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 Post subject: Need Help Delco AM/FM Stereo
PostPosted: Feb Sun 12, 2017 6:26 am 
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New to site, low skills, need help troubleshooting my botched repair attempt.

Working on a 71 Delco AM/FM Stereo 12GFM1.

I took it apart and replaced all the electrolytic caps. I found a blown fusible resistor in the output of one channel, which also had an obviously bad power transistor. I replaced the fusible resistors and installed some 2N3614 that seemed spec'd similar to the original Delco DS-515.

I powered it up, but appears something is shorted. It is drawing 7 Amps from a battery. I probably left the battery on it for 30 seconds, but nothing smoked that I could see, so maybe I just wired something wrong..

Where do I start? I have a Sam's book and can find the numbered test points, follow the schematic, use a meter, but am weak on electronics. Is it possible to isolate a short without disconnecting everything again (ugh)?

I know it's a lot to ask for basic help, but after all the work, I hate to give up on it. Or maybe I should just bite the wallet and send to a pro?


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Feb Sun 12, 2017 4:04 pm 
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Location: South Jersey East of Philly
If this radio is for your vintage car's restoration, and you have little experience on radio reapir...farm it out. You risk trashing the radio more. These car radios aren't exactly made for easy repair, even this vintage. It still may be repairable, so unless you can find a working one thru junkyard networks or eBay... I'd send it out.


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Feb Mon 13, 2017 1:51 am 
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Hi thanks for the wisdom. It is not correct for my car, so hard to justify the cost of having it repaired. I just thought it would be a good challenge, which it is! I view it as a hobby, not an investment, and didn't pay much for it to begin with. Being it was AM/FM Stereo (red dot, expensive option at the time), it would be cool to have, but if I was going to spend a lot on it, I'd probably have it converted with modern guts or just buy an aftermarket "drop in".

As for hard to work on, I'm not having trouble with the soldering, parts replacement, etc., but don't have much experience troubleshooting. Still hoping for either some generic advice or maybe some specific experience with this vintage Delco circuitry.

I thought I'd disconnect power from right and left output blocks, see if maybe I can isolate the short to one channel. I suspect the hybrid amp module feeding one channel might be toast, given the power transistor and fusible resistor were taken out. I guess what was feeding it might also be bad..

If I strike out, I'll investigate the cost for a pro diagnosis, but suspect it may be a tough decision.


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Feb Mon 13, 2017 6:18 am 
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Did you double check the polarity of the electrolytics? Does the schematic have a resistance and voltage chart?

Can you post a link to the schematic?

Matt


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Feb Mon 13, 2017 8:13 am 
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Thanks for the suggestion skylerca.

I'll try to check cap polarities, but can't see them well now that I have it all back together. Is there a way to tell if I have polarity reversed using a voltohmeter on the leads I can get to from the back of the boards?

Here is a copy of the schematic from the Sam's book I have. Let me know if my link didn't work:
https://flic.kr/p/QQZsw4

I removed the wires feeding loads on "1" and "2", then powered up only "3". The blocks powered by "3" pulled < 200 mA, so the short is on the left or right channel output sections, I guess. Or maybe the 2N3614 power transistors I used don't work in this circuit with what appears to be a non-adjustable bias? This is one place I am clueless and could use guidance. When I bought them, I was just guessing at equivalence/suitability.

Is there a way to tell if the AF amp modules are working? For example, can I disconnect the B inputs from the power transistors and learn anything by looking at the amp outputs on pin(s) 7?


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Feb Mon 13, 2017 9:18 am 
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Quote:
Is there a way to tell if I have polarity reversed using a voltohmeter on the leads I can get to from the back of the boards?

Not that I can think of.

Quote:
Is there a way to tell if the AF amp modules are working? For example, can I disconnect the B inputs from the power transistors and learn anything by looking at the amp outputs on pin(s) 7?

There is a feedback path from the collector (C) of the power transistor to pin 3 of the amp, which the chip apparently uses to bias the transistor via pin 7. Disconnecting pin 7 from the base (B) of the transistor would remove this feedback path, so I'm not sure you would get a meaningful measurement at pin 7 under those conditions.

What happens if you connect wire "1" or "2" to power up one output amp at a time? Each one should draw about 1 amp of current if working correctly.

Matt


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Feb Tue 14, 2017 7:06 am 
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Thanks again Matt

I tried one channel at a time, which was my original thought on trying to isolate.

With either line 1 or 2 (power to either power amp block) connected one at a time, either channel pulls about 3 amps in each line... you told me to expect maybe an amp, so seems something isn't right?

I did not measure the current going to the speaker, just overall draw to the radio. I don't understand why there is a coil in parallel with the speaker, yet not a lot of series resistance. Wouldn't this cause the transistor to always be drawing a lot of DC current? What is the expected collector current, given the low value of the fusible resistor?

Maybe the 2N3614s were a bad choice given the Audio Amp module was designed for the original DS-515? Maybe the feedback you mention causes a problem if the gain is different on the transistors?

Another possibility was maybe a bad choice for the caps C16 & C17. If you look at this thread:

http://www.antiqueradios.com/forums/viewtopic.php?f=19&t=242997,

you can see I asked some questions and ended up replacing the electrolytics with non-polarized caps. I used this part:

https://www.digikey.com/product-detail/en/illinois-capacitor/475MWR100K/1572-1145-ND/5344075

Was this a bad idea?


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Feb Tue 14, 2017 6:52 pm 
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The expected emitter current can be calculated from the voltage across the fusible resistor, which is given as 11.65V - 11.18V = 0.47V. Using Ohm's law, 0.47V / 0.47 ohms = 1 amp. The collector current will be almost the same.

There is no problem with using film caps for C16 and C17.

It does seem possible that the new transistors have higher gain than the originals and that the feedback circuit can't compensate. You can introduce additional negative feedback by increasing the value of the fusible resistor (or placing additional resistance in series with it). Try changing it to 1 ohm and see what happens. Measure the emitter, base, and collector voltages before and after.

You can also check the voltages at the amp module pins and compare them with the values given in the schematic.

Matt


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Feb Wed 15, 2017 8:19 pm 
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Thanks again Matt, your sanity check on the emitter current helped.

Making some progress now.

I took a quick look this morning and found the voltage drop across the fusible resistor on one channel was ~1.3V, so obviously the emitter of the power transistor is somehow shorted to ground.

I either really hurt the transistors or I made a wiring error..


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Feb Thu 16, 2017 3:09 am 
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Quote:
I took a quick look this morning and found the voltage drop across the fusible resistor on one channel was ~1.3V, so obviously the emitter of the power transistor is somehow shorted to ground.

That doesn't mean the emitter is shorted to ground, it just means the transistor is conducting about 2.8 amps, which is in line with what you reported earlier.

Measure voltages at the module pins and transistor terminals and make note of any differences from the values given in the schematic. Then change the fusible (emitter) resistor to 1 ohm and measure again.

Matt


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Feb Thu 16, 2017 5:26 am 
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Are you using the proper speakers?

Delco factory speakers are rated at 10 ohms.

You should be using at least 8 ohm speakers and no less.


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Feb Thu 16, 2017 6:25 am 
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The original transistors DS 515, are germanium. The replacements you installed are different, silicon, so .... just sayin', there seems to be a bias or other compatibility issue. NPN vs PNP?

This website says it better than I can.

http://www.hawestv.com/transistorize/germanium1.htm

2N3055 is his recommended sub, but don't take my word for it. He also says things might not work but usually do. 8)

_________________
Watch the doughnut, not the hole.
Burl Ives, RIP, oldtimer.
[:l>)


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Feb Thu 16, 2017 6:57 am 
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westcoastjohn wrote:
The replacements you installed are different, silicon, so ....

That's not correct. The original transistors (DS-515) as well as the replacements (2N3614) are PNP germanium.

2N3055 transistors are NPN silicon and will not work in this circuit.

Matt


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Feb Thu 16, 2017 8:04 am 
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Thanks all.

First, I heard radio! There's hope!

Yes, the replacement I used is a Germanium PNP, in the same TO3 package, and was rated for higher current than original... but I don't have specs on the DS-515, so I cannot compare very well. As speculated, the gain may be higher and messing up the feedback?

I have no idea why, but when I started setting up to measure voltages, channel 2 sprang to life.

In my enthusiasm, I tried to attach an audio file in m4a format, but the forum would not allow this... but trust me, I was excited enough to record these first sounds.

Voltages I measured:

Ch2
Supply 11.48
.47ohm Resistor 10.88, so emitter current is (11.48-10.88)/0.47 = 1.3A
Base 10.50
Collector ~0.50 varying

Ch1 is still messed up, measuring:
Supply 8.92
.47ohm Resistor 6.76, so emitter current is (8.92-6.76)/0.47 = 4.6A
Base 6.14
Collector 6.15

Because the Ch1 supply shows my battery is drooping (high current draw dropped voltage a lot), I think maybe it is losing charge and that may have something to do with why Ch2 started working ??? I have no clue...

Anyway, I did not get to adding more resistance in the emitter leg of Ch1, as it will require some exertion, but I will charge my battery tonight and play with it all again tomorrow.

I also have one of the original DS-515 that I think may be OK. I can try swapping it into Ch1 if increasing the emitter resistance doesn't bring this other channel to life.

I'm feeling much more optimistic now... baffled by the new behavior, but it's good behavior!


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Feb Thu 16, 2017 5:50 pm 
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Congrats on getting channel 2 to work! Since that channel is working fine with the replacement transistor, you don't need to change the fusible resistor in the other channel. The problem is something else.

The sudden springing to life suggests intermittent solder connections. Reflow the solder on all the connections to the amp module and power transistor in both channels, especially the feedback loop from the collector to pin 3 of the module.

Matt


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Feb Fri 17, 2017 8:33 am 
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Thanks again Matt

No luck after reflowing all the solder joints.. one channel still not working, pulling lots of current

I am starting to think the AF Amp module is bad, but given the way my power supply sags, I can't really base it on a comparison of voltages with the working module. In general, this is a barrier to measuring voltages..

Your comments on these options would be appreciated:

- disconnect base from both power transistors, compare DC voltage at the outputs of the two amps
- disconnect emitter lead from the "bad side" power transistor and check voltages, hoping the supply doesn't sag
- replace the "bad side" power transistor with my old one
- replace the module


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Feb Fri 17, 2017 7:32 pm 
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Quote:
- disconnect base from both power transistors, compare DC voltage at the outputs of the two amps

That's a good idea. Or you could disconnect the emitter lead on both transistors. Either way would allow you to compare voltages on the amp module pins between the two channels.

I found this thread at AudioKarma that discusses repairing these modules or replacing them with discrete components: http://audiokarma.org/forums/index.php?threads/looking-for-1969-gto-parts.372348/

Matt


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Feb Sat 18, 2017 8:03 pm 
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I opened emitters, measured voltage at both amp module outputs, but didn't learn anything, as voltage dropped to 0 on both.

I then removed connections between amp outputs and bases, measured resistance from the amp output to ground with power removed, and they are different. The good-channel module measured 120 ohms. The bad-channel module was open. This is at least a difference.

And some really good news for me..

I ordered replacement speakers for my car from a company that also repairs vintage car radios and upgrades them with modern circuitry. I discussed my symptom, and the owner told me these DM-28 module failures are pretty common and the symptoms match mine well. He then, to my surprise, offered to send a replacement module from his stock of "discards". I am humbled by his offer and am now excited to replace the module when it shows up. Actually, he is sending an entire board, so replacing the board may be easier than the module.

I did look on eBay and there is actually a NOS DM-28 module listed right now, so they are scarce, but not unobtainable.

fyi, I had seen the article you linked, thanks.

I also found an article online that showed the attached diagram. Not enough info for me to build an equivalent module, but a functional diagram.

Attachment:
Audio Amp Diagram.jpg
Audio Amp Diagram.jpg [ 130.6 KiB | Viewed 2639 times ]


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Feb Sun 19, 2017 2:49 am 
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Pulled the module, find it is DM-8 instead of DM-28 called out on Sam's schematic.

Either they are the same or someone has been here before me.

Anyone know if they are different?

Attachment:
DM8 from Ch 2.jpg
DM8 from Ch 2.jpg [ 91.92 KiB | Viewed 2624 times ]


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Mar Mon 06, 2017 4:58 am 
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Update - I now have radio out of both channels!

Made progress, have a new symptom I need help with.

FYI, per my last post, I am now convinced that either DM-8 or DM-28 would be fine for my radio. There is some extra circuitry in the DM-8 that my radio doesn't use.

Here's what it took to get the bad channel's power amp working:

- replaced the DM-28 module with a used one
- discovered the speaker and power transistor on the bad channel had been blown
- ordered new power transistors and another disposable speaker

I ended up using two GE-239 transistors I found on e-Bay. All seems good with them. I still have one of the original Delco DS-515 that is good, but figured better to match.

New symptom: the volume was low on one channel, and did not respond to the balance control. While thinking about it, I noticed the red stereo indicator dot was on solid, so tried to adjust it. I found that adjusting the indicator adjust pot R3, it acted like a balance control... I found this weird, but somehow it is interacting through the multiplex detector, IC2.

Anyone able to guide me on figuring out what's wrong?

Since the indicator light is on solid, I figure I'd check the transistor that drives it, Q10. It was on the same voltage rail as the bad amp, so maybe it's all related?

I'll also check the voltages at IC2.

Advice?


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