FG502 problem?


Chris van Lint
 

Hi

I recently tried to fire up my FG502.  The unit was not working and the green power lamp was not illuminated. I discovered that one of the 741 opamps in the power supply was faulty.  I replaced the 741 and the unit now appears to be working OK.  However although the +20V supply is within specs, the -20V is way out at -30V. The +20V supply is easily adjusted with R625, but I cannot find any adjustment pot for the -20V supply. _The +17V and -17V supplies are also within specs._

- Does this mean that I should not be concerned about the -20V being too high?

- Am I correct in assuming that R672 on pin4 of U664 is out of range?

Any advice would be appreciated

Chris


Tom Lee
 

The schematic shows that the -20V supply is derived from inverting the +20V output. So, if the latter is good, then the problem is with the inversion. Check U664 and associated components.

--Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 6/10/2021 17:25, Chris van Lint wrote:
Hi

I recently tried to fire up my FG502.  The unit was not working and the green power lamp was not illuminated. I discovered that one of the 741 opamps in the power supply was faulty.  I replaced the 741 and the unit now appears to be working OK.  However although the +20V supply is within specs, the -20V is way out at -30V. The +20V supply is easily adjusted with R625, but I cannot find any adjustment pot for the -20V supply. _The +17V and -17V supplies are also within specs._

- Does this mean that I should not be concerned about the -20V being too high?

- Am I correct in assuming that R672 on pin4 of U664 is out of range?

Any advice would be appreciated

Chris





Mark Vincent
 

Chris,

Check the series pass for the -20V supply. That could be shorted. It appears that a connector, by looking only at the schematic, to the pins are on a plug that can be removed. See if there is about -21V at 11A (base of the series pass) and about -21,5 to -22V at the base of Q662 if the series pass is good.

Check R672. Replace if bad. Also check the voltage at pin 7 of U664. It should be 1,2-1,4V.

Mark


EJP
 

I would also check all the resistors around the loop leading from pin 6 of U664 back to pin 2, and R670 from pin 3 to ground, and R666, and the two-diode string from pin 7 to ground, and all the transistors.

EJP


Chris van Lint
 

Thanks to all who provided advice/input.  It pointed me to where to look.  In the end it was a faulty U664 741.  All voltages are now spot on.

Chris


Tom Lee
 

Glad it was something simple (and cheap)!

— Cheers,
Tom

Sent from my iThing, so please forgive typos and brevity.

On Jun 11, 2021, at 7:04 PM, Chris van Lint <chrisvanlint@bigpond.com> wrote:

Thanks to all who provided advice/input. It pointed me to where to look. In the end it was a faulty U664 741. All voltages are now spot on.

Chris