter what kinda battery is used, or left in the set. Of course, I've only serviced a few hundred Transistor sets, so maybe you know better...


Yes I am serious, its only possibly any issue while servicing the set. I agree any batteries you use later its no issue. Perform the short circuit test with the meter I suggested and you will see what I mean.

On the remark "I've only serviced a few hundred Transistor sets, so maybe you know better..." :

I'm not necessarily saying that I "know better" about anything than anyone else, it is merely an observation that I made comparing the two types of cells and the hazards during servicing. It is not really about the number of sets worked on, it could be 100, it could be 1000. It is the power of observation that is important. For example, in your work on a "few hundred transistor sets", had you ever observed that the AGC bypass electrolytic, in many transistor radios spends most of its life in a reversed polarized condition while tuned into a station, and it only has the correct polarity applied off station ? Thus shortening the life of and degrading that part. For this reason in many of my transistor sets I fit a bipolar electrolytic in that location. So before slamming someone else's powers of observation, you need to have a look at your own and consider that an opinion about some issue might have at least some merit.


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Fri 07, 2020 3:47 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 292
Back to fault finding the radio...one thing that could possibly cause the fault would be a very leaky or nearly open detector diode. Because there could be some, but low level rectification of the carrier. So you could still hear the stations but they would be quiet.

There are a couple of checks you could do here. One is to see if the AGC voltage changes significantly with tuning into a strong local station, it only would do that if the detector diode was ok.

As an experiment, try placing another diode in parallel with the existing detector diode & see if anything changes, something like a 1N60 diode or a small schottky signal diode is fine.

Another useful piece of "test equipment" if you could call it that, is a crystal earpiece. Which you can connect across the output of a detector, or volume control too, and listen. In a normally working transistor radio, the audio signal level here(depending on the radio) can be anywhere between 30mV and 300mV rms, but is plenty enough to give a loud level in a crystal earpiece. If you you not get a good level on that test, then the problem is likely prior to the vol control.

You can also use the crystal earpiece and a detector diode, like a probe, as you go back through the IF for testing (if you don't have a scope) as a way of demodulating the IF carrier frequency or the RF carrier for fault finding.


Last edited by ACORNVALVE on Feb Fri 07, 2020 3:59 am, edited 1 time in total.

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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Fri 07, 2020 3:56 am 
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Posts: 34465
Location: SoCal, 91387
ACORNVALVE wrote:
fifties wrote:
ACORNVALVE wrote:

Alkaline batteries are unsuitable for low current drain devices like transistor radios.

For this reason I don't let alkaline cells anywhere near my precious vintage transistor radios, especially not during servicing.

Obviously if you have a working transistor radio (not undergoing servicing) there is little risk in putting alkaline cells in it, that is if you don't leave them in the radio too long and forget about them.

Are you serious? Oh please. As long as the power source approaches what the radio needs to operate, it doesn't matter what kinda battery is used, or left in the set. Of course, I've only serviced a few hundred Transistor sets, so maybe you know better...


On the remark "I've only serviced a few hundred Transistor sets, so maybe you know better..." :

I'm not necessarily saying that I "know better" about anything than anyone else, it is merely an observation that I made comparing the two types of cells and the hazards during servicing. It is not really about the number of sets worked on, it could be 100, it could be 1000. It is the power of observation that is important.

No, it's called, "experience", and based on that, my SOP is to replace the usually four Electrolytic caps in a six Transistor circuit. I could care less if one of them, "spends most of its life in a reversed polarized condition". The practical thing to do is replace it, as it's well beyond it's useful life.

ACORNVALVE wrote:
one thing that could possibly cause the fault would be a very leaky or nearly open detector diode.

This is about as far out in left field as one can get. Have you ever encountered an "open Diode"?

_________________
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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Fri 07, 2020 4:15 am 
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Joined: Aug Sat 16, 2014 2:19 am
Posts: 1498
Location: Palos Verdes, CA
Although I am in no way an expert on repairing vintage transistor radios, after watching 50sTransistorRadios (Matt) youtube video on the Sony TR-72, I found that my TR-72 also had an open detector diode and is now working:

https://www.youtube.com/watch?v=EFpF_Pvli0g


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Fri 07, 2020 5:06 am 
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Location: Indianapolis, IN
Richard---I also have found a couple open detector diodes in early transistor sets as well as one which was extremely leaky... It is rare, but does happen...

ACORNVALVE---I've never thought of using a crystal earphone with diode for tracing. Should give it a try one day--- a good & simple idea !

And as Brett mentioned earlier, I've also found PCB cracks and bad traces in these radios causing intermittent issues...

John


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Fri 07, 2020 5:30 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 292
fifties wrote:
ACORNVALVE wrote:
one thing that could possibly cause the fault would be a very leaky or nearly open detector diode.

This is about as far out in left field as one can get. Have you ever encountered an "open Diode"?


Yes, I have had one. It is not too common I agree, and less common than transistor failure. I have a Sony TR-72 that had this problem. The article on the radio is here, diode D2 had failed. Difficult to get as an exact part, so for originality's sake, I simple put another germanium diode in parallel with it:

http://worldphaco.com/uploads/THE_SONY_ ... _RADIO.pdf

In a lot of early American radios of mid to late '50's vintage the electrolytic caps can still in many cases be surprisingly good. Many were built into thin walled white ceramic tubes, with a hard resinous seal at each end. The common one to fail is the AGC bypass cap (due to the polarity issue), often the others are still ok when checked for the three important parameters of uF value, ESR and leakage.

Oddly electrolytic caps from 60's & 70's vintage radios can be more problematic. Though someone once said if an electrolytic capacitor is over 50 years old it should be replaced, no matter what. And as a global argument, that is pretty difficult to disagree with. Though if an original part is still working normally, there is an equally good argument to leave it.


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Fri 07, 2020 5:34 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 422
Location: moreno valley
Thanks everybody, that list of normal voltages get me back on the right track, I had a feeling I was just reading them wrong or just the way some were noted on the schems. I mean I couldn't believe the radio worked as good as it did with such pitiful voltage there. So mine may be okay as far as that and problem is elsewhere. I have ordered some spring loaded leads and clips so I can better measure things and do some of the suggested tests I didn't try yet, will post back.

The Beitman schems are even worse than the ones I posted as far as the exact voltages, a whole bunch more of them are missing decimal points. Weird.


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Fri 07, 2020 7:46 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 292
3Transistors wrote:
Although I am in no way an expert on repairing vintage transistor radios, after watching 50sTransistorRadios (Matt) youtube video on the Sony TR-72, I found that my TR-72 also had an open detector diode and is now working:

https://www.youtube.com/watch?v=EFpF_Pvli0g


That is very interesting, now more than one Sony TR-72's with open detector diodes !


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