C1 Wonderbar Radio Speaker

[Christopher R.]

There is a coil that hangs off a C1 radio speaker. C2s too, I think. It has been posted here that it is important that the resistance of that coil be a certain value. Or, the radio will be damaged.

1. What is that value?
2. With the speaker on the bench, how do you measure that value with a digital ohm meter?

[Jack H.]

Re: C1 Wonderbar Radio Speaker

Speakers for '58-67 Corvette changed P/N over time with the 'matching transformer' captive on the speaker changing from the secondary of a split-winding transformer to a straight choke. The Delco Radio handbook for your year car will show exactly which setup was used in factory production for a given era.

It's not only the resistance of the transformer (or choke/inductor) that's important, but its inductance as well. My copy of the Delco Radio service bulletin 6D-890, from 10-1-61 shows the split winding side of a Delco transformer, PN 7281131, being mounted on the speaker with the opposite side of the xformer unused (floating). The radio's DS-501 audio drive transistor connects to the two end output taps of the transformer while the center tap and common ground tap connect to the speaker's coil.

The total resistance across the center tapped side of the xformer is 1.8-ohms with 0.6 ohms between the top of the center tap winding and the center tap and the balance, 1.2 ohms, between the center tap and the return ground end of the transformer.

One way to measure is to disconnect the speaker from the radio, take it to the bench and temporarily disconnect the lead from the xformer's center tap to the speaker's voice coil. Then measure the DC resistance between the center tap of the xformer and the opposite lead to the speaker's voice coil (SB close to 1.2-ohms). AND, while you're there on the bench, verify you see roughly 0.2-ohms going the other way (from the center tap to the top of the xformer where the radio would normally connect. This should verify the basic voltage divider action of the transformer....

[Richard T.]

Re: C1 Wonderbar Radio Speaker

Does anyone know if the 1958-62 replacement speakers (that come with a transformer) sold by Corvette Central and other suppliers have the correct resistance and induction for an original 1962 Wonderbar? Was it the same for all years 1958 through 1962, since only one speaker and transformer is offered for 1958-62?

[Jack H.]

Re: C1 Wonderbar Radio Speaker

From my reading of various Delco Radio manuals, the basic speaker with it's on-board matching transformer, changed PN and circuitry specifics over it's lifespan ('58 through '67 and into the service replacement era). At times the matching transformer had one P/N with shunt resistor mounted on it, other times another, similar split winding transformer was used without a shunt resistor, and ultimately a simple inductor is shown.

Bottom line: my guess is Delco Radio engineers were 'improving' things over time AND exercizing cost containment with the objective of making each revision downward compatible with what went before.

On the issue of whether or not the 'reproduction' replacement speakers offered by various catalogs are 'correct', there's controversy there. I've yet to hear from anyone expert in the art speak with the authority of a skilled Delco Radio designer familiar with the circuit topology of the Corvette radio, its DS-501 germanium audio drive transistor and its limitations, definitively PROVE the current aftermarket speakers (are there multiple sources?) pose a threat to audio performance and/or upstream electronics longevity given the presense of a DS-501 audio drive transistor that's NOT seen age/abuse....

I HAVE seen 'white paper' articles that 'badmouth' reproduction/replacement speakers with the 'proof' pointing to the DC characteristics of the matching component(s) used on the speaker. BUT, the matching transformer was put there for a reason....

The DS-501 transistor had certain (power handling) performance limitations that modern silicon transistors don't have. The matching transformer primarily acted as a stop-gap to protect the audio drive transistor from failure due to its limitations in the dynamic speaker drive function it was tasked to perform....

Most other cars using the DS-501 audio drive transistor had sufficient behind-the-dash space to package the speaker matching components inside the radio chassis. But, space was tight in a Corvette and the matching components moved out of the radio chassis and up onto the speaker to compensate. So, there's nothing really 'special' about the circuit topology (similar/identical matching xformer can be found inside the chassis of other GM pass car radios) in a Corvette other than space restrictions moved some parts outside the radio and onto the speaker!

But, once the matching transformer was designed it, it formed a low pass filter 'coloring' the audio spectrum off flat. Radio designers knew that and made design compensations up-stream (tone spectrum) to 'cover' the fact the matching xformer was there. That's why you get 'wierd' and 'tinny' sound if you substitute a correct/original Corvette speaker WITH the matching transformer on it for a 'naked' speaker, designed for flat spectral response, even if it's the best money can buy! Also, without the matching transformer between the 'naked' speaker and the DS-501 audio drive transistor you remove the transistor's 'protection' and it's just a matter of time before you either damage the transistor or blow it out altogether!

The Delco radio service literature WARNS technicians of this (running the radio without a matching transformer in circuit). But, there are those who work on radios without being electrical engineers, skilled in design, or technicians with specific Delco training....

You see a LOT of Corvette radios for sale (eBay, swap meets, Etc.) that went through the 'improvement' syndrome:

(1) Original speaker bought the farm due to age/abuse (not uncommon in an open air car with the speaker prone to being rained on).

(2) Instead of replacing the factory original 'Corvette' speaker with a correct substitue, 'someone' figured they's 'soup up' the audio by installing a better quality 6 x 9 that was a flat-response device, lacking the matching circuitry.

(3) What the heck, this approach worked in other GM cars they were familiar with (yeah, but those cars had the matching transformer built INTO the radio chassis itself!)....

(4) Gee, there was a DRAMATIC loss of acoustic output power from the 'better' speaker accompanied by a TERRIBLE 'tinny' sound (yep, the jerk threw out the baby with the bath water by not moving the matching tranformer off the original speaker onto the replacement speaker--STUPID).

(5) The problem CAN'T be with the speaker, hey, it's brand new and works fine on the bench with another radio (guess again, Sam).

(6) In the meantime, trying to understand/troubleshoot the problem, they wound up damaging or destroying the DS-501 transistor by running it to a speaker without the matching circuit components.

(7) Well, it's cheaper to simply 'gut' the original radio and install an aftermarket set.... Yep, that fixed the problem!

(8) Later the 'bad' radio sufaces for resale on eBay, at a swap meet, Etc....

[Richard T.]

Jack, thanks for the explanation. *NM*