Noobie Needs Help with Oscilloscope Fault

[Phil Leigh]

Hello - I've just registered so please go easy on me.

I've just acquired a Philips PM3055 Scope.
It works...mostly. However, it appears to have 2 faults - or maybe 1 fault with two consequences?
1) the Y-pos has to be set to minimum on both channels in order to get the display on the screen...anything above min and the display moves upward and off the top
2) If I connect a signal to channel A (or B) the same signal appears at B (or A). As a result I can only use this as a single channel scope right now - which is fine apart from the Y-pos problem.
Aside from that it works perfectly!
I'm thinking the fault lies in the Pre-amp board - maybe the channel signals are getting shorted/combined erroneously.
I would really appreciate some advice...I've searched to see if this is a common problem but no luck so far
I have the operating manual and about half of the service manual (I have the schematics).
If anyone can help I would greatly appreciate it.
Thanks
Phil

 

[HiTech]

Have you assured yourself that the controls/switches are clean and not "scratchy" from oxidation?

 

[matc]

The mixing of channels might be a clue -
- Is the "channels mixing" switch permanently engaged?
- Might be an earthing problem, this could appear to link channels together.

 

[Phil Leigh]

Have you assured yourself that the controls/switches are clean and not "scratchy" from oxidation?

Thanks. Yes the controls are fine (the controls drive a microprocessor) and not dirty.

 

[Phil Leigh]

The mixing of channels might be a clue -
- Is the "channels mixing" switch permanently engaged?
- Might be an earthing problem, this could appear to link channels together.

Thanks.
The channel mixing switch is off. It cycles through A-B-A&B and both Y-pos controls shift the display no matter what is on the screen!.

I've just been through the service manual and done all of the calibration adjustments. Everything was fine - and it was pretty well aligned apart from this "problem".
I'm now pretty sure the problem is on the pre-amp board.

On closer inspection, what I'm getting is a clone of the A signal on Channel B (or vice versa)...but only at about 80% of the gain of the original...so doesn't appear to be a simple "short"...maybe the signal is bleeding through a low value resistor?

 

[Phil Leigh]

I think I've narrowed the problem down to the Vertical Channel Switch on the pre-amp board.

Looks like the chip is behaving strangely. The logic levels going in are fine (and correspond to the front panel button settings perfectly) as are the analogue signals. But it's not selecting the correct outputs - looks like the internal switching is faulty.

This is an I2C chip - I'm not familiar with this family of devices (although I can understand what they do),

The markings on the chip are unfamiliar to me:

Philips

OQO 020
406 52EC
HSH 9010 2 B

Does anyone have any idea if I can get a replacement/equivalent?
Thanks
Phil

 

[Phil Leigh]

In case it helps anyone, the OQ series Philips chips are unavailable. I ripped out D2202 (Trigger view switch) and used it to replace the faulty D2201 vertical channel switch (see schematics in Service Manual). I've lost Trigger View but got both channels back and fully working!
Will try and fit the faulty chip into the trig view circuit as that only needs half of the switch - which is what I appear to have...

 

[Gordz]

If I remember correctly the PM3055 I used to have came with a full set of schematics. You should be able to 'make up' a replacement part to do that function.

 

[Phil Leigh]

If I remember correctly the PM3055 I used to have came with a full set of schematics. You should be able to 'make up' a replacement part to do that function.

Hi Gordz...
well the chip I ripped out was too far gone - but at least I now have an almost fully-functional 2-channel scope!
It's a shame I can't find an equivalent

 

[john1]

https://www.eserviceinfo.com/downloadsm/18092/Fluke- Philips_PM3055.html

 

[Phil Leigh]

https://www.eserviceinfo.com/downloadsm/18092/Fluke- Philips_PM3055.html

Thanks John...I have the full service manual (the link only has a portion of the manual).
What I need is either a replacement chip (unlikely!) or a working pre-amp board from a non-working scope.
Phil

 

[Gordz]

Great link john1 ! Phil, I still think you may be able to put something together but a replacement board is probably your best bet here. The schematic shows a I2C analog switch which you may be able to trace by looking at the Phillips website ? Tough call...

 

[Phil Leigh]

Thanks Gordz...
The Internet denies all knowledge of this 20-year old chip...
Looking at the layout again, it seems this isn't actually an I2C chip...it has no SDA/SCL lines.

 

[Gordz]

What about something like the CD4053 ?

 

[zevon8]

Not sure if you have seen this website or not in your searching. They carry serviced / refurbished / recalibrated used models of your scope. They may have spares available of what you need, or could help you out in some way.

https://www.sphere.bc.ca/test/index.html

They have a pretty good selection of hard to find bits for many older pieces of gear.

 

[Phil Leigh]

Not sure if you have seen this website or not in your searching. They carry serviced / refurbished / recalibrated used models of your scope. They may have spares available of what you need, or could help you out in some way.

https://www.sphere.bc.ca/test/index.html

They have a pretty good selection of hard to find bits for many older pieces of gear.

Thanks - that's very helpfull. I'll email them to see if they have any boards!
Cheers
Phil