Hi all!

I checked my service manual but I can't find any reference voltages/test points for the adjusting of the idle current of beomaster 1200 power amp. (BM is running hot, so I'll replace trimpots and electrolytic caps and readjust)

I have a working one, so I can take reference voltages from that one if no one knows. But, I don't know where to measure!

Any comments are greatly appreciated!

Correction: trimmers to be adjusted/replaced are the 500 ohm ones, No 399 and 429 in the service manual, right?

I found reference voltages in the schematic too, but which one should I use as guide in adjusting the idle current?

Right. Left channel measured 7.5 mV over R423, using R429. Right channel over R421, using R399 - volume control at minimum. This doesn't quite agree with the values somebody has dabbled over the schematic we have on site (seems like "24 m/A" through those same 0.15 ohm resistors, which would result in 3.6 mV), but I'd rather trust the printed data

Before that, you might want to check the supply voltage; adjust to 33V using R459.

Wow,  Mika, thanks for the quick reply! great, just the info I needed!

Do you happen to know a way to adjust the offset, I can't find any pots for it. It's probably fine but I'd like to check it. It should be as close to zero as possible if I'm right?

I did these adjustments to a BM6000 with great success once, it seems quite the same procedure!Learning new stuff here all the time

There isn't any adjustment for offset - or actually not much of a concept of offset. Like most amps of this vintage, BM1200 is AC coupled (the speaker outputs go through the large 3000 uF caps) and the output stages run off a single sided supply. So at idle state, the outputs float at some fixed value between ground and supply, which is determined by the component tolerances and probably somewhat by the idle current setting - but the speaker, after the coupling capacitor, will only see 0V anyway.

The schematic shows that this voltage should be at 18.6V (I guess, the scan is a bit unclear), but there's no need to adjust it - this is just a test voltage, and if it differs much, there is something wrong with the power amplifier.

Hope yours will be fine after just the idle adjustment

Hi Mika,

Thanks for the clear explanation! I did not know that yet, this made things a lot clearer!

Just like I said, learning new stuff all the time, and it's great! The running hot seems a clear case of drifted idle current, so I guess everything will be fine after adjusting.

Thanks!