Philips PM3070 scope - runs very hot!

[mada3k]

Hi!

I'm kind of new here. Been following the YouTube channel for some years now, and reading in the this forum also lately.

I came over an old Philips PM3070 scope. It works with the exception for LCD backlight bulb (will change it to a LED)

But the XYZ amplifier board runs very hot, and probably done it for a while since it's has a severe brown tint and the flux has boiled.
When it operates I can easy feel the heat radiate from it with my hand. With the cover on I do not dare to guess how hit it gets! This unit has no fan either.


(Sorry for bad picture. It's much browner than it looks)

There's nothing wrong with the picture though. I have no mutli-Mhz signal generator but It has no issues at >10kHz. I measured the voltages that feeds the board, and they are within limits (one 48V rails deviates 1V at most)

- Anyone with a PM30xx series - do the run hot?
- What can be the issue? Supply ripple? Bad caps?
- Should I install a fan and cross my fingers?

Service manuals:
http://files.msys.se/forum/philips_pm3070_pm3072_oscilloscope_sm.pdf
http://files.msys.se/forum/philips_pm3065_pm3067_oscilloscope_sm.pdf

Best Regards
Adam

[EPTech]

Where to start?

It looks like there is a general problem with the active components on the board. I would measure all base-emitter junctions on there, also the ones on final stage as seen on page 3-15. It smells like an over-voltage on one of the rails has taken out a transistor and it cascaded through the rest.
Also check the diodes in series on page 3-15.

Good luck...

[dom0]

I have serviced quite a bit of Philips gear and they all have in common that the designers let the amplifier stages run quite hot (which is because older high speed transistors needed a lot of collector current to reach a high fT, for example many older "general purpose RF" are specified for Ic = 20 or 30 mA. Combine this with a few volts drop and you get enough power to heat a TO-92 up).

I think most of it [Philips gear w/ hot amplifiers] wouldn't make it at elevated temperatures for long, but then again they aren't specified to do so, most of it is specified to run at 30 or 40 °C max.



Oh and, about the fan, make sure the model really doesn't need to have one (and not just someone removed it along the way). PM3070 could be one of the early combiscopes (by the model number, didn't look it up) and they usually had a fan.

[EPTech]

Yes indeed,

Now that you mention it dom0, I am rocking the "more modern" Philips/Fluke PM3394B combiscope and that one gets quite hot as well but it does have a fan so I follow dom0 on this. If you look at the resistors in the collectors of the transistors, they are quite low considering the voltage rails.

Since the device is working there may not be problem after all but I have had partially broken transistors before so...not sure. 

[HighVoltage]

The PM3065 and PM3070 do not have a fan.
The fan started with the PM33xx series.

By coincidence, I have a PM3065 open here in the lab right now from a customer for calibration.
No fan inside.

Here are some pictures
 

[mada3k]

Some better pictures

Where to start?

It looks like there is a general problem with the active components on the board. I would measure all base-emitter junctions on there, also the ones on final stage as seen on page 3-15. It smells like an over-voltage on one of the rails has taken out a transistor and it cascaded through the rest.
Also check the diodes in series on page 3-15.

Good luck...

Thanks for the advice. I will try measure all the transistors for a short.

[amyk]

Those BFW16As are specified for a maximum junction temperature of 200C, so their cases being far too hot to touch may be normal.

[wraper]

most of it is specified to run at 30 or 40 °C max.

?
It is difficult to find any part which have max operating temp rating below 70^oC.
In such cases usually it would be some battery, LCD, mic or some specialized stuff. Not the vast majority of semiconductors and passives.

[dom0]

I was talking about the piece of equipment, not a transistor...

[Shock]

Common to all those heat spots are those big resistors as well.

[EPTech]

Yeah, also seems way too hot for me.

I stick to my original remark. Work your way from the deflector coil back to the pre-amp and see whether there are (semi-) broken transistors. Check everything in the path. I think you will find something there for sure. Either that or one of the rails is too high. You may also want to check for oscillations but check the simple stuff first, I mean the easier to do stuff.

[mada3k]

I'm of a bit of novice in this area. How do I measure the transistors properly and detect the semi-bad? (other than a obvious short) What should I look for?
And I should start from the CRT and "backwards" - Not Powersupply and forward?

I have now removed the both boards for better access.

This is how the CRT-neck board one looks like. Also seems to have running warm, but not close to the first board.



Found something strange. Two transistors that don't match the location of the service manual, and seems to been bodged/repaired(?) on in some way. I guess it's the Darlington-pair in the schematics. Not shorted though.

[grumpydoc]

I have a couple in the "to repair pile" somewhere, the final amplifier boards look toasted in those as well (and in my case bear evidence of not-that-great previous repairs).

I think it might be normal for this vintage of Phillips 'scopes.

The 338x and 339x 'scopes do not run nearly as hot.

[tautech]

I'm of a bit of novice in this area. How do I measure the transistors properly and detect the semi-bad? (other than a obvious short) What should I look for?
And I should start from the CRT and "backwards" - Not Powersupply and forward?

I have now removed the both boards for better access.

This is how the CRT-neck board one looks like. Also seems to have running warm, but not close to the first board.

That I wouldn't worry about, old CRT TV boards sometimes ran hot too.

In your first post you mentioned the flux had boiled, wouldn't this just be somebodies messy rework?

If the scope is running OK, I'd just check the PSU rails meet ripple and voltage spec and maybe source correct replacements for swapped out components.

[Shock]

I'll say it again, on that first board the big resistors look like they have gone toasty at some point, it's common for them to cook slowly before they die. As the board as been reworked you need to check all components in the vicinity, PCB could even be conductive. As was mentioned check supply voltages and ripple.

Transistors tend to fail outright, to test if they meet spec you need to either test them in/a circuit or with something like a curve tracer or an octopus component tester on an oscilloscope. A cheap Chinese $10 ESR/Transistor tester can also be used. But normally testing a transistor with a multimeter on diode test to measure voltage drop is enough to give you a general idea. Search on youtube if you are unsure on how to use the diode test.

If there has already has been a repair first thing you want to check is what rework has been done. Clean the board well and look for mistakes, then if everything appears in spec go back hunting the root cause.

[Emo]

Seeing all the boards being fried more or less, there is also a possibility that this specific scope has run for a long time with blocked vent openings. Otherwise the damage would have been concentrated on just a faulty area/circuit and not all parts

[EPTech]

Hi there,

Besides working your way from the supply back to the Y-amplifier board, also work your way back from the deflector coil amp, seen on page 3-15 to the Y-amplifier, seen on page 3-5. I am almost certain that the V3011 and V3012 are shorted or gone low-ohm. The previous stage would have to handle a lot more current which would cause it to get extremely hot. I do not think this is a supply problem. But that does not mean that ultimately a supply problem may not have caused it.

[mada3k]

I've measured all the transistors and diodes on the CRT-board, the V3011 and V3012 are not shorted (resistance is about ~750 and ~1100 ohms between pins) Couldn't find any shorted or non-conductive transistor och diodes (all measures about ~750 ohms) either.

Right now I'm waiting for some equipment and probes to take a better look at the supply voltages, just to clear that out.

Can resistors go bad? Sure it can burn-off, but can it increase/decrease in resistance?

In your first post you mentioned the flux had boiled, wouldn't this just be somebodies messy rework?

Maybe. I'm talking about the brown gue on the ceramic capacitors. Also the mountholes of the TO-92 and TO-18 transistors has the brown gue.

Seeing all the boards being fried more or less, there is also a possibility that this specific scope has run for a long time with blocked vent openings. Otherwise the damage would have been concentrated on just a faulty area/circuit and not all parts

I guess that to. I ran it with the covers on for about 10 minutes and it got really hot on top. I guess this is the result of a full working day with a couple of books stacked on top or something.

It actually doesn't have any vents on top or under.


Thanks for all the tips and suggestions so far! 

[Shock]

With your transistor testing remember test them forwards and backwards. Commonly resistors burn out, drift high and go open, but they can short.

You got to check everything if it's been messed with (incorrectly replaced) or heat damaged, get in the habit of instinctively measuring straight away. A useful tip is to blow up the screen shot in an editor or print it or the schematic and mark down readings as you go.

It would be nice if you have a thermocouple or infrared thermometer and a Chinese $10 ESR/Transistor tester as then you can measure capacitors and ESR as well as transistors.

I had a look at this photo, and among the rework I can see a suspect bridged solder joint, a suspect burned track and possibly a missing SMD component.

Could benefit touching up the soldering after checking those, but make sure you practice on some rubbish board first, no point in butchering it needlessly.


 

[mada3k]

I'm waiting for an ESR meter to arrive. Looks useful!

Spontaneously I would say that the SMD-transistors is the main heat-problem (all of them in the brown area). The hole mounted transistors sure gets warm, but no problems to touch.

I have now measured the power supply.

All rails look like this, about 100-150mV of ripple:


But then I found that the +48V and -48V rails also have a ugly 100Hz mains ripple on them: (The other others don't)


I suspect that the primary caps would need a replacement. I'll happily change all the caps if thats needed. But it still feels like theres a issue with the XYZ-board.

EDIT: I realize that the 100Hz ripple may simple be caused by overload on the 48V supplies - because the short/overcurrent on the XYZ-module.

[SaabFAN]

In a PM3320A I had last year one of the axial Capacitors on the Powersupply-Board blew its guts out and turned into a resistor, doing some unknown evilness to the rest of the device that damaged something I couldn't figure out.

Maybe there's something similar going on with your device scope: Some problem in the PSU causing the rest to behave erratically.
The PSU is located pretty close to the hot end of the tube, so dry capacitors are most likely a concern here.
Another possibility is that someone tried to hack the unit to get more bandwidth or give the tube more juice because it became dim, increasing the current on this particular board to excessive levels.

From the looks of it, it also might be the resistors next to the SMD transistors. The brown color doesn't reach far enough to be originating from the transistors, I think.
Have you tried desoldering the resistors and measuring them?

[mada3k]

I also have a PM3320A, It's a monster! But it works fine and is in nice condition for it's age. When I got it the X2 mains filter was blown up, so it's replaced. I'm planning to do a complete PSU health check some day. It's so damn complex and probably totally unrepairable (many unobtainable hybrids & ASICs)

But i would rather have this PM3070 healthy, since it's more compact and fully analog. The caps should probably be replaced anyways. Whats a "sane" ripple level for this kind of equipment?

Maybe. But the tube has no issues with brightness or burnin.

Are you talking about R3036 and R3034?

[SaabFAN]

Apart from the CCDs and the 3 programmable Gate Array (which are one-time fuse-programmable - so they'll never lose their content - if I'm not mistaken), most of the stuff inside the PM3320A is relatively easy to find as New Old Stock on ebay, or even new on Digikey. Even the ADC from BurrBrown is still available

The PM3070 certainly is a nice piece of equipment. A good ripple-voltage, in my mind, would be in the area of a few mV. If you're dealing with analog circuits, there cannot be a low enough ripple-voltage. Every mV less ripple is a win.

Yes, I meant those resistors. It looks like the PCB has been grilled by them!
I also noticed that other axial resistors have got cutouts underneath them. I suspect that the cutouts are there for cooling purposes.