1972 Corvette radio repair source

[Patrick H.]

Re: 1972 Corvette radio repair source

Jerry,

Thanks for the advice. It is a stereo model.

The speakers do work, as the 8 track player still plays through them. The radio just stopped playing through them.
I still wonder if the aftermarket add-on 8 track is causing the problem with its internal switch not letting the radio play, but I would not know how to fix that or test it.

I'll pass your info on to the owner. Any suggestions related to the tape player and how to bypass it are welcome. I haven't yet figured out how the player taps into the radio to be able to play when it does.

Patrick

[Jerry R.]

Re: 1972 Corvette radio repair source

If you're not talking about the power switch Gary, then it sounds like someone has already sprayed the volume control segments with WD-40 to remove the scratchy noise that's so common in those controls over time. That caustic chemical was made to clean metal contact surfaces.

The tracks in the two tone segments and the two volume segments of the control are deposited carbon powder. The chemical in contact cleaner breaks it down and turns it into black muck and the metal wiper arm digs a groove and pulls the carbon away from the track path over time. You were wise to not succumb to that poor advice that just won't go away. Some searching on the two main vette forums will find pictures I posted of the tracks after being sprayed. Once you've seen it you know it's horrible advice!

Ed has it nailed (as usual). Either a donor radio is required - as spares just don't exist - or a lot of detective work to find another NOS Delco control as a source of parts from a different model GM radio is required. Rebuilding the control is possible, it's just not easy.

If you were talking about the power switch not working reliably, I can provide a more positive reply (with details). Almost any competent auto radio shop should be able to fix that without blasting the control from the outside.

[Jerry R.]

Re: 1972 Corvette radio repair source

Ah, I missed that part Patrick. Bad speakers are so common in my life I just tend to assume that's the problem. And original Delco speakers rarely let me down "figuratively".

There are two common methods of switching the speakers in tape players. The audio output wires of the radio are passively connected to the speakers through un-actuated mechanical contacts in the tape player when it is idle. That can be through 1.) contacts in an electrical relay or through 2.) contacts pressed or depressed when a tape is inserted pushing on the contacts. Things must be taken apart and wires must be traced. And usually contacts must be cleaned and re-aligned after all of these fleeting decades.

I think you know the answer. Remove the tape player and wire the radio to speaker connections back to original. I bet the radio is still working based on the stereo decoder relay chattering.

The Pontiac stereo and even mono radios (Delco's) used original tape players wired in that configuration. It was the full audio output from the radio that drove the speakers and the full audio output from the tape player that drove the speakers when in use. For all intent and purpose, the wiring was separate in each audio path. The radio speaker wiring just ran into and out of the tape player for switching. I've had to trace those wires many times until I was ready to scream.

Persistence will win that battle Patrick. It worked before thus... it can work again.

Good hunting!

Jerry,

Thanks for the advice. It is a stereo model.

The speakers do work, as the 8 track player still plays through them. The radio just stopped playing through them.
I still wonder if the aftermarket add-on 8 track is causing the problem with its internal switch not letting the radio play, but I would not know how to fix that or test it.

I'll pass your info on to the owner. Any suggestions related to the tape player and how to bypass it are welcome. I haven't yet figured out how the player taps into the radio to be able to play when it does.

Patrick

[Patrick H.]

Re: 1972 Corvette radio repair source

Thanks Jerry.

I'll send you a PM as well.

Patrick

[Jerry R.]

Re: 1972 Corvette radio repair source

Thanks Jerry.

I'll send you a PM as well.

Patrick

___________
PM response sent Patrick...

[Gary R.]

Re: 1972 Corvette radio repair source

Hi Jerry
So it sounds like that pot is no where to found? The radio itself was upgraded to the common electronics, about $400 at the time. It worked until I started to have a problem turning it on. If I very slowing creep it on, it will catch and work fine- unless I turn it off. So I leave it on.

Thanks for any input.

Gary

[Jerry R.]

Re: 1972 Corvette radio repair source

Hi Jerry
So it sounds like that pot is no where to found? The radio itself was upgraded to the common electronics, about $400 at the time. It worked until I started to have a problem turning it on. If I very slowing creep it on, it will catch and work fine- unless I turn it off. So I leave it on.

Thanks for any input.

Gary

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There are basically three answers/solutions to a switch problem Gary.

1.) Ed was spot on! (as usual) That's a five gang control with 2 volume, 2 tone and one dual power switch section. And it comes apart from the front; not that it was ever intended to be taken apart and survive. There are no spares that I've ever
seen and a donor must be from another radio. All that unsoldering of terminals (18 as I remember) from the circuit board is mainly tedious but not something an amateur is going to accomplish. And buying a radio with the hope of getting a good control is tenuous.

2.) If you have nerves of steel and a skilled hand, you can take the control apart one section at a time and then repair or replace the dual power section with a donor from another Delco control. For the most part Corvettes were the only ones that used dual power switch segments (dial lamp and radio). But Delco made radios for all the various GM products and the similarities across models is wonderful for a scrounger. BUT, you only get one, maybe two, attempts at disassembly because the bend over retaining tabs in the controls snap off. Then a guy has to get creative.

3.) Now, you're going to like this part - sort of: I find that better than 19 of 20 controls can be fixed from the outside effectively and quickly, just not by an amateur. Accomplished techs who know the exact layout of the moving components inside the switch segment can do reliable repairs without doing something absolutely stupid like blasting the control with WD-40. So, you did the right thing Gary by not trying to spray it. Your control will be salvageable along with your radio. And most any shop should know how to fix it without a fire hose of caustic chemicals. They should be able to apply the correct solution directly into the switch section.

One drop is more than enough. As how they do it.

You probably want a shop that will do targeted fixes like that. You'll still get a big bill, but it should not be $400. After all it takes 5 minutes to pop the top and 15 minutes in slow motion to fix the control. No, I won't do quickie fixes. They eventually come back and I don't want to see the same radio in less than 30 years. :-)

Why is the control not working? Easy, two reasons. The lubricant they used on the moving parts is now old and hard (like glue) restricting movement and the electrical contacts will have surface corrosion. Solve those two problems and your control will continue to work almost indefinitely.

You asked a good question! There's my two cents worth. Perhaps others have alternate suggestions for you.

Jerry

[Gary R.]

Re: 1972 Corvette radio repair source

Thank you Jerry for taking the time to explain my options. Will the product Ed mentioned work in this application? If so is this available to purchase over the counter.

Since I don't get very much time to work on my own car I will not be removing the radio anytime soon.

[Joe L.]

Re: 1972 Corvette radio repair source

Hi Joe!

Nope, the difference is mostly cosmetic. There are minor circuit component value variations from beginning (68) to end (72) but when you master one, the rest are fun.

All of the amplifiers in the stereo models in that date range can deliver noticeably greater amplitude and a more robust frequency response with very minor component additions. And the aged components, including the intermediate frequency transformers, are repairable if not replaceable.

And the poor 70 through 72's that are restricted by 4X6" speakers can drive a second set of speakers if the bias voltage is re-adjusted. The variable control to accomplish that is already accessible for the radio amp itself and also for the second channel amp in the stereo de-mux (decoder) box on the firewall.

Even the 68-70 models can add a 6"X9" speaker to each channel and redistribute the available amplifier power more effectively in the lower end of the audio spectrum.

The mono's followed the same component pattern variability as the stereo's but changes were minor by my definition and they just didn't have the performance potential of the stereos (and they should by engineering/circuitry comparisons). Still, it's easy to add a 3 to 6 acoustic decibel bump up to them.

73 and after are good performers but when the power switch fails or the DM (Delco Module which was a discrete component configuration in a plastic container - forerunner to an integrated circuit) preamplifier fails, life gets expensive. I'm sure a lot of unused spare parts went to the dump when GM consolidated dealerships. Special original unused parts (with house numbers) are incredibly rare. And 76's used a DM that unique compared to it's predecessors.

Hope that helped. Guess Ill try to answer Gary's question above next. That's the most common problem now that will send original radios to the dump (or eBay to be dumped on someone else). GRIN

Jerry------

For the on-off, volume control switch all you need to do is to find a GM #7304275 which was discontinued without supercession in August, 1971. As far as I can tell, this was a Corvette-only piece. In any event, finding an NOS example is probably about as likely as the people in hell getting ice water.

[Jerry R.]

Re: 1972 Corvette radio repair source

Hi Jerry
So it sounds like that pot is no where to found? The radio itself was upgraded to the common electronics, about $400 at the time. It worked until I started to have a problem turning it on. If I very slowing creep it on, it will catch and work fine- unless I turn it off. So I leave it on.

Thanks for any input.

Gary

If you are referring to the "NOS or service replacement pots" that Ed mentioned, you won't find any without great patience and hours of phone calling and pure luck. I once spent 8 hours solid on the phone until I found an undocumented NOS Delco control for a 62 vette. Pure luck that I found the alternate number in some old Delco documentation. And it was not cheap!

The original part number that Joe mentioned below (or above?) is what you are after for a direct replacement. There were no aftermarket substitutes specified in my original CTS, Clarostat, Mallory or Centralab cross reference aftermarket catalogs (yes, I'm that old, and older :-) ). I've looked many times over the years and eventually always either repaired what was there or, if either one of the sections was still viable, created a circuit to duplicate the original dual switching functions using that one switch section. That keeps the correctness of the dial lamp operation intact (controlled by dash lighting switch).

But if all you need is the on-off switch section, then you can get that off a 7303326 which was the mono version control. The switch sections are the same. As an alternative, a 68-76 control salvaged from an old beat up mono radio will solve your stereo's problem. Plenty of mono radios will always be available. And there were Centralab, Mallory and CTS replacements made to replace the original.

Option #3 is your most cost effective solution. Or, jumper the sections and add an external switch or do as your are now, just leave it on and pray your wife doesn't turn it off.

Jerry

PS: thanks for the input Joe. And I'll take some ice water with me - never thought about that option. heh

[Ken A.]

Re: 1972 Corvette radio repair source

Jerry------

For the on-off, volume control switch all you need to do is to find a GM #7304275 which was discontinued without supercession in August, 1971. As far as I can tell, this was a Corvette-only piece. In any event, finding an NOS example is probably about as likely as the people in hell getting ice water.

Hopefully there will be some new ones by Fall. Will keep you posted.

[Jerry R.]

Re: 1972 Corvette radio repair source

Hopefully there will be some new ones by Fall. Will keep you posted.

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If only 58 through early 62's were in that stash Ken... I'm so tired of rebuilding them from scratch and scrounge.

Gary, this method is not a quickie fix that I'd do but it could last a while and you wouldn't hurt anything if you're careful.

Take both covers off the radio (to allow the heat to pass through), locate the control, locate the switch section at the far rear (four wires, two are brown) and find a shrink tubing gun or a hair dryer with the smallest nozzle available if you have more than one. Turn it on low and aim it at the switch segment and let it warm up. Don't overheat stuff, you just want to soften the hardened lubricant in the segment.

As it warms, start switching it on/off. If it starts clicking crisply, stop. If you have an ohm meter, monitor across the brown wires or the two lamp wires for that matter.

Good luck! _Jerry_

[Patrick H.]

Re: 1972 Corvette radio repair source

Assuming that's what you are referring to, then I would venture a strong suggestion that both speakers are original and therefore they are both shot (open voice coil). There is another late C3 here now and sure enough, the radio works but the owner apparently did not check the speakers as advised. If you want instructions on how to check them Patrick drop me an email. This is a very common problem with the 70-76's. Greg T. can supply excellent replacements made for that original radio or Ron McGee can re-cone the originals.

_Jerry_

Of course, over half the thread has nothing to do with my original question, but...

We have a winner!

Today I went over to try and remove the 8 track stereo system from the equation so that we would know exactly how much it was interfering with the stock radio. I had to remove the storage doors and frame from the rear compartment to access the wiring of the tape player (it is mounted into the bulkhead between the seats). I followed all the wiring and found speaker wires extending to each side of the car, but could not find them at all in the dash or attaching to the speakers. So, I had the owner run the 8 track player to see what was going on...and found speakers mounted behind the kick panel on each side. It turns out that the "speakers" worked when the 8 track was played, but they weren't the factory speakers! So much for assUming.

Needless to say we ordered S&M ElectroTech replacement speakers today and will install them in the next week.

I'll update then, but I would suspect that Jerry is correct in his diagnosis.

[Jerry R.]

Re: 1972 Corvette radio repair source

Of course, over half the thread has nothing to do with my original question, but...

We have a winner!

Today I went over to try and remove the 8 track stereo system from the equation so that we would know exactly how much it was interfering with the stock radio. I had to remove the storage doors and frame from the rear compartment to access the wiring of the tape player (it is mounted into the bulkhead between the seats). I followed all the wiring and found speaker wires extending to each side of the car, but could not find them at all in the dash or attaching to the speakers. So, I had the owner run the 8 track player to see what was going on...and found speakers mounted behind the kick panel on each side. It turns out that the "speakers" worked when the 8 track was played, but they weren't the factory speakers! So much for assUming.

Needless to say we ordered S&M ElectroTech replacement speakers today and will install them in the next week.

I'll update then, but I would suspect that Jerry is correct in his diagnosis.

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Someone thought that install through quite well. Now you have to find where they hid the CB.