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Old 15th Jan 2020, 11:44 pm   #1
high_vacuum_house
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Default Philips PM3217 X deflection problem

Good evening,
I am having issues with the 3217 oscilloscope. The X deflection scan length is only around 5mm in the centre of the CRT and is also running right to left. I have been looking at this and also the X POS control or X10 magnification does very little. The timebase is running correctly with a 5.4V peak to peak sawtooth waveform to the base of V1414. All of the power supply voltages have been checked and all correct.

The X POS control does work correctly and is putting the correct voltage onto the base of V1413 at the CW/MID/AC positions of its travel.

I am getting the same 5.4V sawtooth waveform on the emitter of V1414 as on the base though I am only getting 960mV peak to peak signal on a +11.8V DC offset on the collector of V1414 though the manual states it should be 8.4V.

Both X plates are receiving 106V and 110V with little sawtooth waveform on them. The manual states that it should be approx +26V

I am getting a bit lost as to where to check next as DC coupled stages can cause much confusion.

Many thanks,
Christopher Capener
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Old 16th Jan 2020, 1:46 am   #2
Chris55000
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Default Re: Philips PM3217 X deflection problem

Hi!

Collections of d.c. coupled cascade transistors in oscilloscopes are confusing to fault find at first go, and they don't lend themselves to conventional techniques!

There 8V are three voltage supplies to this amplifier, +38V, +180V and -180V, the +38V is surprisingly important as the X drive preamplifier V1414 will not work correctly if this is low or missing!

I mention this point again as the +38V is obtained from small–signal BAW62 rectifiers rather than the BY206 power diodes of the other supply rails, in the course of fault–findinng this supply could have accidentally disappeared due to burn–out of the BAW62 diodes!

If you're happy the BAW62s are providing the +38V, the next step is to print out the section of the main board layout diagram covering the X Amplifier layouts, using the "Fit To Page" function to enlarge it, then carefully unsolder the following transistors from the board and check them in this order:–

V1423, V1426, V1417, V1418 and V1419.

I strongly recommend one of those little Chinese Colour–Screen Component Testers of the "LCR–T1" type for dealing with multi–transistor direct coupled circuits of this type – they take a lot of guesswork out of fault location!

If any of the transistors only give a reading of one diode or a low resistance indication between any lead(s), bin them and obtain replacements!

If you get good test results from all the six transistors, leave them out at this stage and go on to all the precision value resistors dotted throughout this circuit – Philips precision resistors of this type do have a habit of going high in value or open circuit, and a red and green pen is handy for marking them off as good or bad on a copy of the circuit diagram as the case may be!

These are all E24/E96 series values which you can obtain from RS/Farnell – they will supply individual hobbyists on a pre–paid basis!

The zener diodes V1418, V1421, V1424, V1427, V1428 &, V1431 can partially or completely short to substantially alter bias voltages or cut off transistors, but a simple voltage check across each of them against the circuit diagram should show up a duff one!

Really and truly, the method of removing the active devices, the transistors in this case, then checking the passives left in circuit, is the best way you're going to locate faults in complicated mesh–works of this type – faults in this type of circuit will cause shifts in operating points of the transistors to such an extent that normal fault–finding techniques of measuring voltages can be extremely confusing!

Chris Williams
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Last edited by Chris55000; 16th Jan 2020 at 1:52 am. Reason: B
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Old 19th Jan 2020, 9:57 pm   #3
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Smile Re: Philips PM3217 X deflection problem

Good evening,
A little progress though there are some anomalies. the +38V rail is working fine and within its limits. I pulled V1406 and 1407 as I had replacements to hand. Both tested ok and gains were both matched on my Peak semiconductor tester.

There is a confusion which is there a potential divider connecting both of their bases which should generate -8V from the -12V line. This is made up with R1402 and R1414 which are 4.02K and 8.25K. In mine they are 2.74K and 2.86K producing -6.1V and not -8V as the manual states at the base of the 2 transistors. The resistors have been flow soldered at the factory and therefore haven't been changed at some time. Is there any benefit from changing the value of the potential divider?

There are a few other (official!) mods I can see around the X amplifier which don't fit on the circuit diagram including a 330 ohm resistor in each wire to the deflection plates and some extra capacitors.

Christopher Capener
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Old 19th Jan 2020, 10:59 pm   #4
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Default Re: Philips PM3217 X deflection problem

After replacing V1406 and V1407 the X deflection is back as it was with 5-6mm deflection. After further investigation it appears that V1426 and V1417 are not really turning on or drawing much current at all.

On V1426 the emitter resistor goes to the -180V rail via R1442 (1.15K) and there is virtually no voltage drop across it. Its collector should sit at -85V though it is currently sitting at -5.5V

On V1417 is drawing not much current either, the emitter resistor R1441 is almost the same voltage both sides with the collector currently sitting at +175V and not at +98V

Christopher Capener
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Old 19th Jan 2020, 11:53 pm   #5
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Default Re: Philips PM3217 X deflection problem

Good evening,
The output transistors are completely different from the manual.

V1417, manual shows BFT45 actual transistor fitted is BF470
V1418, manual shows BF238 actual transistor fitted is BF469
V1423, manual shows BFT45 actual transistor fitted is BF470
V1426, manual shows BF338 actual transistor fitted is BF471

Christopher Capener
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Old 20th Jan 2020, 12:14 pm   #6
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Default Re: Philips PM3217 X deflection problem

Philips PM3217
Taken from Mod note:OSC167.
Alternative transistor types for Amplifier Unit PCB, because transistor BF338 is obsolete, component changes:
V1417 and V1423 into BF470 (from BFT45)
V1418 and V1426 into BF469 (from BF338)
R1437 and R1439 into 100E
C1407 into 1.8pf.
For replacement in older instruments, BF338 remains available.

So I suggest that what you have is correct.

Other Mod notes relate to new Y output amplifiers. From both the /02 version and again for the /05 version, the brown look version.
A revised power suppply from /04 version.
No changes apparent for the X output amplifier.
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Old Yesterday, 11:27 pm   #7
high_vacuum_house
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Default Re: Philips PM3217 X deflection problem

Many thanks Bill. I have a few more transistors on order and will see what happens.

Christopher Capener
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