PG506/Repairs

Replacement front plate

• https://www.pcbway.com/project/shareproject/Tektronix_SG504_Front_Plate_a2166d7a.html

Repair hints

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.

Cct ref	Tek Part	Type	Case	Generic type	Notes
Q280	151-0280-00	PNP BJT	TO-39	2N4931, 200 mA, 250 V, 1W
Q290	151-0280-00	PNP BJT	TO-39	2N4931, 200 mA, 250 V, 1W
Q745	151-0446-00	NPN BJT		Motorola SRF2320, Is this the 2N2320? (2N2320 is superseded by 2N834 which is TO-18)
Q435	151-0223-00	NPN BJT	TO-92-3	2N5769, 200 mA, 40 V, 350 mW
Q535	151-0223-00	NPN BJT	TO-92-3	2N5769, 200 mA, 40 V, 350 mW
U460	156-0067-00	OP amp	8-DIP	741CN	There are better performing modern drop-in replacements for the 741
U200	156-0067-00	OP amp	8-DIP	741CN	There are better performing modern drop-in replacements for the 741
U240	156-0067-00	OP amp	8-DIP	741CN	There are better performing modern drop-in replacements for the 741
U610	156-0039-00	Dual J-K Flip-Flop	14-DIP	7473PC
U480	156-0399-00	Opto Coupler	6-DIP	MOC102 / 4N27
U255	156-0399-00	Opto Coupler	6-DIP	MOC102 / 4N27
CR290	152-0141-02	Small signal diode	DO-35	1N4152R, 30 V , 150 mA
CR291	152-0141-02	Small signal diode	DO-35	1N4152R, 30 V , 150 mA

If your PG506 only outputs slightly more than half of the intended voltage in standard amplitude mode, check the DC-AC (S2020) switch on the A2 board for contact issues by slightly wiggling it in a circular motion. If the switch was not moved for a long time, the switch contacts might need to be cleaned.