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Old 18th July 2015, 05:12 PM   #1
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Default Sansui AU-70 hum in one channel

Hello there,

I recently recapped a Sansui AU-70, and I'm getting a hum in the left channel. I think it's a 60Hz hum but I'm not completely sure. It was present before I recapped the amplifier, so it's not as a result of the work I completed.

I have swapped the tubes around and the hum has stayed with the left channel. I have an oscilloscope and I can trace problems I'm just not really sure where to look. It is sort of controlled by the volume pot. If I turn it up, the hum goes away, then at one spot it's completely gone, and turn it up just a little more and the hum comes back. Not sure why that is.

Any ideas?

Here is a schematic: AK Database PDF Gallery - Sansui/Sansui AU-70 Owner's-Service Manual

Also I had a question about biasing. I understand that this is a cathode feedback circuit whereby the cathode is wired to the secondary the OPTs for signal feedback. I had read here that the way to measure the current for bias adjustment would be to know the resistance between the secondary winding and ground. My question is how would I measure that? Sorry, I'm a bit of a greenhorn.

Thanks much.

Jeffrey
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Old 18th July 2015, 05:43 PM   #2
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Maybe output tubes are preloaded and current is more. Check -U on first grids of bad and good channels. Must be similar.
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Old 18th July 2015, 05:45 PM   #3
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I measured the grid voltage (pin 2) and was getting about -12.2V on each side. I readjusted this to -17 based on what I had found online and the hum is still there.
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Old 18th July 2015, 07:31 PM   #4
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Your volume pot might have a loose spot that lost contact at one point of the wiper. Try adding a 2M resistor to ground at wiper to ensure no open circuit. Also check DC leakage at the wiper if there's leak from previous coupling cap.
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Old 18th July 2015, 08:16 PM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by jeffrey88 View Post
I recently recapped a Sansui AU-70, and I'm getting a hum in the left channel. I understand that this is a cathode feedback circuit
whereby the cathode is wired to the secondary the OPTs for signal feedback. I had read here that the way to measure the current for
bias adjustment would be to know the resistance between the secondary winding and ground.
Check the left input sockets very carefully for a bad connection, or a lead there touching the chassis when it shouldn't.
Also make sure the input cables are good by substitution with another known good pair. Sometimes they don't fit tightly enough.
For checking the bias current, it would be best to insert a 1 ohm resistor in each cathode circuit between the tube and
output winding to measure the (floating) voltage across. Bias Tester Probe (mV) Version for Vacuum Tube Guitar Amplifier by AMP-HEAD

Last edited by rayma; 18th July 2015 at 08:22 PM.
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Old 18th July 2015, 10:31 PM   #6
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Try disconnect interstage caps from first grids of output tubes.
Try bad channel without all prestages tubes /only with output tubes/.
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Old 20th July 2015, 02:45 PM   #7
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Hi everyone,

Thank you for the responses I really appreciate it.

@directdriver - I don't have a 2 megaohm resistor on hand to try that with but I will pick one up this week.

@rayma - I had a look at the inputs and everything looked OK.. although, I do know this particular amplifier was dropped at some point and note there is a dent on the bottom chassis where the inputs are. I'm going to take an even closer look but off hand it looked OK to me.

@azazello - I will try that tonight and get back to you.

One thing I do know is the hum is independent of the source, so no matter what source is selected the hum is there.
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Old 23rd July 2015, 02:15 AM   #8
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Hi sorry it took me so long. I removed V2, 12AX7, and the hum came through on both channels.


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Old 23rd July 2015, 02:51 AM   #9
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Default Sansui AU-70 hum in one channel

Okay, this is pretty strange. The hum has moved from the left channel, to the right. I dont know why. I didn't swap tubes and didn't really touch anything at all. Removing V2 still allows hum to come through on both channels (V2 is the left channel) and the hum is unaffected by the volume control. Removing V1, the hum comes through on the right channel. Also, turning up the volume does increase the loudness of the hum. When the volume control is at halfway, the hum comes through on both channels.

What does this mean?


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Old 11th August 2015, 03:34 PM   #10
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Hi all,

I am still chasing this damn hum. I've made some progress though. Here is where I'm at.

Hum is present on right channel.

Remove V1 - hum stays the same
Remove V2 - hum appears on both channels
Remove V3 - hum stays the same

When Tone Defeat is ON, the hum comes through on both channels

On my oscilloscope, I measured Pin 1 of V1 and no hum is present. I measured the wiper of the volume potentiometer and no hum is present. The hum is present on Pin 7 and Pin 6 of V1. I measured the ground point of the grid resistor for Pin 7 and no hum.

Measuring V2, there is no hum present on Pin 1, 2, 6 and 7.

I did measure the length of the wave, and it appears to be a 100Hz hum.. this is odd because I am running it with 117V (using a variac).

Tube sockets have been deoxited and retensioned. Pots and switches deoxited as well.

Any ideas of where to look?
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