lorbas

Hello Sandy, there'a just one trimmer on the PCB (P1, 220 ohm) just before the Mosfets, but rotating in in any direction does not seem to change anything at all... is that trimmer supposed to adjust the DC-offset ? I monitored the voltage on my scope and put it back roughly where it was before. What happens if i adjust it 'wrong' ?GP12 200 watt series 6 poweramp.pdf

Hey 3below, I have a scope, but somehow i'm not able to locate the source of the hum anymore. (Seems to be everywhere... but my days working with electrical stuff is like 30 years ago now, i used to repair TVs and HiFi when i was young...) It's seems already present at T1. Fun thing: when i short the signal-input of the poweramp-pcb to signal-ground the noise is gone. So i think it originates from the poweramp-pcb, though i'm not shure about that at all. I can completely disconnect the power-amp from the preamp-section just be unplugging one connector. Those massive diodes that form the bridge-rectifier seem to be OK. I unsoldered them on one side and tested them with the component-tester of my Hameg-scope and they looked OK. But as you said: they are cheap and i can replace them quite easily. I changed all the electrolytic caps on the power-amp-board, but payed no attention to those other caps so far. Can they cause hum, too ? Ground/earth-connections are done quite massive and well, thick cables going to one central point on the chassis. Have seen that done much worse.

So i've done all that i could: re-soldered all the pcbs and connectors, changed all the relevant caps in the power-supply of pre- & poweramp, but to no avail: still an audible mains hum on the speaker. I measured the mains-hum: it's 'just' 20 milli-volts but is's annoying. My old SWR WorkingMan's 160 got an mains-hum of just 5-10 milli-volts, that's much better. And there are minus 40 milli-volts of DC on the speaker-output that i can't get rid off... Any tips ? Anyone in the UK who deals with that questions ? Thank you.

OK, when i install the new main capacitors i'm gonna re-solder the complete board, Thank you.

Hello and greetings from Germany, bought a Trace Elliot 1215 combo-amp yesterday off Ebay quite cheap, just 50€. It's GP12, 200W, Series 6. Everything works quite fine, BUT: there's a 50Hz cycle noise in the speaker all the time, even when master is turned down and no instrument connected. Bought the manual online, took the amp apart. It's divided into a preamp- and a poweramp-board and even with the preamp-board disconnected the hum is still there, so i think it's the poweramp-board that causes the noise. There's just a very minor hum-voltage on the 4700uf main capacitors which i think is unlikely to cause the hum, but i ordered a new pair anyway. Any other ideas or experiences ? Thank you, Ralf