Philips PM3212 Oscilloscope channel A problem

[SingedFingers]

I dont have another oscilloscope to troubleshoot mine.

This is mistake number one. If you're running old scopes, you need one to fix the broken one. Too much time doing that here:

[Basileus]

I dont have another oscilloscope to troubleshoot mine.

This is mistake number one. If you're running old scopes, you need one to fix the broken one. Too much time doing that here:

Actually i have a dso138 kit i got a while ago, but i dont know if its worth mentioning it..

Anyway, i noticed that when the channels are coupled to ground, when using the CAL pot the trace moves vertically (channel B not much, 1/8th of division max (up), while channel A goes down by a whole division at least).

[Basileus]

Update!

Tried to adjust trimmer pot r543 x1 gain channel A, i cant get the right amplitude. Pot is set at minimum and still the 1.2v cal signal is 5.7 div high (0.2 v/div) instead of six. What is going on? 

[SingedFingers]

I'd check supply voltage to that part of the circuit and check resistors aren't open first. Every DC bias problem I've had with scopes is either a shorted tant pulling the rail down or an open resistor killing bias.

Edit: other option is injecting a known good signal into the front end in X-Y mode and tracing it through the amps one after another until you find the Dicky stage. Your DSO 138 can probably handle that actually if you stay away from the CRT plate drivers.

[boffin]

Update!

Tried to adjust trimmer pot r543 x1 gain channel A, i cant get the right amplitude. Pot is set at minimum and still the 1.2v cal signal is 5.7 div high (0.2 v/div) instead of six. What is going on? 

Check the trimmer hasn't failed to a higher total resistance.  That was the failure on the power supply of my PM3216, a 220R trimmer was 1k5 from end to end.

[Basileus]

Update!

Tried to adjust trimmer pot r543 x1 gain channel A, i cant get the right amplitude. Pot is set at minimum and still the 1.2v cal signal is 5.7 div high (0.2 v/div) instead of six. What is going on? 

Check the trimmer hasn't failed to a higher total resistance.  That was the failure on the power supply of my PM3216, a 220R trimmer was 1k5 from end to end.

All pots seem good. Checked every resistor for offset values/open. Nothing.
I cant figure out where the channel A amps are getting power from. I'll probably try to trubleshoot the whole ps board.

Yep, not having another scope to troubleshoot is a problem.
Thats why i just got a Tektronix 2215 in "seems in fully working order" for 95 € (shipping from uk to italy included). Pretty good score, would say. I'll stop working until it arrives. If i fix the philips one ill might think about selling it.

[Greenray]

Hi.
It's been reading your long discussion, that push me to finally open my PM 3212 (more than 20 years old) trying to repair the Cannel B sync.
This is the reason that move me to write here instead to open a new discussion.
But now I ask you all a right suggestion about it: better to start here or open a new discussion?
I thank you so much.

-------------------------------

Staying on this proper discussion I desire to say to Basileus and SingedFinger some words:

Point 1:  to have only one old oscilloscope it's not really a mistake.... In any case we can use another oscilloscope to help ourselves to repair the one of we are owner. I hope that you could find some appassionated entusiastic people that occasionally help you with his scope.

Point 2: if the spare parts of the REED relays are too expensive or not easy to find, please consider the possibility to buy a broken PM3215, that can be cheeper than a single REED !!!

Point 3: if you are not good skilled with your Iron Solder, please consider an alternative to the normal replacement. Only if as I think the contact is normally opened, you can apply the good part over the fault, without remove the coil from the PCB. The route of the signal will remain short as now: the route of the DC that flow int the coil will be a bit more long, but it will be not matter of problems.
In any case I suggest to remove the wirings to the glass holder of the contacts. Yes I know, we are not talking about Very High Frequency, but this contact it's better that disappear from the circuit.

[Basileus]

Hi.
It's been reading your long discussion, that push me to finally open my PM 3212 (more than 20 years old) trying to repair the Cannel B sync.
This is the reason that move me to write here instead to open a new discussion.
But now I ask you all a right suggestion about it: better to start here or open a new discussion?
I thank you so much.

-------------------------------

Staying on this proper discussion I desire to say to Basileus and SingedFinger some words:

Point 1:  to have only one old oscilloscope it's not really a mistake.... In any case we can use another oscilloscope to help ourselves to repair the one of we are owner. I hope that you could find some appassionated entusiastic people that occasionally help you with his scope.

Point 2: if the spare parts of the REED relays are too expensive or not easy to find, please consider the possibility to buy a broken PM3215, that can be cheeper than a single REED !!!

Point 3: if you are not good skilled with your Iron Solder, please consider an alternative to the normal replacement. Only if as I think the contact is normally opened, you can apply the good part over the fault, without remove the coil from the PCB. The route of the signal will remain short as now: the route of the DC that flow int the coil will be a bit more long, but it will be not matter of problems.
In any case I suggest to remove the wirings to the glass holder of the contacts. Yes I know, we are not talking about Very High Frequency, but this contact it's better that disappear from the circuit.

There is no need to remove the coil from the pcb. I didnt remove it. The problem is that if you want to solder the new reed contact like the faulty one was soldered before, you need to bend the leads in a half circle and its quite difficult without breaking glass part of the contact.
Thats why i'm waiting for a shipment of reed contacts from china i bought a while ago.
I think i will scavenge the broken leads from the old reed contact and solder them back in place, so that i only need to trim the leads of the new one, slide it inside the coil and solder it to the old, pre-bent leads.

I think its probably better creating a new topic for your problem, to avoid confusion and over-cluttering.

[Greenray]

Thank you.

So I wrote the new discussion:
https://www.eevblog.com/forum/index.php?topic=80804.msg1104369#msg1104369

[Basileus]

I'm back on the repair! Now with another scope, i'm checking everything again.

-The DC voltages are all fine on channel A;

With the scope i noticed that the AC signal Vpp is the same on all pair of transistors and similar to channel A UNTIL PAIR V513 - V514. The signal amplitude on V514 collector is twice the one on V513 collector. Now i'm investigating more in depth on that part of the circuit. I'm really tempted to desolder V514, but i feel like i need to check the components near to it. By testing V514 in circuit with the DMM diode mode, it seems perfectly fine.

I might just make a leap of faith, desolder it and check it with a cheap components tester i got. What do you think about it?

[SingedFingers]

I'd check resistors around it first.

[Basileus]

I'd check resistors around it first.

Checked R559, R558, R567, all good.

R558 is quite dodgy because it should be a 17.8k resistor but measures 4.23k ohms. I checked same values resistors across the board and they all give similar readings though.

It is connected to what looks like a voltage reference line but its maked "+12E". E? and there are other places were there are other letters, like C. What are those points? i'm quite confused. Cant find anything on google yet.

I'm still testing components around that transistor but i cant find anything blatantly wrong. 

[SingedFingers]

Desolder one end of it and test again if you haven't already. Other circuit components may be causing this reading to be offset.

[Basileus]

Desolder one end of it and test again if you haven't already. Other circuit components may be causing this reading to be offset.

Checked R662 and R658 17.8k ohm resistors that are the normal and inverted counterpart to that resistor. They give nearly the exact same reading, 4.23 ohms. I dont think desoldering it is worth the trouble also because that area of the board is difficult to reach. Its behind the delay line and another cable that is flush with the board runs through that area.

Still checking components around.

[SingedFingers]

Ah yeah screw that. Good luck

[Basileus]

After a break (after getting a Tek 2215 with a faulty DTB and fixing it)

I went back to fixing the pm3212. This machine is a damm mistery. I checked every reference voltage across both vertical channels and compared them, nothing. I checked every transistor signal with another scope, nothing. I checked the PSU, seems fine. The fault seems so tiny it cannot be found. The A trace just stays 5-7 div higher than normal with slightly lower amplitude. Tweaking the trimmer pots doesnt do enough. I just cant isolate the fault.

Please, does anybody, ANYBODY have a clue on what is going on? I'm so lost.   

[Twoflower]

The 12E is like the other 12A...12K different 12V supplies with own filter caps (Page 127 right half, center). So you should measure there 12V.

Not an expert here, so just thinking loud:
I assume you compared both channels within the pre-amp (see attached picture) with no input and the position knob on lower and on mid position. No need to stress the circuit to go too high, just turn the knob on both channels the same way around. What's the values? Are they identical between the channels?

Yes->Have you checked also the chopper-multiplexer? There both channels are interleaved and this is the last place were both channels have different circuits.
No->Check also the diodes V521/522. Also have a closer look at the source of the wire that just escapes down. The circuit has lots of switches which could be a source of trouble. So Probably compare the voltage there.

[Basileus]

The 12E is like the other 12A...12K different 12V supplies with own filter caps (Page 127 right half, center). So you should measure there 12V.

Not an expert here, so just thinking loud:
I assume you compared both channels within the pre-amp (see attached picture) with no input and the position knob on lower and on mid position. No need to stress the circuit to go too high, just turn the knob on both channels the same way around. What's the values? Are they identical between the channels?

Yes->Have you checked also the chopper-multiplexer? There both channels are interleaved and this is the last place were both channels have different circuits.
No->Check also the diodes V521/522. Also have a closer look at the source of the wire that just escapes down. The circuit has lots of switches which could be a source of trouble. So Probably compare the voltage there.

Ooohh i see. I checked all the electrolytic caps of the power rails on the main board, readings are within +- 0.20 V of the reference voltages. Both channel power rails are quite similar.

I'm checking again both preamps with GND coupling (no input) and both y pos controls at minimum. The voltages on D501 transitors are from 0.10V to 0.20V higher than D601 (channel B). Cant tell if its relevant or they should be spot on. Makes me wonder if its a front end problem. 

[Basileus]

I'm checking with the DMM (no point in using the scope for DC voltages) and i'm feeling like i'm not going anywhere. I cant tell how much difference between the voltages is suspect. 0.20 V, is it a problem or what?

I guess i could make a map with all the readings i get along both channels and post them here. I really cant tell whats wrong.

[Twoflower]

I don't expect a missing supply. Somehow there's a offset hidden. Yes this might indicate to a supply problem, but somehow I have the feeling it's not.

And if the 200mV matter we'll see.

Can you test this again at both the positions I marked in the picture earlier? They come as a pair. So far I can see the signal is amplified differential. If one side fails or the offset between both signal levels is off this could explain what you see. Ideal would be that if the knob is in the middle both marked measurement points has the same voltage. This represents the centre of the screen. A difference will move the trace up/down (regardless if by the pos knob or input signal). The marked spots should contain the signal plus the offset from the position knob.

And can you test the voltages with the pos more in the middle? This gives some feeling how much differential voltage per div is expected.

And please post the results for also for channel B that we can use this data as master. I assume the change would be the same.

[Basileus]

I don't expect a missing supply. Somehow there's a offset hidden. Yes this might indicate to a supply problem, but somehow I have the feeling it's not.

And if the 200mV matter we'll see.

Can you test this again at both the positions I marked in the picture earlier? They come as a pair. So far I can see the signal is amplified differential. If one side fails or the offset between both signal levels is off this could explain what you see. Ideal would be that if the knob is in the middle both marked measurement points has the same voltage. This represents the centre of the screen. A difference will move the trace up/down (regardless if by the pos knob or input signal). The marked spots should contain the signal plus the offset from the position knob.

And can you test the voltages with the pos more in the middle? This gives some feeling how much differential voltage per div is expected.

And please post the results for also for channel B that we can use this data as master. I assume the change would be the same.

Readings with:

GND coupling, AUTO, composite triggering, ALT V mode, Both channels y pos pot roughly centered

CHANNEL A

V522 Anode: 115 mV
V521 Anode: 128 mV

CHANNEL B

V622 Anode: 145 mV
V621 Anode: 145 mV

Wow.

[Twoflower]

It seems we're getting somewhere. One point:
1. Around this point I'm not sure if the voltages are actually DC if in ALT mode. I haven't fully understood the circuit, but the circuit drawn in between both channels might control which channel is displayed. So if you select ALT you might see only one channel show any voltage while the other is disabled. This change every time the trace is fully drawn. Please use either A (if measure on Channel A) or B.

The problem is, that the path through the signal-path is current based. For example the current through R557 is supposed to be 4.2mA and 65µA/div. While the 65µA/div doesn't matter with GND at the input you can ignore that part. I hope your multimeter is doing good at low voltages. Can you measure the following points?

1. Setting: POS = middle; Input: GND, select Channel A --> Voltage across R556 and voltage across R557
3. Setting: POS = middle; Input: GND, select Channel B --> Voltage across R656 and voltage across R657

The voltage calculates like 4.2mA * 301 = 1264mV at this point. The POS-Knob offset is not applied to this. This will happen after the V513/514.

For the good channel I expect at point 2: 1264mV for both resistors. For the bad one it depends if the problem is left or right side. If the error is already there: about 1382mV. If the error is not present there in it will match the one on channel B.

[Basileus]

It seems we're getting somewhere. One point:
1. Around this point I'm not sure if the voltages are actually DC if in ALT mode. I haven't fully understood the circuit, but the circuit drawn in between both channels might control which channel is displayed. So if you select ALT you might see only one channel show any voltage while the other is disabled. This change every time the trace is fully drawn. Please use either A (if measure on Channel A) or B.

The problem is, that the path through the signal-path is current based. For example the current through R557 is supposed to be 4.2mA and 65µA/div. While the 65µA/div doesn't matter with GND at the input you can ignore that part. I hope your multimeter is doing good at low voltages. Can you measure the following points?

1. Setting: POS = middle; Input: GND, select Channel A --> Voltage across R556 and voltage across R557
3. Setting: POS = middle; Input: GND, select Channel B --> Voltage across R656 and voltage across R657

The voltage calculates like 4.2mA * 301 = 1264mV at this point. The POS-Knob offset is not applied to this. This will happen after the V513/514.

For the good channel I expect at point 2: 1264mV for both resistors. For the bad one it depends if the problem is left or right side. If the error is already there: about 1382mV. If the error is not present there in it will match the one on channel B.

Huh...

Yes you are right. The circuit in the middle is the channel multivibrator, which controls which channel and how is fed to the last, fixed gain amplifier. In A and B display modes, the voltage at the diodes is DC only, while in ALT and CHOP modes the signal is a square wave (higher frequency for CHOP mode).

Actually, my scope doesnt have R557-R556 and C512-C514. There are holes for the components, but looks like they removed them and soldered some wire jumpers. I guess they were removed during an hardware upgrade. (Same thing with channel B).

[Basileus]

Voltages on V514, V513 emitters are 3.45 v and 3.45 v +-0.01 v.

On channel B:

3.44v
3.44v

I starting to think that i need a better multimeter.

[Twoflower]

Hrm. The missing resistors doesn't make the measure of the current easy... We can only hope that measure the potential is sufficient. To get better values from your multimeter you can measure the difference between the upper signal path and the lower. For example measure at the wires at R556 (positive tip of your multimeter) and the wire at R557 (negative tip). As the differential value is supposed to very little the MM will switch to the smallest voltage range and thus you get a good reading.

If everything is ideal the measurements would show 0V. Any difference will move the trace up/down. At the R556/557 pair the POS-knob should have no influence, this offset will be applied after the transistors in the signal path.

The problem is to identify what's acceptable and what not. The manuals only show currents and not voltages. Probably the best is to attach an external DC voltage to Ch.B (e.g. 1V). And if you measure the difference on the channel B signal path. This will give you a feeling how much voltage at R656-R657 per div is expected.