Fluke 3092 200Mhz power problem

[JohnMcGivern]

Good news gents I managed to buy another scope for spares.
I tried the power supply in the faulty scope and bingo it fired up and is working at the moment.
Obviously the fault is in the power supply.
What and where is the best place to divide and conquer for analysis purposes.
I cannot do anything till I get the new thermistor.
The scope I bought for spares was working but it was smashed all over the front panel back and side panels inside plastic also display front missing, so bit of luck for a change paid too much for condition but at least I have a functioning scope again.
I will try to repair my old power supply to find what caused the fault so when thermistor arrives I will solder in and try the power supply in the spares scope.
I phoned regarding thermistor and parts order delay due to covid. Many thanks.

[JohnMcGivern]

Hi David sorry for late reply received connectors many thanks but not thermistor or resistors started to solder wires to pins will keep you updated, thanks again.

[factory]

Good to hear the connectors arrived and sounds like you got a good source of spares with the second scope even if it was a little expensive, I managed to buy NOS pot to replace the broken off intensity pot on my older Philips PM3055 too.

The diagram posted on the previous page can't be read, if you could crop the blank bits from the page should be readable (pictures are limited to 800 by 600 pixels).
I wonder if the 5VD is the digital supply and the 5VA (5.2V?) is for analog circuits, but the description before the circuit diagram doesn't mention them.
The 10V ref seems to have a separate ground too.

David

[JohnMcGivern]

Hi David, been going over this schematic for the umpteenth time and still not sure. As you say there is + and -5v digital + and - 5v analogue.
Pins A26 and C26 show different voltages.
A26 shows +5VA
C26 shows +58v
I traced the line from V1212 cathode side it shows continuity with both pins.
Looks like the +5VA on the drawing has been marked wrong (A26).

[JohnMcGivern]

See attachment hope you can read this one David.

[JohnMcGivern]

I was testing Lm339 in diode mode.
Red lead on pin12 ground.
Black lead to other pins.
Most pins were 0.653 one was 0.725.
Pin8 was 0.386 does this point toward partial short?
This was connected to the damaged thermistor.
Received resistors but unable to supply thermistor sent for some from Germany.



Regards, John.

[JohnMcGivern]

Good morning Gents, I received resistors and thermistors today, soldered thermistor.
I would appreciate some help in confirming all connections.
+5vD
-5vD
+5vA
-5vA
+-12, +-18, +-58.
What happens with pin A3 +5v sense.
Also pins C27 ground ref. C28+10vref.
Do all ground connections get connected together?
This board is totally connected from scope and crt.
What happens with the High Voltage section.
The digital and analogue +- 5volts are not clear ie are they separate connections or all together +-Digital and +-Analogue.

Many thanks.

[JohnMcGivern]

Finally plucked up the bottle to test this power supply as I was not sure what would happen.
First switch on was through I/T and 3x60w lamp limiter.
There was very little voltage present on the power supply.
Second switch on I/T and Variac set to 100v.
See attachments for voltages.
Not sure what to do now.
Do I crank up the voltage and monitor the output loads?

Many thanks, John.

[JohnMcGivern]

Took power supply up to 240vac on variac no fail or squeal all recommended loads connected, out of case with no EHT connected.

Voltage Rail Current Values Taken.
+5v = 1.420mA
-5.2v = 513mA
+12v = 967mA
-12v = 817mA
+18v = 330mA
-18v = 191mA
+58v = 63mA
-58v = 83mA
Any suggestions how to procede would be welcome.

Many thanks, John.

[factory]

Were all the voltages there? I can't really suggest what to do next other than trying it back in the scope, who knows it may have been the thermistor, or if your unlucky one of those annoying intermittent problems.

I've never tested a LM339 with diode check, always measured voltages (but on normal through hole parts).

David

[JohnMcGivern]

Hi David thank for replying. I have only changed 3 components Buw12A, 3v Zener diode and the thermistor the transitor and Zener tested good but I had bought new so replaced anyway the thermistor fell apart as I desoldered it as I stated earlier.
I will put it back in the case and give at a go, fingers crossed and post results.

Thanks again David.

[JohnMcGivern]

Success!!!
Put power supply back in case and its working.
Turned on left for 5mins pressed CAL left for about 2 hours all seems good.
I would like to thank everyone for their help and patience.
A special thank you to David for his help, patience and the connectors for the dummy load.
This has been a very good learning curve only problem is when I go to sleep at night
in the morning I have forgot most of it haha.
Don't know what solved the problem maybe transistor was weak under load, or new thermistor.
I never changed any caps.
Sorry David you asked long time ago about cap testing I tested them in circuit with an ESR tester as I said they were all good showing low ESR better late than never, thanks again David I have truly leaned quite a bit from this repair.

Go safe everyone, John