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Help with Tektronixs 492 S.A. PSU fault.


Nick Gregory G0HIK
 

I came to switching on one time and it light up, but nothing on the display.

The +15 v rail is 4.7v
The -15 v rail is -3.1v.

I think the minus rail uses the plus rail as a reference so it may well be just the + 15v rail faulty.

I've disconnected all cards and connectors and the volts are still low, so I dont think is a card fault and it's probably a PSU fault. The 0.3 ohm resistor in the +15v line says its has a draw of 1.0 amps with seems a lot at 4 volts though.

the +/- 17v rails that feed the +/- 15v are fine.

I would have suspected s/c Tants, but there does not seem to be any tants.

Any help welcomed please.

I've included a part of the circuit with some voltages on.

Thanks Nick G0HIK


cricri
 

Hi ,

Open the PSU and test the fuses  !  ( 2 or 3 don't remember )

Seen a similar pb and it was a fuse broken ..


73

F1VL


Le 15/01/2021 à 19:55, Nick Gregory G0HIK via groups.io a écrit :
I came to switching on one time and it light up, but nothing on the display.

The +15 v rail is 4.7v
The -15 v rail is -3.1v.

I think the minus rail uses the plus rail as a reference so it may well be just the + 15v rail faulty.

I've disconnected all cards and connectors and the volts are still low, so I dont think is a card fault and it's probably a PSU fault. The 0.3 ohm resistor in the +15v line says its has a draw of 1.0 amps with seems a lot at 4 volts though.

the +/- 17v rails that feed the +/- 15v are fine.

I would have suspected s/c Tants, but there does not seem to be any tants.

Any help welcomed please.

I've included a part of the circuit with some voltages on.

Thanks Nick G0HIK



Nick Gregory G0HIK
 

Thanks, But unfortunately not this time.

It's a right pain soldering wires to various test points to bring them to the outside would.

I think you can still get 0.15" ( I think) PCB sockets, can you obtain the mating double sided PCB to make a couple of extenders ?


Ian White
 

Nick,

Would an 2x18-way double sided board extender be any use, 0.15in pitch and about 5in long? Could post it on Monday.

A batch was made some years ago for working on FT221 RF boards, so if you really need two there may still be others around to borrow.

73 from Ian GM3SEK


On 15/01/2021 20:49, Nick Gregory G0HIK via groups.io wrote:

Thanks, But unfortunately not this time.

It's a right pain soldering wires to various test points to bring them to the outside would.

I think you can still get 0.15" ( I think) PCB sockets, can you obtain the mating double sided PCB to make a couple of extenders ?


geoffrey pike
 

Hi Nick.
Have you posted this on the Textronics group.io?
Good luck
Geoff
GI0GDP

On Friday, 15 January 2021, 18:55:32 GMT, Nick Gregory G0HIK via groups.io <nickg0hik@...> wrote:


I came to switching on one time and it light up, but nothing on the display.

The +15 v rail is 4.7v
The -15 v rail is -3.1v.

I think the minus rail uses the plus rail as a reference so it may well be just the + 15v rail faulty.

I've disconnected all cards and connectors and the volts are still low, so I dont think is a card fault and it's probably a PSU fault. The 0.3 ohm resistor in the +15v line says its has a draw of 1.0 amps with seems a lot at 4 volts though.

the +/- 17v rails that feed the +/- 15v are fine.

I would have suspected s/c Tants, but there does not seem to be any tants.

Any help welcomed please.

I've included a part of the circuit with some voltages on.

Thanks Nick G0HIK


Mark - M1MPW
 

Is that not a tant by the multiturn preset?
Also... a burnt resistor above the edge connector?

Mark


geoff wiggins
 

I had a similar type of fault on my Tek 466 storage scope. The supply rail was down to 4.7v approx and that was the clue.

I found a huge current drain and the fault was a short cct 2.2uF capacitor fed from a 4.7v zener which was supposed to feed 10v or so to the trigger pots. Believe it or not this little 400mW zener was taking all that current and didn't blow. The capacitor in question was a small yellow type about the size of the zener. Just follow the track around looking for a short to deck.

Good hunting.

73 Geoff. G4XMJ

On 15/01/2021 18:55, Nick Gregory G0HIK via groups.io wrote:
I came to switching on one time and it light up, but nothing on the display.

The +15 v rail is 4.7v
The -15 v rail is -3.1v.

I think the minus rail uses the plus rail as a reference so it may well be just the + 15v rail faulty.

I've disconnected all cards and connectors and the volts are still low, so I dont think is a card fault and it's probably a PSU fault. The 0.3 ohm resistor in the +15v line says its has a draw of 1.0 amps with seems a lot at 4 volts though.

the +/- 17v rails that feed the +/- 15v are fine.

I would have suspected s/c Tants, but there does not seem to be any tants.

Any help welcomed please.

I've included a part of the circuit with some voltages on.

Thanks Nick G0HIK


Nick Gregory G0HIK
 

Thanks all for the helpful comments.

I've just got it licked, well it looks like it anyway.
It was an overload on the +/- 15v rails. I thought I had disconnected every consumer of the  +/-15v from the motherboard, but hidden under the YIG filter was a supply to the 100MHz L.O. Low and behold there were two +/- 12v regulators, decoupled by Tants. One was S/C but both changed out of course.

Several lessons learnt, every day is a school day, I just need to try and retain it all.

Nick G0HIK


Nick Gregory G0HIK
 

I didn't know there was group. I'll join that when it fails next time. The heart always misses a beat when you switch on 1980's test kit.

Nick G0HIK


geoffrey pike
 

Hi Nick,
Aren't Tantalum caps great!, they always fail. There is nothing worse than a board with no tants on it. I assume the 492 is awkward to work at there was a mention of extender boards previously!. The HP analysers of the 80s have fold out boards,
but some of the older ones in HP182 mainframes are not easy to work with. Hope it keeps going for you
cheers
Geoff
GI0GDP

On Saturday, 16 January 2021, 11:07:11 GMT, Nick Gregory G0HIK via groups.io <nickg0hik@...> wrote:


I didn't know there was group. I'll join that when it fails next time. The heart always misses a beat when you switch on 1980's test kit.

Nick G0HIK


Nick Gregory G0HIK
 

Yes Geoff, It's needs a 0.15" pitch extender for the PSU, which I should be able to make. But for the cards on the motherboard it needs something with two row of Pins, how I would make one I don't know. Luckily the faulty 100MHz board was just a module bolted on the underside so easy to get at.

Nick G0HIK