18 Watt TMB Problem

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perplexd
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18 Watt TMB Problem

Post by perplexd »

Hello all,
I am having another issue with my Trinity Amps Plexi Mk II. It is on channel 2 and occurred suddenly. The sound is very thin, very little bass. Channel 1 does not seem affected. I started messing around with the knobs and rolled the tone stack all the way down, there is very little signal passed. The treble control is acting like a volume knob through its entire sweep. The mid and bass controls have more effect once the treble is up. The mid control adds some fullness and bass but not much, and the bass control has an almost imperceptible affect.

Schematic is here: https://www.trinityamps.com/wp-docs/18W ... matic5.pdf

Any ideas? I am wondering if it a bad pot or something shorting in the tone stack?
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JMPGuitars
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Re: 18 Watt TMB Problem

Post by JMPGuitars »

perplexd wrote:
Fri 05/05/23 3:08 pm
Hello all,
I am having another issue with my Trinity Amps Plexi Mk II. It is on channel 2 and occurred suddenly. The sound is very thin, very little bass. Channel 1 does not seem affected. I started messing around with the knobs and rolled the tone stack all the way down, there is very little signal passed. The treble control is acting like a volume knob through its entire sweep. The mid and bass controls have more effect once the treble is up. The mid control adds some fullness and bass but not much, and the bass control has an almost imperceptible affect.

Schematic is here: https://www.trinityamps.com/wp-docs/18W ... matic5.pdf

Any ideas? I am wondering if it a bad pot or something shorting in the tone stack?
Have you tried swapping the tubes?
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perplexd
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Re: 18 Watt TMB Problem

Post by perplexd »

JMPGuitars wrote:
Fri 05/05/23 3:46 pm
Have you tried swapping the tubes?
Yes. All positions.
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Re: 18 Watt TMB Problem

Post by JMPGuitars »

perplexd wrote:
Fri 05/05/23 5:05 pm
JMPGuitars wrote:
Fri 05/05/23 3:46 pm
Have you tried swapping the tubes?
Yes. All positions.
OK, highlighter test CH2 from input through to the PI. Pay close attention to the solder joints.
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perplexd
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Re: 18 Watt TMB Problem

Post by perplexd »

What are all the steps of a highlighter test?

I have traced the channel 2 circuit, measured all resistors in the amp and read values of the capacitors. No errors found in the layout. I also checked all the voltages on the schematic and all are correct, with the exception of V2 Pin1. It reads ~150V as opposed to 180V, and always has.

I also did some chopsticking to try and find it with no luck. I also wanted to clarify, that the issue started suddenly and has stayed this way since, so it is not exactly intermittent. It did the same thing when I was debugging hum on the volume pot earlier year, but was gone by the time I reworked much of the tone-section and solder joints in general.

I wish I could remember how the tone stack interacted when it was sounding right. I believe the signal is cut off when TMB are all the way down in this circuit? I don't recall the treble pot acting like a volume control, but I could be wrong. I am tempted at this point to just starting reflowing every solder joint throughout channel 2. Any other thoughts?
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Daviedawg
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Re: 18 Watt TMB Problem

Post by Daviedawg »

A highlighter test is straightforward if a bit tedious to do correctly. Print both schematic and layout. If you know that the issue is in a particular part of the circuit then concentrate on that.
Start at the incoming signal, inspect every node, measure every component you can and visually check the components and joints. Mark both prints with highlighter as you go to ensure that the layout matches the schematic and vice versa. Look for "whiskers" especially at valve bases.
To isolate the area where some types of issues are, with the amp on and safety measures in place you can tap nodes [with a wooden chopstick] along the signal path until you stop or start hearing noises. Does not work for all faults of course.

Dd
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perplexd
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Re: 18 Watt TMB Problem

Post by perplexd »

Thanks Daviedawg,

I've covered a lot of that, but I will keep probing and probably just start touching solder joints as mentioned.

I'm still curious as to why that V2 cathode voltage has always measure 150V when I get 272V at pin 6? The resistor measures 99.2k.
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Re: 18 Watt TMB Problem

Post by Daviedawg »

When I am testing the signal path I don't use a chopstick. I run the amp, ground the negative probe of my meter as if I was reading the voltages and tap the positive probe to the nodes and pins. This is safe if you use normal precautions but is more likely to create a noise than a non-conductive probe.

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Re: 18 Watt TMB Problem

Post by chaliapin »

What sort of noise exactly are you looking for using the meter instead of a chopstick DD?
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Daviedawg
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Re: 18 Watt TMB Problem

Post by Daviedawg »

Just a clicky break contact noise. If you hold it on you may get hum or nothing. Make and break noises basically. This is only use where you have lost signal or have a silent runner. It is a basic system for fault isolating.

Of course it is better if you have a signal generator then you can listen for distortion or volume loss as well. If you also have an oscilloscope you can see the signal and its qualities. See other posts for advice on that topic.

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perplexd
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Re: 18 Watt TMB Problem

Post by perplexd »

I might be on to something! I realized that I cheated the highlighter test suspecting it was after V2, so I starting poking around from the input jack to V1. I didn't necessarily find anything but when I turned it on and plugged in the amp was back to normal. So I suspect a joint there, however I wish that I could actually find the spot where I could force the issue to switch back and forth.
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Re: 18 Watt TMB Problem (Fixed)

Post by perplexd »

perplexd wrote:
Tue 05/16/23 8:03 am
I might be on to something! I realized that I cheated the highlighter test suspecting it was after V2, so I starting poking around from the input jack to V1. I didn't necessarily find anything but when I turned it on and plugged in the amp was back to normal. So I suspect a joint there, however I wish that I could actually find the spot where I could force the issue to switch back and forth.
I reflowed the solder joints on V1 and put the amp back together a few weeks ago, and no issues since!
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