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 Post subject: Novice Needs Help Adjusting Delco AM/FM Stereo
PostPosted: Apr Tue 04, 2017 4:57 am 
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Joined: Oct Wed 12, 2016 9:11 pm
Posts: 21
Have replaced enough parts to get my radio making noise, need help with basic adjustments and possible troubleshooting, as my electronics knowledge is weak at best.

I'm following the procedure in the Sam's book, had no trouble with the adjustments in the AM section, but things didn't go well when I started into the FM section.

On step 5, I feed my generator into the base of Q2 through a capacitor, and it kills the audio output at the speaker, so there is no signal to minimize by adjusting A11. What p-p amplitude should I measure at the Q2 base? Is my generator overpowering something or is this indicating an issue with my radio? Should I have the antenna in place? Should I be tuned to a station or no station?

Is the "FM Mixer" where the IF gets added to the tuned carrier? If so, is the injected signal intended to overpower the intrinsic IF signal? If I remove the injected signal, where would I see the IF signal on my scope?

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Alignment Instructions Small.jpg
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12GFM1 Schematic Test Point B Small.jpg
12GFM1 Schematic Test Point B Small.jpg [ 94.34 KiB | Viewed 1341 times ]


Attachment:
12GFM1 A11 Small.jpg
12GFM1 A11 Small.jpg [ 42.79 KiB | Viewed 1341 times ]


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 Post subject: Re: Novice Needs Help Adjusting Delco AM/FM Stereo
PostPosted: Apr Tue 04, 2017 7:19 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 17920
Location: Albuquerque, NM 87123
First off, you should never attempt an alignment unless you know the radio is functioning properly. Making noise is not good enough.

You say you fed a signal and it killed audio; was the signal modulated or un-modulated? It sounds like you don't know the amplitude of the signal you are injecting--that's not good. You ask a lot of basic questions that you should already know the answers to before attempting an alignment.

I hate to be blunt, but radios rarely need a complete alignment unless tuned parts have been replaced or someone has been inside making adjustments that were not necessary.


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 Post subject: Re: Novice Needs Help Adjusting Delco AM/FM Stereo
PostPosted: Apr Tue 04, 2017 11:39 am 
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Joined: Jul Mon 26, 2010 8:30 pm
Posts: 24020
Location: Annapolis, MD
Agree that it should be working before attempting alignment.

What parts have been replaced?

In general, the first step in troubleshooting is to make sure the DC voltages are correct. Since AM is working, you can focus on the circuits that affect FM only.

Finally, can you elaborate on "making noise"?--- one useful test is to see it the noise changes when tuning.

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"It's always something". --Gilda Radner (1946 - 1989)


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 Post subject: Re: Novice Needs Help Adjusting Delco AM/FM Stereo
PostPosted: Apr Tue 04, 2017 4:10 pm 
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Posts: 1458
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Someone with weak electronics knowledge should not even think of attempting an FM stereo alignment.

I hate to be blunt, but you are way over your head on this one. The radio must be restored to proper operational functionality first, which includes all electrolytic capacitors replaced, proper bias voltages determined, replacement of any leaky germanium transistors, solder joints retouched etc,

As well as a solid understanding of the circuit functions, adjustment procedures and proper equipment usages.


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 Post subject: Re: Novice Needs Help Adjusting Delco AM/FM Stereo
PostPosted: Apr Tue 04, 2017 4:31 pm 
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Location: Annapolis, MD
Don't let the flak discourage you!!.....here's more

shiny wrote:
Is the "FM Mixer" where the IF gets added to the tuned carrier? If so, is the injected signal intended to overpower the intrinsic IF signal? If I remove the injected signal, where would I see the IF signal on my]

This is not how it works..

The incoming station signal is first filtered in the RF stage(s), and then mixed with the local oscillator. The result is a spectrum of signals that depends on the properties of the RF stages. The band pass of the IF stages then determines what goes through to the final detector.

The most critical part of setting up an FM set is the adjustment of the IF stages AND the detector. The performance is dependent on the absolute signal level, but also on the shape of the curve. Many people feel this is best done with a sweep generator, but others methods are **OK**.

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"It's always something". --Gilda Radner (1946 - 1989)


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 Post subject: Re: Novice Needs Help Adjusting Delco AM/FM Stereo
PostPosted: Apr Tue 04, 2017 10:40 pm 
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Joined: Oct Wed 12, 2016 9:11 pm
Posts: 21
Thanks for the replies, both encouraging and discouraging.

I agree I'm in over my head, that's why I asked for help. I'm trying to learn something, not fix a priceless jewel. It's for fun. If I had your knowledge, I wouldn't ask!

Attached full schematic.

To answer questions:

1. Parts Replaced

All electrolytic caps
Both output power transistors and both fusible resistors
When first powered, I only had sound from one channel. With help from this forum, replaced one of the hybrid audio amplifier modules and got both channels working.

2. As It Sits

Good: Tunes stations on AM and FM up and down the dial. Tuned FM stations sound OK, don't drift. Tone control works.

Bad:

- FM Stereo indicator light is always on, does not extinguish between stations
- Balance control is not working well, seems to be more "on/off" than proportional
- Not yet clear to me if there is any stereo separation.. don't have a controlled encoder/modulator, but if the stereo indicator light wasn't working, I figured the odds of the MPX detector working correctly were low..

These symptoms are why I thought the simple adjustments described in the Sam's book might help before I took on trying to determine if the MPX detector was broken.. understanding this function would definitely be a challenge for me.

3. Adjustments I Have Made

- Stereo indicator adjuster

This is intended to modify the threshold, but did not extinguish the light between stations. I noticed it did have a strong influence on, and/or interaction with, balance control... which made me figure the MPX decoder IC may not be working right.. or the detected signal coming into it may be messed up


- AM adjustment 1

As described in the Sam's book as I posted. Generator at 262 KHz, internal 1KHz modulator, adjusted A1, A2, A3, A4 (L13, L14) for maximum audio output. This worked as expected.. I followed the directions, did not need to understand the circuit function to do this. I set up the generator to 262 KHz using a frequency counter, turned down the signal amplitude, turned down the modulation amplitude, injected on the base of Q6 (Point A on schematic), turned up the amplitude just enough to hear the 1KHz audio output through the speakers. I put my scope on the speaker, adjusted the coils to maximize the p-p waveform on the scope. I was able to find a maximum on each trfrmr, so figured I'd try the FM detector next.

- FM Detector Adjustment 5

I did not touch the FM cans (L13, 14, 15, 16).

Following the instructions for Step 5, which was adjusting L8 on the MPX Detector, did not get what I was expecting, which brings me to my request for help.

I set the generator at 10.7 MHz, minimized amplitude, turned off modulation (I don't have a generator with FM modulation), put my scope and DMM across the speaker. When I connected the generator output to the test point B as described (with a .001mfd decoupling cap in series), the audio noise went away. The generator output was < 10 mV p-p, and I could see it on the scope at the input side of the cap. The directions say to adjust L8 for a minimum between two noise peaks. Without any sound from the speaker, I figured I was doing something wrong and posted this request for help.

So I then disconnected the generator, tuned to static between FM stations, and adjusted L8 until I found a minimum in the audible noise. I also had my DMM across the speaker, and although it wasn't steady, I was able to minimize the AC voltage pretty repeatedly. After this, the FM stations came in more clear, there were more of them, and the tuning was less sensitive to small dial movements.

So I think the FM Detector is working, but may not be adjusted correctly. I'd still like to understand why injecting the 10.7 MHz signal stifled the FM noise. Would I expect to see no AC voltage on the speaker with this 10.7 MHz signal injected? If I can understand what to look for, I'll have more confidence in attempting to adjust the IF cans.

pixellany, thanks for the help!

I read about the sweep generators to tune the IF cans, but I don't have one, and haven't touched my FM cans yet. I may try tweaking them the same way I did the L8 coil, using the peak AC Voltage across the speaker as an indicator, but as of now, the stations are coming in pretty strong, so I think the FM detector is working and I'll leave the FM cans alone... whether the detector is feeding the right stuff into the MPX detector or not is a different question.

I will look at the Q10 transistor driving the stereo signal light next. I can check the DC voltages against the schematic.

I will also check DC voltages at the MPX detector IO pins.


Attachments:
12GFM1 Schematic Small.jpg
12GFM1 Schematic Small.jpg [ 77.09 KiB | Viewed 1251 times ]
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 Post subject: Re: Novice Needs Help Adjusting Delco AM/FM Stereo
PostPosted: Apr Wed 05, 2017 1:09 am 
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Joined: Mar Mon 06, 2017 11:37 pm
Posts: 184
Location: Presque Isle Maine 04769
Shiny,

Good for you for attempting this. I have done a few radios now, and have received helpful advice on this and other forums and I am learning as I go too. It is really hard when so much learning is self directed and online. If it works, great. If not, put it aside, work on a new one, and come back to it later once you have learned more- also, I have connected with a few folks in forums and at recent meet and contact them off line for "special help.".

Keep us posted on your progress!!
Daryl

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Daryl B. Presque Isle, Maine


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 Post subject: Re: Novice Needs Help Adjusting Delco AM/FM Stereo
PostPosted: Apr Wed 05, 2017 7:06 am 
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Joined: Oct Wed 12, 2016 9:11 pm
Posts: 21
Checked DC Voltages:

IC1 (FM Detector) looks OK

IC2 (MPX Detector), not so much..

Attachment:
FM IC Voltages.jpg
FM IC Voltages.jpg [ 51.65 KiB | Viewed 1218 times ]


Attachment:
12GFM1 MPX Detector Block.jpg
12GFM1 MPX Detector Block.jpg [ 114.52 KiB | Viewed 1218 times ]


Pin 6 of IC2 feeds the base of the Stereo Light transistor, Q10. The voltages on Q10 (ECG 159 equiv), with lamp lit:

Sam's, Measured
E 11.68, 10.82
B 11.55, 10.07
C 0, 10.77

Are these voltages expected if Q10 is good and the lamp is lit?

0 volts expected on C suggests the readings are with light off, so I'm not sure how to interpret.

Without removing Q10, I measured using DMM Diode, swapping meter polarity:

CE 153, 80 ohms

I would have expected open in one direction, but not sure if there is another path when it is installed in PCBA.

I guess I can pull it from the board and check it or replace it.

The IC is listed as a Delco DM-14. Any way to tell if it is OK or do the bad voltage readings say it's bad?


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Apr Thu 06, 2017 6:01 am 
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Joined: Oct Wed 12, 2016 9:11 pm
Posts: 21
Pulled Q10, seems OK by DMM diode setting.

So pulled IC2, the DM14, verified all resistors and power connects tied to it.

Have what I hope is a functioning used replacement on the way... will be a few days before I find out.

Attachment:
DM14.jpg
DM14.jpg [ 28.47 KiB | Viewed 1176 times ]


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Apr Tue 11, 2017 7:53 am 
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Joined: Oct Wed 12, 2016 9:11 pm
Posts: 21
Replaced MPX detector IC and Q10 with used parts.

DC voltages on pins 4&5 now look right, so maybe some progress, but still some that are off and stereo indicator light remains lit over full range of adjustment.

I came to the conclusion it would help if I had ability to input a fixed stereo signal on a single channel, but I'm not going to buy an FM stereo analyzer. Being cheap, I downloaded a frequency generator app to my cell phone that let me use left, right, or both channels. I connected the cellphone by bluetooth to a $15 FM transmitter I use in my car as a hands free connection. I set the BT/FM transmitter to a "quiet" frequency, then used the phone to broadcast a 400 Hz tone on only one channel. I then tuned my radio to the tone.

I was thrilled to see this on my scope, connected to test point 26 on the schematic, which I think is the output of the FM detector:

Attachment:
IMG_1622-800x600.JPG
IMG_1622-800x600.JPG [ 71.84 KiB | Viewed 1135 times ]


I was able to adjust the phase control using this trace, seemed pretty straightforward to peak the amplitude.

I then changed the channel on my cell phone signal generator and the trace inverted as I would expect.

When I turn on both channels, I get this:

Attachment:
IMG_1626-800x600.JPG
IMG_1626-800x600.JPG [ 80.75 KiB | Viewed 1135 times ]


As I interpret, this means the stereo information the demultiplexer needs is there, correct?

I'll mess with this more and see if I can figure out why the MPX decoder is not happy.


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Apr Sun 23, 2017 7:11 am 
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Joined: Oct Wed 12, 2016 9:11 pm
Posts: 21
I have it working pretty well now! I found I had made a wiring error, and fed the output of the Stereo Indicator lamp pot into the wrong place, swapping it with what I think was the feedback from the "rear" channel (my 1970 radio was front and back, not left and right) output transistor... yikes.. Took me a while to figure this out, but it finally made sense.

Because I have no fancy FM alignment generator, I used an FM transmitter tied into my cell phone, set it to a dead space on the dial, and tuned the radio to it. I downloaded a signal generator app on my phone and broadcast a 400 Hz tone. I then peaked the audio output by adjusting the IF cans while watching the voltage across a speaker.

To adjust the multiplex detector, I broadcast the same 400 Hz tone, but only on one channel. I put my scope on the "quiet channel" output pin of the multiplex detector. It was easy to minimize the signal (maximizing separation) by adjusting the 19 and 38 MHz coils tied to the Multiplex detector. I found them to be pretty sensitive. I also adjusted the separation pot the same way. It was also pretty touchy.

My balance adjustment is also working!

And finally, I was able to adjust the Stereo Indicator lamp to extinguish between channels !!!

It has been a lot of learning, a lot of false steps, more stupid mistakes than I care to admit, but I am finally feeling like I got this thing resurrected and working pretty well.

I'm sure the radio isn't aligned as well as it could be, and I may mess with the AM and FM IF adjustments some more, but I am calling it good enough.


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Apr Sat 29, 2017 7:05 pm 
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Joined: Oct Wed 12, 2016 9:11 pm
Posts: 21
Here's the red dot working..

Link to Red Dot Video

I'm calling it good enough.


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 Post subject: Re: Need Help Delco AM/FM Stereo
PostPosted: Feb Wed 07, 2018 8:38 pm 
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Joined: Feb Wed 07, 2018 5:09 pm
Posts: 18
I have a 1970's Delco am/fm radio model 23afm1 who's schematic looks exactly like your schematic. And my problem looks to be around the DM14 integrated circuit.

Anyway my difficultly is that I cannot get access to that IC because I don't know how to disassemble the radio. The pc board for that IC is very inaccessible without some disassembly. But that disassembly looks pretty tricky to me and I am looking for some tips.

I have opened up the new forum topic below explaining my problem and I am hoping that someone here can help.

http://antiqueradios.com/forums/viewtop ... 6&t=335573

I apologize for injecting a side issue on this topic.

Thanks
Frank


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