Re: PG506 Repair - I had a Great Day today/


tek_547
 

Hi Michael, this weekend I found the time to check your suggestions from your previous mail. So here are some measurements;
With all the measurements the switch on the front panel is allways on STD AMPL and VAR out.
U400
pin 1, 2 & 3 has a sort of square wave (SW)
pin 4 (+4,3V SW), 5 (-0,4V no SW) & 6 (+4,4V no SW)
pin 8 (+4,3V SW), 9 (+3,7V no SW) & 10 (+5V SW)
pin 11 (+5V SW), 12 (+4,4V SW) & 13 (+2,8V no SW)
pin 7 (0,00V)
pin 14 (+5,49V

As you see several of the pins have no waveforms, so maybe not good.
The remaining 741 opamps are all OK, the uA710 (U470) is replaced by a new one.
The two opto couplers 4N27 I don´t have in my junk box, so these are waiting for replacement
All the transistors are checked on board with a multimeter at "diode test". All transistors have normal b-e junction voltage (0,65V) and no shorts inside Q435 & Q535 are presumably Ok but to be sure they must be desoldered. Better wait as last option.
CR290 & CR291 have a (low) voltage drop of 0,35V (126V at anode CR291 and 125,3V at base Q290) so they must be OK

Q535 b; +6,5V with 50mV square wave on top
Q535 c; VAR at CCW square wave +7Vtt / VAR at middle 0V / VAR at CW 0V

Q560 c; VAR at CCW, at middle and at CW +0,8V no square wave visible
Q565 c; VAR at CCW, at middle and at CW +0,16V no square wave visible
All the diodes on the board I checked with the multimeter on diode test, all looks good

What I see is some strange behaviour around U460 & U470:
U460 pin 6 a triangular wave from +16Vtt with VAR at CCW
U460 pin 6 +16V with VAR at middle (while rotating VAR to CCW increasing to a triangle wave from +15Vtt)
U460 pin 6 +16V with VAR at CW (no triangle)

U470 pin 3 +0,7V with VAR at CCW (with needle pulses to 0V)
U470 pin 3 +0,7V with VAR at CW
U470 pin 7 -0.5V permanent, from CCW to CW with 50mV needle pulses
supply voltage at pin 8 and pin 4 are normal
U460 seems working normal but only the small needle pulses at U470 I can´t explain

If you have the possibility pls check the 5V1 at the collector from Q70, mine is 4,75V. For TTL logic (U670, 671, 673 & 675) this is the absolute minimum.
Maybe this is the reason of the problem so I am interested in the value from your PG506.

And also look that if inside your PG506 T130 in the power supply is about 70°C when the STD AMP switch is activated. So almost not possible to you put your finger on the windings of it. Is this on yours the same?
Thanks so far and hopefully you have some suggestions, Rene

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