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Old 19th February 2007, 08:33 PM   #1
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Default counterpoint sa100 headaches

i bought a counterpoint sa100 15 yrs ago. it was ok.until recently. i have 1 channel making a low frequency whoomp! now and again. i unplug it. wait a day, turn it on and it will work for a while.
i hope it is a capacitor because i plan on replacing the electrolytics on the board.
the transformer windings are 259v for the b+ and 34 0 34 for the output. the board is burnt around the 36v zenerdiodes [d60-d61]. the 10mfd caps [c60-c61] are almost touching the zeners.
for the purpose of reducing the heat, could i change the zeners to 39v and increase the resistance of r-62-63. what would it do to the bias? smoke?
since i am removing this board, i want to take care of these problems hoping to make it more reliable.
i want order the caps from digikey. would it be sonically better if i increase the values,and voltages and temp ratings?
i see several ways to remove the board. which way do you prefer? rev3
if i sound ineperienced, i am. it's been 35 years.
any help would be appreciated.
thank you
pothound
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Old 20th February 2007, 12:43 AM   #2
Leolabs is offline Leolabs  Malaysia
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If you worry about the temperature issue,try something like 105'c.
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Old 20th February 2007, 12:59 AM   #3
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I would not increase the capacitor values if I were you. The designer of this preamp typically used larger than prudent values. I would look at the Panasonic TSHA caps for a replacement if they are 450V or lower... these are 105 deg. C caps and are very good (digikey has them). If this uses 500V caps, then Pana TSUP or TSU would be your best choice from Digikey.

See if you can get anatech to reply to this thread... he ran a Counterpoint service depot and has alot of experience with these and should be able to point you in the right direction.
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Old 20th February 2007, 03:25 PM   #4
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Default counterpoint sa100 headaches

thank you
i want to thank anatech and others, because of their information give me the courage to repair this amp myself.
thankyou
pothound
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Old 21st February 2007, 08:00 PM   #5
anatech is offline anatech  Canada
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Hi Chris (Pars),
Thank you for "grabbing my arm" and pointing me here.

Hi pothound,
Your heater caps will be shot, at least the first one. Get 105° caps.

That DC offset voltage circuit is a problem. Your 10 uF caps are open, those under board resistors are toast and the zeners should be replaced. What was Counterpoint thinking???!

I designed a small PCB for each side (new circuit too) to replace that area of the circuit as that area has been found destroyed due to heat. All you are doing there is generating a + and - stable voltage so you can inject a correction voltage into the fet gates, the bias voltage is generated this way as well. You can remount the caps in the same location if you do this as the heat has been taken off board to another location. You really, really want a rev "C" board. I make these boards one at a time, so they are a pain. There is no pattern for them or any print version of the schematic. What is happening is very straight forward and shouldn't be a mystery.

Remove and short RV3 on each side. You will have to play with tubes to match the circuit gain to each side. Your gain will end up in the +34 dB range, possibly higher.

-Chris
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Old 24th February 2007, 01:35 PM   #6
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Default counterpoint sa100 headaches

hi chris
sorry it took so long. i would like to install these circuits. getting that heat off the board should make this more reliable.
i haven'ordered parts yet. if you will send a parts list and circuit diagram, i will install it.
while i have the board out i'll make any minor changes to improve reliability. it was mentioned to put a resistor ahead of the first cap in the filament supply. what value?
the board is burnt around the filament rectifier.
i need to learn more about the bias control, so i can disable it before i turn the amp on. do i turn it left or right?
thank you
pothound
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Old 24th February 2007, 03:45 PM   #7
anatech is offline anatech  Canada
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Hi pothound,
I don't have the schematic in an electronic format, I can't even locate my paper schematics right now. Just set up a + and - current source from each rail to a pair of 1/2 W zener diodes to replace the originals. Set all this up on a small PCB and replace the capacitors.

For the current sources you can use an led - transistor arrangement or a current regulator diode. I think Digikey has them on hand. Just figure out your desired diode current and watch your dissipations. The actual bias network draws very little current.

If you pull the fuses on the rails, you can measure the gate to gate voltage with the amp plugged in. When it first starts up there are relay contacts that short the gates to ground. Sometimes those contacts can burn open or short when a mosfet blows. If you install 1 ohm resistors in where the fuses go you can measure the bias current. to set the bias voltages. I use a variac when running these up.

-Chris
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Old 24th February 2007, 03:50 PM   #8
anatech is offline anatech  Canada
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Hi pothound,
Quote:
it was mentioned to put a resistor ahead of the first cap in the filament supply. what value?
I haven't figured that out yet. That is yet another thing I want to play with as I identify shortcomings. Play.

-Chris
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Old 24th February 2007, 04:16 PM   #9
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Old 24th February 2007, 05:28 PM   #10
anatech is offline anatech  Canada
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Don't forget to short out VR3 and VR103!

-Chris
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