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Old 30th Nov 2021, 6:08 pm   #1
djm7777
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Default Advice to fix volume control on Trio Amp.

Evening All, hoping someone out there can help me out with a problem on a Trio KA2000A Amp.

The unit turns on alright but the volume is loud no matter what position the volume knob is set to.

I've tried cleaning the pot with contact cleaner spray but it didn't improve.

Anyone know what the issue/fix might be?
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Old 30th Nov 2021, 7:15 pm   #2
Lucien Nunes
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Default Re: Advice to fix volume knob on Trio Amp

Does this apply to both channels? It sounds like the ground connection may be open-circuit at the anti-clockwise end of the pot. Check it for broken connections! Do you have a multimeter with which you could test for continuity to ground from the pot tags?

BTW I think this unit has a tapped pot track for loudness compensation. That won't make any difference to the present situation though.
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Old 1st Dec 2021, 9:46 am   #3
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Default Re: Advice to fix volume knob on Trio Amp

Question, has this been working okay and has just developed the problem or is it a new acquisition?
If you are not familiar with the 2000a, the volume control is in two halves, you need to turn both the front and rear knobs together, after checking the front knob is indexed correctly to the rear section.

If it is a new fault then as Lucien says likely a loss of the ground wire to the volume controls. The ground wires are mostly black on the Trio.
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Old 1st Dec 2021, 10:15 am   #4
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Default Re: Advice to fix volume knob on Trio Amp

If the pot connections are to a PCB, then go over the six solder joints with a hot iron and fresh solder. Otherwise if it's wired look at solder connections at each of the for breaks or dry joints.
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Old 4th Dec 2021, 9:07 pm   #5
djm7777
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Default Re: Advice to fix volume knob on Trio Amp

Thanks very much for all the advice. It is a fault that came with the unit which I only just got hold of. I have ordered a multi meter to do some tests...
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Old 6th Dec 2021, 7:45 pm   #6
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Default Re: Advice to fix volume control on Trio Amp.

Good plan. These are really nice little amps.
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Old 8th Dec 2021, 12:00 am   #7
djm7777
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Default Re: Advice to fix volume control on Trio Amp.

After some tinkering and more contact cleaner the constantly loud volume issue has now disappeared, but there is a constant buzz/white noise coming through the speakers on any input setting.

I have been trying to search for what the issue could be and a couple of potential suggestions that came up were a faulty filter capacitor or a ground loop.

Don't know if it's a factor but originally the plugs were japanese 2 pronged that I swapped for UK ones. There was no earth wire in the cable so just wired up the live and neutral, which another forum said would be ok.

Can anyone point me in the right direction for what to test/replace next to try and eliminate this buzzing noise? I have a digital multimeter now but that's it in terms of diagnostic tools.
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Old 8th Dec 2021, 10:11 am   #8
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Default Re: Advice to fix volume control on Trio Amp.

A very common problem on these old Trios is transistor noise.
The 2SC458 is infamous for going noisy (not just in these amps either).
Have a careful look at your example and look for transistors marked 'C458'.
Often in Q1 and Q9 on the main board, but can be in several other locations and on the tone board and phono board as well. They are unusual shaped rectangular box with a chamfered edge. See the picture at the bottom.

A BC550 (NPN) low noise transistor is a good substitute. Just note the pin outs are different.

There is an excellent restoration thread on Audiokarma : https://audiokarma.org/forums/index....ration.845076/
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Old 8th Dec 2021, 11:17 am   #9
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Default Re: Advice to fix volume control on Trio Amp.

What are the weird black "pastilles" that appear to be wired in series just above the trim pots in the picture?
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Old 8th Dec 2021, 12:00 pm   #10
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Default Re: Advice to fix volume control on Trio Amp.

They are the pair of silicon diodes (De4/6 and De5/7) that feed the bias network. (Type FR1S brings no matches from a web search...)
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Old 8th Dec 2021, 12:08 pm   #11
djm7777
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Default Re: Advice to fix volume control on Trio Amp.

Quote:
Originally Posted by snowman_al View Post
A very common problem on these old Trios is transistor noise.
The 2SC458 is infamous for going noisy (not just in these amps either).
Have a careful look at your example and look for transistors marked 'C458'.
Often in Q1 and Q9 on the main board, but can be in several other locations and on the tone board and phono board as well. They are unusual shaped rectangular box with a chamfered edge. See the picture at the bottom.

A BC550 (NPN) low noise transistor is a good substitute. Just note the pin outs are different.

There is an excellent restoration thread on Audiokarma : https://audiokarma.org/forums/index....ration.845076/

Thanks for the tips. There is a C458 at Q1, Q8 and Q9 and by the looks of it 2 on the vertically positioned board infront of the main board (phono board?).

Some replacement BC550s are now on their way and will be installed. Going to have a read of that restoration thread now and try and get some more insight.
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