Stanford Research SR-620 GHz Universal Timer & Counter repair query

[rickells]

hi,

My Stanford Research SR-620 Display LED's do not turn On when mains power switch is turned On.
The Fuse has not blown, The Fan will not turn On, yet one-small audible "click" from one of the relays can be heard after the power SW is turned On. No audible sounds from any keypad button-presses.

Voltages across the PS Caps read 10V, 10V, 12V, 12V, & 22V, 22V.
Two of the three LM317 PS regulators provide valid regulated voltage at their outputs. The third LM317 gives a -1.2V output, so something is shorted out.

- I will be looking for heat & current surge on the PCB components.

Does Anyone have detailed schematics or detailed repair knowledge of this unit ?

thank you,

[awallin]

Schematic is at around page 140 or so:
http://www.ko4bb.com/getsimple/index.php?id=manuals&dir=06_Misc_Test_Equipment/Stanford_Research
(probably many other sources for the manual also on the interwebs..)

good luck with your repair!

[lowimpedance]

Check the power switch, the contacts inside are most likely oxidized. This switch operates on the low voltage side , see page 140 of the manual, around the lm317's. Thus DC power is supplied to the unit whenever mains is on but the switch will start up the regulators when going from standby to on.

[edpalmer42]

I ran into the same problem on Stanford's FS700 Loran-C Frequency Standard.  The switch they used has a minimum contact rating of 1 ma @ 5 V and they're operating it at only 1V25.  I don't know what the rating is for the switch in the SR620.  I considered rewiring it so that the +-15 actually flows through the switch, but since Loran-C is dead, there's not much point.

I'm surprised that they got caught like that - and on multiple models.

Ed

[rastro]

Check the power switch, the contacts inside are most likely oxidized. This switch operates on the low voltage side , see page 140 of the manual, around the lm317's. Thus DC power is supplied to the unit whenever mains is on but the switch will start up the regulators when going from standby to on.

I have seen problems with a dirty power switch on 2 different DS345 Function Generators.  The SR-620 probably uses the same switch so it's worth giving it a shot of cleaner from the front.

rastro

[rickells]

It appears that the SR-620 switch is a sealed unit.

Will try to jumper it to check operation.

Anyone know what a better (than equivalent) modern DigiKey switch might work & physically fit into the chassis SR-620 ?

The original SW901 does not seem to be listed in the SR parts list.
thank you

[rickells]

hi,

What is a better replacement Power On switch for this unit ?
(part#, Digikey, etc.)

thanks
rick

[rastro]

Possible replacement switch: NE182UEE6AMP

Also some cleaner can enter the "sealed" switch through the slide.  Drip some deoxit on the slide and cycle the switch on and off.  This may take several iterations but I managed to clean the power on switch on my DS345.

rastro