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 Post subject: 1970's GM Delco Car Radios
PostPosted: Feb Mon 19, 2018 7:53 pm 
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I have worked on several early 1970s GM Delco car radios and every one has high distortion when the volume it turned up. I'm working on one right now that happens to be stereo and both channels have that distortion.

Using a modulated 1khz tone my scope measured about 15v P_P at each of the two 8 ohm speakers without visible distortion. I calculate that to be about 3 watts output each chan so it's fairly loud and usable like this. However, the volume control is only at roughly 66% so it can be turned up another 33%. But if I do so clipping starts distorting the sine wave. BTW, I substituted 10 ohm resistors for the speakers and got similar results.

I can see that the clipping occurs at the base of the Delco DS501 germanium output transistors. So either the driving circuit is too weak or the output transistor input impedance circuit is too low. The driver circuit is a strange looking plastic rectangle device that is shown as on I.C. on the schematic.

Can anybody verify that this distortion is typical of these radios?

As I mentioned, I have encountered this problem on every one of these kind of radios and never really found the reason. Although not all of these radios used that strange I.C., but they all had DS501 transistors.

Any comments, suggestions, are appreciated.

Thanks
Frank


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 Post subject: Re: 1970's GM Delco Car Radios
PostPosted: Feb Mon 19, 2018 9:58 pm 
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Since the radio runs of 12-14 volts DC, I would think that would limit the size of the unclipped waveform. I believe they usually used lower impedance speakers (like 3.2 ohms) to get more power.

With an H-driver configuration, you could get as much as 24 Vpp, but I don't think they have it. Then you're only going to get 12 Vpp. With an 8 ohm speaker, this would only provide about 2 watts. With a 3.2 ohm speaker, you'd get 5.6 watts, which is not bad.

If you can drive it right up to the power supply voltage without clipping, there won't be anything further you can do.

Here I'm assuming that the radios don't have anything in them to generate a higher output voltage, like an output transformer that actually steps up the output voltage, or a DC-DC converter that increases the supply voltage. Don't think I've seen anything like that on radios from this era.

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 Post subject: Re: 1970's GM Delco Car Radios
PostPosted: Feb Mon 19, 2018 11:37 pm 
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Thanks, Tom,
I should have posted a schematic which I have now attached.

There is a choke and a 4 mfd electrolytic cap across the shown 10 ohm speaker. That L-C across the speaker seems strange to me but may explain how they get P-P voltage above the 12v supply. Although I can't understand how that combination doesn't hurt frequency response.

The schematic shows DS515 output transistors, but the radio has DS501 transistors. The schematic matches the radio model, but there seem to be other minor differences as well.



Attachment:
radio001.pdf [179.66 KiB]
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 Post subject: Re: 1970's GM Delco Car Radios
PostPosted: Feb Tue 20, 2018 12:40 am 
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That's a new one to me. Maybe this is some kind of early pulsing system that will kick the voltage higher at the output. Need one of the car radio experts to explain it.

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 Post subject: Re: 1970's GM Delco Car Radios
PostPosted: Feb Tue 20, 2018 7:37 am 
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That circuit runs straight off the 12 volt system. The output transistor is running Class A, with a zero signal collector current of about 1 amp.
The choke is in place to carry the majority of the DC current, and the 4 MFD cap is there to prevent voltage spikes, just as in a tube radio.
The module is an IC which drives the output transformer.

Since there is no voltage conversion, the output power to an 8 ohm speaker is limited. We used to repair those sets when people would hook up too many speakers.
Bose got around the power limitation by using 0.2 ohm speakers, but they had a much beefier amplifier driving them. Those speakers would damage your radio.

Delco later went to a bridge amplifier, which could provide more power with a 12 volt supply because both sides of the speaker were driven. Again, we replaced a lot of amplifier chips when people would connect an 8-track player to the same speakers, which shorted out the amplifier.

To get high power, the only practical way is to use a power inverter. This is how those huge power amps work.

Bob

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 Post subject: Re: 1970's GM Delco Car Radios
PostPosted: Feb Tue 20, 2018 5:02 pm 
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Thanks, Bob,

Your explanation for the purpose of the choke makes a lot of sense.

I guess the 4 mfd cap could be for spikes as wires to speakers can be long and cars are noisy. But at say 4khz it's impedance equals the speakers 10 ohms so high frequency rolloff seems pretty severe to me. Perhaps it's some sort pf frequency compensation scheme.

After considering Tom's comments about my clipping problem I'm thinking that there is too much gain into the output transistor. Given that the output is a common emitter circuit with minimal bias (.47 ohm emitter resistor), by definition that means it's limited to relatively small signals. I'm measuring about 2.5v P-P input when clipping occurs and that does sound like it's being overdriven.

Bear in mind this is a stereo radio with two independent audio channels each with the same problem on AM or FM.

And, as I said, every one of these radios that I have seen (about 4) seem to have the same high volume distortion.

If not for my oscilloscope showing the distortion, I would think the problem is my ears.


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 Post subject: Re: 1970's GM Delco Car Radios
PostPosted: Feb Tue 20, 2018 5:12 pm 
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The capacitor connected to the collector of the output (which should be replaced with a non polarized capacitor) is for
1) transient spike suppression should someone foolishly disconnect the speaker load during operation
2) eliminate any residual 10 kHz " whistle" from the output.

This was per original Delco training information.

The distortion you are experiencing is not out of the question, but with any restoration, you must rule out any other weak links. That is why you need replace all of the electrolytic capacitors due to the degradation that occurs with age for reliable future operation.


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 Post subject: Re: 1970's GM Delco Car Radios
PostPosted: Feb Tue 20, 2018 5:40 pm 
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The class A output stage should provide nearly the supply voltage as the peak output voltage to the load.

The big germanium transistor will saturate pretty well, so given some drops here and there, around 10V peak will be the limit. 20V peak to peak at moderate frequencies. That's across your 10 ohm speaker.

You should be able to see about this much output into a 10 ohm load at 400Hz, say. At higher frequencies, that "tone" capacitor takes its toll, and the transistors were remarkably slow, as well.

The maximum output power compares pretty well with the old standby pushpull 6V6 output stage, but the distortion is different.

If you run the "battery" at 13.8V instead of 12.0V, you will get a bit more.

Ted


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 Post subject: Re: 1970's GM Delco Car Radios
PostPosted: Feb Tue 20, 2018 5:53 pm 
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Thanks, Doc,

Delco's explanation for the cap is interesting.

Of course, replacing all the electrolytics is the right thing for a restoration, but my goal is a simple repair.

The radio works really well after I replaced the mux/det I.C. except for this high volume distortion problem. While I haven't replaced any caps I have measured the capacitance of several and paralleled others and found none of them suspicious. I'll probably do some more now. Besides the output cap there's only one other cap in each of the two audio circuits so I'l replace them and see what happens. I already checked the caps in the 11.68v power supply.

BTW, the ripple in the 11.68v power supply seems pretty high when the volume is turned up. but paralleling that cap with much higher value caps doesn't change the distortion problem, but there still is some ripple. I suppose that ripple could be causing some positive feedback somehow resulting in high gain. I'll take another look at that.


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 Post subject: Re: 1970's GM Delco Car Radios
PostPosted: Feb Tue 20, 2018 6:07 pm 
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Usually Lurking wrote:
The big germanium transistor will saturate pretty well, so given some drops here and there, around 10V peak will be the limit. 20V peak to peak at moderate frequencies. That's across your 10 ohm speaker.

Don't see how you get more than 12 V p-p, or 6 V peak, since this is a single ended output stage. Can you explain?

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 Post subject: Re: 1970's GM Delco Car Radios
PostPosted: Feb Tue 20, 2018 8:40 pm 
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Tom Albrecht wrote:
Usually Lurking wrote:
The big germanium transistor will saturate pretty well, so given some drops here and there, around 10V peak will be the limit. 20V peak to peak at moderate frequencies. That's across your 10 ohm speaker.

Don't see how you get more than 12 V p-p, or 6 V peak, since this is a single ended output stage. Can you explain?


The collector load is an inductor, so the output can go as high as twice the supply voltage, at least in theory.


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 Post subject: Re: 1970's GM Delco Car Radios
PostPosted: Feb Tue 20, 2018 8:49 pm 
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Tom Albrecht wrote:
Usually Lurking wrote:
The big germanium transistor will saturate pretty well, so given some drops here and there, around 10V peak will be the limit. 20V peak to peak at moderate frequencies. That's across your 10 ohm speaker.

Don't see how you get more than 12 V p-p, or 6 V peak, since this is a single ended output stage. Can you explain?

I just repeated my measurements and, sure enough, using both a 1khz and 400 hz sine wave I'm seeing 15v P-P across the speaker and according to my scope it's plus and minus 7.5v. I also measured using my true rms voltmeter and it read 5.2V. If I did my arithmetic correctly that translates 14.7v P-P, pretty close to 15v.

I can't explain how it can go negative 7.5v unless it's related to the coli on the output or maybe the coil in the speaker somehow providing a back EMF.

Kind of makes sense that most speakers expect a bipolar signal so that's what you would want your radio to supply.

Hopefully somebody else can explain this to us.

Frank

P.S. As I write this I see Tom has offered some insight. Thanks, Tom...
Edit: Sorry, It was Erich not Tom that offered some insight..Thanks, Erich.


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 Post subject: Re: 1970's GM Delco Car Radios
PostPosted: Feb Wed 21, 2018 6:22 am 
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I'm still a little skeptical about this (although willing to learn...).

If the inductor is supplying the negative-going voltage, then the transistor is cut off during that part of the cycle. Under those conditions, I would expect the current trajectory to be determined by the inductance, capacitance, and resistance in the circuit, and be unrelated to the incoming waveform (except for amplitude). While you might provide a nice 1000 Hz sine wave for the first half of the cycle (if the audio signal were 1000 Hz sinusoidal), the second half of the cycle won't be correct. So I would expect tons of distortion, to say the least.

This would be a little like a Class C output stage on an RF amplifier. Works great, as long as there is a resonant circuit tuned to a single driving frequency. Then the resonant circuit completes the cycle and gets rid of the distortion. Here, you have many frequencies, so that kind of scenario isn't possible.

If you crank up the volume all the way, so that everything is clipping, I imagine you approach 6V RMS. But that doesn't mean the peaks are at 8.5 V (for 17 V p-p) as you would expect for a sine wave. The peaks are square and at 6 V.

Are you really able to confirm that you have a nice clean sine wave, and that the waveform at the speaker is really bipolar (make sure your scope isn't set for AC coupling), and the V p-p is greater than 12 V (on a well calibrated scope)?

If the voltage really were swinging negative, that polarized electrolytic cap wouldn't be very happy. Schematic shows a polarized cap -- although I note that Dr. Radio above recommends a nonpolarized cap. Is the schematic in error?

So far, I'm thinking Elvirafan's explanation makes the most sense to me, but that seems to be at odds with the measurements reported here, if they are correct.

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 Post subject: Re: 1970's GM Delco Car Radios
PostPosted: Feb Wed 21, 2018 8:21 am 
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Tom,

The single ended class A output stage is the most commonly used circuit in old radios. Works the same way with a tube or a transistor.

A single-ended output transformer for a 50L6 or whatever tube also serves as an inductor.

The output device is never fully off for any part of the undistorted output waveform. Once the device current starts hitting zero, you get clipping distortion. The waveform at the transistor collector or the tube's plate goes on up well past the supply voltage as the device current drops below the idle value.

You might go read-up how this works in the Radiotron Designer's Handbook, or in Terman.

Ted


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 Post subject: Re: 1970's GM Delco Car Radios
PostPosted: Feb Wed 21, 2018 8:51 am 
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Well, Ted is correct indeed.

Something I hadn't thought about previously. I had been under the impression that a well matched speaker and output transformer present the equivalent of a resistive load on the plate of the tube, but such is not the case. The instantaneous voltage on the plate spends about half its time above the B+ supply voltage, even for moderate signal values where there is no clipping or significant distortion.

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 Post subject: Re: 1970's GM Delco Car Radios
PostPosted: Feb Wed 21, 2018 4:05 pm 
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Good point about the 4 MFD electrolytic cap being unhappy with the bipolar signal. I was thinking the same thing but the grandkids are keeping me busy and I haven't had time take out the caps and replace them.

And yes they are electrolytes. And they are not identical so one may have been replaced in the past. A few days ago I think I paralleled them with another cap and it made no difference. So I assumed the caps were good thinking if they were leaky the dc voltages would be wrong.

Anyway 'll probably get to replacing them by tomorrow. And if I can find suitable non-electrolytic caps I will use them.

Really odd that the radio designers would use electrolytics with bipolar voltages.

Frank


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 Post subject: Re: 1970's GM Delco Car Radios
PostPosted: Feb Wed 21, 2018 6:00 pm 
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The tone capacitor in these sets is not a regular electrolytic. The original can will be marked with more than a normal DC voltage rating.

A non-polar electrolytic, or a film cap might be used as a replacement.

It might be prudent to use an electrolytic with higher than just enough voltage rating. 105C and lots of hours of rated life are also good things to look for. Two DC caps back to back in series would also be a good choice.

If it were my radio, I might investigate using a zener-type TVS across the transistor, plus a lower value capacitor.

Ted


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 Post subject: Re: 1970's GM Delco Car Radios
PostPosted: Feb Thu 22, 2018 6:12 pm 
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I disconnected one of those electrolytic caps and substituted a non-polarized capacitor and got exactly the same results. That is, distortion at high volume settings. I measured the old disconnected capacitor and it seems good.

I did not try the other channel's capacitor, but it's kinda hard to get at and my guess is that it's okay.

At this point i'm thinking of adding some emitter resistance to the output transistor, maybe 1 ohm. This would reduce the overall gain of that stage and give more headroom before the base bias is overrun by the signal driving it. I'm hoping that the result will be higher undistorted volume.

Like I previously mentioned, I have seen about four other Delco radios of this vintage and they all have this problem. Some were AM only, some AM/FM, and one stereo. The pre-output stages are different, but they all share a similar germanium transistor output stage.

I am under the impression that transistors do not degrade over time, but I'm not sure about germanium transistors. However, if the beta of these transistors did degrade over time that might be an explanation for what I am experiencing.


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 Post subject: Re: 1970's GM Delco Car Radios
PostPosted: Feb Thu 22, 2018 7:34 pm 
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frank2644 wrote:
I am under the impression that transistors do not degrade over time, but I'm not sure about germanium transistors. However, if the beta of these transistors did degrade over time that might be an explanation for what I am experiencing.


Replaced a lot of transistors in Delco and FoMoCo radios back in the '70s. The germanium outputs did get weak. They were hard to replace as they were hard to get and the cross referenced parts could not handle the heat. I think the high heat and current was hard on them.

Up though about '80, bad transistors were common. There were some that were well known to fail. Probably the most common was a silicon osc trans in the late '70s FM radios. Apparently, impurities caused the trans to later become unstable.

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 Post subject: Re: 1970's GM Delco Car Radios
PostPosted: Feb Thu 22, 2018 10:46 pm 
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SHenion wrote:
frank2644 wrote:
I am under the impression that transistors do not degrade over time, but I'm not sure about germanium transistors. However, if the beta of these transistors did degrade over time that might be an explanation for what I am experiencing.


Replaced a lot of transistors in Delco and FoMoCo radios back in the '70s. The germanium outputs did get weak. They were hard to replace as they were hard to get and the cross referenced parts could not handle the heat. I think the high heat and current was hard on them.

Up though about '80, bad transistors were common. There were some that were well known to fail. Probably the most common was a silicon osc trans in the late '70s FM radios. Apparently, impurities caused the trans to later become unstable.


Thanks for the information. I could buy a replacement transistor on Ebay, supposedly new, old stock, but I'm wondering if age might still cause a problem. Googling around some folks speculate that the transistors were not sealed well and moisture causes the problem.

I hate modifying the circuit like I stated in my last post. I'll have to think about it some more.


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