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Old 7th October 2005, 06:26 AM   #1
koumbar is offline koumbar  Canada
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Join Date: Feb 2005
Location: Toronto
Default Threshold 800A Help!

Hi,
I have the 800a we had discussed on the forum some time ago with Mr. Pass.
I went ahead and replaced all the electrolytics as well as some of the smaller disc cap's. I have also replaced the RCA's, the coax signal cable from the RCA's to the small "control"(?) board sitting on the base of the amp,and from there to the driver (?) boards.
The amplifier played with new life and sounded excellent at normal volume, however it became harsh louder and would go into stand by mode. I was happy since most of my listening is at lower levels. Mr. Pass you were right when you suggested to me last time that the trannies were not the problem.
Yesterday while attemting to replace the film cap's .15uf 250v ITT/GB (brown) on the "control" board (around the red pulse engine PE 5437 and going to ground from the speaker terminals in series with a resistor) I goofed large and ESD took out the lower VU meter. Well at least that's the only thing I could detect as fried. Now, the stand by light comes on and then the power on light comes on, however there is no voltage, no real power going anywhere.
I am wondering if anybody knows what this red pulse engine is all about as I think this may be the cause ( I may have fried it) of the problem. Any input would be appreciated.
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Old 7th October 2005, 06:32 PM   #2
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It's difficult to fry that transformer - it's a pulse transformer driven
at high frequency to drive the TRIAC which is the main power
switch. I wish I had a schematic to give you, but to break it
down simply, the control board had input buffers and volume
controls and also a limiting function that could be set from the
front panel. The rest of the board used an AC sense touch
button to drive an on/off flip flop (made from an op amp with
positive feedback) which then drove an oscillator which in
turn drove the TRIAC control.

If you see a high frequency AC signal driving the pulse
transformer, then the logic is working. Be careful - that board
is AC live.

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Old 7th October 2005, 07:45 PM   #3
koumbar is offline koumbar  Canada
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Join Date: Feb 2005
Location: Toronto
Mr. Pass thank you for taking the time to look at my question.
I will try again tonight to pinpoint the problem with this info.
By the way I have ( well had prior to my !@#$%&*) 87Volt rails; is that normal? I think it is a bit high, compared to what I am used to seeing on the forum for other amps.

Big thanks to the Pass-Labs forum for making everyday people, like myself, with vintage Threshold amplifiers feel welcome and not alone.
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Old 8th October 2005, 03:11 PM   #4
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I second that emotion. I gained much info from this place when rebuilding a 400A and it turned out very groovy!
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