News:
March 07, 2021, 04:31:58 AM

Author Topic: Single-Chanel Problems with Stereomour II [resolved]  (Read 1383 times)

0 Members and 1 Guest are viewing this topic.

Offline mjhara

  • Jr. Member
  • **
  • Posts: 11
Single-Chanel Problems with Stereomour II [resolved]
« on: May 19, 2016, 06:43:55 PM »
Hi everyone -

I just recently completed the Stereomour II kit, went through all the voltage checks and resistance checks and everything worked great...for about a day. Then, when I fired it up on day 2 - the left channel gave a weird buzzing-whooping-humming sound that was not constant, and did not output hardly any volume at all (I could still - barely - hear the music). I unplugged the speakers, flipped the kit over, checked a few things, re-soldered more joints than I care to admit, but I am still left with one anomaly - when I do the voltage check, I get a voltage of something like 99-105 DVC out of terminal 7, instead of the normal 170-230 VDC to be expected. For comparison, I get something like 180-190 VDC out of terminal 14, which is the mirrored terminal and is of normal range. All the resistance checks are more or less normal.

Despite my better judgement I fired up the amp anyway - and for a moment, both channels seemed to work fine; however, whenever I bumped the desk the amp was sitting, I would get a momentary strong static out of the left channel. I then gave it a few minor disturbances - walking on the floor, dropping books on the desk, etc, and each time I would get a strong static out of the left channel only.

I have almost zero electrical experience and have been staring at the backside of this amp for hours, so any help or advice you could offer would be greatly appreciated. Thanks so much in advance!
« Last Edit: May 24, 2016, 07:14:45 AM by Caucasian Blackplate »

Online Paul Joppa

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 5239
Re: Single-Chanel Problems with Stereomour II
« Reply #1 on: May 19, 2016, 07:35:15 PM »
The movement sensitive noises indicate an erratic connection, and the low voltage T7 indicates a problem involving the driver tube. There is only a small chance it's the tube itself, but if you swap them side to side you can be sure.

Assuming the problem remains in the left channel, we can talk then about measurements to clearly identify it. Meanwhile, a careful look with good lighting and magnifying lens at all the solder joints around the problem tube socket and the volume and balance controls would be the best first thing to try - it's fairly quick and very often effective.
Paul Joppa

Offline mjhara

  • Jr. Member
  • **
  • Posts: 11
Re: Single-Chanel Problems with Stereomour II
« Reply #2 on: May 20, 2016, 04:20:04 AM »
Thank you for the quick reply.

I tried swapping tubes but it had only a nominal (about 5V) effect on the measurement.

I will resolder all of the tube connections when I return home tonight and report back what I find. Thanks -

Mike

Offline mjhara

  • Jr. Member
  • **
  • Posts: 11
Re: Single-Chanel Problems with Stereomour II
« Reply #3 on: May 20, 2016, 05:53:04 PM »
Ok, so I went back and reflowed all of the tube joints and checked all my AC voltages once again (all normal and within range), but am still getting the voltage anomaly at terminal 7. At Terminal 7 I'm getting a voltage of 116 V, whereas I'm getting a voltage of 195V on terminal 14 (the opposite side). I've tried to trace my way through the circuit but haven't had much success so far rectifying the problem.

Also, I just noticed that the LED nearest IA (the top left LED looking from the top) is not lighting up, which I suspect probably has something to do with my problem as well.

Any suggestions?

Mike
« Last Edit: May 20, 2016, 05:54:54 PM by mjhara »

Offline Doc B.

  • Administrator
  • Hero Member
  • *****
  • Posts: 8482
    • Bottlehead
Re: Single-Chanel Problems with Stereomour II
« Reply #4 on: May 20, 2016, 06:25:17 PM »
I'd suggest unscrewing, carefully lifting and looking closely at the underside of the PC board. The problem could be due to a bad solder joint somewhere on that board.
Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.

Offline mjhara

  • Jr. Member
  • **
  • Posts: 11
Re: Single-Chanel Problems with Stereomour II
« Reply #5 on: May 21, 2016, 04:48:41 AM »
Ok, I did a bit more reflowing of joints, examining what I have and making sure it's all hooked up correctly. I also swapped the tubes once again to see if that was the problem, but other than slight variations in the tube voltage the problem remains the same - Terminal 7 is approximately 50% of the voltage of terminal 14 (about 100V vs. about 200V), and one of the LED's on the board is not lighting up. I tried to take measurements of as many different connections as I could, and the anomalies I found are attached.

I think it has to be in the tube / board somewhere, but I cannot figure out where. At this point I've reflowed all the joints but it would appear that I am still losing about half my voltage to somewhere on my right channel.

Offline mjhara

  • Jr. Member
  • **
  • Posts: 11
Re: Single-Chanel Problems with Stereomour II
« Reply #6 on: May 21, 2016, 06:35:21 AM »
I just noticed another thing looking at the high-res photo of the tube connections - it looks like I may have cracked the 3650F metal film resistor that runs between the center plug and pin 8 on the pin socket, and indeed when I measure voltage between terminal 8 and the corresponding pin plugs on both tube sockets, I get a discrepancy that is similar in proportion to the current discrepancy I have between terminals 7 and 14 (something like .95V vs. 2V). Could I have potentially damaged this film resistor? If so, where do I go for such parts now that Radioshack is closed?

Online Paul Joppa

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 5239
Re: Single-Chanel Problems with Stereomour II
« Reply #7 on: May 21, 2016, 08:40:46 AM »
You can measure the resistance of that 365-ohm resistor to be sure, but definitely replace it - even if it's the right resistance, the protective coating is broken and it will corrode. Email [email protected], and Eileen will get it in the mail probably on Monday.
Paul Joppa

Offline Doc B.

  • Administrator
  • Hero Member
  • *****
  • Posts: 8482
    • Bottlehead
Re: Single-Chanel Problems with Stereomour II
« Reply #8 on: May 21, 2016, 08:55:51 AM »
Actually Josh would get it in the mail. Emailing [email protected] is the way to go.
Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.

Offline Paul Birkeland

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 14547
Re: Single-Chanel Problems with Stereomour II
« Reply #9 on: May 21, 2016, 08:58:21 AM »
You have a backwards transistor on your driver PC board.

-PB
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline mjhara

  • Jr. Member
  • **
  • Posts: 11
Re: Single-Chanel Problems with Stereomour II
« Reply #10 on: May 21, 2016, 01:23:10 PM »
Well...I feel quite stupid.

I flipped the transistor around boom, works like a charm.

Thanks for all your help - sorry it turned out to be such a trivial little thing.

Sounds amazing!

Mike