Faulty Analogue scope (Crotech 3132) - suggestions on where to look next - Page 1
Forum section: electronics / testgear

author: timbos
author_rank: Contributor
timestamp: April 21, 2014, 04:57:41 pm
content: Hi,I originally posted this in the beginners forum, but was advised to try and post it here...I've got an old Crotech 3132 20MHz scope that stopped working yesterday. After having it switched on for a couple of hours, it's released an acrid burning smell and isn't producing a trace on screen. I've taken the covers off and think that I've diagnosed that there's a fault on the EHT board (see page 4 of the attached diagram):http://www.tjstephens.com/images/Crotech3132 _circuits(wm).pdfObservations:I've got the circuit diagram for the board after trawling the web, and I think that the board consists of an oscillator (a Wien oscillator?) and a two-stage (Cockroft-Walton ?) voltage multiplier. That's where my understanding kind-of ends.The NPN transistor Q601 is on a heatsink and was very hot when I first checked it after unplugging the scope (and making sure that the various caps on this board were discharged).The two resistors (R613, R614) on the output stage of the transformer have clearly been very hot. Their paint is peeling and the surfaces are charred, although they measure 5.8K and 6.1K, so they're still performing at about specification.Can you suggest where I might look next? I'm not sure how to confirm whether these observations are a symptom or the actual fault (my guess is that something else may have failed upstream)? Is there anything else that I should be looking for?Summary of Advice so FarCheck/replace the caps in the PSU board (I'll look at getting some caps, but I don't have enough suitable ones in my parts bin at the moment).Look for blown components everywhere (I have, and I don't see any obvious ones apart from the two resistors mentioned above).Check voltages elsewhere on the board (I'm not hugely keen to try this, because I got a little puff of smoke the last couple of times that I applied power to try and see what was happening).

author: N2IXK
author_rank: Frequent Contributor
timestamp: April 21, 2014, 06:27:42 pm
content: Those resistors are on the secondary side of the HV transformer, so I would be looking for shorted caps/diodes in the multiplier, or shorted components in the CRT circuit (including the CRT itself). The zeners on the blanking board would be prime suspects.If you lift the 2 burned resistors, does the oscillator run OK? Do you get high voltage out of the transformer?

author: timbos
author_rank: Contributor
timestamp: April 21, 2014, 06:40:45 pm
content: OK, I've desoldered the EHT board from the scope and then powered the thing up again. The regulators to make 5V, 12V and 24V on the main PCB all produce voltages within spec. Unfortunately I only have the schematic for the scope (not the manual), so I don't know what voltages should be on some of the other test points around the various boards.On the EHT board, there are 4 big (0.1uF) polystyrene caps that look like they're not original. They sit over the resistors that have obviously overheated, and whilst a couple of them appear to be a little marked on the outside by the heat from the resistors, they appear to be working (I'm testing them with the Ohms range on my multimeter, and the resistance measurement increases for a few seconds and then indicates open circuit).I've desoldered one end of C601 (which appears to be a Ceramic disc cap), and it seems to be open circuit (no charging up). Should I expect this, or might this be a culprit?Additionally, the board shows a little sign of scorching around R604, which is in parallel with this capacitor. R604 still measures the correct resistance.The Tant cap on the schematic (C606) looks for all the world like an electrolytic, but has a little yellowing on the side. I can't tell whether this is because it's been smoked on by the stuff, or it's overheated.QuoteI've not tried that (but I will). The burned resistors measure to have about the right resistance though (which is why I thought they may be a symptom rather than a cause).Going to check the various diodes and then see if I can find a bench supply to run the board outside of the scope.

author: oldway
author_rank: Super Contributor
timestamp: April 21, 2014, 07:09:51 pm
content: Quote from: timbos on April 21, 2014, 06:40:45 pmDon't do this, you risk to kill the EHT transformer by over-voltage.It can't work without EHT regulation and without load.

author: N2IXK
author_rank: Frequent Contributor
timestamp: April 21, 2014, 07:23:12 pm
content: Yes, the burned resistors are a symptom. They are burned because they dissipated too much power.This would be caused by excessive current draw through them, from shorted/leaky diodes or caps in the multiplier stack or an excessive load on the HV supply. All the power on the HV rail is ultimately drawn through those 2 resistors....Again, if you remove the 2 burned resistors (R613 and 614), you effectively disconnect the entire HV multiplier and everything downstream from the HV transformer and oscillator/driver. With those resistors pulled out, do you get high voltage AC out of TR601 (pins 5 and 7)? If so, you have a short downstream of the transformer somewhere.Next check would be to reinstall the resistors, and disconnect the output lead from the EHT board (Terminal 10-4, one end of R615). This is the EHT output, and should give a high negative voltage if all is working well.

author: timbos
author_rank: Contributor
timestamp: April 21, 2014, 09:38:31 pm
content: I've checked the Zeners on the blanking board and the EHT board; they measure 0.7v drop in forward bias, and open-circuit in reverse. The diodes in the HV voltage multiplier stage on the EHT board seem to be open circuit, but I haven't found specs for them on the net (part-number RY16LC) so my current assumption is that their forward voltage drop is more than my multimeter can push.I'm going to see if I can source some high value resistors to make a voltage probe to look at the output from the EHT board - my multimeter will only go to 1kV and I doubt that's enough.

author: tautech
author_rank: Super Contributor
timestamp: April 21, 2014, 11:54:32 pm
content: Your assumption about the HV diodes is correct

author: TonyPh
author_rank: Regular Contributor
timestamp: April 22, 2014, 09:20:34 am
content: You can, to an extent, check those HV diodes by using a power source higher than the expected Vf of the diodes.30v ------>|--------\/\/\/\/\/\-----------0vThe resistor should be high enough that you don't allow too much through the diode, I chose 10k. Measure across the diode with your DMM to give you the forward voltage drop.Reverse the diode and measure across the resistor to check for any reverse leak. These are the things I did with the BY187 in my Gould scope recently, which incidentally meaured 17v Vf.Obviously this doesn't give you absolute confidence as the diode could be breaking down at high voltage but it is a start.The post you made on Vintage radio now has some detail about the high voltage in a reply given by WME_Bill who has stated is -1.8kv which will need a high voltage probe of some kind, he has also pointed out a few other voltages for you.

author: timbos
author_rank: Contributor
timestamp: April 23, 2014, 09:38:45 pm
content: Thanks for the suggestion regarding measuring the diodes. I'll try that.QuoteYes, I saw that post. I'm in the process of sorting out some resistors to make a HV probe so I can try the things he's suggested.

author: timbos
author_rank: Contributor
timestamp: June 22, 2014, 09:06:37 pm
content: I've finally managed to get time to go back to this scope and try some further diagnosis.On the advice of tautech, I connected up my bench supply (which only goes to 18V) through a 100K resistor to each of the diodes. They each measure about 8.5V in one direction and 18V in the other (i.e. in reverse bias). I'm guessing that this means that they're not damaged.On the advice of a chap on a vintage radio forum, I disconnected the HT output of the board, and don't see the voltage on the test-point (TP17) return to its specified -1.8kV, so I'm assuming that the fault remains on this board.The op-amp in the oscillator is running at about 30kHz (I pulled another scope out of a skip a couple of weeks ago), 5V pk-pk. The output from the transformer is only getting to 90VAC (rather than the ~450VAC that I was expecting), which suggests something is wrong with that side of the circuit. I measured the resistance of the transformer windings, and the input side measures 1R, but the output side is 150R which seems a little high. Is this normal?The capacitors on the multiplier circuit don't appear to be shorted out when I test with my DMM. Is there a sneaky way to check them with stuff that I might have here (i.e. not an ESR or LCR meter).Any suggestions on where to look next gratefully received.

author: tautech
author_rank: Super Contributor
timestamp: June 22, 2014, 09:49:41 pm
content: Is the 30KHz signal getting to the base of Q601?Tantulum C606 could be a suspect especially if of the bead type... always check bead tants.R613 614 have drifted enough that they will need to be replaced but leave them there for now.Check throughout the Blanking PCB and other boards that are feed HV for faulty components.Probably just a resistance check might do to find low resistance paths.Remember they will be reversed biased paths(-1.8KV)

author: timbos
author_rank: Contributor
timestamp: June 28, 2014, 04:52:54 pm
content: Quote from: tautech on June 22, 2014, 09:49:41 pmYes. There's about 1V pk-pk on the base, which is amplified up to about 40Vpk-pk on the emitter.Is the resistance of the secondary winding on the transformer (150R vs. 2R for the primary) within spec? I think I expected that to be lower.Quote from: tautech on June 22, 2014, 09:49:41 pmThe burning smell and smoke definitely came from the HT board, and I've disconnected the HT out for this board, so if the fault were elsewhere, I'd expect that the quadrupled would be producing 1.8kV. It's not, so there's definitely something wrong with this board.Question is: transformer or caps?

author: timbos
author_rank: Contributor
timestamp: June 28, 2014, 05:50:52 pm
content: Quote from: tautech on June 22, 2014, 09:49:41 pmAm I looking for short-circuit? The cap seems to test OK.QuoteI had a quick probe around the blanking PCB, and C703 seems to be short circuit. My reading of the circuit diagram is that this will provide a direct route to ground for the HT feed from the EHT power supply, which would explain why the resistors overheated. C703 is an El-Ci-Ar FMD2 with radial leads. I think that this is some sort of polypropylene film cap, but i've not found the exact part on the net yet.So Ithinkthat I've found the cause of the fault. Now I've got to sort out replacement parts.Can anyone see if I've missed anything -- particularly regarding the EHT power supply board?