Oscilloscope Philips PM 3212 fail Sync B - Page 1
Forum section: electronics / repair

author: Greenray
author_rank: Contributor
timestamp: January 02, 2017, 10:09:43 am
content: Hi.I jealously keep a PM3212 Philips Oscilloscope buyed as used in the '80 years.I've use very much this instrument during my hobbyist and study phases.Several years ago the sync it's not looking at the Channel B.It doesn't matter if this problem is for me and for you small or big, because I want to repair it.At the begin I was imagining that having two channels this task could be simple respect what really is. I was convinced to apply the same "CAL" signal to both channels and compare it: yes, but using what?Going to the point: what I can record?The two channel are quite the same, with very small difference.The Y input signals A as B, pass to several stages, to finally go to the deflection system on the CRT.This is all OK, for both my channels A and B.What don't go well is the Sync signal B that (as for A) is derived from the stage called "Impedance Converter", by the coaxial cable called "Channel B Pick Off".It's a very silly thing: the Y signal from his input run to the CRT, without any fail, but the "Channel B Pick Off", derived from it it's not good if checked on the S16 Group, the Sync Source Selector.So I was hoping that the problem was the coaxial cable, that connect the origin of the "Channel B Pick Off" to his destination board, but also this hypothesis fall through because the cables are not in short circuit and there are no interruptions.I have also exchanged the coaxial A with B and the problem turns now on A. This is an important verification (now A coax is returned on A as B is returned on B).I have measured many voltages inside the circuits with V611-V612, but all the measurements on the Channel B are very similar to the same on the Channel A.I have just one oscilloscope, this. So it is not so easy to look "inside" the circuit.I need some suggestion to exit from this "stall" phase.Thank you.I decide to write this, after reading the interesting discussion:https://www.eevblog.com/forum/repair/philips-pm3212-oscilloscope-a-channel-problem/msg1096835/#msg1096835

author: Basileus
author_rank: Regular Contributor
timestamp: January 02, 2017, 10:24:06 am
content: Hi!First thing first, i suggest you to look at this guide Tektronix made on troubleshooting oscilloscopes. Its good theory:https://www.google.it/url?sa=t&source=web&rct=j&url=http://www.smcelectronics.com/DOWNLOADS/TEK%2520TROUBLESHOOTING.PDF&ved=0ahUKEwjNkLW9naPRAhXKXBQKHdkxAY0QFggrMAM&usg=AFQjCNF_st2t9-_YJalugcLXDG8v5U6Wig&sig2=0NuZM9l-Bp8fJn7C5YGRLAIf i have understood correcly, you problem is that you cant derive triggering signals from channel B, am i correct?You tried swapping the coax cables from both channels and now the problem is on channel A: i guess the coax is faulty then.

author: Greenray
author_rank: Contributor
timestamp: January 02, 2017, 10:33:25 am
content: You have right understood the described fault.Exchanging the coaxial cables swaps the problem by the channels, as I was imagining.But this don't means that the coaxial cable of B is the cause.The coaxial derived sync cable B (as for A) is not interrupted and not in short circuit.This don't means that the coaxial cable B is surely good, but I don't see any reason to doubt furthermore on it.I will read with attention your suggested text from Tektronix. Thank you.

author: tautech
author_rank: Super Contributor
timestamp: January 02, 2017, 10:41:03 am
content: Link to schematic:http://elektrotanya.com/philips_pm3212_oscilloscope.pdf/download.htmlWait forprocessingto change toget manual.Better manual:https://doc.xdevs.com/doc/PHILIPS/PHILIPS%20PM3212%20Instruction.pdf

author: Basileus
author_rank: Regular Contributor
timestamp: January 02, 2017, 10:50:17 am
content: The triggering circuit is common for both channel a and b pickoff, apart from their trimmers. I dont see why it cant be the coax cable faulty. I would try to replace it if its not too big of a trouble.

author: Greenray
author_rank: Contributor
timestamp: January 02, 2017, 11:05:35 am
content: You're right. The real Sync circuit is common, but before the selector S16 the signals are obviously separed, coming from different sources (A, B, EXT, LINE).I think that the Sync circuit works well: this is demonstrated choosing A as Sync source;I think also that the Coaxial cables are well working: this is not very easy to demonstrate, but I trust on it;So, if the Sync is good, if is good the "vertical attenuator circuit" (S16 on board) and both coaxial are good, my conclusion is that the the fault stay before all these item.I'm convinced that the derived signal called "Channel B Trigger Pick Off" is not as the rest of my scope needs.There is a new:Putting CAL on both channels and selecting COMP(osite) as Sync, my scope sync well A as B even when I chose for Y, A or B.While I was investigating, I've found a scketch of the Vertical Attenuator Unit, where appears 3 diodes that are not present into all the schematics, but just in one. I think that can be useful remember also this particular.Note that Using EXT sync no problem are detected because the signal, from the BNC input jump directly to the DC/AC switch, without enter in the Trigger Preamplifier.

author: Greenray
author_rank: Contributor
timestamp: January 03, 2017, 10:01:54 am
content: Now I want to fix one point.I put CAL into the B input, AC coupled, 20 mV/Div.I put CAL also into EXT Trig,I chose EXT as Trigger and CHOP as Y,on the screen appears a square wave with about 1.2 V of amplitude (the signal on B).I've done this to try how works the B channel amplifier examining it with the A channel.I want see where the signal is present and where it stop its path to the trigger.So now I'm seeing that the CAL signal, from the BNC goes to the collector of V607 where the measured amplitude is about 1 mV pp. (depending from the gain of B).I'm seeing also that the same waveform, appears reversed on the V606 collector.I think that this is right until here.Going forward, I see similar signals on the emitters of V609 and V 608 (about 0.4 mV pp), and the same signals amplified on their collectors (about 120 mV pp).While these collector signals run to their destination, their emitter signals (about 0.4 mV pp in this case) are going to the base of V611 and the base of V612.I think to be finally near the core of the problem of my scope.There is a thing that confuse me many times in my measurements.Normally a transistor as V612, when is polarized and drived as I've described, present on his collector the same signal present on his base, reversed and amplified.But the collector load resistor of this transistor is not on the main board!The collector load resistor of this transistor is on the board where stay S16.This collector signal also called "Channel B Trigger Pick OFF" coming from V612, is connect with its load resistor R362 through a small coaxial cable.The complex schematic diagram don't give us with enough evidence that some combinations of S16 can kill closing at GND our signal "Channel B Trigger Pick OFF".So it's natural that if I chose A as Trigger input, S16 kill to GND the B Trigger signal.In this condition we will see nothing on the collector of V612 even if B Channel works well.

author: Greenray
author_rank: Contributor
timestamp: January 03, 2017, 02:02:15 pm
content: About S16.The behavior of S16 is slightly different from the ideal behaviour.3When A and B are both pushed in, the Trigger circuit will receive the signal from theComposite source;(in this condition A and B trigger source are both shorted to GND)2When B is pushed in, and A is released out, the Trigger circuit will receive the signal from theBsource;(in this condition A and Comp trigger source are both shorted to GND)1When A is pushed in, and B is released out, the Trigger circuit will receive the signal from theAsource;(in this condition B and Comp trigger source are both shorted to GND)0When A and B are both released out, the Trigger circuitstill continue to receive the signal from the Asource;in this conditionBand Comp trigger sourceare both shorted to GNDThis means that the only one condition permits to the B Trigger Signal to go to the Trigger circuit and this condition is when "B" is selected.OK, this is right, but during the troubleshooting we have to keep in mind that the B Trigger Signal can survive only when we select "B".This is a complication in my case because having a fault in the Trigger B of my scope, until now I've try to use the trigger coming from A.Mumble mumble!

author: Greenray
author_rank: Contributor
timestamp: January 04, 2017, 10:33:20 am
content: I have extract and reproduced the part of the schematic where my attention is now focalized.This help me to view better the "electrons", but is also a way to study the circuit while I write it down.Now it's more easy for me and my old eyes, to read references and follow them in the layout to do my measurements.I have to thanks to Schematics.com that allow me to drawn quickly what I need.Now I show you this work.

author: Greenray
author_rank: Contributor
timestamp: January 04, 2017, 11:02:31 am
content: Now I will go directly to the point of my strong suspect.The vertical mode is on CHOP;B input is connected to CAL, coupled in AC; 200 mV/div.The Trigger selected is B (because all the other selections will ground the signal Channel B Pick Off)I have my DMM and the probe on A, available for my measurements.Now I see the B channel showing the square wave of CAL occupiing 5.7 divisions of the screen.This is not synchronized and so is free running.If I look and measure the BASE of V612, I see a square wave with 20 mV of amplitude with much noise, due to the high sensitivity and the probe hanged.The medium voltage on the BASE is about 7.34 VThe medium voltage on the EMITTER is about 8.22 V.On the EMITTER I see quite the same signal saw on the BASE.Now I will look at the COLLECTOR.The COLLECTOR of the transistor V612, by a coaxial cable, is connected to R362 on the assembly where S16 select the Trigger Source.Keeping selected "B" as trigger source, I can be sure that the signal it's not put to ground.Measuring with my DMM the voltage on R362, I read 2.88 V, the same value of the COLLECTOR V612.But the no one effect of the signal present on the BASE of this transistor, appears on the Collector!!!My conclusion is that the V612 emitter voltage change with the same phase and amplitude of his base voltage. This means also that the Emitter current don't receive any contribute from the Collector current.The Collector current in this configuration must be "beta" times greater than Base current.Why in my scope the Ie is equal to Ib? Why the Ic add 0 mA to the Ie?My suspicion are now direct on V612.What reason there is for V612 to keep his collector fixed to 2.88 V, without any variation due to the signal measured on his base terminal?The first thing I've though is that V612 could be broken and open, but I've verified that on his Collector resistor R362, there are about 3.46V, that means that in the resistor flows about 4 mA.Now is to be verify if this current flows into the V612 or elsewhere.

author: tautech
author_rank: Super Contributor
timestamp: January 04, 2017, 07:32:36 pm
content: When you narrow down to a small area of interest I find it is useful to pull the active devices and test them individually, many cheap DMM's have a transistor tester and it's easy to check their Hfe meets their datasheet spec.If the trimmers are of an open frame type they need be checked too as decades of exposure to humidity can start corrosion on the wiper contact or even the resistive track itself. Sometimes I mark their set position and adjust them some small amount each way to regain the wiper contact.

author: Greenray
author_rank: Contributor
timestamp: January 04, 2017, 08:48:40 pm
content: I feel agree with you.My very cheeper DMM permit to measure the HFE of transistors.But even if I trust in my hands, my solder, my vacuum one shot pump, before to pull out of the board I want understand better than now where is the core of the problem.Pulling out a component it's always a risk, due to many element of the entire operation.The PCB it's in fiberglass material (FR4 or similar), but is very old and I remember that in that far age, often the trace follows the removed component, or the "via" loss his metalization.There is also the risk that microscopic drop of lead could go around in the circuit.So I will manage with caution and precautions, thrusting in myself, but only when the number of the "candidates" become very, very small.To be clear, before of this kind of operation I plan where will possible, to make with clips and crocodiles jumpers;i.e. between base and emitter to inhibit the suspected transistor;or between emitter and collector, to see and follow the behavior of the following part of the circuit.I think also, after to have inhibited the suspected transistor, to connect soldering with short rigid wires Emitter and Base of a new transistor in a kind of "piggy back" on the suspected and finally connect the Collector to his own collector resistor.In this way I will look the different behavior of the two transistor under compare.Yes I know that all this can be see as a way to waste my time. This is not totally wrong!About the trimmers, I'm agree with you.In some of my cases, to avoid to damage the fragile carbon film of an old trimmer, I have lightly change the shape of the wiper, to obtain that it turn on a new arc, concentric to the damaged (or oxidated) one.This is an operation to do paying very much attention because also the cursor after some decades of year became fragile.Changing its shape there is the risk to reduce the pressure of the cursor on the carbon, so, we need to be careful!

author: tautech
author_rank: Super Contributor
timestamp: January 04, 2017, 09:34:54 pm
content: When I got a temp controlled station the risk of damaging pads and traces reduced dramatically and I now have NO hesitation to remove any component for out of circuit tests irrespective of the age of the PCB.Suck, resolder, suck, resolder, suck all with new solder will release old stubborn joints and afterwards a clean with IPA will ensure there is little risk from any rework.While may faults can be diagnosed with a DMM some circuits and current flows can make this challenging without extensive experience and often it's just simpler to pull a component.IMHO if a preset can be damaged by movement it is untrustworthy and need be replaced anyway.

author: Greenray
author_rank: Contributor
timestamp: January 05, 2017, 10:11:34 am
content: Simplifying a lot, and wanting to forget the part of the circuits far from the problem (following my opinions) I've draft this schema to help myself to follow the signals, do some measure and report them with my pencil on the printed sheet.One confession: when I was a boy and I was beginning to work with transistors, there were in use the germanium type and the PNP type. After some year I've begin to work with Silicon NPN transistor, for all the rest of my life.Now, I have some difficult to think to the behavior of the circuit when the transistors are PNP.So my steps are in this case very slow, I'm sorry.Please remember that I've omitted to draw many contact of S16, assuming only the condition where "B" is selected a Trigger source.

author: Greenray
author_rank: Contributor
timestamp: January 05, 2017, 02:57:04 pm
content: Now (after have removed an error) in the simplified schema appears also the DC voltages measured with my DMM.These value will surely help me to understand what it's happening inside it.

author: Greenray
author_rank: Contributor
timestamp: January 05, 2017, 03:40:52 pm
content: Looking at the voltages in the simplified schematic, I see that across the R362 there are about 0.73 V (3.61-2.88).This means that the R362 current it's around 36.5 uA.I'm convinced that this current is just the Base Current of V351 and that the contribute of V612 collector current it's near to Zero.The trigger Source Board it's the same for the other Trigger Sources.All the other sources of the Trigger seems to go well, but not "B".So my opinion that the failure depends from V612 became more strong.Furthermore, I have non other hypothesis about other possible causes.

author: Greenray
author_rank: Contributor
timestamp: January 05, 2017, 05:45:19 pm
content: Hi people.I've just replaced the transistor V612 that was BF450 with another PNP: the BC556,that has another package!Before to do it, I've placed the BC556 over the BF450 using his long leads.This it's been possible because my hipothesys was that V612 were opened and what follow demonstrate that this was right.The BF450 it's much dead: there is no one junction good!Powering my scope and putting CAL only to B BNC, then using B as Trigger source, I finally see the desired result of my long study on the PM 3212.Be quite guys, the problem is now solved, but now I can follow another bad thing on the Trigger of my scope.Applying CAL to A and B, I can now see both these inputs, using A or B as Trigger source, but with one restriction: I can couple the Trigger in AC or DC for A, but for B only AC.Why?Now we go to discover the reason. Would you follow me?

author: Greenray
author_rank: Contributor
timestamp: January 06, 2017, 08:47:38 pm
content: After have read the chapter "Trigger Balance" this morning I had "touched" the trimmer of the Trigger B.So now my scope shows B as A using as thrigger source B, coupled in AC of DC: good.Now that the most important things are OK, and my scope seems as new (!?) I want spend some time on the illumination of the graticule, where one of the two lamps is burned out.I've followed the good example of a famous video on youtube, but introducing a 18K resistor (in parallel to the 22K potentiometer) to permit to the regulation to cover the LEDs intensity starting from the full light and going to a quite "dark" of them.The voltage on the cursor of my potentiometer was never falling under 0.9 V causing a current into the Base of the transistor always excessive for a wide regulation. But if I subtract current to the potentiometer, placing a 18K in parallel to it, the regulation become more appropriate for my desires.I like to see my reticule very light, but what I often need is to see waveform so speed that the trace became not so light. In this condition asking more intensity the trace became not so much focused, so I prefer increase contrast darking my room and the intensity of the reticule.Resuming, I need to regulate the LEDS (that replace the original lamps) until the minimum intensity.