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Author Topic: Stereomour II - Right Channel Issue Troubleshooting  (Read 144 times)

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Offline vtecths

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Stereomour II - Right Channel Issue Troubleshooting
« on: February 15, 2021, 09:44:20 AM »
Hello-
 
Working to troubleshoot an issue on my SII. We recently moved so the amp was packed up. After re-installing the tubes and firing it up, I noticed the right channel was producing only light static but the left channel functioning normally. Tried swapping the tubes around but the right channel remained only producing a light static, which was not volume dependent.

I'm thinking I may have installed the power tubes incorrectly and took something out. I took a round of resistance measurements, which I have attached. First thing that stood out was #10 measures open regardless of volume position but #11 varies like it should. This may explain the dead channel?

There are a few other measurements that aren't quite right (#15, 36, 37, 39, 40 and 45) Each of these measure in millions of Ohms and continuously go down. Not sure if these have anything to do with the volume pot issue.

Any assistance would be greatly appreciated.

Thank you,
Erik

Offline Paul Birkeland

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Re: Stereomour II - Right Channel Issue Troubleshooting
« Reply #1 on: February 15, 2021, 10:23:03 AM »
You're likely going to find a loose connection that side somewhere between pins 1 and 4 on the 4 pin socket and where the big white biasing resistors mount.
Paul "PB" Birkeland

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Offline vtecths

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Re: Stereomour II - Right Channel Issue Troubleshooting
« Reply #2 on: February 15, 2021, 12:36:43 PM »
Thanks, Paul.

I didn't notice anything physically loose but freshened the solder connections in that entire area.

This didn't change any of my previous measurements, unfortunately.

Testing the volume pot itself, I'm unable to get a reading at all from the point that would go to connection #10. Is there a way to temporarily bypass the volume pot
completely to rule out this being the issue?

Offline Paul Birkeland

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Re: Stereomour II - Right Channel Issue Troubleshooting
« Reply #3 on: February 15, 2021, 04:05:12 PM »
What DC resistance do you get on the volume pot with one probe on the lower rear lug and one probe on the lower middle lug? 

How about the the DC resistance between the lower rear lug and the lower forward lug?

A bad volume pot will not cause your resistance problem at terminal 15.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline vtecths

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Re: Stereomour II - Right Channel Issue Troubleshooting
« Reply #4 on: February 22, 2021, 01:43:20 PM »
If I'm understanding you correctly, with the volume in the 'o' position I measure the following:

Lower Rear lug and Lower Middle Lug = 34.87k Ohms
Lower Rear lug and Lower Forward lug = 34.87k Ohms

Offline Paul Birkeland

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Re: Stereomour II - Right Channel Issue Troubleshooting
« Reply #5 on: February 22, 2021, 02:51:20 PM »
How about lower forward and lower middle?  (Turn the volume pot all the way down for this one)
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline vtecths

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Re: Stereomour II - Right Channel Issue Troubleshooting
« Reply #6 on: February 23, 2021, 01:48:11 PM »
With the volume pot at 0, I’m reading 2.1 Ohms.

Offline Paul Birkeland

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Re: Stereomour II - Right Channel Issue Troubleshooting
« Reply #7 on: February 23, 2021, 02:22:35 PM »
With the volume at 0 (all the way down), lower forward and lower middle should read much higher than that.  With the pot all the way up, that reading would make sense. 

If you'd like, you can set your balance and volume pots in the middle of their rotations and play a 60Hz tone into the amp (with the amp off) from your phone with the volume all the way up, then measure the AC voltage between ground and the upper middle, as well as ground and the lower middle lugs of the volume pot.  It should be roughly the same and from most phones it will show several hundred millivolts of AC voltage.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man