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Old 28th May 2019, 3:58 pm   #41
WME_bill
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Default Re: Hameg HM307-3 Cleaning and Repair

HM307
Source of high voltage transistors. I forgot to say that one of the pair of MJE13002-3 from a failed fluorescent light bulb is much the best source of high voltage medium power transistors, centre collector, TO126.
Serve as replacement for MJE340 or BF257, the traditional one for this use, and for your BF459 in the EHT regulator or BF458 in the amplifier output stages. If you have to buy them might even be cheaper.
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Old 28th May 2019, 4:24 pm   #42
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Default Re: Hameg HM307-3 Cleaning and Repair

I replaced TR301 with a BD157 as they looked to have similar characteristics. I was able to adjust the -1000V rail once again. If the BD157 turns out not to se suitable I can sub one in from my other scope.

The BF458 did not seem seem to get warm in operation. In fact it seemed to be cold.

I performed the CRT brightness adjustments as outlined in a post by WavyDipole. This was done in the dark to make sure that things were spot on. With the brightness set as best as I could get it the results were disappointing. With the intensity set at max and the room light being on the trace was very faint; practically unusable.

I hope that the 3RP1A CRT is not duff and low emission. Knowing my luck it is dead!

Could this be a problem elsewhere? I've checked the intensity pot and it is good. It does read a bit high though at 565KΩ.
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Old 28th May 2019, 9:51 pm   #43
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by WME_bill View Post
Hameg HM307.
I attach a scan of the missing page to save you searching.
I attach a Simple FET tester from Wireless World 1972 on measuring Idss with your Avo.
Bill, thank you for those. I have saved them both for future reference.
As you say, peculiar for the manufacturer to omit pages from their manual!

Quote:
Originally Posted by OldTechFan96 View Post
The BF458 did not seem seem to get warm in operation. In fact it seemed to be cold.
I think you mentioned that it tested OK out of circuit. Your substitute now works so evidently the BF459 was at fault, but a curious mode of failure. Perhaps as Bill surmised, the failure occurs as it warms up and maybe a junction goes o/c.

Quote:
Originally Posted by OldTechFan96 View Post
I performed the CRT brightness adjustments as outlined in a post by WavyDipole. This was done in the dark to make sure that things were spot on. With the brightness set as best as I could get it the results were disappointing. With the intensity set at max and the room light being on the trace was very faint; practically unusable.

I hope that the 3RP1A CRT is not duff and low emission. Knowing my luck it is dead!

Could this be a problem elsewhere? I've checked the intensity pot and it is good. It does read a bit high though at 565KΩ.
I don't think the pot resistance being a bit high would make much difference in this instance. Its just acting as a simple potential divider. Sorry, but I don't know enough about CRTs to comment on whether this one might be low emission, but as for thoughts about a problem elsewhere, thinking about your report that focus voltage was quite high (numerically), is that still the case now you have replaced TR301? If so, then are R353, R354 (390k) Ok and not gone high? Also is R350 (180k) not o/c? Note Bill's comments about the resistor chain and the 33v zenner in post #35 under "Lack of spot". Come to think of it, are you getting 100v across the zenner Z351?

It seems that beam intensity was a recognized issue and in the next version (HM307-4) of the oscilloscope they changed the CRT and revised the supply arrangement to provide -1.8kV. Just quite how much of an issue it was I'm not sure. However, I came across this project where the author used the same CRT:

https://www.labguysworld.com/ES_CRT_DEFLECTION.htm

Most of the images are in subdued light and these look OK, but in the only one that was taken under normal workshop lighting conditions the CRT display is quite faint. It can be difficult to tell from photos as they don't always reflect the true lighting conditions, but perhaps something to compare with.
Attached Files
File Type: pdf Notice_oscilloscope_HAMEG-307.pdf (885.6 KB, 7 views)

Last edited by WaveyDipole; 28th May 2019 at 10:20 pm.
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Old 29th May 2019, 6:55 am   #44
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Default Re: Hameg HM307-3 Cleaning and Repair

Actually this image doesn't look too bad, so I guess at least this level of intensity should be achievable.
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Old 29th May 2019, 8:21 pm   #45
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Default Re: Hameg HM307-3 Cleaning and Repair

Further investigations:

PSU - EHT section:

All resistors good bar R310. Drifted to 829KΩ. Has been replaced.

CRT focus voltages:

R355, R354 and R351 are good.

R352 and R353 have both drifted a little bit. Between 2.5% and 4.6%. Will be replaced. Look to be 0.5W/1W.

Z352,ZPD33, is S/C. Hence, -900V on each side. 100V is measured across Z351.

R370 drifted and was replaced.

R367 is O/C. Hence 97V at E of T354. Replaced.

Attached is a photograph of the CRT in the dark with the intensity set to max. It is barely visable in normal lighting conditions.
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Old 29th May 2019, 9:14 pm   #46
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Default Re: Hameg HM307-3 Cleaning and Repair

Hameg HM307.
I am glad my suggestion of voltage tests insitu is producing results.
The slightly high voltage at the accelerator anode, pin8, emitter T354 due to that fault may slightly increase the brightness. It will give you an oval spot. Not important at this stage.
But you are making progress.

The zener Z352 having failed is significant, as means that your beam blanking has failed, and that might be why you have low brightness.
Set the two preset R356 R357 to allow maximum voltage across the intensity control. When it is working is the time to do proper job with these presets as Wavey Dipole says. You want the grid pin2 slightly negative of the cathode pin3 to maximise brightness. Probably best not to test at the grid itself, as that can make it positive of the cathode.
It is always very uncomfortable working with these high voltage points, but use your high voltage probe, which I hope you have made now, or just move the intensity control pot.

Have you checked the voltage change at the cathode, collector T353 as I suggested. You are looking for 40v change or so, but sitting at -900v.
Beam blanking is a very frequent trouble spot.

The other variations in resistor values of 5% or so are not significant in this circuit, and so is not going to cause your low brightness.

If the tube emission is going, then you will have low contrast rather than low brightness. A line will look sort of woolly.
The later model uses a different tube with a pda, which will give a bright trace at very fastest sweep speeds. But otherwise will not make much difference. A properly operating mono-accelerator tube like this one will give you all the brightness you could want even under bright lights.
I suspect because the original tube 3RP1A is of early 1950's vintage it just got superceded. Often still available on the second user market. Though mono-accelarator tubes are now the main market as they are the cheapest CRT for use in digital scopes.
wme_bill

Last edited by WME_bill; 29th May 2019 at 9:28 pm.
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Old 31st May 2019, 1:37 pm   #47
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Default Re: Hameg HM307-3 Cleaning and Repair

The replacement parts have arrived today. Foolishly I did not think about the physical size of the replacement potentiometer. I bought a 20MM one whereas I should have bought a 16MM potentiometer.

ESR sells one that fits the bill. It's working voltage is only 250V. Would it be worth trying one despite the the much higher voltages that would be connected to it?
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Old 1st Jun 2019, 3:38 pm   #48
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Default Re: Hameg HM307-3 Cleaning and Repair

The aforementioned potentiometer has been installed along with a replacement 33V zener diode.

Now the -1000V rail is low! Adjusting the HV pot won't rein the voltages in. Adjusting the pot yields -923V on the high side and -723V on the low side.

A few days a go we established that the -1000 section as working as it should, especially after replacing TR301. The faulty BF459 was replaced with a BD157.

The collector of TR301 measures 324V. This is double of what it should be! I'll try and find a BF459 to see if it makes a difference.

Any ideas about what is causing this problem?
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Old 1st Jun 2019, 6:53 pm   #49
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Default Re: Hameg HM307-3 Cleaning and Repair

Given that only two things have changed i.e. the pot and the zener, I would be double checking those changes. Is the potentiometer wired correctly? Is the zener the right way around? Is the pot is definitely 470k not 470Ω? Might seem silly but....

If you disconnect the -1kv line where it comes in at R357 and measure it then, what voltage do you get?
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Old 1st Jun 2019, 9:23 pm   #50
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Default Re: Hameg HM307-3 Cleaning and Repair

I've made sure that the diode is the right way around by looking at old photos I've taken and looking at the surface manual. Cathode faces the IC. Pot is fine too. Marked 470K and is definitely connected up OK. I even put the old parts back in and the -1000V rail was still low!

I've lifted the wire link that connects the HV to R357. The voltages present here are the same as in post 48. Since there is no change I imagine that the fault is in the -1000V section of the PSU. There is a BF459 in my other Hameg scope.
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Old 1st Jun 2019, 11:59 pm   #51
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by OldTechFan96 View Post
I've made sure that the diode is the right way around by looking at old photos I've taken and looking at the surface manual. Cathode faces the IC. Pot is fine too. Marked 470K and is definitely connected up OK. I even put the old parts back in and the -1000V rail was still low!
Ok, so everything double checked and OK. Good thinking regarding the photos. Always useful to do that to have something to refer to.

Quote:
Originally Posted by OldTechFan96 View Post
I've lifted the wire link that connects the HV to R357. The voltages present here are the same as in post 48. Since there is no change I imagine that the fault is in the -1000V section of the PSU. There is a BF459 in my other Hameg scope.
So we have confirmed that the output of the -1kV line is still low without the load attached. Furthermore since we have +324v (I presume that's +) at the collector of T301 and <-723v at the output, we do appear to have >1kv output from the rectifier/doubler.

In the previous post you mentioned that you can adjust the voltage to within certain limits although generally the output level was low. The fact that you can make an adjustment seems to suggest that the regulator circuit is actually functioning, but you can confirm that as Bill suggested:

Quote:
Originally Posted by WME_bill View Post
Does the voltage (to chassis) on the base of the BF459 T301 vary as you alter the HV adjustment pot R318. If so, then 741 is likely to be OK.
Then do the same at the collector of T301. That will show both are working.
I would also check that both the 24v and -12v rails into the op amp uA741 are OK. If one of those reference voltages is incorrect or missing, then the op amp may still possibly function, but the output level from IC303 to base of T301 would be incorrect which, in turn, would affect regulation. If either of those rails is wrong, then investigate the relevant supply line, including the electrolytic caps, regulators and rectifiers. That includes C310 and/or C314 as well as C301, C303 and C302, C304. Electrolytic caps can sometimes have intermittent faults that manifest themselves as they warm up.

BTW, I wouldn't be putting that BF459 from the other scope into this circuit just yet. If the original one failed, and the BD157 substitute failed, then there is a risk that the replacement will also fail. We first need to confirm that the BD157 has, in fact, failed, and if so, then why.

Last edited by WaveyDipole; 2nd Jun 2019 at 12:28 am.
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Old 2nd Jun 2019, 11:27 am   #52
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Default Re: Hameg HM307-3 Cleaning and Repair

Further PSU Investigations:

B of T301 (BD157): ranges from -3.38V to -4.11V when adjusting HV pot.

Collector of T301 (BD157): ranges from +315V to +525V when adjusting HV pot.

IC voltages:

Pin 4: -7.2V. The -12V goes through a 3k3 resistor so the -7.2V figure sounds about right.

Pin 7: +13V. Correct.

EDIT: the above measurements were taken with the HV rail disconnected from the CRT section. I thought this would be worth mentioning.

Last edited by OldTechFan96; 2nd Jun 2019 at 11:47 am. Reason: More info
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Old 2nd Jun 2019, 10:43 pm   #53
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Default Re: Hameg HM307-3 Cleaning and Repair

Check the diodes in the rectifier/doubler. The designation EM513 on the diagram suggests a module of some sort and if its potted then a repair might be somewhat more difficult. However, it should still be possible to measure across D301/D302 and D304/D305 pairs. The results should be the same for both pairs. Two diodes in series should test the same as a single diode, just that the resistance or voltage drop across two would be double that of a single. If the two readings vary very significantly (say by about 50%) then one of the diodes in the side with the lower reading is s/c.

I spent a little while this afternoon drawing the circuit in the LTSPICE simulator - simply because I had nothing better to do and hadn't played with it for a while - to see whether I could replicate the problem. For example, the simulator showed 6.8v at pin 4 of the IC, which is close to what you measured. On the other hand I couldn't match the voltages at the base of T301.

Since you seem to have ruled out just about everything else, I tried simulating a shorted diode in the rectifier/doubler. It didn't seem to matter which one I chose, but so long as I only shorted one, the simulator produced a result that might explain what you are currently getting, although there is also a horrible amount of ripple.

I have uploaded a couple of screenshots. The first one shows the PSU working as expected. The yellow trace is the output, while the blue one is the voltage at the collector of T301. The simulator shows around 190v-200v at the collector of T301 so a bit higher than the 163v on the diagram, but the output is -1kV as expected. The difference might possibly stem from the characteristics of the transistor and diodes used for the simulation. Playing with the values of the two parts of R318 to reflect different wiper positions does vary the output voltage as expected.

The second screenshot shows what happens when one diode is shorted. I'm not quite sure how a DMM set to DC volts would behave under those circumstances but those oscillating voltages do seem to average out into the right ballpark.

NOTE: The reason that the amplitude of the AC wave generator is set to 680v is because the simulator expects the peak voltage rather than the 481v RMS voltage shown on the diagram. To derive the peak voltage, the RMS voltage is multiplied by 1.414.
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Old 2nd Jun 2019, 10:52 pm   #54
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Default Re: Hameg HM307-3 Cleaning and Repair

Ripple might account for the horrible trace (bright spots etc) in the picture in post #45.
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Old 3rd Jun 2019, 10:33 am   #55
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Default Re: Hameg HM307-3 Cleaning and Repair

Thanks for the simulation Wavey Dipole. I'll look into that program.

I've checked the EM513 rectifier diodes out of circuit and they all test OK.

Voltage across D301/D302: -625V

Voltage across D304/D305: -621V

I've also made a few PSU measurements which are included with the attached picture.

Buggies, funnily enough, that horrible trace only appeared once! I have not seen it since. All of the large smoothing electrolytics were tested with my Hunts tester and they were found not to be leaky.
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Old 3rd Jun 2019, 2:42 pm   #56
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Default Re: Hameg HM307-3 Cleaning and Repair

Well its good news that the rectifier diodes are OK, but that still means there is a problem somewhere else.

Quote:
Originally Posted by OldTechFan96 View Post
Further PSU Investigations:
B of T301 (BD157): ranges from -3.38V to -4.11V when adjusting HV pot.
I'm puzzled by that, as a negative voltage to the base of the NPN T301 would simply shut it down and current would not flow between C and E resulting in no output. The sim shows between 628mV and 629mV there and the voltages at the inputs to the 741 are between 400-500 microvolts.

Its possible, I suppose, that the 741 OP amp might be mis-behaving in some peculiar way but I would suggest checking everything carefully again including C311 and C312. Substituting both the 741 and T301 together might also be worth a try although the information you have posted in #52 would seem to suggest they are working in principle. Given that this was working prior to power down for some repair work and suddenly does not work again on power up, and we have ruled out any work that was done on the CRT circuit by disconnecting it, this does seem to be looking like a thermal issue somewhere. A cracked track, cracked/dry joint, corroded ground point or something similar.

I also have a question: The two resistors R312 and R313 are marked 1M5B. I read that as 1.5 mega-ohms, but what does the 'B' stand for?

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Old 3rd Jun 2019, 6:20 pm   #57
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Quote:
Originally Posted by WME_bill View Post
Hameg HM307.
Varying EHT. Usual culprit is the chain of resistors controlling the feedback - R313 toR309. They can change value under voltage stress. Change the 3 high value ones I suggest.
wme_bill
OldTechFan96, might be worth considering this as well if you haven't already done it.
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Old 4th Jun 2019, 2:43 pm   #58
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Default Re: Hameg HM307-3 Cleaning and Repair

I'm going to have a good look at the previous posts and do some more troubleshooting of the -1000V rail.

Does anybody think that it would be worth completely rebuilding the -1000V section of the PSU? I'm thinking about all new metal film 1% resistors, new HV caps, new electrolytics, new diodes, a new 741 op amp and a modern equivalent for the BF459. There has been a few failures here so I think that it may be worth doing.

What is T301 and the IC actually doing in the circuit? I guess that it is a form of voltage regulator. How does it do this without being fed -1000V? I imagine that only a fraction of the voltage is going into the IC.
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Old 4th Jun 2019, 3:25 pm   #59
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Default Re: Hameg HM307-3 Cleaning and Repair

Hameg HM307.
Some progress with the EHT regulator. But T301, the regulator transistor must have failed, as it is rated around 200v Vc, and you have up to 500V. It will be acting as a poor quality resistor, so all voltages around it will be very odd.

For the fault remains, as your replacement BD157 has failed. So change it.
But not yet, as it appears it will burn itself out again. You could use a really high voltage transistor like the MJE13003. Have you tried my substitution from an old light bulb, as a cheap and immediatey available source of MJE13003.

I suggest leave the EHT load connected, as that will give the regulator something to regulate down to and will stop the HV line going higher. As it is so low, it is unlikely to damage any other parts of the circuit.

The voltages suggested by Wavey Dipole in his simulation look about as expected.
But what about the drive from the OpAmp output pin6 to the transistor through R306, 1k. I wonder if this is operating correctly. That 1k resistor is connected at both ends, is it?

I expect the base current of T301 to be say 30ua, for a collector current of 1ma and gain of 30. So the voltage across the 1k should be 30mv - almost too small to measure sensibly. But if it is muh higher, then that willreinforce the diagnosing that the BD157 / T301 has failed.

Have you looked at the voltages around the lower part of the EHT chain. The voltage across R316, 43k, will be about +13v, as junction R316/318/309 will be slightly positive, and junction other end of R318/R309 will be negative, to allow the SetHV Pot to work.

Have you compared the input voltages Pin2 and pin3 to the OpAmp with the output on pin6. Output should vary with movement of the SetHV adjust pot. But be prepared to disconnect R306 the driver resistor, for if the Transistor is failed, the base current going all ways will affect measurements of the output of the OpAmp.
This style of EHT regulator usually gives no trouble and is usual with main powered EHT supplies.
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Old 4th Jun 2019, 3:49 pm   #60
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Hameg HM307
Picking up todays post by OldTechFan.
1. I'd suggest not rebuilding. There is a single major fault somewhere. We just have to find it, as then the whole machine will spring to life.
Do remember my comment about removing each component in turn, testing it and soldering it back. I would hesitate to do that. You have already seen how one transistor tested fine, but when back in circuit, failed under stress. Diodes are the ones where you generally need to lift one end, as often the fault current will show a nice 0.6V forward and you will assume a near by transistor Emitter-Base explains the 0.6V reverse voltage.
Remember a diode or a transistor Base-Emitter forward conduction is about 0.6v, and should be infinite in reverse.

2. The regulator. quite an interesting circuit. Take a bridge rectifier. The negative side of the bridge goes off to the supply. The positive side of the bridge, instead of going to chassis, goes to the collector of a transistor which acts as if a variable resistor (by changing the current it can pass) to drop the bottom of the positive rail before it gets to chassis. It is as if the chassis point is tapped between two resistors, and the bottom one is varied.
So the net output is the main supply of 1176V, with the chassis point tapped in by the variable transistor at +163V. Leaving 1013V between negative rail and the chassis.
A small part of the 1000V across the EHT resistor chain is tapped off by SetHV R318 and fed to the OpAmp. So that it can move each side of zero, the Resistor chain is extended to +24v rail by R316.
The OpAmp is connected as a follower and the output will be close to the net of the two inputs. One input (negative on p2) is about chassis level. The other to Pin3 will vary and thus vary the output on pin6, attempting to keep the voltage on pin3 close to zero as well.
If the 1000v rail goes more negative, then pin3 will go -ve, pin6 go -ve, and this will tend to switch off the transistor T301, which then seems like a greater resistance and thus absorb a greater proportion of the 1173v across the EHT supply, so the 1kv rail will drop back to its set value.
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