Author Topic: Help With Some Basic Voltage Checks  (Read 164 times)

HiFiFun

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Help With Some Basic Voltage Checks
« on: May 10, 2017, 03:42:36 PM »
Hello all,
I am in a brain fog today.
I'm troubleshooting my RCA SHF-6 amp, which developed an
volume increase to maximum volume when volume pot was
set at 4/10.
 
1. How do I check the cathode resistors' voltages ? e.g. (R22 in schematic)
2. How do I check the screen grids' voltages ?

(meaning where do I place the meter's test probes)

I have a good DMM, and I think I remember, but....
HFF

AlexanderMartin

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Re: Help With Some Basic Voltage Checks
« Reply #1 on: May 10, 2017, 06:46:33 PM »
You'd probably be better off measuring the volume control and seeing if it's completely shorted at 4/10.
Not sure why you'd need to measure cathode voltage, just measure the cathode resistor.
For screen grid, one probe to ground and one probe on the point.

TC Chris

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Re: Help With Some Basic Voltage Checks
« Reply #2 on: May 10, 2017, 09:13:52 PM »
Hey, that schematic shows some negative feedback in the power amp.  I'm impressed. 

Chris Campbell

AlexanderMartin

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Re: Help With Some Basic Voltage Checks
« Reply #3 on: May 11, 2017, 12:27:41 AM »
Did most of those amps back then NOT use NFB circuits  ??? ? I know the low end line sets didn't due to gain purposes.

TC Chris

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Re: Help With Some Basic Voltage Checks
« Reply #4 on: May 11, 2017, 11:43:42 PM »
Did most of those amps back then NOT use NFB circuits  ??? ? I know the low end line sets didn't due to gain purposes.

Yeah, the garden-variety amps didn't.  I've got a GE F-107 console radio from around 1937 that uses feedback for tone control purposes.  It's always interesting to see what the manufacturers did.  I've got  V-M Model 566 console phono that uses P-P 6BQ5 outputs and an ultralinear circuit.  That was a surprise--"What is that winding on the output transformer, anyway?  This is a V-M consumer grade device!"

Chris Campbell