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Old 12th Dec 2020, 7:57 pm   #1
JohnMcGivern
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Default Fluke 3092 200Mhz power problem

Hi gents, I just bought the above scope. I turned it on and it started for a split second high pitch squeal then shutdown. I was tired and left it till today.
I removed the case looked for any signs of scorching or bad caps all looked ok.
Switched on and it stayed on for approx 10secs then shutdown followed by high pitched squeal again.
Can anyone advise me how to approach this scope to repair whatever is wrong.
How do I remove the boards for inspection properly?
It is the connectors at the bottom of the boards I refer to they are plastic and appear to have some kind of side retainers but I don't want to force them.
I have looked at several threads with a view to fix the scope. I have to remove the 2 boards for closer inspection.
Any help would be welcomed.

Kind regards, John
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Old 12th Dec 2020, 8:01 pm   #2
JohnMcGivern
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Default Re: Fluke 3092 200Mhz power problem

Sorry guys forgot to attach pdf.
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File Type: pdf PM3082 PSU schematic.pdf (331.5 KB, 126 views)
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Old 12th Dec 2020, 8:30 pm   #3
trh01uk
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Default Re: Fluke 3092 200Mhz power problem

Quote:
How do I remove the boards for inspection properly?
Surely that question is answered by the service manual? It appears to be freely available from ManualsLib.

Richard
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Old 13th Dec 2020, 1:50 pm   #4
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Default Re: Fluke 3092 200Mhz power problem

PM3092 power supply.
Philps seemed to have much trouble in the 1980's with switched mode power supplies. They kept changing the design in each new model.
The one finally settled upon, which you have, was also used in the PM3055, PM3065, PM3320, PM3382-92 and others with minor variations.

It contains a catch with that innocuous 3 volt zener diode at the bottom of the chain through the transformer and the FET. If the FET fails, then the zener is burnt out, and when you next switch on, having replaced the FET and the fuse, the circuit fails to switch off, and burns out the FET again.

The only warning about this circuit in any of Philips manuals is in that for the PM3320.
I attach extracts and the circuit for PM3320 from that manual, which seems easily available on the web. If you are stuck I can scan my copies.
Your present fault however seems to be somewhere else. Be aware you are working at full mains voltage. You will need some form of isolation transformer, or else be sure none of your test equipment is earthed. I hate working on switched PSU. Otherwsie study the operation description from the manual, and check out all the critical components like the FET and transistors, the opto coupler etc. Also check there is no excessive load on the secondaries. Disconnect the loads, but put some sort of load across one of the secondary circuits, say +5v or +12v, say 500ma and disconnect the EHT oscillator feed +56v.
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File Type: pdf PM3320 PSU warning.pdf (412.7 KB, 108 views)

Last edited by WME_bill; 13th Dec 2020 at 2:02 pm.
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Old 14th Dec 2020, 1:16 pm   #5
JohnMcGivern
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Default Re: Fluke 3092 200Mhz power problem

Hi WME Bill, thank you for your reply and advice. I downloaded 3320 and 3092 service manuals. I have been busy but I managed to remove the 2 boards for inspection. Ther are hidden location tabs at the bottom of the power board. Anyway I will test 3v Zener and fet/transistors and fuses etc. And post back the results, many thanks, Bill.
Go safe.
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Old 14th Dec 2020, 5:51 pm   #6
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Default Re: Fluke 3092 200Mhz power problem

Further to WME_bill's warning about switchers, that particular one will operate down to 100V or so which means you should be able to just feed DC from a current limited PSU into L & N, if you have one with enough current & volts! This is somewhat safer, from an energy point of view, and you will have somewhere to ground a 'scope. I've done this on switchers a few times as I have a supply that can do 350V DC at 1A (still lethal, it should be said).

dc
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Old 14th Dec 2020, 7:26 pm   #7
JohnMcGivern
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Default Re: Fluke 3092 200Mhz power problem

Hi Dave, I have a 30v 3A PSU. I am using an Isolation TR and Variac I also have a dim bulb setup. I will take my time to try to understand this setup.

Many thanks Dave.
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Old 14th Dec 2020, 7:43 pm   #8
dave cox
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Default Re: Fluke 3092 200Mhz power problem

Hi John,
The variac / isolation TX will give you a safer environment plus the possibility of a real ground for a scope

It does rather sound like you have a down stream short and the PSU is saving itself from a meltdown. It may actually be fully operational, once you find / fix the short!

Higher voltage DC supplies are not that common but I mentioned it in case you had one to hand.

dc
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Old 14th Dec 2020, 9:19 pm   #9
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Default Re: Fluke 3092 200Mhz power problem

The high pitch squeal is probably related to the voltage protection (crowbar) circuit, if the PSU board is similar to the PM3055 them this is on the secondary side of the PSU on the 5V supply, it triggers an SCR across the +/-48V supply, sending the PSU into short-circuit mode.

I have a PM3055 that does exactly this waiting in the repair queue, one at work had the same fault too, but people didn't care and kept cycling the power until it started up, eventually after a few months it blew the mains fuses.

Also it's worth measuring the values of the small blue Philips capacitors, from experience these seem to be a high failure rate part, they tend to lose a lot of capacitance, even some NOS ones.

David
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Old 14th Dec 2020, 10:27 pm   #10
JohnMcGivern
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Default Re: Fluke 3092 200Mhz power problem

Hi factory, thanks for your help. The blue caps you mention do you mean the ceramics?
There are only 3 blue ceramics on the board. All electrolytics are brown. Did esr check on them all appear good.
Regards, John.
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Old 15th Dec 2020, 7:30 am   #11
JohnMcGivern
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Default Re: Fluke 3092 200Mhz power problem

Factory just forget what my previous post read I was only thinking of the through hole side of the board doh!
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Old 15th Dec 2020, 8:40 am   #12
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Default Re: Fluke 3092 200Mhz power problem

Apologies I missed out "electrolytics", does the PSU in your scope not use the blue Philips axial electrolytics? If it does check the capacitance hasn't dropped with age, you may need to lift them from the board to do this.

David
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Old 15th Dec 2020, 12:34 pm   #13
JohnMcGivern
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Default Re: Fluke 3092 200Mhz power problem

Factory, all the electrolytics are radial.
This 470pF 500v ceramic measures 102ohms to gnd. See attachment. Copy it into paint then you can see it not great with computer either,haha.
Could this be the problem?
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Old 15th Dec 2020, 1:40 pm   #14
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Default Re: Fluke 3092 200Mhz power problem

If it wasn't disconnected when you took the measurement, you could be measuring the 102 ohms through the resistor & the winding of the transformer, I can't quite read the value though.

Will have to see if I can download the manual when I get home, as it seems quite a bit newer than the PM3055 PSU, which was referenced as being similar.

David
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Old 15th Dec 2020, 3:19 pm   #15
JohnMcGivern
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Default Re: Fluke 3092 200Mhz power problem

This is the service manual I have.
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Old 15th Dec 2020, 3:21 pm   #16
JohnMcGivern
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Default Re: Fluke 3092 200Mhz power problem

PDF is more than 4Mb, sorry.

Last edited by JohnMcGivern; 15th Dec 2020 at 3:26 pm.
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Old 15th Dec 2020, 9:14 pm   #17
factory
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Default Re: Fluke 3092 200Mhz power problem

Had a quick look at the PM3082 manual and while the primary side of the PSU is mostly the same as the older PM3055, apart from a different switching transistor & opto-coupler and a few passive component values.
The secondary protection circuits have been redesigned to add thermal, over & under-voltage protection and a power fail circuit. The 10V reference differs too.

David
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Old 15th Dec 2020, 9:56 pm   #18
JohnMcGivern
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Default Re: Fluke 3092 200Mhz power problem

I have the same service manual have not found anything obviously wrong will just plod away hopefully I will learn from this hunt.
Found suspect Schottky diode one side ok one side open this was near the battery.

Many thanks gents.
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Old 16th Dec 2020, 6:36 am   #19
JohnMcGivern
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Default Re: Fluke 3092 200Mhz power problem

I thought my luck was in put scope back together turned on all seemed OK display was lit text at the bottom about 7seconds then shutdown squeal I assume from transformer and back to scratching head why has it lasted longer possibly something getting hot?
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Old 16th Dec 2020, 4:11 pm   #20
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Default Re: Fluke 3092 200Mhz power problem

Philips / Fluke 3092.
The squeal could be the power FET heavily overloaded lowering the frequency. This suggests that your problem lies in excessive secondary loads. Try disconnecting them and see what happens. Anyway disconnect the +56v feed to the eht oscillator, as it makes it much more comfortable to work upon the scope without the high voltage. It will not harm the CRT for a short time.
You should always put a load on a switching power supply. I see the manual suggests a complex panel on all the outputs, but a small load on the -12v (to stop the over-voltage circuit) and +18v (to control the feedback loop) and say on +5v something more hefty to draw a current load should be enough to get some idea. It need not be right up to the currents mentioned in the manual.
Happily, this suggests your primary (the uncomfortable side) is operating correctly.
It is a pity there is no explanation at all upon the over-voltage or temperature protection circuits, in the PM3092 manual or any of the other manuals which use similar circuits.

Quite how you get 102ohms across that snubbing capacitor V1291 on the +56v rail is a mystery as Factory says.
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