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I am trying to get this instrument, serial prefix 1632, back in operation. Depressing the line switch does nothing, no light, no fan, etc. I read 122 VAC inside the box. All of the test points on assembly A16, low-voltage power supply, show abnormal values, mostly fractions of the power called for. In the case of TP4, adjusting R26 has no effect. The manual that came with instrument, dated August 1976, does not have a troubleshooting flowchart. I would appreciate suggestions on proceeding forward from anyone who has encountered this problem before.

Joe
KW4YW

On Sat, Jan 29, 2022 at 11:06 AM, Joe wrote:

I am trying to get this instrument, serial prefix 1632, back in operation. Depressing the line switch does nothing, no light, no fan, etc. I read 122 VAC inside the box. All of the test points on assembly A16, low-voltage power supply, show abnormal values, mostly fractions of the power called for. In the case of TP4, adjusting R26 has no effect. The manual that came with instrument, dated August 1976, does not have a troubleshooting flowchart. I would appreciate suggestions on proceeding forward from anyone who has encountered this problem before.

Joe
KW4YW

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Not based on experience but review of the schematic:

+15V is the reference to other supplies, if it is off the other supplies will be impacted too. I recommend focusing on getting +15V fixed, probably other supplies will recover too.

It is relatively easy to debug it since it is a linear supply. First step is to make sure U1 gets proper power supplies. If you have another scope look at pin 7 (VC) for minimum voltage. There will be some ripple but valleys shouldn't dip below 18V. If you see a lower voltage suspects are C8 and CR4. If you don't have another scope, note says 21.3V is expected across C8.

If pin7 voltage is fine, then next step is pin 8 (V+) voltage. It should be at least around pin 7 voltage and nominally about~ 3.5V above pin 7 voltage. If it is not even above ~ 18V then VR1 is bad.

Also note pin 6 (Vout) voltage. If a short is detected Vout will be lower than what is written (16V).
Pin 3 (non inv) should be ~ 7V.

If Vbe of Q4 is above 0.7V then Q4 is bad. ?

The most likely case is a short at +15V supply. Measure the voltage across R24, if it is close to ~ 0.6V then there is a short down stream from the regulator and regulator is in current limit. Easiest way is to measure +15V to ground resistance, less than 50-ohm should be considered a short.?

Depending on what you find we can further narrow down the debug.
Ozan

Also check the bridge rectifiers near where the wire harness plugs in from the transformer. I've have quite a few of the 1700 series that had shorted rectifiers causing similar issues.
That and the various .200" headers for the board to board interconnects would have bad solder connections on the PCB and the header pins as if something over time was acting like an insulator between the gold plating and the solder.??

Yes check the rectifiers, I repaired a 172aa that 1 rectifier had broken into 2 pieces,something I've never seen before