Philips PM3212 Oscilloscope channel A problem

[Basileus]

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.

Oh i feel so dumb  cant even use a dmm.

I probed here and there and the most suspicious thing i ve found is a difference of 90 mV at the bases of V512- V511.

Same transistors on channel B have a difference of 4 mV.

Doesnt change by moving the trace or toggling the triggering source control (checked because close to trigger A pickoff). Maybe the "second" stage of the preamplifier is so unbalanced that the last stage (with pos pot) cant rebalance it again. Still, the v difference between the other transistors pair are "reasonable".

Probably the fact that channel A is miscalibrated doesnt help too. I fiddled too much with the trimmer pots 

[Twoflower]

No worries. as long as your DMM and scope didn' did this:  that could be fixed.

One good rule is to remember the original pot settings before changing them or take a picture (which might help for simple pots, not for 10-turn trimmers). If it's too late: You might be lucky, you have a complete manual of your scope. The bad thing is, depending how many trimmers you touched this can take some time and good understanding of the meaning of the individual knobs.

You should try to understand the schematics a bit. Moving the POS doesn't change anything there as the POS poti is located later in the signal path. So Here the poti R543 and 547 might change this. But this looks like only the trigger supposed to be affected. That should have no impact to the traces on the screen. We should save this for later (but you should keep this in mind).

Can you please post what you measure at TP4 - TP5 (see picture)? As you see we're down to few mV/div offset. Unfortunately the 7.8mV/div don't say to GND or to the 'other side', so please measure the voltage between TP4 and TP5 as this might be double the value thus easier to measure. Because you wrote earlier that the values on the anodes V521/522 showed some offset (13mV) this might match with the offset you see. You 'only' need to figure out the first place of this mismatch and check what could be the cause there.

[Basileus]

No worries. as long as your DMM and scope didn' did this:  that could be fixed.

One good rule is to remember the original pot settings before changing them or take a picture (which might help for simple pots, not for 10-turn trimmers). If it's too late: You might be lucky, you have a complete manual of your scope. The bad thing is, depending how many trimmers you touched this can take some time and good understanding of the meaning of the individual knobs.

You should try to understand the schematics a bit. Moving the POS doesn't change anything there as the POS poti is located later in the signal path. So Here the poti R543 and 547 might change this. But this looks like only the trigger supposed to be affected. That should have no impact to the traces on the screen. We should save this for later (but you should keep this in mind).

Can you please post what you measure at TP4 - TP5 (see picture)? As you see we're down to few mV/div offset. Unfortunately the 7.8mV/div don't say to GND or to the 'other side', so please measure the voltage between TP4 and TP5 as this might be double the value thus easier to measure. Because you wrote earlier that the values on the anodes V521/522 showed some offset (13mV) this might match with the offset you see. You 'only' need to figure out the first place of this mismatch and check what could be the cause there.

I have found something *interesting*

The v difference between tp4 and tp5 (A) is 8.5 mV;

The v difference between tp4 and tp5 (B) is 4.4 mV;

WHEN SETTING THE CAL POT TO MAX POSITION:

The v difference between tp4 and tp5 (A) goes to 18.0 mV, nearly doubling;

The v difference between tp4 and tp5 (B) is 3.3 mV;

Mmh maybe its caused by the A preamp being miscalibrated..

[Twoflower]

That doesn't look good to me. Why does R2 have such a huge influence there on channel A. I would assume something like you see on B or even no visible change at all. Again the signal traces are current based, but that doesn't feel right.

Idea: Could it be that one of the V508/509 is blown? Can you check TP4 to GND and TP5 to GND how do they behave if you change the pos? Please do the same on the B TPs. Please post the readings. That helps me to get a better understanding what happens.

By the way, the documentation dan3460 posted (3rd post) shows the full calibration process. This can fix the misaligned trimmers. Still it does not note a faulty offset.

[Basileus]

That doesn't look good to me. Why does R2 have such a huge influence there on channel A. I would assume something like you see on B or even no visible change at all. Again the signal traces are current based, but that doesn't feel right.

Idea: Could it be that one of the V508/509 is blown? Can you check TP4 to GND and TP5 to GND how do they behave if you change the pos? Please do the same on the B TPs. Please post the readings. That helps me to get a better understanding what happens.

By the way, the documentation dan3460 posted (3rd post) shows the full calibration process. This can fix the misaligned trimmers. Still it does not note a faulty offset.

Not R2, R7.

I dont have a function generator and other stuff to calibrate it. Still,i have an uncalibrated cheapo fun gen. I could measure the amplitude and frequency with another scope and calibrate the philips one with it.

V508 and V509 are socketed and i think i already checked them. Ill check them again.

[Basileus]

That doesn't look good to me. Why does R2 have such a huge influence there on channel A. I would assume something like you see on B or even no visible change at all. Again the signal traces are current based, but that doesn't feel right.

Idea: Could it be that one of the V508/509 is blown? Can you check TP4 to GND and TP5 to GND how do they behave if you change the pos? Please do the same on the B TPs. Please post the readings. That helps me to get a better understanding what happens.

By the way, the documentation dan3460 posted (3rd post) shows the full calibration process. This can fix the misaligned trimmers. Still it does not note a faulty offset.

Not R2, R7.

I dont have a function generator and other stuff to calibrate it. Still,i have an uncalibrated cheapo fun gen. I could measure the amplitude and frequency with another scope and calibrate the philips one with it.

V508 and V509 are socketed and i think i already checked them. Ill check them again.

Huhhhh.. V508 readings are higher than V509.

My dmm is a cheapo with the *diode mode* that is also the 2000 range ohms. Cant tell if the values it shows are resistance or voltage drop like a proper diode mode.

Still, V509 junctions measure about 850... something and V508 1100. Ill swap their place, lets see if the problem flips.

[Twoflower]

Ah different poti, OK that explains that you see changes there at all. Completely my fault.

Probably it's best to calibrate the scope as good as you can before we continue. This would hopefully reduce the risk that we're chasing some misaligned trimmer. For the amplifiers you can do a lot without a signal source.

In 'Diode mode' the DMMs usually show the voltage drop. You can try that: Measure a silicon diode --> about 0.7V.

And if you haven't touched the trimmers for chan B you should swap the trannies back after this test.

[Basileus]

ITS THE GODDAMM TRANSISTOR 

Always check things twice. I cant belive i didnt spot it the first time i checked the transistors. Oh well. I made it. Thats all what matters. Looking for replacement now!!

[Twoflower]

Sure that's the trannie or could it be oxidized leads causing bad contacts in the socket?

But at least it is narrowed down to a simple clean or replacement. It looks like your scope gets better by the date! 

[Basileus]

Sure that's the trannie or could it be oxidized leads causing bad contacts in the socket?

But at least it is narrowed down to a simple clean or replacement. It looks like your scope gets better by the date! 

I tried to clean the leads with contact cleaner and scrape them with a knife. Didnt do anything. Trace is still offset, if i swap the transistors the problem flips. I cant recognize the faulty transistor anymore, the readings seem quite similar now.. I guess ill get two new BF450 or similar. Meanwhile, ill try to clean these i have with some fine sand paper. Thank you Twoflower, im so high right now ^_^.

Yes the scope is slowing getting better. I replaced the reed contact, but seems like the switch doesnt work anymore. Same stuff as the other one. Ill have to open it, glue it, spend 2 hours straight trying to fit the springs again. At least i know what to do, right? 

Then some cal, and we are good.

[Twoflower]

OK, the oxidized leads was just a last straw. It seems that you need to get hands on some replacement BF450.

Crossing my fingers that you don't need so long to get the switch back together. 

[Basileus]

OK, the oxidized leads was just a last straw. It seems that you need to get hands on some replacement BF450.

Crossing my fingers that you don't need so long to get the switch back together. 

Huuhhhhhhhh....

Sand paper didnt do the trick but...


Moving it to V511 did. Now it works fine. Still, i should probably get a replacement 

Now the trace is PERFECTLY CENTERED, but i wonder if that transistor placed in V511 will mess up the triggering.