C3 Radio output problems 69 AM/FM

[Tom Bishop]

I have output transisitor problems with my original 69 AM/FM. I have 2 radios that both produce very low output with the Delco 501 output transistor I have. Does anyone know what is the right transistor & where I could source one? I hate dancing without the music.
Thanx

[Joe L.]

Re: C3 Radio output problems 69 AM/FM

Tom----

I believe that the D-501 is the correct transistor. However, yours might have failed for whatever reason. I don't know of a source for these. There might even be a modern-day replacement that would be as good or better. Jack Humphrey might know.

[Jack H.]

Thanks for the intro, Joe!

Yes, there is a source for replacement of the Delco DS-501 germanium transistor. Right off the top of my head, I don't remember the mfgr/supplier but it's NOT a household brand name. If memory serves, the list price on their exact replacment germanium clone is something like $25-30; ouch!

Due to that issue (price), what most of the classic radio repair shops do is get/gut vintage Delco radios from the scrap yards (an 'as-is' radio chassis sells for $5-20 at the yards) for their source of DS-501 transistors. Then, since they're in the business of repairing old classic radios, most of these shops have a vintage Tektronix (HP, Etc.) transistor curve tracer machine.

They simply 'unplug' the used DS-501 transistors, clean up the leads and test verify 'em on their curve tracer machines. The DS-501 was used in a LOT of other Delco Remy radios of the period (Olds, Caddy, Etc.) and most are installed on heat sinks so the back side of the xsistor is visible (it's ink stamped 'DS-501' on the can) and easy to verify. Beware, there was another, somewhat lower power 'cousin' to the DS-501 used in that era, the DS-503, so you should verify the donor radio set you're contemplating actually has the right speaker drive xsistor before you buy it.

If you're not an EE or electrical tech with bench experience and/or access to a transistor checker, you can probably phone around to local radio/TV shops to find someone who can test the 'suspect' DS-501 for you (along with your replacement). I think I'd start by verifying the integrity of the existing speaker drive xsistor before going on what might be a wild goose chase for a replacement....

Last, depending on the age of the car/radio, it takes two to tango! In straight axle and mid-year Corvettes, there was insufficient room inside the radio chassis to mount the speaker's impedance matching transformer and this component was installed on the speaker itself (rather unique to Corvette) while most other GM cars/trucks tucked the transformer inside the radio chassis.

If you try to run a replacement speaker that lacks the matching transformer with the raw output from a Corvette radio and its DS-501 drive transistor, two things will happen:

(1) The sound quality will be terrible! You'll lack volume and it'll sound REAL tinny. That's because designers knew the matching xformer was in-circuit and 'colored' the audio spectrum in prior feed stages to match.

(2) Without the matching transformer, 'flyback' from the speaker drive magnetic can be expected to VERY QUICKLY destroy your radio's DS-501 transistor!

Point two is generally why we see so many used Corvette radios on eBay and at swap meets being sold in 'as is' condition.... The factory original speaker became damaged due to moisture (especially true of convertible cars), someone replaced it with an off-the-shelf substitute speaker (one that lacked the matching transformer), and took out the radio's original DS-501 speaker drive xsistor!

Delco radio service manuals of that era DO have a basic 'go/no-go' test proceedure for the DS-501 that you can conduct using a basic volt-ohm meter. But, this will not FULLY verify the dynamic range of the xsistor like a professional curve tracer....

[Roberto L.]

Great post, thanks! *NM*

[Henry J.]

Jack: Impedance Matching Transformer Spec?

Excellent post Jack. I am looking for appropriate speakers (not necessarily original) for my 68 with U69. I have managed, I believe, to find all the other pieces of the radio system. Are you able to provide the relevant specifications for the correct impedance matching tranformers on the speakers so that I know exactly what I should be looking for? Thank you.

[Tom Bishop]

Re: Jack: Impedance Matching Transformer Spec?

Thanks for the help, I was unaware of the speaker impedance issue, as was the Classic car radio shop I sent these radios out to. What is the correct speaker impedance & can you add a resistor to simulate the correct impedance on a typical 8 Ohm speaker or do you recommend only going with OEM?

[Jack H.]

No need.....

The radio's circuit topology was changed in Shark era from that employed in SA (straight axle) and MY (mid-year) times. The matching transformer was moved off the speaker and tucked away inside the radio's chassis components. You'll find original Shark era speakers have no matching transformer on 'em and you can safely substitute with just about any off-the-shelf, modern speakers that fit.

Factory orignal speakers had a 10-ohm nominal impedance, so modern 8-ohm speakers substitute very well. With their higher efficiency (better magnets and cone design), you simply won't notice the modest impedance mis-match.

For earlier cars (SA/MY), you 'can' use off-the-shelf speakers IF you jury rig them to add the impedance matching transformer. Take it off a damaged original Corvette speaker that's beyond repair via re-coning or get one from an EARLY GM passenger car (say '61 or earlier Cadillac) at the scrap yard. While the scrap yard car's original speaker may be 'beyond hope' from sitting/rotting for years in the yards, the transformer SB A-OK for donor transplant....

[Henry J.]

Thanks, Jack! *NM*