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Bottlehead Kits => Stereomour II => Topic started by: vtecths on February 15, 2021, 09:44:20 AM

Title: Stereomour II - Right Channel Issue Troubleshooting
Post by: vtecths 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
Title: Re: Stereomour II - Right Channel Issue Troubleshooting
Post by: Paul Birkeland 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.
Title: Re: Stereomour II - Right Channel Issue Troubleshooting
Post by: vtecths 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?
Title: Re: Stereomour II - Right Channel Issue Troubleshooting
Post by: Paul Birkeland 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.
Title: Re: Stereomour II - Right Channel Issue Troubleshooting
Post by: vtecths 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
Title: Re: Stereomour II - Right Channel Issue Troubleshooting
Post by: Paul Birkeland 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)
Title: Re: Stereomour II - Right Channel Issue Troubleshooting
Post by: vtecths on February 23, 2021, 01:48:11 PM
With the volume pot at 0, I’m reading 2.1 Ohms.
Title: Re: Stereomour II - Right Channel Issue Troubleshooting
Post by: Paul Birkeland 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.
Title: Re: Stereomour II - Right Channel Issue Troubleshooting
Post by: vtecths on January 12, 2024, 05:10:55 PM
Apologies, I'm finally coming back to troubleshoot this. Using my MacBook, I've played a 60 Hz tone, full volume with the balance and volume pots in the middle and am only reading 50 mV on both top and bottom middle lugs of the volume pot. Between ground and the outer upper and lower lugs on the volume pot both read 250 mV.
I did notice this evening, the temperature of the 2A3 tube on the faulty
size is running ~100 degrees F cooler than the functioning right side, if this would help isolate the issue.
Title: Re: Stereomour II - Right Channel Issue Troubleshooting
Post by: Paul Birkeland on January 13, 2024, 01:08:56 PM
Were the voltage checks OK on this amp?  I see some resistance readings but if you've turned the amp on and tried to use it, a voltage check would preceded that and should have caught what you're describing if it does have to do with operation of one 2A3.
Title: Re: Stereomour II - Right Channel Issue Troubleshooting
Post by: vtecths on January 14, 2024, 11:50:59 AM
Hey Paul, I appreciate the follow-up!
I took a fresh round of voltage checks this afternoon. I’ll attach them for review.
I did notice [#7 and #14] along with [#1 and #2] have roughly a 30 volt delta.
Title: Re: Stereomour II - Right Channel Issue Troubleshooting
Post by: Paul Birkeland on January 14, 2024, 03:50:01 PM
The 85V where you're supposed to have 58V is alarming.  I would be looking at the two sand cast 5W resistors on that side of the amp up front to see if they are still well connected.  Maybe also check that the 249K resistor is well connected and soldered in that area. 

This is almost certainly just a loose connection that has made itself known during the move.
Title: Re: Stereomour II - Right Channel Issue Troubleshooting
Post by: vtecths on January 15, 2024, 05:50:02 AM
I've re-soldered all of the 5W sand colored resistors along with the 249K resistors. The 249Ks measure correctly [248k ohms].

I'm able to get audio out of the left channel but now it is softer than the right and it distorts as you raise the volume close to half. Right channel sounds normal to full volume.

I've freshened up the other solder points on that side as well but no dice  :-[
Title: Re: Stereomour II - Right Channel Issue Troubleshooting
Post by: Paul Birkeland on January 15, 2024, 05:50:51 AM
Could you post some build photos? 

I would 100% not listen to an amp that's this far off on the voltage checks. 
Title: Re: Stereomour II - Right Channel Issue Troubleshooting
Post by: vtecths on January 15, 2024, 06:08:43 AM
Gotcha, ok. You bet--
Title: Re: Stereomour II - Right Channel Issue Troubleshooting
Post by: Paul Birkeland on January 15, 2024, 06:16:32 AM
Does this voltage follow one of the 2A3s if you swap it?

It also jumped out at me that some of the STP heater wiring feeding the 9 pin sockets is up above the rest of the wiring, and one of the bare shield wires looks perilously close to shorting out one of the plate connections.  This wiring should be installed against the chassis for quietest operation, but I wouldn't address this until the other issue is resolved.
Title: Re: Stereomour II - Right Channel Issue Troubleshooting
Post by: vtecths on January 15, 2024, 08:47:10 AM
Appreciate the callout on the heater wiring. I made sure none of them are shorted but will address after this issue is resolved.

Swapping the 2A3s net the following updated measurements:

#1: 368.5V [from 370.3V]
#2: 397.2V [from 403.5V]
#7: 210.2V [from 173.3V]
#14: 208.0V [from 207.5V]

The distorted output doesn't appear to be following the tube.
Title: Re: Stereomour II - Right Channel Issue Troubleshooting
Post by: Paul Birkeland on January 15, 2024, 01:17:33 PM
You've posted all the voltages that were OK to begin with, what we'd be most interested in is seeing if the 85V readings switched sides.

Oddly the problematic 2A3 voltages on the left channel...

Can you measure the DC resistance at terminal 36?  It looks like one of your 5W resistors is missing the lettering, and really your amp is acting like only one of those resistors is present.
Title: Re: Stereomour II - Right Channel Issue Troubleshooting
Post by: vtecths on January 24, 2024, 11:48:55 AM
I ended up replacing that 5W resistor. This resolved the static/distortion in the left channel, however I'm still experiencing the right channel tube running noticeably warmer [~100 degrees F].

I ran through another round of resistance measurements after swapping the resistor. I'll attach these.

Terminal 36 and a few others still don't measure quite right.

Title: Re: Stereomour II - Right Channel Issue Troubleshooting
Post by: Paul Birkeland on January 24, 2024, 02:30:16 PM
2.9K on terminal 15 means that only the 3K/5W resistor is appearing there, and the 2K/5W resistor just isn't connected or well enough soldered to be doing anything.  This is what you'll need to address to even everything up.