Weak left Channel - What to check? [resolved]

jrhunter62 · 5705

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Offline jrhunter62

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Reply #15 on: February 10, 2021, 02:16:56 PM
I got the input signal as close to 5mV as I could.  I'm not sure this meter does well at that low of a reading.  With the unit on I got 3.46v on the left output and 6.96v on the right output.


Offline jrhunter62

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Reply #16 on: February 10, 2021, 02:29:16 PM
Sorry, I forgot to mention it was 3.46v on L and 6.96v at 1,000hz and .035v L and .077v R at 10,000hz


Offline Doc B.

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Reply #17 on: February 10, 2021, 02:48:27 PM
OK, now swap EF86 tubes, do those measurements again and see if anything changes from left to right.

Dan "Doc B." Schmalle
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Bottlehead Corp.

Offline jrhunter62

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Reply #18 on: February 10, 2021, 02:53:57 PM
Swapped EF86 tubes - 3.11v on L and 7.76v at 1,000hz and .033v L and .086v R at 10,000hz



Offline Doc B.

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Reply #19 on: February 10, 2021, 04:41:34 PM
Looking at the left output RCA jack in your image. I can't tell for sure because of the overhead angle, but it might be that there is some solder that has run down from the center pin cup and is touching the outer shell. 

Dan "Doc B." Schmalle
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Bottlehead Corp.

Offline jrhunter62

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Reply #20 on: February 11, 2021, 02:30:19 AM
I checked the left output and I can't find any solder touching the outer shell


Offline Doc B.

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Reply #21 on: February 11, 2021, 05:21:34 AM
Do you have a spare 6922 to try?

Dan "Doc B." Schmalle
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Bottlehead Corp.

Offline jrhunter62

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Reply #22 on: February 11, 2021, 05:35:16 AM
I do not. 


Offline Paul Birkeland

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Reply #23 on: February 11, 2021, 06:32:45 AM
Quote from: jrhunter62 on February 10, 2021, 02:16:56 PM
I got the input signal as close to 5mV as I could.  I'm not sure this meter does well at that low of a reading.  With the unit on I got 3.46v on the left output and 6.96v on the right output.
Gain at 1kHz of the Eros is about 50dB, and 5mV with 50dB of gain applied to it would give 1.5V of output.

It would help to know what you saw on your meter that was as close to 5mV as you could get so we can know whether the lower volume channel is the issue or whether the higher volume channel is the issue.  So far it is looking more like the higher volume channel may actually be the issue, but it's not possible to say without concrete data.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline jrhunter62

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Reply #24 on: February 11, 2021, 09:27:42 AM
I have attached pictures of the meter and what I am measuring.  Previously I was concerned the meter might not have been up to it because the values were fluctuating, but I am able to get .005v consistently now.


Offline jrhunter62

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Reply #25 on: February 11, 2021, 09:30:34 AM
more pics


Offline jrhunter62

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Reply #26 on: February 11, 2021, 09:35:20 AM
I took another picture of the right input measurement at .005v, but it isn't playing nice with the forum, you will have to take my word for it.


Offline Paul Birkeland

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Reply #27 on: February 11, 2021, 10:08:36 AM
OK, taking 0.005V in and getting 3.16 out is 56dB.  That's a little more than expected, but not impossible.  If you are getting 7V out of the other channel under these conditions, I would be looking toward the EQ as the source of your issue.  It could be something simple like the two little brown caps being swapped, or it's possible the 10.7K resistor got swapped for a different part by accident.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline Doc B.

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Reply #28 on: February 11, 2021, 12:13:27 PM
If a visual inspection doesn't turn up some swapped EQ components it might be helpful to measure at 100Hz, 1kHz and 10kHz again and note the voltage difference between channels at each of those frequencies. If the percentage of disparity is larger at one end of the spectrum than the other that might help to narrow down what components to check for a cold solder joint.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.

Offline jrhunter62

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Reply #29 on: February 11, 2021, 02:01:37 PM
I can't find any components swapped or mis-wired

100Hz: R- 27.8, L- 18.63
1KHz: R- 7.68, L- 3.14
10KHz: R- .091, L- .026


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