Philips PM 3232 repair

[Heribert Hechtersheim]

Hi,

I just bought an old Philips PM 3232 on ebay for 23,10€ (30,96). It was probably not the best deal, because it is only a 0-10 MHz scope and doesn't work. When I powered it up, the Backlight worked, but I could not see any trace on the screen. I checked the 0.6V calibration squarewave with my other oscilloscope and it was not working. Then I measured every voltage rail on the powersupply board and found out that the the +20V rail was far too low. I added another cap in parallel and it was stable. However, the trace still did not appear.
What should I do next? Take out the PCBs and test every single component? Is there any way to check if the CRT is working?









Luckily, the transistors are plugged in sockets, so I can easily substitute them.

schematics:
http://i881.photobucket.com/albums/ac18/Hechtersheim/Philips%20PM%203232%20repair/oszilloskop_zps5cb651a6.png

service manual:
http://de.scribd.com/doc/82277579/Philips-Pm3232-Service-Manual-ET

[Heribert Hechtersheim]

I measured all the voltage waveforms in the time-base generator which were shown on the Schematic. Evereything was ok. Then I unplugged the blanking input of the CRT and I saw the trace(see picture below). What me makes me wonder is that both points 11 and 12(see the schematic in my first post) which are very close to the blanking input have the right waveform. In addition, turning the intensity knob does not make the trace less bright or brighter. It only horizontally streches the picture a bit.
Does anybody has an idea what could be wrong or what else should I check?

I would really appreciate any help.



First trace !

[PA4TIM]

Have you set the trigger to auto ?

First make the PSU perfect. Not only a cap parallel because the original can leak DC. Measure voltage and ripple. This must be spot on.

No reaction on the intense pot can be simple. Just measure on the wiper of the potentiometer to see if it reacts. (max the same as one of the outer pins, min the same as the other one.
Many Philips scope problems are related to dirty switches or pots..

[jancumps]

Have you set the trigger to auto ?

First make the PSU perfect. Not only a cap parallel because the original can leak DC. Measure voltage and ripple. This must be spot on.

No reaction on the intense pot can be simple. Just measure on the wiper of the potentiometer to see if it reacts. (max the same as one of the outer pins, min the same as the other one.
Many Philips scope problems are related to dirty switches or pots..

That's my experience too. And in addition to switches, pots and caps: the rectifier bridge that sometimes gives up when the PSU caps go the way of the dodo.

[Heribert Hechtersheim]

Thank you very much for your answers!
I measured all DC voltages with my digital multimeter. It cost only 20€ so the accuracy isn't the best but I think for this purpose it is enough. I wasn't really sure though how to measure the ripple. I used the AC mode of my scope and tried to make the ground lead as short as possible. I will add a picture below so you can tell me if I measured it correct. In the table are my measurements. The reference values come from the service manual I linked in my first post.

required value	measured value	max. allowable ripple	measured ripple
+ 12V, +-30mV	12,00V	4mV p-p	10mV p-p
+ 12V, +-400mV	-12,00V	6mV p-p	5mV p-p
+ 20V, +-900mV	19,27V	40mV p-p	40mV p-p
+ 80V, +-2V	78,5V	250mV p-p	90mV p-p
+160V, +-3V	159,7V	100mV p-p	75mV p-p

I removed the cap I had added on the 20V rail and measured a good value. I probably measured something wrong first. But the ripple of the 12V rail is to high. Here is apicture which shows how I measured the ripple:



The cap is very close to the switching transformer so maybe the measurement wasn't so accurate.



The ripple is 0,2cm * 0,005V/cm * 10(because of the 10X probe) = 0,01V = 10mV. Is that correct?
Max. allowable ripple is 4mV so I will replace the cap, right?

I had set the trigger mode to auto.
The pot of the intensity is ok, I tested it.

[tekfan]

In addition, turning the intensity knob does not make the trace less bright or brighter. It only horizontally streches the picture a bit.

Sounds like the power supply isn't regulating very well. When the power supply is loaded the HV for the CRT also drops. This causes the electrons to move slower through the deflection plates. The plates have more time to deflect the beam and that causes the trace to stretch out a bit.

The voltages and ripples seem reasonable. Check for something loading the power supply and causing a drop in regulation.

These Philips scopes almost always have a problem with the power supply. Be sure to check for shorted capacitors and non socketed metal can transistors on the HV board.

[jancumps]

These Philips scopes almost always have a problem with the power supply. Be sure to check for shorted capacitors and non socketed metal can transistors on the HV board.

Why do you point to the metal can transistors? Is there a known problem with them?

[tekfan]

Why do you point to the metal can transistors? Is there a known problem with them?

I realise that I sounded like this is an obvious fault. It is not, but it's still something worth checking to confirm that's not the problem. You never know what a shorted capacitor or a HV pulse can do to transistors even if they are rated for HV.

[Heribert Hechtersheim]

Thank you very much for your help!

These Philips scopes almost always have a problem with the power supply. Be sure to check for shorted capacitors and non socketed metal can transistors on the HV board.

I checked the PSU again. The whole scope has only socketed transistors except the two beefy ones for the switching power supply. The only thing I found was a weird transistor on the PSU. It is a BSS68(TS 800 on the schematic). I measured the resistance between base-emitter and base-collector in both directions. The only resistance my multimeter showed in 20M? range was 15M? from collector to base. So I think the transistor is dead.
I substituted it with a 2N3906, it was the only PNP transistor I had lying around. The voltage rating of the 2N3906(Data: http://alltransistors.com/transistor.php?transistor=3751) is much lower than the BSS68(Data: http://alltransistors.com/transistor.php?transistor=29000) and I think that the characteristics are very different. Therefore I will have to buy a transistor that fits better later. Has anyone an idea what transistor would be a good replacement for the BSS68 or is it save to leave the 2N3906 in the socket?
Anyway, I turned the scope on with the 2N3906 and I saw the trace! I can even adjust the intensity of the trace a bit, but it could be darker when I turn the knob all the way down. Maybe I can fix that by adjusting the pots, I have to look at the manual. I also noticed that the triggerlevel pots are very dirty. The resistance between the wiper and one of the outer pins jumps randomly and is sometimes even higher than the resistance between the outer pins. Can I fix that with contact spray? The other knobs and switches feel a bit dirty, too.
Another thing I have noticed is that you can see the trace on the left side of the screen even when it is not triggering. How can I fix that?
I forgot to mention that the scope makes a very high-pitched sound when tourned on. It is not very annoying but you notice it.

[Heribert Hechtersheim]

The only thing I found was a weird transistor on the PSU. It is a BSS68(TS 800 on the schematic). I measured the resistance between base-emitter and base-collector in both directions. The only resistance my multimeter showed in 20M? range was 15M? from collector to base. So I think the transistor is dead.

I measured the BSS68 transistor again with the diode test mode of my multimeter and it shows a continuity voltage of about 670mv between emitter-base and collector-base. That is why I think the transistor is not broken and I will have to search for other bad components. Do I have to unsolder all the caps before testing them?

[tekfan]

When testing ESR of capacitors they can generally stay in the circuit since when you connect the ESR meter the lowest impedance path will be the capacitor you're testing. When testing capacitors for leakage you have to disconnect at least one leg so other components don't interfere with the high resistance reading. When searching just for shorted capacitors an ohmmeter is enough and no need for disconnecting the cap from the circuit. Some of the Philips caps will probably have a higher ESR than normal so it's best to do that check first.

I've never had good experience when using contact cleaners (maybe used the wrong stuff). I usually completely disassemble a scratchy pot and manually clean the contacts. This is good practice especially on high resistance pots which may be found in the HV supply. One or more pot is usually a high resistance type (>1Mohm). You can always try some contact spray and if it's not better try to disassemble the pot. You've got nothing to loose. Just don't use WD-40 as a contact cleaner like most of the genius people I know do.

You said that the trace brightness doesn't go low enough. Usually there's a trimpot somewhere to set the cutoff voltage of the CRT.
Usually the front panel intensity knob is set to somewhere about 1/4 open and then the trimpot is turned until the trace is barely visible on the screen.

This should also cure the problem when you see a dot on the left side of the screen even before it triggers. That's because the blanking circuit isn't providing the necessary cutoff voltage to fully dim the spot.

Remember to check also emitter-collector for shorts. Recently I had a transistor which tested good from base to emitter and collector but was shorted completely between e-c.

I've recently also repaired 8 Philips PM3211 scopes and all of them emit this high pitched whine when starting up. After a few seconds the caps charge up and the frequency is beyond the hearing range so it seems quiet. Looks like all of these Philips scopes had the same power supply topology. The audible whine in your scope may be a problem with a power supply overload.

Hope I'm not rambling on too much.
Good luck at repairing the scope.

[Heribert Hechtersheim]

I've never had good experience when using contact cleaners (maybe used the wrong stuff). I usually completely disassemble a scratchy pot and manually clean the contacts. This is good practice especially on high resistance pots which may be found in the HV supply. One or more pot is usually a high resistance type (>1Mohm). You can always try some contact spray and if it's not better try to disassemble the pot. You've got nothing to loose. Just don't use WD-40 as a contact cleaner like most of the genius people I know do.

I used the contact cleaner and the result was not perfect so I wanted to try your method. Before I could unscrew the pot, I had to remove knobs on the front panel.
Unfortunately, my spanner socket was to big to fit into the knobs:


Therefore I had to modifiy the spanner socket with my professional equipment 

The result was an egg-shaped socket, but it was small enough to fit into the knob:

Then I could take out the time-base unit:

I cleaned the contacts of the time/cm switches with Q-Tips and unscrewed the pot. Here you can see the wiper and the arc:


I also cleaned the panels and knobs. The white paint on two knobs was not good so I had to repaint them:




The result:


The right one is the original painted knob.

When testing ESR of capacitors they can generally stay in the circuit since when you connect the ESR meter the lowest impedance path will be the capacitor you're testing. When testing capacitors for leakage you have to disconnect at least one leg so other components don't interfere with the high resistance reading. When searching just for shorted capacitors an ohmmeter is enough and no need for disconnecting the cap from the circuit. Some of the Philips caps will probably have a higher ESR than normal so it's best to do that check first.

The electrolytic capacitors of the PSU are not shorted and don't leak. I don't have an ESR meter and I am not sure if I want to spent money for it. I think it would be easier for me to just buy new caps and replace all of them on the PSU. Are there any differences between radial and axial ones except shape? In addition, there is no tolerance on the parts list of the service manual for the electrolytic capacitors. What do I have to take into account when buying these?
I already replaced the low voltage caps with some I had in stock. I didn't make a difference, the sope showed no trace.

I've recently also repaired 8 Philips PM3211 scopes and all of them emit this high pitched whine when starting up. After a few seconds the caps charge up and the frequency is beyond the hearing range so it seems quiet. Looks like all of these Philips scopes had the same power supply topology. The audible whine in your scope may be a problem with a power supply overload.

8 scopes? Really? Do you do that for a living? Did they all have problems with the PSU caps?
The whine when the scope is turned on is only barely noticeable. Maybe if I was 10 years older, I could not hear it.

Hope I'm not rambling on too much.
Good luck at repairing the scope.

Thank you very much! You are not rambling too much. It's great that you share your knowlegde and experiences.

[tekfan]

Nice tip on grinding away the socket. I always find using pliers and that usually leads to ruined screw heads.

At least the time base switch doesn't have a giant mass of wires soldered to it like most Philips scopes. The only sensible way to clean those is to rotate the controls for a bit until the trace becomes stable.

Is that damage on the carbon element of the pot? If it is you can always slightly bend the wiper contacts so that they don't slide across the bad area. If the carbon element has a crack in it there will be no conductivity between the far end contacts of the pot. I've saved a few weird HP pots that had a hairline crack in the resistive element by applying a tiny drop of conductive silver paint in the crack. I'm impressed that it still works like new.

The electrolytic capacitors of the PSU are not shorted and don't leak. I don't have an ESR meter and I am not sure if I want to spent money for it. I think it would be easier for me to just buy new caps and replace all of them on the PSU. Are there any differences between radial and axial ones except shape? In addition, there is no tolerance on the parts list of the service manual for the electrolytic capacitors. What do I have to take into account when buying these?
I already replaced the low voltage caps with some I had in stock. I didn't make a difference, the sope showed no trace.

There's no electrical difference between axial and radial caps. It's just the physical construction. The caps are probably 20% tolerance like most older electrolytics. Probably every new electrolytic cap is better than those old ones so don't worry about their ratings too much. If you want some extra headroom you can buy 105°c caps which will probably last longer because of the higher temperature rating. It's probably a better idea to just replace the caps than investing in an ESR meter. The caps can also be replaced as a last resort if everything else seems to work as it should.

8 scopes? Really? Do you do that for a living? Did they all have problems with the PSU caps?
The whine when the scope is turned on is only barely noticeable. Maybe if I was 10 years older, I could not hear it.

I got 14 scopes from my school. They wanted to throw them away and were in really bad condition. Some had missing knobs and front panels and all were very dirty, but all had the complete electronics inside. I gave some to a friend and the rest I kept. I don't do this professionally. It's just a hobby (that takes up a lot of space). Once I found what the common problem was, I repaired all of them in 2 days.

The power supply in these scopes runs at about 30-40kHz so it it may be possible for it to be audible if it runs lower than that.

[Mr Simpleton]

I would make sure that the semiconductors are making contact in the sockets. Often you only have to juggle the transistor to clean off the oxide. Seems you are making good progress, good luck.

[PA4TIM]

I always unplug an plug transistors in sockets a few times. I had a Tek 7704 that had a trace problem that was only caused by a transistor/socket problem.

Your psu values look good. I always test caps on DC leakage, capacitance and DF with one leg desoldered.I have designed and build several ESR meters, I have a professional ESR meter ans can measure it in other ways. And I almost never measure ESR in repairs. Only if it involves a SMPS, and I never measure it in circuit. Done a lot of tests comparing in situ and desoldered caps and they differ much to much. Bad ones can look good so that is allready  a no-no for me.
The best check is measuring ripple current and DF
http://www.pa4tim.nl/?p=3775 about ESR
http://www.pa4tim.nl/?p=1728 build an ESR meter with adjustable frequency and able to measure as low as 100 nF caps
http://www.pa4tim.nl/?p=1385 DC leakage and testing
Measure C with a LCR meter, most multimeters use DC and are more then 100% wrong if caps are bad. The high ESR makes the meter think (RC time) the capacitance is still good,

I repaired a few Philips scopes but older models (including two 1 GHz sample scopes) most problems: switches and leaking high voltage cascade caps.
The old blue Philips caps from the 70's/80's are not the greatest in my opinion. From all my gear I only had a Tek SA that needed all caps replaced,  In a HP LCR meter  al caps smaller as 10 uF were bad but for the rest no problems, except for Philips gear . There I hade to replace many caps, but that are most silverface models, so older then your scope.

I use a pipe-socket/rohrschluessel.   http://upload.wikimedia.org/wikipedia/commons/7/7a/Rohrschluessel.jpg

The best way to test transistors is measuring the voltage in situ. Vbe must be around 0.6-0.7V or 0.35 for germanium. And with a scope you see amplification or switching. And if voltages are good, the transisator is working.

Nice documented restauration. You have a good eye for detail.

[ElectroIrradiator]

I always unplug an plug transistors in sockets a few times. I had a Tek 7704 that had a trace problem that was only caused by a transistor/socket problem.

The white Mini-Latch style connectors used in newer Philips scope models are worth exercising as part of a restoration. They also tend to cause weird problems simply due to making poor contact.

[Heribert Hechtersheim]

Again, thank you for your help! It seems that my audience is growing

The power supply in these scopes runs at about 30-40kHz so it it may be possible for it to be audible if it runs lower than that.

I looked at the service manual and found out that it runs at approximately at 18kHz, so it's just below the upper end of the audible spectrum.

Is that damage on the carbon element of the pot? If it is you can always slightly bend the wiper contacts so that they don't slide across the bad area. If the carbon element has a crack in it there will be no conductivity between the far end contacts of the pot.

Yes it is. I opened it again and fixed it the way you said.

It's probably a better idea to just replace the caps than investing in an ESR meter. The caps can also be replaced as a last resort if everything else seems to work as it should.

I replaced all electrolytic caps on all boards. Sadly the board is not so colourfull than before with the new black caps. The trace still does not appear. Now at least we know that the caps are fine and not causing the problem  

I would make sure that the semiconductors are making contact in the sockets. Often you only have to juggle the transistor to clean off the oxide. Seems you are making good progress, good luck.

Thank you! I moved all transistors in the sockets a bit. Still no trace.

I always unplug an plug transistors in sockets a few times. I had a Tek 7704 that had a trace problem that was only caused by a transistor/socket problem.

Your psu values look good. I always test caps on DC leakage, capacitance and DF with one leg desoldered.I have designed and build several ESR meters, I have a professional ESR meter ans can measure it in other ways. And I almost never measure ESR in repairs. Only if it involves a SMPS, and I never measure it in circuit. Done a lot of tests comparing in situ and desoldered caps and they differ much to much. Bad ones can look good so that is allready  a no-no for me.
The best check is measuring ripple current and DF
http://www.pa4tim.nl/?p=3775 about ESR
http://www.pa4tim.nl/?p=1728 build an ESR meter with adjustable frequency and able to measure as low as 100 nF caps
http://www.pa4tim.nl/?p=1385 DC leakage and testing
Measure C with a LCR meter, most multimeters use DC and are more then 100% wrong if caps are bad. The high ESR makes the meter think (RC time) the capacitance is still good,
I repaired a few Philips scopes but older models (including two 1 GHz sample scopes) most problems: switches and leaking high voltage cascade caps.
The old blue Philips caps from the 70's/80's are not the greatest in my opinion. From all my gear I only had a Tek SA that needed all caps replaced,  In a HP LCR meter  al caps smaller as 10 uF were bad but for the rest no problems, except for Philips gear . There I hade to replace many caps, but that are most silverface models, so older then your scope.

Wow, you have a very nice and usefull website. Thank you for sharing your research. It's too bad that it wasn't on the first pages when I googled for ESR. If i had not ordered the new caps before you wrote, I maybe would have ordered the parts for building an ESR meter. Perhaps that will be a future project.

I use a pipe-socket/rohrschluessel.   http://upload.wikimedia.org/wikipedia/commons/7/7a/Rohrschluessel.jpg

The best way to test transistors is measuring the voltage in situ. Vbe must be around 0.6-0.7V or 0.35 for germanium. And with a scope you see amplification or switching. And if voltages are good, the transisator is working.

The pipe-socket/rohrschluessel looks like a usefull tool, I have to look for it in my local hardwarestore. Do you also say "Rohrschluessel" in the Netherlands or did you translated it for me into german?

Nice documented restauration. You have a good eye for detail.

Thank you very much!

The white Mini-Latch style connectors used in newer Philips scope models are worth exercising as part of a restoration. They also tend to cause weird problems simply due to making poor contact.

I think my scope does not have these sockets, because there were no latches on them. I only found this three different types:


Orange.


Black.


And white.


So I think, if all caps are okay, all testpoints in the circuit are ok and the voltage levels on all the low voltage rails are high enough, the problem has to do with the high voltage circuits.
I think either the acceleration voltage is too low or the blanking voltage is to high when it shouldn't blank.
But I assume that the acceleration voltage is too low because the waveform I measured of the circuit which drives the blanking was like it is displayed in the schematics.

Here are the parts of the schematics that have to do with the -1500V rail:





I could not find any shorted caps or diodes in the circuits. Maybe the five fold voltage multiplier is damaged? The problem is that I cannot check the single components of the multiplier because its in a box filled with some (isolation?) material:



So I thought I could maybe hook it up to the 12V squarewave of the transformer and measure the output disconnected from the crt with my scope. Is that a bad idea? I would do it like this:



Does anybody know what ciruit could be inside this "black-box"? Could it be an Cockcroft-Walton Multiplier like Dave explained in that video: ?

http://youtu.be/ep3D_LC2UzU

I have one last question. Do I have to care about x-Rays when the scope is turned on and the body housing is open? The CRT is partially covered with metal:

[Heribert Hechtersheim]

I connected the 5 fold voltage multiplier to the 24Vptp and the 40Vptp sqarewave from the transformer. Here is what I measured at the output:

	24V ptp	40V ptp
10M input impedance	8V	25.4V
1M input impedance	1.3V	4.9V

Can you detect with this values if the voltage multiplier is damaged or not?

[PA4TIM]

Problem with those cascades is the often the caps that leak to much under high voltage. So testing under low voltage will not be enough. Do not fire it up without a load because the voltage can rise far above the SOA. But to much load and it will get to low. These cascades often go bad on old Philips. In the older ones they use an open cascade and there I replaced the doorknob capacitors because they leaked and that pulls the voltage down. It is just a classic combination of R and C but I do not know witch one. That is also not that important as long as you get the right voltage, it is more a static fied as that it must deliver  a lot of current. A read about a Philips repair that was fixed by placing an modern inverter but I can not rember what kind he used and what voltage. But If you know the input voltage and output you can make a cascade like Dave did.  I altered the kathode HV in a HP calculator because the new tube needed an other voltage. I only had to change the value of the resistive feedback divider.  But be carefull to keep within the SOA of the parts used in the whole chain.

But is your -1500V OK ? The -1500 is the base that must be OK and the multiplyer makes the accelleration voltage out of the 1500V.
If intensity and focus are not good most times there is a problem with the cathode voltage (and then also the 8KV5)
If you do not have a HV probe you can often find a resistor in the feedback tthat is not to big (remember you place the impedance of your multimeter parallel to that resistor so correct your measurement) and measure the voltage over it. Then with the calculated current and the value of the other resistors you can calculate total voltage.


We call it a pijpsleutel, a pijp is a rohre and sleutel is schlussel, so we use the same word.

[Heribert Hechtersheim]

Thank you for your answer.

But is your -1500V OK ? The -1500 is the base that must be OK and the multiplyer makes the accelleration voltage out of the 1500V.

I don't know.

If intensity and focus are not good most times there is a problem with the cathode voltage (and then also the 8KV5)
If you do not have a HV probe you can often find a resistor in the feedback tthat is not to big (remember you place the impedance of your multimeter parallel to that resistor so correct your measurement) and measure the voltage over it. Then with the calculated current and the value of the other resistors you can calculate total voltage.

Can you show me where the feedback is in the picture of the schematic I linked in the last post?

I measured the voltage across resistor R807, which has a resistance of 1.8M?. It connects ground with the -1500V Rail. It was -431V.
Then I calculated the current. With the 10M? input impedance the total resistance is (1/10+1/1.8 )^(-1)*10^6=1.525 M?
The current is U/R. So the current should be 431V/1.525M? = 282.6 µA

How can I calculate if the supply voltage is -1500V? Can you explain it a bit more detailed please?

[PA4TIM]

OK, I was wrong, I'm to much used to Tek schematics  in your case this is hard to do because that transistor and things like no value for R804. You can not do much further without a high voltage probe. The manual must tell how to check and where to adjust the -1500V. Probably that potentiometer under the transistor.

Do you have a bunch of 10 M or much better 100M resistors to make a high voltage probe ?
Three things are more or less vital for scope repairs, a HV probe, a second scope, a good multimeter  and a accurate stable function generator for calibration.
Ideal is also a  timemarkgenereator  and amplitude calibrator.

First things to do always: check power supplys including HV, then you look at the rest.

[Heribert Hechtersheim]

OK, I was wrong, I'm to much used to Tek schematics  in your case this is hard to do because that transistor and things like no value for R804. You can not do much further without a high voltage probe. The manual must tell how to check and where to adjust the -1500V. Probably that potentiometer under the transistor.

Sorry I forgot to mention that R804 is 820k in my scope. I could not find anything about the HV checking in the manual. I think you can only adjust all Voltages with R823. The potentiometer R801 under the the transistor is for adjusting the brightness. If you are interested in looking at the whole schematic and the service manual, you can find them in my first post of this thread.

Do you have a bunch of 10 M or much better 100M resistors to make a high voltage probe?

I only have 11 1M resistor in stock
Maybe I will order some because I already found some good information about HV probes:

Basics of High Voltage Probe Design: http://www.repairfaq.org/sam/hvprobe.htm
EEVblog #85:
Homemade High Voltage Probe (> 40kV): http://rimstar.org/equip/hv150kvprobe/hv150kvprobe.htm
The secret world of oscilloscope probes by Doug ford:  http://dfad.com.au/links/THE%20SECRET%20WORLD%20OF%20PROBES%20OCt09.pdf

I also found one HV probe on the german ebay I added to my watchlist: http://www.ebay.de/itm/321217522586?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2648

Did you build yourself a HV probe? How did you do it?

Three things are more or less vital for scope repairs, a HV probe, a second scope, a good multimeter  and a accurate stable function generator for calibration.
Ideal is also a  timemarkgenereator  and amplitude calibrator.

I only have a second old HP 1741A scope and a very cheap multimeter. 

[PA4TIM]

Nice video, a good source is also the concept series from Tek. There are two books about probes, one you find here http://www.davmar.org/concepts.html

But that is about scope probes. For the -1500V you need a multimeter HV  probe.

9 x 11M , a 100K trimpot and a 1M restistor in series. The top of the first 11M  is hot, the bottom (1 M) is gnd. Connect a black wire to the 1 M resistor and a aligator clip , make a tip on the 11M top resistor. Put this in a plastic tube and make a probe from it. (isolate the place you put your hand extra with a second tube or so) (if it is just for this repair and only 1500V you can box it and use two good isolated wires with clips so you do not have to touch it at all. (that was my first HV probe, i put silicon hose around good isolated wire and that came out a plastic box so I did not have to touch anything.)

Now you have a voltage divider and you connect the black lead of your DMM to the gnd side where you made the black wire and the red of your meter to the wiper of the trimpot.
The problem is it is not very high impedant so you will have an error because the 1500V also is a high impedance point. But you can calculate that by measuring the resistance if the scope is off and the crt connector on the back is disconnected. Then do the math. calibrating the probe can be done with some known low impedant high enough voltage that will give a usable reading on your meter. (maybe in the scope)

I have two Tektronix HV scope probes and a few medium voltage probes (1500 and 2500V)
For my multimeters I have 2 HV probes. A 20 kV and a 40 kV. (and the one I made myself using  nine 100 M resistors, a 1M trimpot and a 10 MOhm resistor.

I will try to look in the manual later tonight

[Heribert Hechtersheim]

I purchased 100 10M resistors(±1%) by an ebay auction for 3€
Since I do not have any reliable low impedant high voltage source I decided to build the probe without a trimpot. Here is my schematic:



The resistance of the paralell resistors is (1/10 + 10/11)^(-1)M = 991k
The ratio of the second series resistance to the total resistance is 0.991/100.991 ? 0.01 which is also the ratio of measured voltage to real voltage. This means that when I measure a 100V voltage my multimeter should display 1V.

I first looked for a tube I could use as a container for my resistors. I found this pvc tube in the basement:



Then I soldered the resistors on each other. I used a needle as a tip. But it was not so optimal because the solder does not stick very well on the metal. Therefore I pushed the lead of the resistore through the hole of the needle and twisted it very tight. After that I soldered it and then the junction seemed to be robust.



After that I put heatshrink tubing all over the leads and the resistors except the tip and the end. I left it uninsulated because I read that it is good to have the current forming an arc there if the voltage is to high so the multimeter does not get damaged. I also soldered the wires to the ends. For the ground connection I used a black alligator lead. For the connection to the multimeter I used a TV coax cable.





Finally, I sealed the ends of the tube with hot glue. In addition, I had to build my own connector to my multimeter with hot glue which looks a bit messy.




I measured the voltage of the negative high voltage rail. I marked the point where I measured on the schematic.



 The multimeter showed me -15.27V, so I think the rail is fine or do I have to do more calculations? Can I now presume that the voltage multiplier is broken?

[PA4TIM]

Nice job. I think you are right, the -1500V is a low impedance point so -1527 could be correct. Most times the optimum is a certain value but in practice they work still  plus/min 100V Volt. Not that I advise you to do that. But just for diagnose .