Can you help me decipher this schematic?

Hi,

I am a total newbie so please forgive my ignorance. I am working on repairing my Adcom GFA-555 and I am trying to understand why I am getting a voltage drop of ~70V across R13.

According to the service manual "D3 and D4 forward-bias transistor Q6 at 1.3 volts from current provided through R13. This creates about 0.6 V across R11 so that Q6 sources about 1 milliamp through Thermostat T1."

Here is the schematic:

It looks to me like R13 sits directly between the positive and negative rails, with only D3 and D4 above it. Given my limited knowledge, I don't see how this forward biases Q6 at 1.3 volts. I have reviewed the circuit board carefully and it matches the schematic. I have replaced Q6 to be safe and the voltage drop across R13 was unchanged.

What worries me is that it appears R13 is being subjected to about 70V and about .0055 amps, which is 0.38 Watts, which is more than the transistor is rated for.

What am I missing here?

Much apprecaited!

Brad
 
R13 is limiting the current between the diodes and ground, not the -V. You should have +V minus 1.3V across R13.
 
R13 is limiting the current between the diodes and ground, not the -V. You should have +V minus 1.3V across R13.

Yes, I should have said ground instead of negative rail. But that is basically what I have: positive rail voltage minus 1.3V. Does it worry you that the wattage appears high for a 1/4 Watt resistor?
 
Correct. R 13 sees ((V+) - 1.2V). R13 forward biases he B-E junction of Q6, causing collector current to flow and turning on the current source for the input differential pair Q3 and also the bias circuit current source Q4. The collector of Q6 is in series with a thermal breaker that if it opens will shut down the amp by removing signal and bias by turning off Q3 and Q4.
Tom
 
Hi Brad,
I get a power of 0.22W with 70 V across R13, 22.1k. Marginal for a 1/4 watt resistor but probably ok.
Tom
 
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