GW Instek GPS-4303 power supply - CH2 set always low

[calin]

If anyone has the service manual or the schematic for GPS-4303 or GPS-3303 please please .. share
Now here is why  ....I have an GPS-4303 supply which was pretty much my work horse for the last few years and it finally started acting up a few weeks ago.

Here is what is happening, the supply "works" just fine, I mean it regulates now ripple etc etc, all is dandy on all channels. So I still can use it if I wanted to no issue as is but .. I have one nagging thing and I cannot get to the bottom of it.

The supply has a "Load ON/OFF" button , when that is off it displays the SET value for voltage and current (with reasonable precision), but it does help a lot in day to day use to not have to short the outputs to do that.  This works fine for all channels except channel 2. On channel 2 the SET it constantly reads high, about 2V high,  .. for example i set it to output 10V but when I switch the output ON it puts out something around 8V. Mind you that the font panel meter works perfect, is spot on .. if it says the output is 5V that's what I measure with my meter also, so that part works; just this darn annoyance with SET on channel 2. 

BTW, it is not calibration, I have the calibration procedure/manual for it and I tweaked the thing that everything is spot on. Except I cannot get the damn set value for channel 2 so something else is iffy on that "set circuit for channel 2".

Another odd thing I noticed is that the set value if left say I am @ 10 with output ON, I switch output OFF and SET value jumps to 12V but then if I leave it for a while it starts dropping slowly towards 10... like a capacitor discharging slowly. I noticed this "slow discharge" on CH4 also, even if channel 4 does not have the set high behavior if I switch the output off it teh SET value initially jumps but then it gets in about 3 seconds to the good value. 

Mind you the panel meters on this supply are shared CH1 & CH3 ann CH2 with CH4.

If anyone wants I will post the Adjustement and Calibration Manual here.

BTW !!! this is GPS-4303 .. NOT GPD !!! GPD is the programmable series, the GPS is fully analog.


UDATE: I dropped a mail to GW Instek support also, maybe they are nice

[calin]

Well,


GW Instek got back to me with an half arse capture of an incomplete schematic for channel 2 and recommendation to flip around relays. They will not share the schematic for an "active product" ... "please send to technician" 


Not really too much help at all as I flipped around the relays from CH2 to CH2 without any change. I suspect that all GPS series have similar channel configuration ... digging digging

[JFJ]

They will not share the schematic for an "active product" ...

In the past, your power supply was sold under other badges, e.g. ISO-TECH IPS4303 and Tenma 72-6905. As those products are now discontinued, their former suppliers may be more willing to release service information.

[calin]

Wow, indeed .. they do look alike  ... unfortunately I am in the Google digging phase even for these two. I will drop a mail to RS , maybe maybe ...

[calin]

Yay !!!  .... I think I fixed the thing, a seems like it was a suppression diode on a relay that switches SET voltage in/out on CH2. Changed that and all in working order. 

[DogP]

Just wanted to say I had a similar problem with my Tenma 72-6905 (rebadged GPS-4303) where the output off voltage and current readings on CH. 2 weren't accurate, and repaired it by replacing both RL503 and RL504.  One of them had an open coil, the other probably had burned contacts (sometimes worked better than others).  I noticed when I tapped the relays with a screwdriver, the readings would bounce around, so that gave me a clue that I was on the right track.  Thanks to calin for pointing me to the relays... they wouldn't have been my first guess for this kind of problem.

Unfortunately, drop-in replacement relays seem to only be available from China, so it took a few weeks to arrive, but they were cheap.  The original P/N was KDE-HT-124C5, (Coil:24VDC, 2.8K resistance; 3A 120VAC/24VDC).  I replaced it with P/N HK4101F-DC24V-SHG, and it looks like P/N HJR-4102E-L-24V would work as well.  You just need to confirm the pin configuration is the same.  Of course you could hack in other relays with the same specs with jumper wires.

On my PS, CH. 1 was also all over the place, but that was an obvious problem... it had a leaky cap (C806).  I replaced several other marginal caps while I was in there as well.  It was also missing a couple knobs, so I replaced them with some cheap 20mm dia. knobs (pots have 6mm shafts).

BTW, I never found a service manual for this specific PS, but it looks like much of the circuit is the same as the Tektronix PS280/PS283, which does have a service manual available. 

All around, a great deal for $50.

Thanks,
Pat