1994 GMC K2500 Stereo: Minor Tuning and Noise Issues

[cc333]

When I bought the truck in 2017, it had a Pioneer aftermarket head unit installed which didn't work very well, primarily due to very bad AM/FM reception. It was also very inconvenient to use, being way out in the middle of the dash.

I noticed that the stock system was still intact, and thought it would be much nicer to use, given that the controls are much easier to get at, especially while driving.

I don't drive it much, so it took a few years to get around to it, but when COVID hit, I had nothing but free time, so I decided to dig in.

I began by buying a replacement stock center console bezel thing, as the original one was gone, and then I set about putting everything back together.

Fortunately, whomever installed the aftermarket system did so using adapters, so nothing was cut, and everything just plugged back in with zero trouble.

When it came time to power it on, I was pleasantly surprised to find that it still worked. It still had bad reception though, which turned out to be a bad antenna cable; I replaced the cable and that solved the no-reception problem.

I have since rebuilt the tape deck and upgraded the speakers (I posted about the speakers earlier this year) and wired in an aux input piggybacked off the tape deck (I soldered the aux cable directly to the deck's circuit board where the harness plugs in so I didn't have to modify any of the harnesses), so the system is, at this point, almost as good as it can be.

I say almost because there's a couple minor issues I'd like to resolve somehow, which leads (finally) into the subject of this thread.

First, reception is noisy across all bands, and in all conditions except when the engine is off and I have the ignition turned to the accessory position. I have read that this can happen when one of three or four mounting brackets for the CDM is missing, because it provides an important ground. Can I just put a bolt in there and run a wire to a ground point, perhaps on one of the other brackets, or do I need to somehow find and install the original bracket?

Second, tuning selectivity is quite bad, even for a stock tuner. Say I'm on 740 AM (for me a local all-news station) and I try to tune around. I can hear 740 clearly on both 730 and 750, and maybe also faintly on 720 and 760, depending on how close to the towers I am. It's not just slop, it's the same signal bleeding over. No other radio I've used, stock or aftermarket, new or old, does this. Could this be something a recap could fix? The CDM seems to work fine otherwise, so I'm hesitant to do a recap if I don't have to, lest I break something and make it worse.

That said, it works well enough that I've just been living with it, but I want it to be the best it can be and I want it to last, so I realized that I should probably look into fixing these minor annoyances now, since they may get worse in time to the point of rendering the stereo useless, which I don't want. I could upgrade to a better aftermarket system, but I like to keep things as stock as I can, since it's been my experience that stock systems tend to work best in the vehicles they were made for, and have all the nice integration features that many aftermarket systems – particularly the cheaper ones – often lack. And besides, if it works mostly fine as is, why replace it when it's easier and more cost effective to fix it? My only complaint is that the acoustics aren't too great, but that's a speaker and cab layout problem, not so much a stereo system one.

Anyway, I apologize for the fairly long winded post, but I want to be thorough.

Thanks!

c

 

[H2814D]

What year is your truck? If it has the stock stereo system, which components do you have other than the head unit (where the buttons and display is) and the CDM (under the dash near the accelerator pedal)? CD player? Cassette? EQ?

In any case, the CDM (under the dash) has very old capacitors, that, if they have never been replaced, have likely reached the end of their service life. This will cause reception and tuning issues. They are usually fairly easy to source and replace, if you have fairly good soldering skills. If not, you may want to allow someone else to do it for you.

But to answer your questions, you can certainly add a ground strap to the metal part of that CDM. The CDM is negative ground and the case is grounded to the electronics inside of it.

The usual issues with reception in these radios is with the antenna cable. If you still have your original antenna cable it usually starts to rust out up near the antenna mounted on your fender. Depending on which model you have, there is usually an access panel near the antenna connection that will allow you to get to the antenna cable. That connection in the line is usually what rusts out. Sometimes you can get away with trying to twist the connection back and forth to improve things, but if it is too far gone, replacement of the cable is in order.

 

[cc333]

What year is your truck?

1994

If it has the stock stereo system, which components do you have other than the head unit (where the buttons and display is) and the CDM (under the dash near the accelerator pedal)? CD player? Cassette? EQ?

It's a stock 5-band EQ system with Cassette (no CD Player).

In any case, the CDM (under the dash) has very old capacitors, that, if they have never been replaced, have likely reached the end of their service life. This will cause reception and tuning issue. They are usually fairly easy to source and replace, if you have fairly good soldering skills. If not, you may want to allow someone else to do it for you.

I see. One of my other hobbies is restoring vintage electronics, and old, failing capacitors are super common.

That said, I'm pretty good at changing them out, so all I'd need to know is how many there are and what their values are so I can get started.

But to answer your questions, you can certainly add a ground strap to the metal part of that CDM. The CDM is negative ground and the case is grounded to the electronics inside of it.

OK, I'll have to do that. Perhaps when I replace the capacitors.

The usual issues with reception in these radios is with the antenna cable. If you still have your original antenna cable it usually starts to rust out up near the antenna mounted on your fender. Depending on which model you have, there is usually an access panel near the antenna connection that will allow you to get to the antenna cable. That connection in the line is usually what rusts out. Sometimes you can get away with trying to twist the connection back and forth to improve things, but if it is too far gone, replacement of the cable is in order.

The cable is new. The old one was rotted, and somehow came into contact with a live wire in the engine compartment and actually melted, so needles to say, the radio was receiving nothing at all until I replaced it.

Everything else (the antenna itself and the connector at the CDM) seem solid, so I'm pretty sure the issue lies with the CDM. Probably the capacitors, as you suggest.

With strong signals, I find that this system actually sounds quite good even as is, especially on AM (lousy speakers and bad acoustics notwithstanding, of course). I actually was able to tune into an AM Stereo station recently (a nearly extinct species it seems), I noticed that its bandwidth opens up and sounds even better. I wish modern car radios could still sound this good on analog.

c

 

[scott2093]

I wish modern car radios could still sound this good on analog.

Delco have consistently been one of my favorite tubes as well. Not sure if GM had a stringent selection process..

 

[H2814D]

1994


It's a stock 5-band EQ system with Cassette (no CD Player).


I see. One of my other hobbies is restoring vintage electronics, and old, failing capacitors are super common.

That said, I'm pretty good at changing them out, so all I'd need to know is how many there are and what their values are so I can get started.


OK, I'll have to do that. Perhaps when I replace the capacitors.


The cable is new. The old one was rotted, and somehow came into contact with a live wire in the engine compartment and actually melted, so needles to say, the radio was receiving nothing at all until I replaced it.

Everything else (the antenna itself and the connector at the CDM) seem solid, so I'm pretty sure the issue lies with the CDM. Probably the capacitors, as you suggest.

With strong signals, I find that this system actually sounds quite good even as is, especially on AM (lousy speakers and bad acoustics notwithstanding, of course). I actually was able to tune into an AM Stereo station recently (a nearly extinct species it seems), I noticed that its bandwidth opens up and sounds even better. I wish modern car radios could still sound this good on analog.

c

OK. Thanks. It is quite possible you have a 16169561 CDM in that truck with that year and those components. If your new stereo has the larger antenna fitting (3/8"), it will not fit the CDM. Prepare for that too. Our CDM's have a smaller plug (1/4"). Yellow arrow is the original CDM size. See the pic.

There are two 470uF 16volt caps (these are the most common ones that cause most of our problems). They leak first and damage traces if not caught soon enough. Two 100uF 16volt caps. One 4000uF 16volt cap (hard to find, but you can substitute a 3900uF 16volt). The 4000uF is usually still good though. One 47uF 25volt (usually the orange one). And one 47uF 16volt near the orange one. In the tuner section, there is one 220uF 10volt and one 47uF 16volt. Replace the caps with good quality replacements. I attached a pic of what you should see when you open the CDM. This one is a different model but the components are in the same places.

Be very careful removing the old caps, since the board is multi-layered and removing the old ones too fast may disrupt the middle sandwiched layer. Try and get as much of the old solder out as you can. I am most successful with copper braided solder wick and flux. Note that you will be working through a rubberized conformal coating when removing that solder. And while you have the CDM out, make sure you go over the soldered side of the board and look for poor solder joints.

After re-reading your reply, I see you already have the antenna plugged into the CDM, so disregard that part of it, but in the event anyone else is considering the same thing, I went ahead and left the antenna plug size difference information in there.

Let us know how it goes please.

 

[cc333]

OK. Thanks. It is quite possible you have a 16169561 CDM in that truck with that year and those components. If your new stereo has the larger antenna fitting (3/8"), it will not fit the CDM. Prepare for that too. Our CDM's have a smaller plug (1/4"). Yellow arrow is the original CDM size. See the pic.

Yep, that's likely the one. My research suggests that the CDMs in the 94 trucks is generally considered more robust and reliable. I wonder why, exactly? Presumably, it's just a better, more refined design.

Looking at the photo, I'm impressed by what appears to be a very well designed tuning section that should be very much better than what I'm experiencing. Thinking about the problem, it almost seems like there's something going on with the the thing that filters out adjacent frequencies above and below the chosen frequency (I'm not sure what it's called), and I'm sure new capacitors will help.

Anyway, the cable I installed has the proper small connector, as did the rotten original; the aftermarket stereo that came with the truck was connected to the stock cable via an adapter.

There are two 470uF 16volt caps (these are the most common ones that cause most of our problems). They leak first and damage traces if not caught soon enough. Two 100uF 16volt caps. One 4000uF 16volt cap (hard to find, but you can substitute a 3900uF 16volt). The 4000uF is usually still good though. One 47uF 25volt (usually the orange one). And one 47uF 16volt near the orange one. In the tuner section, there is one 220uF 10volt and one 47uF 16volt. Replace the caps with good quality replacements. I attached a pic of what you should see when you open the CDM. This one is a different model but the components are in the same places.

OK, thanks for the info. I probably won't do it right away since I wiped out this month's budget on some upgrade parts for my computer. I'll likely get to this next month though.

Be very careful removing the old caps, since the board is multi-layered and removing the old ones too fast may disrupt the middle sandwiched layer. Try and get as much of the old solder out as you can. I am most successful with copper braided solder wick and flux. Note that you will be working through a rubberized conformal coating when removing that solder. And while you have the CDM out, make sure you go over the soldered side of the board and look for poor solder joints.

Good advice, thank you! To make de-soldering a bit easier, can I carefully scrape the conformal coating away from the capacitor joints?

This also reminds me that I should get some more solder and wick. I wish it wasn't so expensive *sigh*

Let us know how it goes please.

I will try!

c

 

[H2814D]

To make de-soldering a bit easier, can I carefully scrape the conformal coating away from the capacitor joints?

Yes. It is very thin anyway. And it appears to be silicone based. It will actually melt when the solder wick and flux is applied over it with the soldering iron, but the thing to consider is in NOT breathing those fumes. If you are going to scrape it be careful you don't damage the nearby traces. I typically just use the wick, flux, and soldering iron over the joint. Like I said, it melts away pretty quickly.

 

[cc333]

Good to know, thanks!

Can someone point me to an illustration of the CDM mounting setup? I am curious to see exactly where the missing mount is supposed to go.

I also noticed something interesting while driving the other day.

I was listening to the radio, and it seemed a bit noisier than I remembered, then I thought I'd turn the headlights on to see what would happen.

The noise was reduced, but not gone.

I'm supposing that turning the headlights on somehow reestablished the missing ground connection, but I'd have to study a schematic to know for sure.

c

 

[H2814D]

If you happen to find a schematic for the 16169561 CDM, or any of the models after 1990, you will be the luckiest person on Earth. But I don't think you will.

Attached is a picture of a CDM with the mounting brackets installed. There are metal clips that are attached to the CDM with little bolts that hold those rusty brackets to the CDM.

So if you haven't yet done so, I would suggest you open up that CDM you do have and inspect the capacitors inside of it. If they have never been replaced, they are 30 years old. The two 470 caps, especially the one 2/3rds of the way up on the left side and just before the tuner section shown in that pic up there ^^^^, are the first to go bad. If that one by the tuner section leaks its electrolytes, it WILL damage the traces and then the repair is a bit more involved after you recap it. And the capacitors may be the reason for your poor reception. Bad caps limit current and voltage, which the rest of the board needs to lock onto and fine tune the stations. They also filter noise, so they may be the reason.

Do you have LED bulbs in the truck anywhere?

 

[Donwan]

I would also mention that running it without the bracket you are likely experiencing ground issues. Be sure to check the ground under the dash on the driver side by the flasher.