sajjad amer
Active Member
I have purchased this amplifier in protection mode. I have followed the instructions below from this very forum, to adjust the offset and Bias after recapping the amplifier board and changing trimmers.
However the amplifier is still stuck in protection mode on DBT. Would someone be kind enough to guide me to next steps?
Many Thanks in advance
the 5-pin plastic terminal,
Connect the DC voltmeter between "out" and "out", on the each channel (those two points are derived vrom NPN-PNP emmiters of each channel)....adjust at minimum possible. Do it for both channels
The adjustment trimmers are located on the power amp PCB left and right, on the corners.
They are two: HOT-COLD and COLD - EARTH
Pins 1 and 4 should be adjusted with hot-cold one
If those values are OK (<500mV DC), then check between pin 1 and 3, and pin 4 and 3 (Ground) and, also adjust to minimum using the second trimmer (COLD -GND)
At the time of less than 500mV, the relay should be released, and then proceed as per manual, if you want.
Connect the DC voltmeter between "out" and "out", on the each channel (those two points are derived vrom NPN-PNP emmiters of each channel)....adjust at minimum possible. Do it for both channels
The adjustment trimmers are located on the power amp PCB left and right, on the corners.
They are two: HOT-COLD and COLD - EARTH
Pins 1 and 4 should be adjusted with hot-cold one
If those values are OK (<500mV DC), then check between pin 1 and 3, and pin 4 and 3 (Ground) and, also adjust to minimum using the second trimmer (COLD -GND)
At the time of less than 500mV, the relay should be released, and then proceed as per manual, if you want.
1. the service manual is wrong in printing; you should measure between HOT (red) and chassis; put the DMM on the highish voltage selector, in volts range (e.g, 2V, or 20V). Some modern DMMs has also VU meter, similar to many decks, using LEDs or something like that.
2. adjust DC between the HOT and GND and HOT/COLD; although the leakage is present in milivolt-range on the DMM, it can be done stable using volt-range mode.
3. adjust bias, as mentioned previously; 8 mV is OK but only if that value is between central and any lateral leg of the big rectangular emitter-resistors
If you are following exactly the service manual description, then you are measuring between two LATERAL legs on the rectangulars, and in this case, the bias should be 17-19mV (= 36-40mA, since 17/0,44 = 8,5/0,22=38,6mA (the later is between central leg and any lateral).
4. All adjustments must be done with the volume pot at minimum. This is important, since we are adjusting only power amp, and we don't want the DC from the preamp section to be measured now. (This, different part of adjusting is done in the opposite side - volume pot at maximum, and adjusting some other VRs, which are usually on the PCB behind the front plate)
5. recheck the DC HOT/COLD and HOT/GND
6. recheck again the bias several time (no signal, no music after first warming up, just left it for some 15-30min in steady state), and recheck DC after any correction for bias. Adjust as necessary.
2. adjust DC between the HOT and GND and HOT/COLD; although the leakage is present in milivolt-range on the DMM, it can be done stable using volt-range mode.
3. adjust bias, as mentioned previously; 8 mV is OK but only if that value is between central and any lateral leg of the big rectangular emitter-resistors
If you are following exactly the service manual description, then you are measuring between two LATERAL legs on the rectangulars, and in this case, the bias should be 17-19mV (= 36-40mA, since 17/0,44 = 8,5/0,22=38,6mA (the later is between central leg and any lateral).
4. All adjustments must be done with the volume pot at minimum. This is important, since we are adjusting only power amp, and we don't want the DC from the preamp section to be measured now. (This, different part of adjusting is done in the opposite side - volume pot at maximum, and adjusting some other VRs, which are usually on the PCB behind the front plate)
5. recheck the DC HOT/COLD and HOT/GND
6. recheck again the bias several time (no signal, no music after first warming up, just left it for some 15-30min in steady state), and recheck DC after any correction for bias. Adjust as necessary.
However the amplifier is still stuck in protection mode on DBT. Would someone be kind enough to guide me to next steps?
Many Thanks in advance