EDC 520A repair

[feedback.loop]

I found this thing on eBay listed for $69.99, offered $40, and they accepted.
I have not seen the following topic until now https://www.eevblog.com/forum/buysellwanted/multiple-edc-520s-voltagecurrent-calibrators-on-ebay/
Check out the repair video:
https://youtu.be/pkMySlkpTJk

[dacman]

If you try to control that unit from the IEEE, I would suggest using addresses 0-10.  Address 16 (which is set in the video) is bugged on those units.  Also, if the IEEE controller is fully 488.2 compliant, it will not send it commands due to the device not acknowledging its presence.  You would need a controller that would send it commands anyway.

[Kleinstein]

The BU208 look like they are not over-specified, but slight to weak in the SOA curve (at least with the old Siemens data-sheet I looked at). So with 150 V of DC at the transistor they might blow at more than 50 mA due to 2nd breakdown.

[feedback.loop]

The BU208 look like they are not over-specified, but slight to weak in the SOA curve (at least with the old Siemens data-sheet I looked at). So with 150 V of DC at the transistor they might blow at more than 50 mA due to 2nd breakdown.

I found schematics for EDC 522. The output amp seems to be identical to 521 on the first glance, but BU208A + 2N2219A in the output stage.

http://www.ko4bb.com/manuals/24.23.183.55/EDC_522_DC_Voltage_Calibrator_Service_Manual_schematics.pdf

[Muza]

Thanks again!
Inspired by your repair video i decided to fix my two units. After following your repair log, the first one got fixed quite easily.
Apart from the 2N5682 transistors you replaced, mine had a burnt MJE350 as well, so I replaced all three. On my other unit, two of these had a common 10K 5W resistor leading to them, which was charred. This exact resistor was missing on the unit under repair - not for longer though.
Recapped the whole power supply, but the 5V digital rail kept dropping too low. The 5volt LM309K regulator has been tested to be okay. Rectifier bridges seems ok....refusing to deal with the transformer I just slapped an external 5V PSU hacked from 2A USB phone charger (HTC make).
I realized that almost all of the OpAmps were OP07 to which I luckily had a better upgrade. I replaced these with five precision Burr-Brown OPA177 opamps.

The second calibrator has a shorted primary on the transformer...so after all I might have to deal with winding one...

Muza

[SeanB]

Scope on the input to the regulator will likely show a lot of ripple. Cure will be to change the rectifier diodes to ones with a higher current rating. Very common for rectifier diodes to fail long term by having an increased forward voltage drop under current, as the die inside cracks or breaks partly loose from the wire leads.