71 radio not working

[Vincent F.]

I have power and ground to the radio, not sure exactly what the amplifier does, but after doing some testing on it the 3 prong plug they all have continuity together which dose not seem normal. Is there a resistor inside? Im lost on this.

[Les G.]

Re: 71 radio not working

There are alot of possibilities I don't know how long you have own the vehicle or if the radio worked but then stopped. To simplify things check the speakers first after 40 years they are usually the first to go. The car came with 10 ohm speakers and you should replace them with 10 ohm. to test you can just hook up 2-8 ohm speakers and see if that solves your problem.

[Keith L.]

Re: 71 radio not working

If it doesn't light up I doubt it's the speakers.

[Vincent F.]

Re: 71 radio not working

what bis the function of the amplifier?

[Jerry R.]

Re: 71 radio not working

It is the last audio amplifier transistor that drives the speakers and it is extremely rare for them (DS-501's) to fail.

Suspect the speakers for sure! The 70-76 4X6" originals all experience open voice coils sooner or later. Past that the usual culprits are internal electrical corrosion and failing electrolytic capacitors. Not rocket science - more like changing spark plugs.

By the way, it's common for one or both halves of the power switch to fail leaving things pretty dead. That can be repaired.

[Alan S.]

Re: 71 radio not working

Hi Vince,
Jerry R. does a nice job repairing 71 radios, and others, at a fair price. I had a pleasant experience wth him. If you out run out of possibilities iat home consider sending it to him.
Regards,
Alan

[Jack H.]

Re: 71 radio not working

In GM radio parlance, the part you call 'amplifier' is refered to as the 'Convector' GM # 7311901.

It's nothing more than a DS-501 transistor, heat sink and cable interconnect. It's job is to translate final audio drive waveforms from voltage to current.

However, I diagree with the advice that the DS-501 transistor is a hearty critter and almost never goes bad. It's a form of germanium transistor (RIGHT from the original Bell Laboratories technology transfer)....

There's more than one reason for why the rest of the world embraced silicon transistor technology when Texas Instruments introduced it under their patent!

[Jerry R.]

Re: 71 radio not working

Here's a bit of hard data on the DS-501 topic. If anyone else has any measured reliability numbers on this device or on the failure rate of the FM oscillator transistor in the 65-67's, please share it. (Yes, I actually find this stuff interesting...)

The DS-501 debate caught my attention quite a while back as it arises regularly on the various forums. Most of the conclusions have been speculative/subjective at best so, having switched to a spreadsheet format a couple years back to document the various radios that cross the bench, it was pretty easy to compile the number of radios (mostly vette) containing either one or two of these devices and compare that to the replacement parts inventory showing the number of DS-501's used.

So, having learned to never trust my assumptions, I ran the numbers last month on the sortable 1300 Delco radios most recently repaired and entered in that document. In all, a total of 1534 DS-501's were tallied. Of those devices, 6 were defective which I computed as a failure rate of 0.39%. That's 1 in every 256. Not too shabby considering that Delco never bothered to apply thermal grease between the device and the heat sink. Additionally, the extremely high number of today's Corvettes with replacement speakers having grossly incorrect loading coil values (which stress those transistors) indicates an excellent survival record.

From memory, the DS-501 is rated for 60 watts (not audio watts) @ 77F and 50 volts so it should handle 10 amps or more with that big heat sink. The 63-67 radios, as an example, typically only draw a fairly stable 1 amp of amplifier current when loaded with a correct Delco speaker. Not much of a stress (12 watts)... For reference, the whole radio pulls around 1.5 amps.

The aftermarket vendor speakers (other than Greg's at turnswitch.com and Corvette America's) stress those device rating figures significantly and degrade the DS-501 physically (due to heat) over time. This is quickly observable by the measured deterioration of the low end audio response. But if all works as designed, the fusible resistor in the radio will fail and protect the DS-501 long before its parameters are exceeded. That is its specific function. I've observed that even that failure is very limited and more correlative to adding loads of extra speakers and similar tinkering.

On a related subject, the battery buss (all 12 volts) from the radio power switch is being fed through the DS-501 with only a .47 ohm fusible resistor in series between it and the extremely thin wire in the speakers voice coil. The loading coil is there, in very simplistic terms, to shunt some of the current away from that fine wire. If you ever tinkered with model engines, you know what a glow plug does when power is supplied to that thin wire. BEWARE and use a correct speaker WITH a loading coil!

For what it's worth, now you understand from whence my comment came...

[Vincent F.]

Re: 71 radio not working

great posts, I wish I knew more about the electronics as well.
on the 3 prong plug for the Amp, 2 wires go to the quarter sized resistor I assume insulated from the fins, and the 3rd wire looks like ground.
Does it make sense all 3 ring contunuity together. is this a short in the resistor?

[Jerry R.]

Re: 71 radio not working

That's the PNP type DS-501 transistor Vincent, not a resistor.

Here's the information you need to test it:

1. the blue wire is the collector, the green wire is the base and the yellow wire is the emitter.
2. the instructions are here -> http://www.allaboutcircuits.com/vol_3/chpt_4/3.html

[Jack H.]

Re: 71 radio not working

An interesting approach, but here are a few critiques:

(1) The candidate radios you're studying are WELL burned in which neglects infant mortality effects.

(2) At this point in time, the daily use profile of these sets is WELL skewed from the exercise profile they saw when new and the donor car was driven on a daily basis.

On the speaker voice coil 'loading' up with current to mimic a diesel glow plug, audio spectra is AC without a DC content. The time averaged voltage applied to the speaker coil is ZERO and heating effects result from the true RMS value of the audio waveform's ensemble...

[Dan D.]

Re: 71 radio not working

Back in the early 60s I repaired a few of these radios. Almost every one was the DS-501 and its fusible resistor. Jerry, I don't know if your spreadsheet recorded age or not, but it is possible, even probable, that very early devices were not very robust and the device was improved over the years. In the late 50s and early 60s transistor technology was in its infancy and reliability left something to be desired. Especially true with Germanium power transistors.

As for the small signal PNP transistors used in the front end, yes, I have seen them fail too, but I don't have any idea what the failure rate might have been. But I do know that high frequency Germanium PNP transistors back then were also very fragile, regardless of who made them. In the early 60s the company I worked for used them, and they would fail without warning. One device we used back then was a 2N1742 - made by Philco no less. I still have some of those critters (as Mike C. would say). I will never throw them out, but I will never use them either. -Dan-

[Jerry R.]

Re: 71 radio not working

Excellent and I'm glad to see that you agree with me regarding the DS-501's Jack.

So many of the various forum guys asking for guidance or for info on how to test their convectors are being led to believe that the DS-501 component is a high failure item today. As their units are indeed among those you describe as 'in the survivor category and that have been burned in over several decades', their mortality is not high on the suspect list. Thanks for confirming this. That was my point.

And thanks for the critique although it is not relevant to the data I compiled and referred to earlier. I can't say I've ever talked with anyone who was asking for help with their unused/un-exercised C1 through C3 radios. My world is the mud wasp filled, rusty bricks that guys inherit in third and fourth owner cars. New radios would be boring... :-)

You misinterpreted the point of the speaker loading coil comment Jack.

I fully understand the flow of the DC and AC components in the pseudo primary and secondary of that odd engineering output circuit solution. That's not in question. The point was simply to advise 58 through 67 guys that if they find a replacement speaker in their cars that DOES NOT have a loading coil, they need to get that speaker out of there. Not just from the technical/safety aspect, but a refreshed radio is just not going to deliver its best possible sound if the speaker is hampering the operation.

Additionally, 58-67 owners need to be aware that the units still being sold by LIC, CC, and several other aftermarket vendors today do not work correctly for their Corvette application. (I've sent emails to some of them but responses never arrive.) If only the bias controls in our radios had enough adjustment range to compensate for the aftermarket speaker's engineering error... Sad!

To digress to the first item above for a moment, the abnormally high current load of the incorrect speaker loading coils -IS- stressing the DS-501 thermally (as the forum experts allude) but the fusible resistor is designed to fail before the high current transistor does. Guys should check for an open .47 ohm 'fusistor' before suspecting the output transistor.

Finally, to clear it up for the non-techies, the few feet of wire in a voice coil wound on a paper form and slipped over a magnet is not meant to handle a large current source at 12 volts WITHOUT a loading coil being installed in parallel.

To convey the message more clearly, I should have said "what would happen if you took 6 to 8 feet of very thin voice coil wire and put it across your car battery?" I guess that would give a new meaning to the phrase "fire it up!". Thank heaven for the current limiting properties of the two devices between the battery and the speaker voice coil when no inductor (loading coil) is present. Plastic and paper speaker cones can burn.

Happy holidays to all who suffered through this techie talk!

[Jerry R.]

Re: 71 radio not working

Hi Dan and yes, the sheet records the model year of each radio repaired. So they are sortable, but the date codes of the devices are often not legible.

I too go back to the early days of transistors and kept a few of the CK721's and 2's that the guys at Bendix offered me in the mid 50's. Strange looking by today's standards and I also would never use them. They had a max voltage of 1.5 as I remember. They are destined for one of my kids with an MSEE. I was two days old when the first successful transistor was tested. Scary... but fortunately my Germanium percentage is very low.

And yes, you nailed one of the common failures of the 63-67 problems so prevalent today. The germanium units just stop working. But, when the soldering pencil touches a lead, the device springs back to life. In goes a re-biased silicon sub, at least on this bench.

That's exactly what the FM oscillators do except that the recommended substitute moves the injection frequency so far off that 108 usually ends up at 104 on the dial and that's where lots of shops left them. They didn't know how to modify the circuit to compensate for such a major shift I guess. No, I don' know why this minutiae is so interesting. :-)

Just from observation of so many Delco radios, I'd guess that the DS-501 WAS an upgraded device as a part of the DS series. I've accumulated an odd assortment of NOS (same TO36 style) Delco transistors which pre-dated the 501 such as 2n278's and 2N174's. The weakness (apparent from dissection) seems to be in the contact wire bonding to the wafers. The Ford Germanium transistors from that era are horrible by comparison - my opinion. I don't even like to work on Ford radios anymore. Delco's had failures with clear patterns and old Ford semiconductors just croak randomly. What a pain to trace when MUX decoding is involved...

Although not forgotten, I haven't taken time to update the thread on the trunk lid connectors for the 53-55's. I'll do that with pictures one day and describe how to make a water resistant connector whose center pin actually touches the sides of the mating socket. One of the GC Corvette cables came in with a 55 and he sent the trunk lid connector along. As mentioned by someone, the connector threads do not match and there is only a bare center conductor wire extending from the cable. Gutting a TNC connector yields the parts to solve the problem and a grommet minimizes the cross threading damage. I'll share the info as time allows.

Good to hear from you Dan! Stay busy...

[Dan D.]

Re: 71 radio not working

Hi Jerry, good to hear from you too. I'm keeping busy for sure, assume you are also.
I have not forgotten about our last thread with these old radios and the early C1 antennas. I did get my old 54 rear deck lid out of mothballs. The lid is torn from the accident and I have a clear view of the mesh antenna. Interesting, I have measured the mesh (dimensions - not electrical) and photoed it. Will post it shortly.

I have moved my Corvette effort from the garage to the basement workshop for the winter. This was planned, seeing how we are basking in a foot of snow and sub-freezing temps up here in NY. I have been working on the Cadillac radio. Got it all apart for cleaning and re-building. Even got the clock gear train out, cleaned, re-lubed and re-installed. When I get it running I plan on testing 3 different coax's and 2 antennas. Will see how well the trimmer peaks with the various antenna stuff.

One thing I have thought about here with the series cap in the coax is precipitation static. For everyone, precipitation static is static electricity that builds up on antennas. It is caused by wind passing over the antenna and can generate thousands of volts. This can be a big problem on ships as ships tend to have very large antennas (a lot of surface area) and there is always plenty of wind on the open sea. More recently it has become a problem on military vehicles in Kuwait and Iraq. They have long (16 foot) whip antennas and the desert wind and sand generate huge amounts of static electricity. I have seen a steady stream of sparks jump a 1/2 inch or better gap on coax cables.
This is not a problem if the antenna input circuit in the radio has a DC path to ground. That will prevent static from building up. Car radios all have this DC path through the tuning coils. But if the circuit is AC coupled, as with a series capacitor inside the coax, now we have a problem. Car antennas are much smaller, and not as much wind, but I think the problem will still exist. What harm will it cause? Well static in the speaker for sure, and I would think it would trash the capacitor. The supply houses that sell these coax cables claim this capacitor is quite fragile and easily damaged. If it blows short, which would be the probable failure mode, the radio would still work and static would be gone as the circuit is now DC coupled. But the sensitivity will go all to pot as the trimmer would now be out of tune. Jerry, what do you think?
I have much more interesting stuff to talk about on these radios, but we have hijacked this thread (again) and this is getting too long winded.

So I will sign off for now, but will post more in the future - probably after the holidays. Jerry, very good to hear from you again. -Dan-