arthurdb
Member
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FINAL EDIT: Got it fixed by replacing the 6H6 tube that rectifies the side-chain signal. This was what I initially suspected but turns out the first replacement tube was a dud which needed a much higher AC signal to start developing a DC voltage. Its behavior in circuit was similar to the original one so I though it might have been something else....
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Hello folks.
I've been given a SA-39B compressor for repair and I can't find out what's wrong.
It seems to be the same as the one at the beginning of this thread and working off the schematic linked in this post (attached below)
Power supply voltages are more or less in line with the values on the schematic as well as signal AC values. The neon bulb was at a much higher voltage (70VDC instead of 55VDC) but I still could get the 275VDC output from the voltage regulator. I tried with zener diodes for 55 VDC (and adjusted the regulator to suit) but didn't really change anything.
The audio tubes are well matched (within 0.1mA) except the 6V6 output tubes which have a few mA difference.
R3 is adjusted to show 0dB compression with no input signal.
Applying an input signal I do get signal at the output however there is no compression unless I drive the signal hard enough, way beyond clipping (clipping starts at app. 45Vpeak on the jack output on the front panel). If I feed a really crazy high input signal I can get the needle to drop, so the meter seems to be working fine. I can get an indication of beyond -20dB compression with a high enough input signal and the output is really completely distorted at that point.
So anyways, this led me to believe that the 6H6 double diode valve, which rectifies the AC signal into the DC control voltage for compression, was not working properly.
Indeed, the schematic seems to indicate that with 35VAC on each side of the rectifier input, I should get a -24VDC* control voltage, however I had 0V with 35VAC.
Received a new 6H6 valve today and it's not working any better. Same behavior. With both the old tube and the new one, I need to drive the input of the rectifier with 80VAC on each leg to get -1VDC on the output (meter indicating -5dB). This identical behavior leads me to believe it is safe to assume that they are both working okay, right?
Checked the whole circuit at the output of the rectifier, no shorts or anything, everything seems to be just as indicated on the schematic. There isn't much to it so I don't really see what could go wrong. S1 (release rotary switch) and associated resistors, R31, R3, C1 and the input pot all test ok. I checked heater supply for the 6H6 valve and its fine, if a bit high (6.6VAC). I have 41VDC on both legs of the rectifier input (schematic says 39VDC) and yes, as mentioned above, the audio signal is also present and symmetrical on both legs.
I have tested for continuity between the 6H6's socket's solder lugs and the tube pins with the tube half plugged in.
What am I missing? Is it possible that there is some sort of leak on the screens of the 1612 tubes explaining that I'm not getting the expected CV voltage (connected to screen of the 1612 - PIN5)? I have measured resistance between pin 5 and all the other pins on both tubes, unplugged of course, and they all read open circuit.
*Should that be 2.4VDC?
EDIT: Forgot to mention but this unit had already been fully recapped
EDIT2: I originally attached the wrong schematic, this has been corrected
FINAL EDIT: Got it fixed by replacing the 6H6 tube that rectifies the side-chain signal. This was what I initially suspected but turns out the first replacement tube was a dud which needed a much higher AC signal to start developing a DC voltage. Its behavior in circuit was similar to the original one so I though it might have been something else....
--------------
Hello folks.
I've been given a SA-39B compressor for repair and I can't find out what's wrong.
It seems to be the same as the one at the beginning of this thread and working off the schematic linked in this post (attached below)
Power supply voltages are more or less in line with the values on the schematic as well as signal AC values. The neon bulb was at a much higher voltage (70VDC instead of 55VDC) but I still could get the 275VDC output from the voltage regulator. I tried with zener diodes for 55 VDC (and adjusted the regulator to suit) but didn't really change anything.
The audio tubes are well matched (within 0.1mA) except the 6V6 output tubes which have a few mA difference.
R3 is adjusted to show 0dB compression with no input signal.
Applying an input signal I do get signal at the output however there is no compression unless I drive the signal hard enough, way beyond clipping (clipping starts at app. 45Vpeak on the jack output on the front panel). If I feed a really crazy high input signal I can get the needle to drop, so the meter seems to be working fine. I can get an indication of beyond -20dB compression with a high enough input signal and the output is really completely distorted at that point.
So anyways, this led me to believe that the 6H6 double diode valve, which rectifies the AC signal into the DC control voltage for compression, was not working properly.
Indeed, the schematic seems to indicate that with 35VAC on each side of the rectifier input, I should get a -24VDC* control voltage, however I had 0V with 35VAC.
Received a new 6H6 valve today and it's not working any better. Same behavior. With both the old tube and the new one, I need to drive the input of the rectifier with 80VAC on each leg to get -1VDC on the output (meter indicating -5dB). This identical behavior leads me to believe it is safe to assume that they are both working okay, right?
Checked the whole circuit at the output of the rectifier, no shorts or anything, everything seems to be just as indicated on the schematic. There isn't much to it so I don't really see what could go wrong. S1 (release rotary switch) and associated resistors, R31, R3, C1 and the input pot all test ok. I checked heater supply for the 6H6 valve and its fine, if a bit high (6.6VAC). I have 41VDC on both legs of the rectifier input (schematic says 39VDC) and yes, as mentioned above, the audio signal is also present and symmetrical on both legs.
I have tested for continuity between the 6H6's socket's solder lugs and the tube pins with the tube half plugged in.
What am I missing? Is it possible that there is some sort of leak on the screens of the 1612 tubes explaining that I'm not getting the expected CV voltage (connected to screen of the 1612 - PIN5)? I have measured resistance between pin 5 and all the other pins on both tubes, unplugged of course, and they all read open circuit.
*Should that be 2.4VDC?
EDIT: Forgot to mention but this unit had already been fully recapped
EDIT2: I originally attached the wrong schematic, this has been corrected
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