ISO Tech IPS 603 Display Issue

[Musicmanager]

Hi Guys

A new member, please be gentle .. .. .
I'm a retired teacher taken to electronics to keep me out of the way ! Very much a novice .. .. .. .

I recently acquired the Power Supply from Ebay for a song. It was explosive at the outset, but using some earlier posts I found on this forum and a lot of patience I've managed to get it working; It powers on, outputs as it should, but the digits on the screen are only visible 'at dusk with the light behind her !'

I've done some research and think that  the problem is related to the ratio of Vlcd to VDD ( at the moment they match )

The power connector to the process board ( +15; -15; 5v; 5v & 0v) is stable at the SMPS end, but if the Process Board is connected the 5v's remain stable, but the +15vdc rapidly drops to about 1.5v and then the -15vdc is all over the shop.

I've searched well but I cannot find anything else out of place .. .. .. .

Is there a chance anybody has faced and solved a similar issue .. .. . ? Any ideas ? ( Apart from drop kick it over the garage, I mean )

Thanks

S

[AVGresponding]

Sounds very much like a failed component (probably a crapacitor) is pulling the voltage down.

I just acquired a 405 variant of this supply, which is probably very similar inside.





I'd suspect these smd jobbies first, if your 603 is the same as my 405 in substance.

[Musicmanager]

Hi AVG

Thanks very much for your response .. .. .. .




As you can see the content of mine is slightly different.

I was expecting a cap problem, not least because of the reading I did of some previous experiences on this forum by other members; I have renewed every cap on both the SMPS and the process board.

I don't have those SMD canned caps in my setup .. .. .. .

Have you seen a schematic ?
I have two .. one that is similar to mine and another that is not the same neither are much use .. .. .. .

Thanks

MM

[AVGresponding]

Could do with a better quality pic so I can zoom in a bit more.

If you've already shotgun-replaced all the caps it's unlikely that's the issue unless you're really unlucky. Do you have a DMM with decent low ohms resolution? If you do then barring visual evidence of magic smoke escaping that's often your best bet for finding a shorted component. Pretty sure Dave made a video on this some years ago. You definitely want to check the resistance between the +15V rail and ground on the control board, and between the -15V and ground.

You say the PSU is stable when it's not connected to the control board, have you checked if it remains stable under a controlled load, by putting, say, a 150Ω resistor across the +15V to ground?

Might be worth reading this thread too: https://www.eevblog.com/forum/repair/a-gw-instek-psp-405-lab-bench-power-supply-repaired/

These PSUs are branded variously as Iso Tech, GW Instek, RS Pro to name but three, I don't know who the OEM is. It could be worth broadening your search parameters to include the other brands, though I couldn't find any schematics for them either. My google-fu powers are inconsistent, so I wouldn't take that as definite evidence of absence however.   

[Musicmanager]

Hi AVG

Thanks once more .. useful thoughts .. .. .

I'll try an organise a better quality pic tomorrow - currently under flourescent lights which are awful for pics.

I did load the PSU for a test using a Constant Load device I built a while ago - I guesstimated the load of the process board and screen @ 2 amps and loaded accordingly .. the PSU +15v remained stable. I didn't know how to test the -15v .. but if I'm interpreting the schematic I have got correctly, the -15v is dependent on the +15v, so I made an assumption.

I'll try and find the video about low ohms testing .. .. I think my DMM is capable.

The thread you reference is one of the previous threads I referred to earlier - it's the content of that thread and a couple of others that has got me this far .. .. ..  it has to be said, given my abilities the fact that the thing is not explosive and outputs correctly is a small miracle.

The schematics I've got are both mentioned in that thread or  one of the others that are linked to it.

Thanks again for your thoughts

MM

[AVGresponding]

For testing the -15V under load, connect the 0V output to your load bank +, and the -15V to your load bank 0V/ground.

If it's holding up under a 2A load, that looks good. I'd definitely be looking for a short on the control or display board.

Though the + and - 15V supplies may be nominally independent, they are probably on the same secondary winding on the transformer, and if one is being loaded down so much it collapses, it'll cause the other to be affected too.

[Musicmanager]

Hi AVG

Firstly I'm sorry for the silence .. .. I'm disabled and the current heatwave is giving me a few problems.

I think I have made some progress .. .. .. I used a DC - DC module to provide an independent + & - 15vdc, along with a 5vdc from my bench PSU but got the same problem of +15v collapsing; There are 3 x 470uf caps right next to the connector which I had replaced, but I removed them again so that I could see the circuit clearly, found no problem so I put them back - +15v is now stable !   Don't ask 'cos I don't know !

It made no difference to the screen display however .. .. .so I did some circuit tracking and realised that apart from the power connections to the screen board most of the other tracks go to an 18 pin IC marked AD7541AKN which the datasheet describes as a DAC which uses a V. Ref from a TL431 nearby.

There is a ferrite toroid in the vicinity which had been allowed to bounce, smashing some of the decoupling caps and it had flattened that TL431, shorting it out, so I replaced it; maybe the shorting has damaged the DAC too .. .. .. .

Anyway, I await the arrival of a replacement AD7541A and then I can answer that one !


Thanks

MM