Hello all, I picked up this Crest at a swap meet in broken condition. It’s very clean on the outside, but one channel was causing it to stay in protect. Someone took a stab at it, after I pulled the board out is saw many lifted pads and some of the worst flux residue I’ve ever seen in my life. There was a sticker on the back of a local audio “repair” shop and it wouldn’t surprise if they did the work. I fix at least a dozen items they see before me, last being a pair of McIntosh 500 watt monoblocks that they said couldn’t be fixed. Those poor things, but they were fixed an have been going strong daily for a couple of months now.
Anyways, end of rant. In the bad channel of this Crest all of the PNP outputs were shorted as was one of the drivers. There was evidence of one of the predrivers going up in smoke as there was residue on the heatsink. The last person that attempted a repair on this used nothing but NTE components. I’m not a big fan of NTE.
So pre-drivers were changed to Onsemi MJE340 and MJE350 which is what the schematic calls for. For the drivers I put in new Onsemi MJE15032 and MJE15033 which is what the schematic calls for. For the outputs I replaced the Toshiba 2SA1302 and 2SC3281 with Onsemi MJL1302 and MJL3281, I wanted both channels to be the same so I did the good channel as well, plus the Onsemi are better.
Got it all back together, powered up great. No issues at all. I need to set bias though and while I have somewhat of a service manual that is mostly schematics, it gives no information on bias. I’m measuring across the two outer most emitter resistors on the bottom (there are a row of transistors and emitter resistors on the bottom of the heatsink and a row on the top).
Measuring across a single 0.33 ohm emitter resistor I have bias at about 1.9 mV, I am still on my dbt until I get a proper number to shoot for. I’m at what, a little over 5-1/2 mA right now? Measuring across a singe resistor what should I go for? This amp has a healthy fan so it does have adequate cooling.
Thank you,
Dan
Anyways, end of rant. In the bad channel of this Crest all of the PNP outputs were shorted as was one of the drivers. There was evidence of one of the predrivers going up in smoke as there was residue on the heatsink. The last person that attempted a repair on this used nothing but NTE components. I’m not a big fan of NTE.
So pre-drivers were changed to Onsemi MJE340 and MJE350 which is what the schematic calls for. For the drivers I put in new Onsemi MJE15032 and MJE15033 which is what the schematic calls for. For the outputs I replaced the Toshiba 2SA1302 and 2SC3281 with Onsemi MJL1302 and MJL3281, I wanted both channels to be the same so I did the good channel as well, plus the Onsemi are better.
Got it all back together, powered up great. No issues at all. I need to set bias though and while I have somewhat of a service manual that is mostly schematics, it gives no information on bias. I’m measuring across the two outer most emitter resistors on the bottom (there are a row of transistors and emitter resistors on the bottom of the heatsink and a row on the top).
Measuring across a single 0.33 ohm emitter resistor I have bias at about 1.9 mV, I am still on my dbt until I get a proper number to shoot for. I’m at what, a little over 5-1/2 mA right now? Measuring across a singe resistor what should I go for? This amp has a healthy fan so it does have adequate cooling.
Thank you,
Dan
I designed my own amps so no bias instructions for it.
1/ Turn bias right down.
2/ Apply 100mV sine wave to input.
3/ Monitor output on a scope.
4/ Turn bias up slowly until cross over distortion goes.
Thats probably safest way but I a sure others will chirp in with other ways.
1/ Turn bias right down.
2/ Apply 100mV sine wave to input.
3/ Monitor output on a scope.
4/ Turn bias up slowly until cross over distortion goes.
Thats probably safest way but I a sure others will chirp in with other ways.
