PG506/Repairs
If your PG506 Fails to show a display of the Variable Value on the display, check the wave forms at U400 and U470. In case of failure in the "STD. AMPLITUDE_Variable" mode, where the display and HIGH_LOW indicators do not operate; check for these wave forms at U470 and U400. This assumes that the display momentarily shows "8.8" when the variable switch is pressed "in". If the display does not show "8.8", then there are other issues related to the display and not the DVM circuit.
U400= SN74LS00N Link to datasheet here: https://www.ti.com/lit/gpn/SN74LS00
U470= TL710CP or equivalent
Here is a link to photos of these wave forms. There are two photos of a legal pad and hand notations which are an index key to associate the photos to the various pins. Photos of U400 were taken at each pin and with the variable in Full CCW. ZERO and Full CW positions. With few exceptions, there are 3 photos per pin. Exceptions are Vcc, Ground and a few others. Please refer to the proper PG506 Service Manual for more details. These photos were taken on the RELAY Equipped Version of PG506 (Not "A")S/N is B040000 and above.
PG 506 U400 AND U470 Wave forms https://www.icloud.com/sharedalbum/#B0e532ODWGRrNkO
If you have questions contact me at >mlynch002@gmail.com< or >mlynch003@yahoo.com<.
PG506 Settings:
STD AMPLITUDE: SELECTED AMPLITUDE VARIABLE: OUT AMPLITUDE SWITCH: 1V
WAVE FORMS Photographed on TDS460A. See photos for scope settings as these can vary from photo to photo.
During this repair, I found these parts defective. The repairs were much more than a non-functional display and some of the components below were related to those additional problems. The display was the last thing to get working.
In the course of the repairs, I found these components bad in the unit.
Q280 151-0280-00 PNP
Q290 151-0280-00 PNP
Q745 151-0446-00 NPN
Q435 151-0223-00 NPN
Q535 151-0223-00 NPN
U460 156-0067-00 OP amp (741CN)
U200 156-0067-00 OP amp (741CN)
U240 156-0067-00 OP amp (741CN)
U610 156-0039-00 Dual J-K Flip-Flop
U480 156-0399-00 Opto Coupler
U255 156-0399-00 Opto Coupler
CR290 152-0141-02 (1N4152R)
CR291 152-0141-02 (1N4152R)