BK Precision 1760 not suplying enough current

[Celectronicamx]

Hi, I am new in this Forum

I represent a electronic repair shop in Mexico city.

I Have a Regulated PSU (The ones with enormous and heavy transformers). It's a Lab PSU, Triple output 0-30V 0-2A.

The issue is at follows:

It powers up, it allows to set Voltage level, current limit, endeverything. If i Measured voltage, it was present and the same as the gauge.
Whenever I connected a Load, the Voltage Dropped. Let's say I want to power a small air compressor (the ones you connect to the car lighter) with 12 Volts. When I connect the load, the Voltage dropped to around 8V and no current was flowing.

So i opened it up, and found many small C1815 transistors, wich drive a group of relays, this relays connected to some BIG Rectifier Diodes....
Around that circuitry, it seems someone tried a 3rd party repair, although it looks horrible, everything is correctly done. What they did was cut a pair of  FAT lines on the PCB (they seem to carry BIG CURRENT), and then they re-connected them using standard stranded cable cal 12 or something...
One of those C1815 around the repair was damaged, with no measure reading when testing from B to E on direct polarization, so I replaced it and checked EVERYTHING around this transistor, finding anything else damaged. I also checked all the big transistors, MOSFETS, DIODES, small and big resistors, MEasured all the Big and small capacitors' ESR and Capacitance, and did not found anything out of order.

NOW the PSU delivers Voltage AND current. But whenever i Plug in the LOAD, the Voltage comes down to 8V again, this time, suplying the correct demanded current. Can someone tell me what can be hapening or if this is normal? i have not been able to plug in any other LOAD as i dont have anything else laying around.

Any aditional info can be required just let me post it

Help will be HIGHLY Appreciated-

Thanks

[ArthurDent]

From what I recall a 12 volt compressor probably draws much more than 2 amps. To check to see if the power supply repairs really fixed the problem, use an 12 volt auto bulb that draws about 2 amps and increase the voltage from 0 to 12 volts and see what the ammeter indicates and if the bulb lights.

[The Soulman]

The relays are for switching different voltage secondary winding's to minimize the voltage drop across
the linear regulator circuitry and therefore minimizing power loss/ heat build up.
Maybe one of those relays is stuck or something is wrong with their drive circuit?
Reading again it seems you've fixed something there, maybe it's ok now.

A air compressor isn't advisable as a load for trouble shooting, use light bulbs as mentioned before or a proper dummy load.

[Fludo]

I purchased a 1760 which had the same problem on the 5V channel.  I could set the voltage, but as soon as I connected a load the current would be close to zero. 

The fix was to replace the internal fuse on the channel, it did not look blown when I looked at it but a continuity check revealed an open circuit.

[Celectronicamx]

I purchased a 1760 which had the same problem on the 5V channel.  I could set the voltage, but as soon as I connected a load the current would be close to zero. 

The fix was to replace the internal fuse on the channel, it did not look blown when I looked at it but a continuity check revealed an open circuit.

some days Ago i read something about some PTC fuses, maybe i'm confused. They said this PS use some resettable fuses for disabling the outputs when in short circuit, and they said this fuses reached a time in their life that they just did not came back to normal state after a failure, and even though they were not open or fused, they also did not worked correctly (a.k.a they did not let pass enough current or induced voltage drops). the thing is, i have not been able to find this fuses in the circuitry...

i Measured all the obviuosly looking fuses and low value resistors to check they were not open. but sill nothing...

[Celectronicamx]

Yes, i was Thinking the same thing, maybe the compressor needs more than just 2amps. The PS has a Paralell mode so i will try to give 4 amps or:

I found some 50W resistors laying around some friend's shop, so I'll build some dummy loads and test what happens. i will post results later.

Thanks!!!

[Celectronicamx]

The relays are for switching different voltage secondary winding's to minimize the voltage drop across
the linear regulator circuitry and therefore minimizing power loss/ heat build up.
Maybe one of those relays is stuck or something is wrong with their drive circuit?
Reading again it seems you've fixed something there, maybe it's ok now.

A air compressor isn't advisable as a load for trouble shooting, use light bulbs as mentioned before or a proper dummy load.

Yes, I did dismounted the relays to check they were switching, by injecting Voltage on the coil. They did work.

I will try with some 50W resistors i found and then with bulbs.

Do you think the one's on the microwaves would work? they are 110/220V rated. I use them to discharge large capacitors hehe