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Old 4th Jun 2019, 5:50 pm   #61
OldTechFan96
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Default Re: Hameg HM307-3 Cleaning and Repair

Thanks for the explanation, wme_bill. I'll have to get my head around that!

I don't have a source of the HV transistor you mentioned. I do have another BD157 or two just in case the others have went bad. I won't install them yet.

I have been going through the measurements you recommended. R306 is connected correctly.

Junction of R316/R318/R309: +1.4V

Junction of R318/R309: -1.8V

IC voltages:

Input V of pin 2: 0V

Input V of pin 3: 0V

Output V of pin 6: -3.5V. Varies with HV adjustment.

The fact that the input voltage on pin 2 and 3 is 0V is concerning.

R306 removed:

Input V of pin 2: 0V

Input V of pin 3: -1.9V

Output V of pin 6: -5.2V. Varies with HV adjustment.

I won't rebuild the PSU. It could make things worse. Making voltage measurements with the scope on is the way to go but because of the high watt resistors getting hot I have been keeping the scope turned on a little as possible. When checking components out of circuit I have been making sure that they are installed the right way around and soldered properly.
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Old 4th Jun 2019, 9:03 pm   #62
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
R306 removed:

Input V of pin 2: 0V

Input V of pin 3: -1.9V

Output V of pin 6: -5.2V. Varies with HV adjustment.
That seems to back up Bills observation that T301 is leaky. The only way you should get a negative voltage at the non-inverted input to the 741 IC is if T301 is turned on. In that case there will be a current flow to ground providing a ground reference to the rectifier. The opposite side of the rectifier then becomes negative at around -1kv output relative to ground. With R306 disconnected, T301 should be turned off. Instead it appears to be leaking slightly more current allowing the voltage on the -1k rail head further into the negative which in turn shifts the voltage at the wiper of R318 slightly further into the negative.

WME_Bill is right. The 200v rating of the BD157 is evidently not sufficient to survive the possible transient voltages in the circuit and so failed under stress. The +163v noted on the circuit at the collector of T301 is the nominal running voltage, but the simulator confirms a much higher transient does occur for a fraction of a second at power up. The MJE13003 sounds like a good option.

BTW, RS sell a KSE13003 which appears to have similar characteristics.

Last edited by WaveyDipole; 4th Jun 2019 at 9:33 pm.
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Old 4th Jun 2019, 9:23 pm   #63
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by OldTechFan96 View Post
Junction of R316/R318/R309: +1.4V
Junction of R318/R309: -1.8V
What is the EHT voltage when you took these readings. I assume about -700v. If so they look about right.
Quote:
Input V of pin 2: 0V
Input V of pin 3: 0V
Output V of pin 6: -3.5V. Varies with HV adjustment.
The voltage at p3 will be only millivolts, as the OpAmp has a gain of 1000, set by ratio of resistors 2M2 to 2K2. But as output p6 varies, suggest OK, but not negative voltage. Should be positive, to switch on the transistor , as WaveyDipole suggested, so as to increase the current through the transistor and thus lower the voltage across it and thus increase the voltage on the EHT rail..

Removing R306 will have very odd effects, as the EHT rail will go more negative, which is why p3 goes more negative than in your first quoted reading.
While as the transistor base now floats, the collector will go off anywhere and as it is suspect anyway, we just cannot guess. I suggest put R306 back, but try connecting base quickly to chassis and see what happens to the pin3 input and p6 output.
Have you replaced the OpAmp. I rather suspect that should be tried.
What views from WaveyDipole? Are we narrowing the fault condition down. It seems we have an original fault and some consequential failures.
Just seen WD's posting #62. We both seem to agree. I always feel happier when a greater expert agrees with me.
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Old 4th Jun 2019, 10:53 pm   #64
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by WME_bill View Post
Just seen WD's posting #62. We both seem to agree. I always feel happier when a greater expert agrees with me.
wme_bill
Well I don't consider myself by any means an "expert" and I am continually learning from others! Your comment about the base floating leading to unpredictable behaviour of the transistor is a case in point and I guess we cannot really be certain of anything except that the behaviour of the transistor is not as it should be. In my mind the jury is out on the 741, but with the relatively low cost of them I would be tempted to throw one into the next order and replace it anyway just to rule it out. I definitely wouldn't waste time or good parts trying another BD157. Cricklewood Electronics is a possible source for a BF459, but given that we don't know the reason for the original failure, I can see that your suggestion to try something with an even higher breakdown voltage, such as the MJE13003, makes sense.

One possible source for a genuine MJE13003 is this eBay seller who is generally considered to be trustworthy:
https://www.ebay.co.uk/itm/MJE13003-...r/382825942544
However one can get 5xKSE13003 cheaper from RS!
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Old 5th Jun 2019, 12:16 am   #65
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Default Re: Hameg HM307-3 Cleaning and Repair

I agree that the next step should be to replace the BD157 and the 741c op amp. Maybe a new socket.

Just been looking at the transistors offered by ESR. They stock BD232 transistors with a Collector-Base Voltage of 500V and a Collector-Emitter Voltage of 300V. Or a BF259?

Are any of these worth trying?
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Old 5th Jun 2019, 10:04 am   #66
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Default Re: Hameg HM307-3 Cleaning and Repair

If it helps at all, 'Vintage Parts' look like a cheaper option for BF459 - I've never had a problem with anything from there, but otherwise I have no connection.

http://markhindes.easywebstore.co.uk/BF459_A1KIG6.aspx
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Old 5th Jun 2019, 5:38 pm   #67
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Default Re: Hameg HM307-3 Cleaning and Repair

Thanks for that Goldie99; I've never heard of them before. I have bookmarked them.

I've got some op amps and a few transistors to try. If they don't work I'll patiently wait for the MJE13003 that Wavey Dipole has kindly sent out.
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Old 5th Jun 2019, 8:06 pm   #68
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Default Re: Hameg HM307-3 Cleaning and Repair

I've installed the new 741 op amp, socket and tried both new transistors. The -1000V rail is still too low!

With BD232:

-1000V rail: ranges from -960V --760V.

C of T301: 400V

Input V of pin 2: 0V

Input V of pin 3: 0V

Output V of pin 6: -5.3V.

With BF259:

-1000V rail: ranges from -960V --760V.

C of T301: 310V

Input V of pin 2: 0V

Input V of pin 3: 0.65V

Output V of pin 6: -5.3V.

What should the voltage be at the inputs of the IC?

Does anybody wonder if something connected to the -1000V rail is pulling it down?

I'm going to lift one end of R305 and see what I measure.

Last edited by OldTechFan96; 5th Jun 2019 at 8:07 pm. Reason: More info
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Old 5th Jun 2019, 9:03 pm   #69
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by OldTechFan96 View Post
Does anybody wonder if something connected to the -1000V rail is pulling it down?

I'm going to lift one end of R305 and see what I measure.
Is this with the CRT circuit connected or not? As far as I can tell, Z212/R237 on the CRT circuit is the only point that the -1kv line connects with. I initially suggested that you disconnect at this point so as to determine whether the CRT circuit was unduly loading the PSU down, but after doing so you reported that the output from the PSU was still low, which pointed to a problem with the PSU itself. Now that you have replaced the IC and the transistor, it might be worth re-connecting it to give the PSU a load to regulate and measure again. If it is still low either way, then something else must be going on.

What voltages do you measure across the following individual resistors: R310, R312 and R313?
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Old 5th Jun 2019, 9:42 pm   #70
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Default Re: Hameg HM307-3 Cleaning and Repair

The measurements in post 68 were taken with the the CRT circuit connected to the -1000V rail.

Voltage across:

R310: 0.241V

R312: 422V

R313: 422V
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Old 6th Jun 2019, 8:02 am   #71
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Default Re: Hameg HM307-3 Cleaning and Repair

The voltage across R310 should read slightly more than half of what you read across one of the 1.5MΩ resistors. I would say from that reading that capacitor C311 is shorted.

Last edited by WaveyDipole; 6th Jun 2019 at 8:25 am.
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Old 6th Jun 2019, 8:58 am   #72
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Default Re: Hameg HM307-3 Cleaning and Repair

BTW, I have just tried that scenario on the simulator model to see what conditions it would generate. It showed an output of -790v and the collector voltage at T301 oscillated between +200 and +400v. Furthermore, there was a negative voltage at the non-inverted input of the 741 op amp, although not quite as low as that reported. However, the conditions do seem to approximately correspond to that reported.

If this is indeed the problem, then the output returns to -1kV once the capacitor is disconnected from the circuit which should suffice for test purposes. In the long term it would naturally need to be replaced.
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Old 6th Jun 2019, 10:26 am   #73
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Default Re: Hameg HM307-3 Cleaning and Repair

I have removed C311 and it tests fine. It is not shorted. With it removed the -1000V rail still reads low.

I have also checked C312 and C13 and they are both fine.
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Old 6th Jun 2019, 11:55 am   #74
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Default Re: Hameg HM307-3 Cleaning and Repair

Then there must be another explanation for that very low voltage reading across R310/C311. Assuming that your probes made good contact, then such a low voltage would indicate to all intents and purposes a short or at least a very low resistance under running conditions. Back in post #40 wme_bill mentioned that resistors can sometimes change value under high voltage stress conditions. Maybe R310 is OK when examined using a low test voltage, but breaking down under running conditions where much higher voltages are present. I would suggest replacing that resistor.

Last edited by WaveyDipole; 6th Jun 2019 at 12:01 pm.
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Old 6th Jun 2019, 12:41 pm   #75
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Default Re: Hameg HM307-3 Cleaning and Repair

I replaced R310 last week because it had drifted a bit high. I have replaced it again and we are back in business! The -1000V rail is now what it should be. You were both correct about the HV breakdown.

This shows that the new BF232 and op amp are working correctly.

If I remember correctly, the last thing I did before the PSU fault was replace a zener in the CRT section. We were also investigating the lack of brightness of the CRT.
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Old 6th Jun 2019, 3:37 pm   #76
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by OldTechFan96 View Post
I replaced R310 last week because it had drifted a bit high. I have replaced it again and we are back in business! The -1000V rail is now what it should be. You were both correct about the HV breakdown.

This shows that the new BF232 and op amp are working correctly.
That’s good to hear. You mention that you had already replaced the resistor and it was this replacement that failed? If you have replaced this with another from the same batch, then bear in mind that although the current running through it is quite small, the composition of the replacement may not have been suitable for handling the voltage across it, in which case the second replacement could fail again in a similar manner. My estimate suggests that the voltage across it should be in the order of 215v and you would want a comfortable margin, so maybe one rated for 350v or higher.

Quote:
Originally Posted by OldTechFan96 View Post
If I remember correctly, the last thing I did before the PSU fault was replace a zener in the CRT section. We were also investigating the lack of brightness of the CRT.
Yes, how is that looking now?

Last edited by WaveyDipole; 6th Jun 2019 at 3:56 pm.
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Old 6th Jun 2019, 5:26 pm   #77
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Default Re: Hameg HM307-3 Cleaning and Repair

The resistors are from a Vellemen resistor pack. The working voltage given is 250V. 188V was measured across R310. I agree that this is a bit close.

Z352 had -900V on each side. Checked C352 (1uf 25V tantalum) and its resistance varied between 10KΩ and 20KΩ. Replaced it with a ceramic capacitor and the voltage on the cathode of Z352 was closer to what it should be.

The voltages on T354 were higher than expected. Measured 140V rail and it was a bit high at 148V. The output of the rectifier was also high at 198V. I noticed that C305 was leaking electrolytic on the PCB. It was removed and the board cleaned up with IPA. R304 is good at 1200Ω. This is one of the hot resistors mentioned earlier in the thread.

I replaced it with a 47uf 450V radial capacitor I had around. I probably will replace it in the future with a 250V axial capacitor.

This only brought the 140V rail back to 146V, still a bit too high. I noticed in my notes that in the past the rail measured 133V. I wonder is something attached to the 140V rail is drawing less current? 140V goes to the astigmatism control and the Y final amp.

Attached are some diagrams with some more voltage measurements.
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Old 7th Jun 2019, 12:05 pm   #78
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by OldTechFan96 View Post
Z352 had -900V on each side. Checked C352 (1uf 25V tantalum) and its resistance varied between 10KΩ and 20KΩ. Replaced it with a ceramic capacitor and the voltage on the cathode of Z352 was closer to what it should be.
Sounds like someone replaced it at some point with an unsuitable part. There is bound to be 33v across the zener diode and therefore also across the parallel capacitor, which is why the circuit diagram indicates that a capacitor rated for a minimum of 40v should be used. Tantalum capacitors are very particular about excessive voltage being applied to them and tend to fail short.

Quote:
Originally Posted by OldTechFan96 View Post
The voltages on T354 were higher than expected. Measured 140V rail and it was a bit high at 148V. The output of the rectifier was also high at 198V. I noticed that C305 was leaking electrolytic on the PCB. It was removed and the board cleaned up with IPA. R304 is good at 1200Ω. This is one of the hot resistors mentioned earlier in the thread.

I replaced it with a 47uf 450V radial capacitor I had around. I probably will replace it in the future with a 250V axial capacitor.

This only brought the 140V rail back to 146V, still a bit too high. I noticed in my notes that in the past the rail measured 133V. I wonder is something attached to the 140V rail is drawing less current? 140V goes to the astigmatism control and the Y final amp.

Attached are some diagrams with some more voltage measurements.
Those two rails are unregulated so the voltage will vary depending on whether there is a load connected or not, and what the mains input supply is doing at the time. AFAIK, mains voltage in the UK is 230v +10%/-6% so the voltage on the transformer secondary can be expected to vary by a similar amount. As a result the rectified DC will also vary, so some variance is to be expected. You have cleaned up the leaking electrolyte and replaced the capacitor so if C306 and C307 are OK then it sounds like you are good to go for now.

With regards to R304 getting hot, the fault in the Y-preamp may have been a contributing factor, stressing the final stage and putting excessive demand on the supply. That might have been sufficient to make R304 run a bit hot and drag the voltage down. However, faulty smoothing capacitors can also cause such problems. With repairs having been made to the Y-amp and preamp, it one might expect it to run a bit cooler now. Of course, with a 4W resistor some heat under running conditions is only to be expected, but if it is still running excessively hot, you might also want to make sure that C307 is not electrically leaky.

With regards to the voltage reading at the emitter of T354, this will vary somewhat with the input voltage, but will also very much depend on the position of the 100k preset R369. The diagram shows circa +85v, which is approximately what might be expected when correctly adjusted, but 100k provides quite a wide range of input to the base of T354 so the voltage at the emitter (i.e. to pin 8 of the CRT) will also vary considerably as the preset is adjusted. I imagine that the preset is positioned close to one end of its possible travel? May also be worth checking that R370 is not gone high. For now it might be sufficient to simply adjust the preset to obtain approximately +85 volts at the emitter, or just leave it as it is. Once you have a running trace and can display a square wave, you will then be able to adjust this control visually for an evenly spaced trace.

It does sound like you are making good progress. Has the brightness of the trace improved now that you have replaced that tantalum capacitor and now that the voltage across Z352 is closer to what it should be?

Last edited by WaveyDipole; 7th Jun 2019 at 12:12 pm.
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Old 7th Jun 2019, 5:16 pm   #79
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Default Re: Hameg HM307-3 Cleaning and Repair

I've checked all of the smoothing capacitors in the PSU (both HV and LV) and they are not leaky (Hunts tester) and show low ESR.

I tried the brightness setting procedure and the level of brightness did increase slightly. Unfortunately the trace is still barely visible in normal lighting. Lots of ripple I think!

wme_bill suggested in post 46 that low CRT emission manifests itself as low contrast and blurriness. Despite being dim, the trace can be adjusted to be quite sharp! A glimmer of hope for the CRT?

The ripple in the trace is another problem to diagnose. The main problem at the moment is the lack brightness from the CRT. I'll start checking the CRT circuit. Especially around the blanking circuit. What does this do?
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Old 7th Jun 2019, 6:53 pm   #80
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Default Re: Hameg HM307-3 Cleaning and Repair

The voltages measured on T352 (BF440 PNP) are all -870V. It tests good using the diode check on my DMM. With the component tester I get curious results. In my experience, if the tester shows anything but a transistor the transistor under test is bad. T352 could be the cause of the low brightness.

I'll have to look for a substitute.

See attached photograph.
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