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Old 12th April 2008, 03:22 AM   #1
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Default Dynaco ST-120 runs hot

No amp has ever kicked my butt like this "class B" POS. This is an eBay "treasure" factory wired ST120A, latest version with glass boards and TIP mods. Replaced all semis both channels, all electrolytics. After rebuild it seemed to work OK, but ran a bit hot. Came home one day soon after to find smoke-filled room and big hole burned in power supply board. One channel blown again, 300 ohm 7w resistors had gotten so hot they unsoldered themselves. Should have stripped chassis at this point and used it for a Chipamp. Instead I bought a FET regulator board from the eBay guy, and rebuilt blown channel. Replaced 7w resistors with 10w. Again, both channels run way too hot. Over 100c at heatsink after 10 minutes (boils spit). I am not sure how the bias on this works, other than it is not adjustable. Can anyone give me an idea how to cool this down before it blows again? BTW I have checked for oscillation with scope. Output chokes and zobels are in place. B+ exactly 70v, but center of output pair is 32.2v. I swear, if this blows again it's not getting another chance........

Bob G.
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Old 12th April 2008, 04:44 AM   #2
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looks to me like you are having bias problems.

check voltage accross R27, anything more than 30mV is suspect....
same with D1, if voltage is more than 5.1 volts, then you have problems...

or your amp could be oscilating at hf, if you have a pocket sized am radio, bringing it very close to the amp will reveal it..
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Old 12th April 2008, 04:47 AM   #3
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Quote:
Instead I bought a FET regulator board from the eBay guy, and rebuilt blown channel. Replaced 7w resistors with 10w.
the original regulator circuit is easy to repair, i believe you should stick to the original, in order to preserve the original flavor of the amp...
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Old 12th April 2008, 05:04 AM   #4
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Thanks for the reply, Tony. I can't repair the original regulator without replacing the burned PC board. Voltage across R27 is 323MV one channel. 491mv other channel. That would be almost 1A across .47 ohm resistor? 5.1v across D1 is OK. This is with ST 2N3772 outputs. I have some good originals I will try next to see if it makes a difference.

Bob G.
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Old 12th April 2008, 05:08 AM   #5
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check D3 and D2, they could be leaky....if these are good, then you could be having more Vcc supply than the amp was designed for...that is why it is important to get the original regulator going again, else try to lower your voltage to the amp..
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Old 12th April 2008, 05:14 AM   #6
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Quote:
This is with ST 2N3772 outputs. I have some good originals I will try next to see if it makes a difference.
they won't matter if you have bias problems....
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Old 15th April 2008, 05:20 AM   #7
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Default Finally....fixed

Well, I have enough hours in this to buy a Krell, but it was my fault. I "Upgraded" the RCA jacks and ran nice twisted pair wiring to pins 12 and 13 of the PC14 board. Unfortunately the original RCA jack is the only point for signal ground to connect to chassis ground. My fancy unobtanium plated jacks are isolated from ground, leaving signal ground floating at 3.5v above chassis ground. A 10 ohm resistor between pin 12 and 15 fixed things nicely. Of course it still sounds like a ST120, but it's for my boy's room and with the BR-1 speakers he got from Santa it is a big step up from his boom box.

Bob G
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