Dead Philips oscilloscope PM3055

[Brasto]

Today my dearest PM3055 appeared to be dead when pushing the power-on switch.
Found 1.6 Amp fuse blown and linecapacitor X2 exploded. Thought this to be an easy fix by replacing defective items. Second chance pushing powerbutton fuse burnt immediately, further troubleshooting revealed BUT and BUZ transistors mounted on heatsinck in primary circuit were shorted to all 3 pins.
Installed 60 watt lamp in series with powerline, and checked for short with BUT and BUW still removed.
No shorts detected and a decent 340VDC was measured at the filtering capacitors.
Before I will install new BUT and BUZ, I wonder whether there are other known culprits to be replaced.
Any help is highly welcome

[singapol]

Welcome,

You have to test the components of the power supply yourself to make sure the new transistors you put in does not fail again. See Fig.9.5 power supply schematic of PM3055 service manual. Good luck.

http://www.qsl.net/vk5bar/AHARS-Resources/Philips%20PM3055/PM3055%20Service%20Manual.pdf

[sahko123]

You should also make sure when you remount the new transistors that they are not shorted against the heatsink as this was a problem for me and my HAMEG HM203-4

[Brasto]

Hi Sahko, you are correct, clearance is minimal due to offset of center pin....
Circuit traces are very vulnerable to soldering new transistors in and out, so I have installed little sockets and I am using replacemts all plastic, so I do not need the insulation mica's

Unfortunately the FET still is allways fully conducting as indicated by my series 20 watt light bulb...
I am still puzzled as to cause of problem, replaced little thyristor, not solving my problem....

[kxenos]

I think you will have to replace the controller IC.

[vk6zgo]

Be wary of T 6001.
I had a problem with a Sony  KX27 Monitor's SMPS where a similar transformer primary had been "cooked" by being left on for an extended time after the switching transistor had failed short cct.

The result was the transformer had developed a shorted turn (or several) .

This was not evident with resistance tests,& the transformer was varnished black,so didn't look cooked.but I got suspicious when  all the other associated components I replaced  went "bang"!

Luckily we had an LCR meter,so when another Monitor came in with an unrelated fault,we checked its transformer .
The resulting reading of Inductance & "Q" showed how dead the faulty transformer was.
Needless to say,we added the L & "Q" information to the Service Manual!

[oldway]

The SMPS of this oscilloscope will not start oscillate if you use a lamp in serie.
No way to use this protection !

[Brasto]

I think you will have to replace the controller IC.

Aah, good thinking.....but this model has NO controller IC

[kxenos]

There must be a voltage/current controlled oscillator of some sort. Even a 555 could be used as a switching regulator. Can you check for pulses in transistor's base?

[Brasto]

There is a fancy circuit with a BC337 transistor possibly supposed to control the FET.
When probed I see a steady DC of 15V at its emitter, NO pulses.

[oldway]

This circuit works only if all the components are good.

Otherwise, it will not oscillate and destroy the power semiconductors (BUZ80 and BUW12).

If you have 15V at the emitter of the BC337 and therefore also on the gate of the mosfet BUZ80 / V6014, the BUW12 (V6018) must be conducting and the converter must begin to oscillate.

Without the BUZ80 and / or without the BUW12, it is normal that there is no oscillation.

Be aware you can't feed this SMPS with a variac starting from 0V. You must start from the min voltage of 90V.

[kxenos]

N6001 probably is an oscillator with frequency controlled by the REF pin, so the optocoupler's LED should be switching on-off during normal operation (if voltages +10REF and -12V are OK) and so the switch V6013 should be switching X45 - X48 voltage between about 15.7V and 0V. You can check for a rising voltage across R6009 when the PS starts. That would mean that opto is probably OK and that V6013 should be closed bringing X45-X48 down to about 0V.
My gut feeling is that V6013 is open (most probably) or that something in the opto's circuit has failed (V6031, N6001 or H6001).
Can you measure these 4 components?

[oldway]

Oscillator is made by thyristor BRY39 who became conducting when current in BUW12 reach a value varying with information from the voltage regulator and not by N6001.
This power supply burns the BUZ80 and the BUW12 if not oscillating.
If Brasto said he has measured 15V on emitter of BC337 and that it is not oscillating, that's means that he has removed the BUZ80 and the BUW12 or that they are open.
Without the BUZ80 and the BUW12, this power supply can't oscillate...

Brasto wants to check the power supply without the power semi-conductors, but this is not possible with this kind of schematic.
The only thing he can do, it is to check each component with multimeter or component tester and if everything is allright, power it on with the power semi-conductors in place and a voltage higher than 100Vac.

[Brasto]

I have replaced the little thyristor BRY39 by a brandnew one with no results.
You are correct in stating that BUZ and BUW will die in a second when not being pulsed, my lightbulb of 25 watt in series with the main power cord prevents their premature end of life......(does it?)

During this weekend I will remove the transformer from the board and check for weird inductance.
Will be using my little 3-pole component tester to check individual components, at least one shall be found to be defective 

 

[oldway]

NO, NO, NO you make it absolutely wrong....this power supply will never start oscillating with a lightbulb in serie....The BUZ and BUW stay fully conducting and your lightbulb glow as it is directly feeded on 230V.

There is no other way than to risk to power on this SMPS directly on 230V.

[Brasto]

I have been using these pages as a guide to troubleshoot SMPS
http://www.giangrandi.ch/electronics/smpsfix/smpsfix.shtml#light_bulb_trick
Not using the lightbulb trick has already cost me a handfull of fuses and BUZ-BUW components.

As per the attached functional diagram, I am thinking as a last resort to apply my own +10V reference voltage from an external lab supply to see whether some clue can be derived from that.....

[oldway]

You are going nowhere because:
- you don't understand how this SMPS works, however this is clearly explained in the service manual.
- you deny us important informations about what you have done.
For exemple, you never said us that you have tried several times to power on this power supply without lightbulb in serie.
If you want that we would be able to help you, you must give us all the informations you have.

You can't simply apply what you read on Internet without thinking a little.
Lightbulb in serie can't be used with auto-oscillating SMPS, nor with SMPS with PFC.
And at least, choose a 100W lightbulb, not a 20W one !!!

Read the technical specifications of the power supply:
line input voltage ac: 100-240V
limits of operation: 90-264V
This power supply has a strange and bad behaviour when voltage on C6007 and C6008 increase very slowly from 0V : input current increase quickly and go to short circuit.

Nevertheless, if you have checked all the components of the primary side and that they are ok, if you have checked all the secondary diodes for shorts and all is ok, if you have tried several times to power on the power supply without lighbulb protection and every time, the BUZ and the BUW blows up, there are only 2 reasons possible:

- you bought a "handfull" of fake BUZ and BUW "made in china" who does not withstand the high voltage of this kind of flyback.
- the flyback transformer is faulty and has a very low primary inductance (internal short circuit between turns or layers)

[Brasto]

You are right....I do have great problems to understand "this type of SMPS with auto oscillating and PFC ckts"
The flyback recharge path as said to be T6001-1, V6012, V6009, R6004, C6009, L6002, T6001-2 as well as a mixup between R6007 and R6009 in fig 9.2 and the schematic does not help me much to understand the engineers description.
Basically there are two options left to me, I will remove Transformer T6001 from the PCB and check its condition, at the same time will once more check the discrete components and finally will make sure my replacement BUZ and BUW have come from a reliable manufacturer......dV/dt obviously is critical.

I'll keep you all posted after my weekend diagnostics....

[oldway]

From Service Manual:

NB: there is no PFC in this power supply.
NB2: as it works on the current mode principle (control of peak current of V6018 each cycle), it should be a very secure power supply if inductance of T6001 is ok.

[Geekfix1706]

Hi Brasto, did you get anywhere repairing this scope?
I recently purchased an intermittent starting one. The PSU completely died a few days later, but it's fixed now. I may have tips that can help, some of the suggestions give although wise sounding aren't correct.

[Shock]

Hi Brasto, did you get anywhere repairing this scope?
I recently purchased an intermittent starting one. The PSU completely died a few days later, but it's fixed now. I may have tips that can help, some of the suggestions give although wise sounding aren't correct.

Geekfix1706 just post your findings anyway the more info the better.

[Geekfix1706]

OK, so I bought a PM3055 with an intermittent fault, occasionally wouldn't power up.
The day after it blew, fuse black removed and checked V6014 & V6018 BUZ80 and BUT11 not BUW12A as diagram suggests...both s/c dead. Managed to acquire some BUT11 but no BUZ80 found a jap device that looked better on Paper 2SK1460. Before fitting these I checked everything on primary and secondary, no s/c diodes or secondary rails, small caps on primary side allmeasured fine once isolated from the PCB. SCR across +&-48V was fine. BZX79C3V0 V6021 was s/c, but that was it.
After reading comments here I set up a Variac for 90V AC fitted the two new devices and with a T1.6A fuse and PSU out of the scope applied power, not immediate and not violent but fuse blew, checked on secondary and there was virtually no voltage on any of the supplies from the failed start. The BUT11 was dead again but the 2SK device was fine, re checked again found nothing decided to change the secondary caps (just in case) fitted another BUT11 and a smaller fuse T1A and tried again, fuse blew (not violent) and second BUT11 was dead....2SK FINE.?
Around this time I was worried that I had a transformer with S/C turns or bad Q, I got a (working) power supply form a modern LCD Tv and hocked it up so that it drove the transformer on the PM3055, only ran it briefly while checking secondary and supplies appeared albeit over voltage...
scratches head. At this point I wondered where the BUT11 had come from, it had been changed in the past and the diagram says it should be BUW12A, I got my hands on a BUW13A but it's in a different (larger) package... I shoe horned it in, cut the trace to the transformer from V6017 V6019 isolating it and fitted a 40w bulb, looking at the action of the BRY39 firing realised a substantial current would be needed to develope 1.34V across the two parallel resistors in the BUW emitter connection, I removed them and put a 10R in place,fitted a new fuse turned variac to zero. Applied power gradually bulb glows brighter and brighter and then abruptly goes very dim, fuse OK. Tried again same result, measured voltage across gate and cathode of BRY39, it rose with the mains and lamp brightness got to 0.6V and then the lamp went dim. Clearly the BRY device was working, Hooked up my other scope and checked what was happening at the gate, voltage goes up and then at 0.6V ping the circuit bursts into oscillation, I carried on increasing the mains all that happened was the frequency got higher. Put a 60W bulb in parallel with the other and tried the same the lamps did not light at all?.. but checking with the scope on the gate of the 2SK1460 showed the circuit had started oscillating virtually straight away, long before I got to 90V on the variac and showed a good strong pulse.
Removed bulbs, reconnected the transformer and again with variac at zero applied power, secondary supplies appeared quickly and stabilised... power supply has now worked perfectly for days and has been fine refitted to the scope and left on.

Im still confused as to wether the BUT11 should have been there in the first place, but replacements did not work, maybe one of the secdary caps although measuring fine went very bad as power was applied, I did go over the old Ines with my (ancient) Dick Smith ESR meter and although not good none were tragically bad.
Hope this helps, get the bulbs and the variac out! Ignore above advice about not using it.

[oldway]

You should be more careful before asserting that the advice of others is wrong ....

Brasto lives in Holland where the mains voltage is 230V and the advice that I gave takes it into account because he use bulbs of 230V

You probably use 110V bulbs, so the current is double for the same power.

Moreover, apart from replacing the BUW12 with a 2SK1460, you have not found any faulty components and you have no valid explanation on what happened, on the fault and on its "miraculous" repair.

You do not seem to have understood how this power supply works or why problems appear when powering this power supply with a low voltage and limited current source.

Quite simply, a sufficient peak current must be reached in transistor BUW12 to trigger thyristor BRY39.

If this current is not reached due to the bulb current limitation, the thyristor is not triggered and the power supply does not start.

With 230V bulbss, the current is very low and does not allow the startup of the converter.

[Geekfix1706]

Sorry, but I am also on 230V, I did not replace the BUT11 with a 2SK1460 and understanding how the circuit works enabled me to ensure that the bulb provided enough current through the resistor pair in BUW12A emitter to fire BRY39...which it did, more to the point the variac and slow starting enabled me to check what was going on without destroying bucket loads of BUT11.
I have always thought trying was a good way go, being told not to because 'I know better' is not so good, nothing is learnt, truth is the power supply starts quite happily from a slowing rising supply, I supplied this information through experience and a wish to help others, and the BUT11 I used were 'new old stock' and not from a far eastern supplier.
Please do not have the ordacity to insist that I (or others) do not understand, it's rude and unhelpful.

[oldway]

Current in V6018 must reach 3A to trigger V6013 and to start the power supply to oscillate....and a 100W 220V bulb limit the current at 0.45A...At lower temperature of the filament, current may be higher but not as hight as 3A...