Question for DS 501 transistor experts

[Matt Innocenzi]

Hello circuit heads, I have a detailed question here that applies to all 1959 through 1962 Cadillac radios.  These radios feature a Delco DS-501 transistor for the amplifier.  According to the schematic, I should be getting 10.5V at the base, 10.7V at the emitter, and 1.5V at the collector for the speaker output.  I am getting the correct values at the base and the emitter, but 0V at the collector and therefore no output.  I know the speaker is good because it works on a known good radio.  I tried other DS-501 transistors that I had in stock that tested good, but have the same problem with 0V at the collector.  I replaced all of the wax capacitors with brand new ones.  Does anyone have any recommendations on what could be going on here?  Thank you!

[J. Gomez]

Hello circuit heads, I have a detailed question here that applies to all 1959 through 1962 Cadillac radios.  These radios feature a Delco DS-501 transistor for the amplifier.  According to the schematic, I should be getting 10.5V at the base, 10.7V at the emitter, and 1.5V at the collector for the speaker output.  I am getting the correct values at the base and the emitter, but 0V at the collector and therefore no output.  I know the speaker is good because it works on a known good radio.  I tried other DS-501 transistors that I had in stock that tested good, but have the same problem with 0V at the collector.  I replaced all of the wax capacitors with brand new ones.  Does anyone have any recommendations on what could be going on here?  Thank you!

Matt,

This will be very hard to provide a few hints via this format with just very limited data to go by.

You stated that you replace "all wax capacitors" with new ones but have you also tackle the electrolytic ones? There is a small electrolytic 4mf on the collector side which needs to be check.

Have you tested the transistor?

Did you check the resistors are within specs, depending on the % value on each?

Which out the transistor have you check for resistance on any of the legs on the transistor socket?

Assuming the transistor(s) tested good and you have the main speaker with the choke connected to the radio, and the power voltage is between 12.6V – 14.4V DC.

There is two method on checking the correct bias of the output transistor checking the voltage or the output "Amp" at the collector (1A @ 12.6V or 1.2A @ 14.4).

There is a bias variable resistor to control and adjust the correct "Collector bias", this resistor is on the secondary side of the output transformer to a ground on the main wiper side. Not sure if you check this one?

Hope this can help you..!

[Matt Innocenzi]

J., thank you this was actually quite helpful.  I did replace the 4mf electrolytic one as well, but I thought that was on the base side and not the collector.  I will check again.  I did not check the resistors, but I will do that as well.

As for the bias resistor, it is my understanding that is on the input side of the transistor, which was getting the correct input voltage, so I did not make any adjustments to it. 

[J. Gomez]

J., thank you this was actually quite helpful.  I did replace the 4mf electrolytic one as well, but I thought that was on the base side and not the collector.  I will check again.  I did not check the resistors, but I will do that as well.

As for the bias resistor, it is my understanding that is on the input side of the transistor, which was getting the correct input voltage, so I did not make any adjustments to it.

Matt,

This is from the '59 7272505 Delco model you can see there are several other caps around the "B, E and C" of the transistors just as FYI.

Yes the bias control is on the "B" side of the transistor on the output transformer side, so the bias adjustment is made on the "B" side of the transistor. Since the DS501 is a germanium type the "B" to "E" bias voltage needs to be around 0.2 V-0.3 V to overcome the forward voltage drop.

Good luck..!

[bcroe]

The base and emitter voltages are listed using a ground reference,
but what really matters is the base-emitter voltage.  That system
cannot function unless the 4 pin speaker-transformer arrangement
is plugged in.  Then, get a handle on emitter current by measuring
the voltage across R32, 0.47 ohm.  I would expect a fraction of a
volt, which varies with the bias adjustment.  If not, the transistor
is suspect.  good luck, Bruce Roe