FIXED HP Integral PSU repair

[Hiwi]

Hi all,
i have a non working psu from an HP Intergal on my bench.
I did a sucessful repair of another PSU but this is a very hard case.

So is there an engineer with some more experience?

What i already did:
The psu has no output voltages, i just ticks in an intervall of a second. I think it wants to start, but maybe there is a short, but i can't find it.

High rectified voltage (324V for 220V Europe) is ok
There is the start peak voltage, which i can see on my oscillocope.

The start circuit delivers a voltage for a period of time to start the PWM MC34060A.
The MC34060 is then powered over a special aux voltage.

I checked nearly every diode and transistor, the optocouplers and the thyristor in the crowbar circuit and the triac in the mainline. I desoldered every component and made test circuits for the optocouplers and triacs/ thyristor, so i can be sure they are working.

I changed the opamps and now waiting for a MC3460A from China, this pwm circuit is out of production. I hope they are no fake chips.

Has anyone experience with this psu. I have heard of many dead psu's in this machine.
A service manual can be found in the net.

Ralf
   

[Hiwi]

Hi all,
I made some more tests and at least I fixed the PSU.
May be it is interesting for others.

1. Applied adjustable voltage 5-6.3 Volts to the 5 V rail. => the tyristor fires as expected at 6.2 Volts. There is no short and the opamp is working.

2. Tested all the other rails for shorts => good.

So the transformer T1 or the PWM Chip must be the reason for the defect.

3. Tested the coils of T1 for shorts => no abnormalities.

4. Treated the PWM IC with cooler spray => no diffeernce.

I desoldered the PWM IC (MC34060A) and build a circuit for testing => works but not very stable.

The chinese MC34060A have not arrived yet. So I went to a TL494. It is no problem to get these through our distributor. It is still in production, i suppose.

The difference between a MC34060A is that it was developed for pushpull operation.

There is a second FET output on pin 8 & 9. The housing is a DIP16, the MC34060A has a DIP 14 housing. But they are pin compatible. I soldered pin 8 (input) to ground at Pin 7 and left the pin 9 open. Pin 8 & 9 are up in the air and the other pins are connected to pin 1 -14
through a socket.

I had to adjust the opamp threshold to get the correct feedback to the TL494 and set the output to excact 5.1 Volts at the 5 Volts rail.

Now the PSU is fixed and waiting for assembling & installation in my HP Integral

Ralf

[Hiwi]

I am not a great fan of recapping without any reason. So I made measurements of the ESR of the main big caps only and exchanged the X2 and the 2 caps in the startup circuit.
They are very important for starting the PSU. And replace with the excact value.
Ralf

[HP67]

Great

BTW if you didn't already know, eevblog has really forums on electronic topics including repair.

But it seems you did not need it this time

[Hiwi]

Hi HP67,
yes this is not the right forum.

I know that eevblog blog, but the hints there are often not very useful. The common answer is recapping. I didn't have to change any cap at the end.

I was hoping to find a psu designer or electrical engineer here. I am only a simple physicist :-)

And I find my finding great, replacing the MC34060A with a TL494 aka KA7500.
So I wanted to share this.

I found a great video from curious mark repairing a PSU of a Hp test equipment.
That's my understanding of a good tip!

Ralf