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Author Topic: Hallicrafters SX-96 cathode voltage problem  (Read 7734 times)
N1AN
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« on: February 18, 2015, 05:33:08 PM »

My SX-96 has decided to quit.

Measured voltage on the Sensitivity circuit connected to the cathodes of the RF amp, 1st mixer and 1650KC IF amp is 65 volts.  Schematic indicates I should be seeing 1.7 volts.

Any suggestions on what may have failed or where to look?

Thanks & 73

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AC2EU
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« Reply #1 on: February 18, 2015, 06:09:07 PM »

you probably have an oscillating IF stage that is kicking the AGC into panic mode.
Turn off the agc and the voltage should drop, but the actual problem will still be there.
Probably bad bypass caps... 
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KD0REQ
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« Reply #2 on: February 18, 2015, 06:49:33 PM »

I immediately said bypass caps reading the post.  you have an obvious bias point rise.  also check the DC bias resistors on all the cathodes.
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W8JI
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« Reply #3 on: February 19, 2015, 09:08:45 AM »

The cathode probably has an open resistor, if the other voltages on the tubes with cathodes connected to that point are OK.

Old composition resistors age up from moisture.

You also could have a grid being held too positive, or a shorted tube, but I would be more inclined to think an open cathode resistor.
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WA1RNE
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« Reply #4 on: February 19, 2015, 09:55:14 AM »

I doubt cathode resistors are the problem - there are 4 stages with 4 separate cathode resistors which are raised above or brought to ground by the "sensitivity" control R59, a.k.a RF Gain. The 4th is V6 the 50.5 kc IF Amp.

Unlike most receivers which vary AGC voltage to the grids of various stages for RF gain control, this one simply raises the cathodes of 4 stages above ground reducing their gain.

The complete circuit is the series combination of R59 and the Receive/Standby switch S7A. (When S7A is open, R60 a 330K resistor is in series with R59.)

Therefore your problem is likely an open in the line running to the pot and/or an open at the pot or switch and associated wiring.

 ...WA1RNE
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KM1H
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« Reply #5 on: February 19, 2015, 03:23:21 PM »

Quote
I doubt cathode resistors are the problem - there are 4 stages with 4 separate cathode resistors which are raised above or brought to ground by the "sensitivity" control R59, a.k.a RF Gain. The 4th is V6 the 50.5 kc IF Amp.

Agree and an open low value resistor with very little circuit voltage/current is an extreme long shot. Leaky cathode bypasses will affect performance but not cause an oscillation, AND all the bypasses in the first 3 gain controlled stages are disc ceramics anyway. The V6 cathode and screen bypasses are paper and both are likely quite leaky and the one for the screen could drive that stage into oscillation. Try removing those 4 tubes one at a time to try to localize the problem.

Id still bet on the switch or pot as the most likely culprits but replace all those apeper caps and electrolytics while youre at it.

Carl
KM1H
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N1AN
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« Reply #6 on: February 24, 2015, 08:24:06 AM »

Turned out the Sensitivity pot was bad.   Replaced it and voltages are normal and the receiver is playing nicely again.

Thanks to all for pointing me in the right direction.

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