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 Post subject: Recapped Zenith 500H - barely audible
PostPosted: Feb Tue 04, 2020 10:59 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 422
Location: moreno valley
Stuck on this Zenith 500H, it only gave tiny amount of static when turning vol pot but that's all. Recapped it and cleaned the pot and double-checked all the polarities with the existing caps and the component layout schematics. The green 100v .22 is temporary til I go find a smaller one that fits that tight space.
The vol control functions and the dial tunes, but even at max volume it is only barely audible, either thru the speaker or headphones, on the few powerful stations I can actually *hear*. Any suggestions? Thanks!


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Tue 04, 2020 12:16 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 292
The fact that you can hear something when the volume control is turned, means likely the audio stages are working. The static you hear is the coupling capacitor to the volume control being charged and discharged as you rotate the control. This can be a little irregular, especially if the control is worn.

If the radio is not working (receiving stations) it could be multiple causes.

You need to apply some logic to it, based on the standard design of a Superhet transistor radio.

So lets look at what has to be happening for the radio to be working:

1) The main tuning gang (the larger capacity one) has to be tuning the main ferrite rod coil across the MW band radio frequencies.

2) The small coupling coil on the rod must be feeding the tuned signal into the mixer or mixer oscillator stage.

(any broken wire connections = no go)

3) The output of the mixer or mixer/osc stage feeds into the IF amplifier, so not only must the IF amplifier stages be working, but they must be tuned to the right frequency.

4) The local oscillator must be running the IF frequency above the tuned radio station.

5) The output of the final IF must be detected, by the detector diode, the diode has to be good.

6) The AGC voltage also developed by the detector diode must be correct.

If any of this is not right , it will stop the radio working.

Having said all that, at a practical level, you have replaced the electrolytic caps. The next most unreliable part is the transistors themselves. A quick check on them is to look at the voltage across the transistor's emitter resistors, with a meter.

Usually if the transistors are ok, these emitter voltages are correct. The emitter current represents the sum of the bias current (base current) and the collector current which is at least 20 to 30 times higher than the base current if the transistor is working (has a reasonable hfe). So if the emitter voltages are way high or way low, the transistor has likely failed.

So the next step (without any sophisticated gear like scopes & signal generators); check the transistor emitter voltages compared to the values on the manufacturer's schematic.

PS: I wouldn't use those nickel metal hydride rechargeable batteries in a vintage radio like this. The short circuit current is enormous, due to their very low internal resistance. If you have a slip up, and accidentally short something, you could easily destroy components, transistors & coils. My advice is always use zinc-carbon cells in vintage transistor radios, especially while working on them. Just a helpful tip.


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Tue 04, 2020 12:51 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 422
Location: moreno valley
So does it narrow it down to a particular area by the fact that at max volume, I don't get even get the usual relatively loud hiss/interference you would hear between stations? I.e. the vol control does change the volume, and it does tune, just at an almost zero final amplification or whatever it's called. :)

On the emitter resistors, are the two indicated examples of which ones you mean? Please explain exactly what you mean by measure across them, where am I putting the probes? Thanks!


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Tue 04, 2020 1:24 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 292
frenchmarky wrote:
So does it narrow it down to a particular area by the fact that at max volume, I don't get even get the usual relatively loud hiss/interference you would hear between stations? I.e. the vol control does change the volume, and it does tune, just at an almost zero final amplification or whatever it's called. :)


Well if it is tuning in stations the local oscillator must be working and there is at least something coming out of the detector. To check the audio amplifier you need for example to set the volume mid way and inject an audio signal into the the center leg of the volume control. Typically this would be a 1kHz signal from a test generator.

Without test equipment, if you could get a crystal earphone, connect that across the outer leg of the volume pot, or the detector diode, and listen to see of the stations sound loud & normal.

So its a matter or working out if the signal out of the detector is too low, or the audio amp has lost its gain.

It might pay to double check the connections around the volume control and the capacitors that got replaced in that area in case the actual signal from the detector is not getting coupled to the volume control, or there is an inadvertent short in that area.

To measure the emitter voltages you put the meter probe (positive) on "ground" (common) which is positive of the battery terminal will do. The negative meter lead will go on the transitor's emitter, so you will be measuring the voltage across the emitter resistors. The voltage you measure is an indication of the current passing through the resistors as in V = IR.


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Tue 04, 2020 2:08 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 422
Location: moreno valley
Okay I'll check this stuff. I have ohmeters, tube testers etc. but I don't have a generator for the amplification check, is there something else I can use as input into the volume pot, like the audio output from another device?


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Tue 04, 2020 2:29 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 292
frenchmarky wrote:
Okay I'll check this stuff. I have ohmeters, tube testers etc. but I don't have a generator for the amplification check, is there something else I can use as input into the volume pot, like the audio output from another device?


Yes you could. If you have another working transistor radio, simply connect the grounds of the two volume pots together (which is one outer leg) and the other outer legs of the two pots together, so a two wire link between two radios putting the volume pots in parallel. Have the good radio tuned into a station. Turn its volume pot down so its not distracting. If the audio stages are working on your Zenith 500H, then when you turn its volume control up you should get good loud audio from the other radio coming out of the 500H's speaker.

Another option could be the output of an iPod's audio jack, with the iPod volume set very low and couple that into the volume control top leg, probably best with something like a 4.7k to 10k series resistor, obviously with the common connection from the ipod jack connected to the radios ground (+battery supply in this case).


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Tue 04, 2020 2:37 pm 
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Hi I noticed that you have rechargeable batteries in it, first try some new alkaine batteries. the rechargeable ones put a lower voltage (1.25 vdc)
Rich


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Tue 04, 2020 10:18 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 422
Location: moreno valley
Hmm I was just setting up to measure the transistor voltages and suddenly there was decent volume! At max it's not nearly as loud as it should be, but much better than almost zero. If I hold the corners of the board and gently flex it slightly back and forth the volume gradually goes from nearly zero to pretty good but in smooth, graduated way. Not acting like a pure bad connection, but seems like I must have one like a weak transistor socket connection, bad trace etc. someplace so will check that out too.


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Tue 04, 2020 11:37 pm 
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Joined: Aug Wed 24, 2011 4:35 am
Posts: 4795
Location: Sunnyvale CA
frenchmarky wrote:
Hmm I was just setting up to measure the transistor voltages and suddenly there was decent volume! At max it's not nearly as loud as it should be, but much better than almost zero. If I hold the corners of the board and gently flex it slightly back and forth the volume gradually goes from nearly zero to pretty good but in smooth, graduated way. Not acting like a pure bad connection, but seems like I must have one like a weak transistor socket connection, bad trace etc. someplace so will check that out too.


This is the classic "cracked trace" issue and the 500H might be the worst-ever subject for that. The boards were the worst of the worst, and also, have very fine traces that are absolutely notorious for cracking. Fixing it is tedious but straightforward.


Brett


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Tue 04, 2020 11:39 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 422
Location: moreno valley
Okay here's what I measure between ground and the transistor emitters. Little hard to read the schems but looks like maybe the driver transistor is way off? Reads 0.29 but should be 37V? I think the flexing thing was just moving the tuner off whatever station I have on it. Other than that I don't know why the volume magically came back but at max it is still not nearly as loud as my other 500H. Can't find whatever was killing it or make it happen again, also I pulled all the transistors and cleaned the leads.


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Wed 05, 2020 6:14 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 422
Location: moreno valley
Also I tried testing the driver transistor with the Hfe function on my meter, PNP, equiv. is NTE 102A. I read a '27' but a couple of sources I looked up seem to say it should it be a minimum of 63 up to 200+. I don't know if this means it's bad or not or if the datasheet Hfe should be about what I should read on this meter but just tossing out what I'm finding in case it helps.


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Wed 05, 2020 1:00 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 292
I can barely read the schematic as the resolution is poor. But it appears that probably the total resistance in the emitter of the driver transistor probably is in the 160 to 200 Ohm region. So, if the emitter voltage was 0.37V as per the diagram, then the emitter current would be 0.37/200 = about 1.8mA, which is a sensible value for the driver stage. It would still be ok if the voltage across the emitter resistor was a little lower , like 0.29V.

The quiescent current in the output stage looks ok too at 0.04/4.7 = 8.5mA. So the DC conditions in the audio stages appear ok. You may just have a fractured pcb track/ connection if the problem is coming & going with physical flexion of the pcb.


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Wed 05, 2020 3:22 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 422
Location: moreno valley
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.

On the flexing I think it was just causing a gradual, temporary drift off of a station from the slight tweaking, can be retuned in mid-flex to same volume by slightly retuning the dial so I don't think that was actually causing the 'break'. I am hoping maybe it was just a dirty transistor lead because now I can't get it to go 'almost dead' anymore like it was at first, but still poking around.


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Wed 05, 2020 5:50 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 422
Location: moreno valley
By sheer chance the *only* transistors for radios I ever bought was years ago was for a micro radio, some 2SB54s, and low and behold they can be substituted for 102A in a pinch. On my meter it gets 200 Hfe compared to 27 of the one in the radio. Maybe a little louder but not that big a change. At max it still sounds more like 1/3 or 1/2, only good for strongest stations. And still same emitter voltage using the sub. I don't understand why the schems say -37 v emitter and -44 v collector but I'm only getting < 1 v and the transistor there at least appears to not be the cause.
Correction, readings on driver in circuit with radio on are -0.29v emitter,-0.38v base, -4.5 collector.


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Wed 05, 2020 8:34 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 422
Location: moreno valley
Darn, the flexing DOES cause it to actually cut out and get flaky at the far end of the tweaks so looks like there is a weak joint or break somewhere, will start tracking it down and make sure it's not just the AA battery terminals' connections.


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Thu 06, 2020 12:25 am 
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Joined: Dec Wed 25, 2013 7:57 am
Posts: 4187
Location: USA
Is it possible someone before you tinkered with it and sent the radio out of alignment? Regardless, do check all wiring and traces. Alcohol is good for cleaning the board and exposed contacts.


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Thu 06, 2020 12:38 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 422
Location: moreno valley
I have no idea if anyone misaligned it but it doesn't seem to be molested in any way inside, so not sure. Is there a way visually tell if things have been turned? I poked every path I could follow along the traces and only come up with 0 ohms. The flexing issue, maybe was the batteries just shifting and breaking contact, can't duplicate it now. This recapping business is so much easier when you do it and the thing just plain works, I think I've been way too lucky over the years in that category. :)


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Thu 06, 2020 12:48 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 422
Location: moreno valley
---The AGC voltage also developed by the detector diode must be correct.---

Can the diode be defective partially and cause this, or would it only work, OR fail totally and in that case I would get zero reception or other issue?


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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Thu 06, 2020 12:49 am 
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Joined: Jan Tue 31, 2012 1:55 am
Posts: 12012
Location: Mountains of Mourne. Ireland.
As mentioned...

Clean the trace side of the circuit board with some IPA and a soft toothbrush. A single pencil line between traces can wreak havoc or shut a circuit down completely.

Before and after soldering...
I clean (and dry) with IPA (isopropyl alcohol) using a small acid brush or an old toothbrush. I use 70% (strength) isopropyl alcohol... mixed 50/50 with regular nail varnish remover (acetone), it leaves no residue.
In a past life I have used Zippo lighter fluid (naphtha) or denatured alcohol in a pinch.

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


Last edited by egg on Feb Thu 06, 2020 12:57 am, edited 1 time in total.

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 Post subject: Re: Recapped Zenith 500H - barely audible
PostPosted: Feb Thu 06, 2020 12:55 am 
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Joined: Jan Tue 31, 2012 1:55 am
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Location: Mountains of Mourne. Ireland.
Are you in the ballpark?
Image
.. from→ Signal tracing transistor radios
.. viewtopic.php?f=4&t=278833

Greg.


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