A little background... I have owned my 1970 Corvette for over 15 years, so am quite familiar with it. It is 'unrestored original' that I would personally consider a surivior, if I were allowed to use a trademarked term.

My radio has worked 'as expected' through those years.
This spring when I got my car running from storage my radio appeared to not be working.
After doing some research here and digging into the car I realized that the radio hadn't actually stopped working entirely but appeared to be playing VERY quietly in the back of the radio itself (nothing from the speakers) - this is with the passenger dash removed my ear about 12 inches away and the motor off. The volume did go up and down as I adjusted the radio volume. I could recognize songs (volume full, ear 12 inches away from radio) but could not actually listen any further away.
After doing some reading I decided this must be the transistor, so I sourced a NTE105 to replace my original Delco (7005 date coded) transistor.
Today I replaced the transistor, but to my surprise nothing has changed - exact symptoms as before.
What's my next course of action in troubleshooting and repairing this problem?
I have no desire to modify my car from original, but I would like to listen to the radio 1970 style.

 

Hi 70/46,
You may want to give Jerry Rudbeck a call. He's in S.C..
He does a nice job repairing these radios. I was pleased with the repair he did on my 71's am/fm radio a few years ago.
You can describe what's going on, and perhaps he can give you some advice or suggestions.
His repair costs are very reasonable if you decide to send it to him.
864-363-0947
Regards,
Alan

 

Jerry Rudbeck
Electrodesign.US
864-363-0947

Got my AM/FM stereo U69/U79, back from him about two months ago. It works perfectly. Nice guy. Very reasonable cost about ($250). He had it about three weeks. Looks great.

Dan

 

Thanks for the advice...
I'm in Canada so logistics of cross border repair might not be straightforward... (although I do benefit from GM's export data)
Anyway, if possible would like some input on troubleshooting myself or Canadian repair options before shipping cross border... Thanks again.

 

I had all of the OEM radio equipment in my 74 when I bought the car and the radio did not work. After the radio came back from repair the radio still did not play in the car. The radio repairer suggested the speakers were bad when I discussed the situation with him. I thought it would be unusual for both speakers to be bad. He said it was not that unusual for both speakers to have failed. He was correct.
I suggest you hook up a spare speaker to the passenger side as that side is easy to access and see if the radio play from that side.

 

Normally when you send something from Canada to the USA for a repair you need to fill out paperwork prior to it leaving. I forget which form so I will check and get back to you.

 

Agreed - mine kinda worked on one speaker (kick panels on'68) the other side just buzzed a bit - inspected and speaker cone on both were shot. Haven't bothered to repair in 12 years - wouldn't be able to hear it anyway over the sound of engine 'music'

 

Try a different speaker that is not in the car. What you are hearing is most likely the sound coming from the output transformer or some other component that is being driven by the radio, but it is not making it to the speaker.

 

Your radios problem is typical and for the sake of sharing the bliss, here's why you heard what you did.

First, 70-76 4X6" vette speakers are notorious for failures. The cone and black sealant over the hair thin voice coil wires harden and when the volume is turned up, they just don't flex like they once did. Thus, the wire shears off and no more sound. Look for electrically correct low power (made for the old radios) speakers. Don't stick modern high power speakers in as they are made for the higher power new radios (and are less efficient with the available output power).

From 58-67 Delco mounted an inductor (half a transformer) on the speaker proper. It had several functions but think of it as the audio output transformer primary and the voice coil as the secondary. In 68, Delco moved the inductor inside the radio and also onto the stereo decoder where it belonged.

You could hear the radio playing at a low level because the audio power was traveling through that inductor (open voice coil) and setting up a strong magnetic field. That field saturated the sandwiched flat metal laminations that the inductor coil is surrounded with and the moving pieces of metal acted as very tiny speakers. Electrical vibration converted to acoustic vibration...

The original transistor was in great shape to allow that but you were close to the core problem.

Now that you changed the transistor, be sure to re-adjust the radios internal bias control so that the idle voltage between the green and black wires is close to 1.5 vdc at the affected speaker. And just FYI, it normally draws about 1.5 amps with the volume turned down.

Greg Thompson (877-780-2861) sells an excellent replacement speaker which was made specifically for 68-76's.

To bring the audio quality back up to snuff, remember that the electrolytic capacitors have aged far beyond their design life specs. Change all those and you'll be surprised how good those U-69's can sound.

That model radio has a lot of untapped potential! It's one of my favorites. You should hear it with 6X9's added in parallel with the 4X6's. Just remember to re-adjust the bias to help correct for the impedance change.

Good to hear you kept the original and avoided the rice box replacement.

 

Wow, thanks for the detailed info Jerry.
It sounds like you're saying you believe my radio is fine since I'm hearing what I'm hearing in the back, but that it wouldn't be a bad idea to replace the caps inside it at some point.

It also sounds like you're suspicious of the speakers... one piece of info that I didn't include is that about 12 years ago I had a bad passenger side speaker and replaced it with a speaker I found in surplus at Princess Auto (it's a canadian thing). The speaker was labeled as a Delco 9.5 ohm (iirc), which I think is exactly what I want. And the sytem worked and sounded 'okay' for many years since then.
As someone else stated I find it hard to believe both speakers would go at once, but will see if I can find a speaker to test with this weekend.

Last question for Jerry - how do I go about adjusting the internal bias control to fine tune the output voltage - where is this adjustment?

Thanks again for all the help. I will update again once I've had a chance to troubleshoot further.

 

Hi Jerry,
VERY nice to see you on the Forum.
My radio is still working fine, (Chapter Top Flight last fall with ALL the Operations points), AND I'm still recommending your expertise and services!
Best Regards,
Alan

 

1970L46 -

Yes, that's what your description seems to indicate. But remember, you have two inductors, one for each channel and they are in different locations. Check both for sound. And, the laminations in some inductors are tighter than others. The more tight, the less the sound level.

But a better test is to unplug the speakers and test each one with an ohm meter. Of course the continuity could check good and still the voice coil could be stuck in it's chamber, but it's rare. If the inside of the magnet frame cylinder wall has rusted, the coil won't move and the paper cone won't vibrate but you will read continuity. Still, the 4X6's invariably have open voice coil windings.

There are no absolutes with these old radios, especially vette radios. Whether from neglect or manufacturing quality, the 1500+ vette radios I've seen aged poorly compared to other models. So you have to be prepared to check everything. Multiple problems are also common.

As long as the speaker you install is 8 to 10 ohms, don't worry although one designed for cars is the logical choice. The radio isn't going to be that touchy (if it's amplifier parts are still in tolerance. The bias control will compensate for any normal variance anyway.

And yes, I see it all the time that both speakers will go at once, especially with new installations of radios hooked to speakers that have not been used in a long time. Everyone invariably cranks them up all the way to see how loud they will be and the dried speaker components can't take the stress and the coil wires snap.
Additionally, since the coil wire has been sealed into the black (liquid tape like) compound on the front of the cone, moisture is also sealed in at the time of construction. If you follow that extremely thin wire and scratch the black stuff off until you find the break (allow a lot of hours with a high magnification lens and fiber optic glass repair tools), you'll eventually find a track of green. That's what's left of the copper wire where it decomposed.

Anyway, this is all interesting trivia to me and part of the reason I don't get bored working with them.

Regarding the bias ajustments, go to Ebay and search for AR-78. That inexpensive manual will give you pictures of the radio and demultiplex boxes and way more techie info than you'll need to set the bias and locate the caps and other parts that tend to decay over time. Also, remember, there are two bias controls and two inductors and two amps and you have two speakers with no sound. Finally, if the speakers do check good, start looking for corrosion at key electrical points within the radio circuits and focused in the power amplifiers.

Have fun!
___________
To Alan and everyone else I recognize here in the thread: Thanks! It became so rare when I could get on the forums! I missed it. Last months vacation time was great and the first time in years that I was away from the bench and all the radios. It was time for a break for sure and I'm going to slow down a little bit on the radios to have time to enjoy the car side of our hobby!

Let me know when it quits Alan but I'm hoping it won't fail until long after I do. ( <-I couldn't find one with horns.)

And please all, help me spread the word - don't use RadioShack tuner cleaner to spray volume controls to get the scratchy noise out! That corrosive stuff is a metal (contact) cleaner for old television set switches. It stops the scratch for a while but if you open the switch, the carbon deposited track is being turned to black mud and being wiped off a bit more each time the control is turned. Actually, the guy who suggested using 30 weight oil was doing less damage (don't do that either).

Have fun, that's what it's all about!

 

That basic radio [and Im talking about the internals, NOT the bezel,dial,etc]was used through 1976 Camaro and X body (Nova, Skylark, etc) complete with flip dial, etc., if I remember correctly. I have one right here coded for a '75 X body, in fact. I agree; it is a very fine unit- the first one I had was in my '71 Chevelle (from the factory: manual drum brakes, but an FM radio, lol).

Anyway my point mostly is that some of the Delco service manuals down the line will also include this radio as well as newer designs. I have at least one myself that does. So if you have trouble finding schematics, etc., look in less obvious places....

I think my Chevelle also used those 4x6 speakers, too- 10 ohm. I added Jensens in the rear and an under-dash Alpine cassette unit.

 

Oh and something else: Guys who are missing the "convector" can easily build a replacement simply by scrounging up a couple of old AM radios that used that same DS-501 transistor. You can usually get them for next to nothing. Just find one if Mono or two if Stereo and keep them with the attached heat sink, find the suitable mounting location, and fab an appropriate cable to connect them. You may want to make adjustments such as those referred to in this thread earlier by Jerry - Radio Guy regarding bias, etc.

 

Update:
On the weekend I pulled out my DMM to verify Jerry's suggestions.
Testing from the 3 wire connection behind the radio (2xGRN, 1xBLK), I found that to the speakers I was getting an open - both sides (testing continuity between black wire and either green).
I also found that I was getting 1500 mV (1.5V) from the radio between black and either green.
I didn't have a speaker around to test for sound, which I would have liked to do before shelling out a good chunk of money for replacements... BUT I found this Canadian link (great for me) that was selling Delco 4x6 10 ohm speakers for a song...
http://www.fcsurplus.ca/shopping/pro...-Car-Speakers/
It does appear that they will ship to US if anyone needs...

 

Jerry,
hello,
hope no one gets mad,
I did not want to start a new thread for a simple speaker question.
What are the correct replacement speakers for my '68 coupe?
I have 1 in each of the kick panels. The left side sounds shot.
Right one plays nice and loud.
Thanks in Advance,
Marshal

 

If you still have the stock Delco radio head in the car, you will need to find speakers that have a satisfactory impedence value. The original speakers were 4" x 10" [low wattage] speakers with 10 ohm impedence. You may be able to find some "original configuration" replacement speakers, but they will be of old design, poor sound quality.

Look for 4" x 10" speakers with 8-10 ohm impedence that will be rated for 25 watts (or so). That may be a tall order for you to find, but that's what you need. If you can't find a good quality car audio speaker with those specs, the direct replacement speakers sold by the Corvette aftermarket vendors is probably your next best alternative.

Another option would be to install two 4" x 6", 4 ohm speakers in each location...wiring the speakers in series. Those speakers are readily available, and when wired in series would yield an 'effective' impedence of 8 ohms. If I were in your shoes, that's would be my "Plan B" instead of buying the "replacement" speakers.

 

I replaced both speakers on my 74' about two years ago. I bought the replacements from Volunteer Vette Products. The speakers, which were reasonably priced, were electrically correct and had the proper end plugs on the wires to connect to the car speaker harnesses. I did have to slightly enlarge the 4 mounting holes to fit over the speaker mounting studs. I would say the quality of the speakers is about equivalent to what GM installed originally. Radio plays fine with them.

 

Hi Marshal!

7T1vette is right.

And here's an easy way to solve it. Contact Greg Thompson at 877-780-2861 and ask him to tell you about the 4X10" -Delco Style Special Rectangle- speakers he had reproduced. They are exact replacements for your car (and quite a few other GM products). Save the brackets from your original speaker(s) to use on the new ones. If the wallet will handle it, change them both and have it done with. Greg's speakers are well built! _Jerry_

 

Hope its ok for me to jump int here too...

Jerry,
my 74 the radio will work then quit.......work then quit.......the light does not come on when not working........bad power source?

speakers when working not the best sound...
just bought the car so had not been used last couple years.....your thoughts?

thanks,