Solartron 7075 - Page 1
Forum section: electronics / testgear

author: wiss
author_rank: Frequent Contributor
timestamp: May 14, 2014, 08:08:13 pm
content: Got my first rack-multimeter last week.On first power-up every second digit did not light up. The display is multiplexed, driving 2 digits at a time, so of course the driver-chip was dead (DS8880). Dropped in another and the entire display fully works!I let it sit powered up for a while to see how much it drifts and stuff.After some 15 minutes: BANG, loud! and smokethe power supply is full of soot,the main reservoir-cap blew... but it looks all fine, what's wrong?It was the mains filter that blew!Can they do that?Got another one and plugged it in on its own while measuring the voltage, steady 227 V, seems ok, after a few minutes that one goes fizzz!Could it be that they might have been stored in moist environment for a long time and paper-caps have gotten moist and leaky?

author: krivx
author_rank: Frequent Contributor
timestamp: May 14, 2014, 08:12:00 pm
content: The mains input filters in Solartrons are notorious for this kind of failure. This is a good reminder to replace mine before it goes...

author: wiss
author_rank: Frequent Contributor
timestamp: May 14, 2014, 08:15:48 pm
content: What do you use as a replacement?

author: krivx
author_rank: Frequent Contributor
timestamp: May 14, 2014, 08:26:46 pm
content: This thread has details on a replacement for the 7150, I don't know if the 7075 uses the same filter. I would guess that they are the same.https://www.eevblog.com/forum/testgear/solartron-7150-replace-mains-filter-now-and-calibration/There is another thread on 7075 repair herehttps://www.eevblog.com/forum/reviews/solartron-7075-%28or-will-i-be-able-to-reassemble-it-%29/

author: lowimpedance
author_rank: Super Contributor
timestamp: May 14, 2014, 11:56:06 pm
content: nice destruction there !.Any IEC filtered inlet that has the same rating and will mechanically fit will do the job.

author: Vgkid
author_rank: Super Contributor
timestamp: May 15, 2014, 03:25:34 am
content: Here you go, did a lot of narrowing down. You have the big brother to my unit.http://www.mouser.com/Connectors/AC-Power-Line-Filters/_/N-71w5mZscv7?P=1z0x8giZ1z0x8ehZ1yztz1uZ1z0x87jZ1z0x8ggZ1z0x8eeZ1yztz1sZ1z0x65nZ1z0x8glZ1z0x8fvZ1yzshl4Z1yzr6egZ1yzv48vZ1z0x8e0Z1z0x8eaZ1yzr697Z1yyqswnZ1ywtkrvZ1z0z5fmZ1z0z810

author: wiss
author_rank: Frequent Contributor
timestamp: May 15, 2014, 07:08:33 am
content: Oh, only 4 EURI expected a few times more!Right now it is running without a filter.It had a 150 uV offset. That could be trimmed out with the AD-zero, but only for a single range (of course).First I suspected a leaky JFET somewhere and started measuring voltage over all series-resistors, nothing.Measuring voltages relative ground, I soldered a small pin to the exposed ground-plane on the underside and measured from the back of the card. When I measured the input to the copper-amp, the display voltage sometimes changed, but it was the same as the measured voltage... except on the chopper-side of R431 (between the amp-input and the optical chopper). I replaced the LED-card and the short-circuit-offset was gone.But still a large 1Meg-offset ("shorting" the input with a 1M resistor).No drop over the 100k series input res...But it changed when I measured the voltage on the AC-reed-relay. Removed the reed-tube and found some gunk under it.Now I only have the last 10 uV short term drift to fix. Probably dirty board.