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PostPosted: Wed Mar 18, 2020 10:25 pm 
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When I first built the TMB18, the TMB channel worked fine but the Normal channel did not work at all (either input). I check every connection and component for an error in the build but could not find anything wrong. Then after about 30mins of use the Normal channel started working fine.

After about 3 months of use the Normal channel has stopped working again (either input). The Normal channel isn't hissing, crackling or intermittent - it is completely silent. The amp has been used a lot but has not been moved. I have tried rotating the three preamp tubes and that has not made any difference.

Any ideas on what might be the cause of the problem?


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PostPosted: Thu Mar 19, 2020 9:46 am 
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CharlesD wrote:
Any ideas on what might be the cause of the problem?

I'd be willing to bet that there's a bad solder joint in that section of the amp that makes intermittent contact. I'd start by reviewing the amp schematic & layout and re-flowing all of the solder joints that are unique to the Normal channel.

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PostPosted: Thu Mar 19, 2020 9:12 pm 
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That's all I can think of trying but it doesn't make sense to me that there is no crackling or intermittent signal. Maybe I will try pluggin in to the Normal channel and whacking it a bit to see if anything happens :-)


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PostPosted: Thu Mar 19, 2020 9:18 pm 
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CharlesD wrote:
Maybe I will try pluggin in to the Normal channel and whacking it a bit to see if anything happens :-)

You could try doing this with the chassis out of the cabinet (HIGH VOLTAGES, BE CAREFUL!!) and poke around at the connections with a non-conductive probe (a chopstick is popular for this purpose). Good way to find a bad connection. Did I mention to BE CAREFUL?

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PostPosted: Thu Mar 19, 2020 10:10 pm 
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Ok so I reflowed as many joints as I could that seemed to pertain to the Normal channel. I tried the amp again. All the solder joints looked pretty much perfect already to be honest. No change. Then I whacked the chassis a couple of times and it crackled a bit and then started working.

I can't think what could have caused this - maybe a preamp tube socket connection? That's all I can come up with! All the tubes are lighting up but that just means they are getting power from the heater wires right? Could the pins carrying audio be connecting poorly perhaps?


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PostPosted: Fri Mar 20, 2020 10:37 am 
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You might want to try swapping V1 and V3 to see if the problem follows the preamp tube to the TMB side.

Something else to consider: Possibly a faulty Volume pot on the Normal channel? The signal path goes directly through it.

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PostPosted: Mon Apr 06, 2020 11:09 pm 
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I had already rotated the tubes twice to try to eliminate that as an issue. Thus I THOUGHT it was not a preamp tube.

Checked the amp again today and the problem was back. Normal channel wasn't working. Put DeOxit contact cleaner on pins of V1, inserted it a few times. No difference. Whacked the amp again, it works. Then there was some shrieking at sudden insane volume. Shut the amp down. Then turned it on again. Amp now working, but there is a big thump when I tap V1 or V2. V3 seems to be fine. Does that mean both V1 and V2 are bad? That would explain it. I couldn't find anything wrong with the soldering on any of the tube sockets (or anywhere else).


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PostPosted: Tue Apr 07, 2020 9:35 am 
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CharlesD wrote:
I...Amp now working, but there is a big thump when I tap V1 or V2. V3 seems to be fine. Does that mean both V1 and V2 are bad? That would explain it. I couldn't find anything wrong with the soldering on any of the tube sockets (or anywhere else).

Two bad tubes would be quite unusual, but not impossible. But since V1 feeds into V2, I'd try replacing just the former, to start.

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PostPosted: Wed Apr 08, 2020 10:27 am 
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Intermittent problems like what you are describing are almost always caused by a faulty connection. Usually it's bad soldering. But it could also be an internal problem with the volume pot. Also possible, but less likely is that one of the capacitors or resistors in the normal channel are physically damaged internally. A faulty tube seems unlikely here.

You can try bypassing the volume pot. It's going to be very loud though.

It could also be a short that you can't see on the under side of the circuit board.

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