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Old 12th Jun 2019, 6:43 pm   #101
WaveyDipole
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Default Re: Hameg HM307-3 Cleaning and Repair

Ok, so with the new OC installed, T353 collector voltage is now switching between about -870v and -900v? Then that is good. Since it was previously stuck at around -870v it is curious that this has had no effect on brightness though. You are sure that the connection to pin 3 of the CRT is good? It seems unlikely that R51 would have gone o/c but might be worth disconnecting pin 3 of the CRT and just checking it though.
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Old 12th Jun 2019, 7:54 pm   #102
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Default Re: Hameg HM307-3 Cleaning and Repair

I've resoldered pin 3 on the CRT base and socket. There is continuity between the socket and the PCB. The socket has been retensioned a bit. R366 is good.

With the auto trigger on the C of T353 is -867V no matter the position of the hor. ext button. The voltage only changes with the AT off.

I wonder if we are coming to the conclusion that there is a problem with the CRT?
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Old 13th Jun 2019, 9:42 am   #103
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Default Re: Hameg HM307-3 Cleaning and Repair

Not sure. Maybe WME_bill or someone else has some further ideas?
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Old 13th Jun 2019, 1:27 pm   #104
WME_bill
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Default Re: Hameg HM307-3 Cleaning and Repair

Hameg 307.
We are making progress, as you have now got the blanking circuit working correctly.
But it is difficult for WaveyDipole or me to advise you when you shower us with lots extra voltage readings when we don't know if you have changed the operating conditions and control settings. I suspect it confuses you as much as us.

I have advised you to stop the timebase working, by switching Auto Trigger off, and having no signal input. With AutoTrigger left on, your meter readings are meaningless.

Now, to go back a bit, can you confirm you have got the EHT regulator working so that the voltage is -1000v at the top of the intensity control, and -900V at the bottom of that control. (within 10%).
Do please use the high voltage probe as I suggested, rather than the top range on your meter. These circuits handle very small currents, and an Avo8 or even a 10M DVM take a lot of current and can alter the operating conditions.
The voltage across the bright-up circuit must be valid, as your Edit note shows the blanking is working when you operate the ExtX control. Leave it at ExtX.

Can you say whats the spot on the face of the CRT doing. I assume you have it centralised (by the X and Y shift controls).
1. Does the brilliance now alter as you move the intensity control.
2. Does the focus control make the spot change from small intense to larger fuzzy. Leave it at fuzzy to avoid burning the screen.
3 Having established this, then does the intensity changes when you switch the Bright-up circuit. (ExtX to Int, not triggered).

Then report what you find.
It could be that the tube is faulty, but we must establish constant known operating conditions and settings of the controls so that we can tell.
Take heart that getting a sophisticated machine like an oscilloscope repaired is a very good learning exercise, and you will be surprised later how much knowledge you have picked up.
wme_bill

Last edited by WME_bill; 13th Jun 2019 at 1:34 pm.
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Old 13th Jun 2019, 2:30 pm   #105
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by WME_bill View Post
Take heart that getting a sophisticated machine like an oscilloscope repaired is a very good learning exercise, and you will be surprised later how much knowledge you have picked up.
wme_bill
I can heartily second that! It was WME_bill (among others) who helped me get a Telequipment D61a going some 6 years ago:

https://www.vintage-radio.net/forum/...ad.php?t=99387

The knowledge that I gained from that experience taught me a great deal and the principles grasped while doing so helped me repair a number of other oscilloscopes over the intervening years.

Last edited by WaveyDipole; 13th Jun 2019 at 2:43 pm.
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Old 13th Jun 2019, 4:03 pm   #106
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Default Re: Hameg HM307-3 Cleaning and Repair

I agree about this being a great learning exercise. I feel more confident than I did 2/3 weeks ago.

I do have some one meg resistors but they are only 0.25W. Would they be suitable?

The -1000V measures 1007V

Intensity control measurements at the middle of R358 (TB off, AT off and no signal):

Fully clockwise: -910V

Fully anticlockwise: -941V

Since the above are not quite right I'll look into what the trace is doing a bit later.
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Old 13th Jun 2019, 5:12 pm   #107
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Default Re: Hameg HM307-3 Cleaning and Repair

Observations for WME_bill (TB off, AT off and no signal):

1. The brilliance does alter from a low state to a high point as the potentiometer is turned. Overall, the trace is unviewable in normal lighting conditions.

2. Yes, the focus control does change the spot from sharp to fuzzy.

3. There is no change in brightness when the hor. ext. is pushed in/out. The spot just moves a bit.

Attached is a photograph of the spot I'm getting.
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Old 13th Jun 2019, 6:45 pm   #108
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Default Re: Hameg HM307-3 Cleaning and Repair

HM307.
Thanks for amazingly quick reply.
1. So tube is getting a variable EHT voltage and responding.
2. Tube operating correctly, as focus works.
3. Now this is trouble. Tube is not responding to change in cathode voltage.
Yet we know that the bright-up pulse of 40v is there.
So new theory: the heater cathode insulation of the Tube has failed, so that the cathode is pegged to the 900v rail.
Or transformer insulation of heater winding failed.
4. Test tube insulation - un-plug and use ohmmeter.
5. Test transformer heater windings to earth, while base unplugged.
6. Repport votlages when you have your HV probe.
What does WD think?
wme_bill
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Old 13th Jun 2019, 8:40 pm   #109
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Default Re: Hameg HM307-3 Cleaning and Repair

The resistance of the heater winding is 1Ω.

The resistance between the chassis and both pins 1 and 2 of the CRT socket is 4.57MΩ.

Resistances were measured with the CRT removed.

Is this the kind of thing where an insulation tester would come in handy? I have a Wee Megger but it does not work.

What DIY HV probes would you reccomend?
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Old 13th Jun 2019, 11:54 pm   #110
WME_bill
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Default Re: Hameg HM307-3 Cleaning and Repair

Hameg 307
Still a bit more to understand I'm afraid.
4. I suggested the heater cathode insulation. Measure between the cathode pin and one of the heater pins on the CRT base. Use your usual Avo or a DVM on highest ohms. Socket removed from CRT base. pin 1 to 3
5. One of the heater windings from the CRT socket to the chassis. You say 4.5Mohm, so that's OK.
Both should be infinite.

High Voltage probe. Make it up yourself. Take say 8 to 10 resistors of 10M ohm and a 1M in series in a plastic felt pen case. Solder a point on to the top 10M, and two wires from across the 1M at the other end. Glue the point into the end of the tube. Other end, bottom wire to chassis and to one side of meter and the top of the 1M to your meter. This will be fine with a DVM across the 1M. You will have approx 100:1 ratio, so 1kv will read as if 10v.

Adjust the probe (if you want to bother to tune the ratio) by selecting the resistors you use and then calibrate it by measuring a high voltage elsewhere, such as the X Amplifier HT rail, with and without the probe in use. That gives you the probe voltage ratio. Then get your calculator out.

My concern here is that when you measure the negative grid voltage of the CRT, the current through the meter + probe to chassis will move the grid positive. If it goes more positive than the tube cathode, then the tube will pass a large current and could destroy itself. Exactly the same as with a valve if you make the grid more positive than the cathode, or even the gate on a FET.
That is why I said use a high resistance probe, as then the grid voltage movement is less.
The Avo on 2.5kv range will absorb 20ua on 1kv, which could move the grid 10v positive in this circuit. Not enough to destroy the tube (that requires 50v/100v with this CRT), but certainly enough to distort your analysis of the circuit.
Use less resistance in the probe with the Avo, set it on 1Kv range(equivalent to 50M) and just use the probe with 5 resistors = 50M in series with 1kv terminal, giving a 2:1 ratio. Not so satisfactory, as the readings will be at the bottom of the scale, and difficult to calibrate.
wme_bill
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Old 14th Jun 2019, 11:52 am   #111
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by OldTechFan96 View Post
Intensity control measurements at the middle of R358 (TB off, AT off and no signal):

Fully clockwise: -910V

Fully anticlockwise: -941V
Given that voltage range, it looks like the wiper of the 'min' preset is set away from the -1kv line, probably at the opposite end of the track. The Intens pot range would then be limited to between less than -950v and -900v. To get the full range on the Intens control, the wiper on the 'min' preset needs to be at the -1kv line and the wiper on the 'max' preset needs to be on the -900v line.

There also appears to be a few kΩ of resistance between the max point and the -900v line giving rise to a voltage difference between fully clockwise point (-910v) and the -900v line. This might be because the preset wiper is not all the way across on the -900v line or the wiper contact is dirty. Could also be down to a dirty Intens pot I suppose. This can all be checked by measuring resistance between wiper and end contacts on the presets and the pot.

Of course, this still does not explain why the CRT does not appear to be responding to changes in voltage at the cathode and in this regard the resistance measurement between pin 1 and 3 on the CRT might be informative.
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Old 14th Jun 2019, 3:01 pm   #112
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Default Re: Hameg HM307-3 Cleaning and Repair

I've been looking at the intensity preset pots as suggested by Wavey Dipole and have adjusted them so that I get -1002V with the intensity control f. anticlockwise and -901V with the control f. clockwise. These voltages were measured on the middle pin of the intensity control. I've attached a photograph of the current positions of the pots. The brightness is overall still quite low.

H/K resistance (at CRT socket): 6.5MΩ

H/K resistance (at CRT pins): O/L

I'll get some 10MΩ resistors ordered and make myself a HV probe.
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Old 14th Jun 2019, 4:54 pm   #113
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Default Re: Hameg HM307-3 Cleaning and Repair

Hameg 307
Still progressing. Very helpful comments from Wavey Dipole about range of brilliance control in #111 (exactly as I suggested in #46).
1. When something is not working, you can forget all these fine calibration and set up adjustments as you mention in #112. You must get the machine working first. Then you can do the fine adjustments and calibration. The makers manual always assumes the machine is working , and just needs tweeking. After all, no machine they ever made fails or suffers from faulty components, even after 20 years.

We now have:
1.No change brightness when ExtX is moved in/out (3 #107). Which should be caused by change in cathode voltage p3.
2. Yet 40v change at R366 (Edit # 100).
3. So somehow variation you report at R366 does not reach CRT cathode p3.
4. Cathode p3 is not locked to heater and 900v rail, as cathode-heater insulation appears good (#112).
5. We have voltages around CRT:
cathode -900V unblanked (#100)
grid -910V (assuming by TB off you mean ExtX)
so grid on -10V with respect to cathode. This should give a fully bright spot.
6. I seem to remember you have confirmed the anode of the CRT p8 is somewhere around 0 to +100V and the focus control (-600V to -700V on p4) changes spot from fine to fuzzy.

7. But as WD says in last para of #111, CRT not responding to voltage changes at cathode p3.

8. Confirm you are changing the cathode voltage by switching ExtX on and off. And that when in the off position, the TB is not operating by AT off and no Y input signal, so there is just a spot. (para 2 #104 and para 2 #96).
9. To be sure, check voltages at tube base itself.
10. We have to resolve point 7 above, as WD says.
If all voltages are valid, then the only conclusion is that the tube is faulty. Though I am still doubtful.

At this stage, I should not worry too much about a HV probe, provided you are using your Avo on 2.5kv range, which restricts the current drawn. And I suppose a 10M DVM will draw a similar current when measuring 1 kv.
wme_bill

Last edited by WME_bill; 14th Jun 2019 at 5:05 pm.
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Old 14th Jun 2019, 9:09 pm   #114
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Default Re: Hameg HM307-3 Cleaning and Repair

With AT off the cathode voltage varies between -867V and -900V when the hor. ext. button is pressed in/out.

CRT voltages (AT off, hor. ext. in, no sig, intensity minimum):

1: -907V
2: -1005V
3: -904V
4: -615V
5: N/A
6: +95V
7: +88V
8: +100V
9: +109V
10: +109V
11: N/A
12: -899V

The X/Y amp voltage will be unbalanced due to factors mentioned earlier in the thread.
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Old 14th Jun 2019, 10:09 pm   #115
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Default Re: Hameg HM307-3 Cleaning and Repair

Hameg 307.
I am sorry, I don't understand the table of voltages you give us. It does not seem to relate to my table, most of which can be answered with words. What about my point 8 -effect of switching the bright-up.

Oh, I see, it may be my point 9, and is the voltages on the pins of the CRT. If you can say which electrode it refers to, it makes it so much easier for other minds to read and understand and contribute. Those all look about right, except for the grid to cathode.
I believe I have already posted the maker's data sheet on the tube which names the electrodes.

And the brilliance still remains very low and does not brighten up with the ExtX in/out?
It appears that the voltage between cathode p3 and grid p2 is 100v, (grid negative), so I am not surprised that the tube is dim, as that will have turned the brilliance down to virtually nil. For a bright trace, expect about 10v as I have already suggested.
How have you set the Intensity and ExtX controls.

I was going to say the tube is failing, but with the voltages you quote, I am not surprised it is dim.

What does Wavey Dipole think.
wme_bill

Last edited by WME_bill; 14th Jun 2019 at 10:27 pm.
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Old 14th Jun 2019, 10:23 pm   #116
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by OldTechFan96 View Post
Observations for WME_bill (TB off, AT off and no signal):
3. There is no change in brightness when the hor. ext. is pushed in/out. The spot just moves a bit.

Attached is a photograph of the spot I'm getting.
Just re-reading some of the previous comments to review the information and this just stuck out. Are you saying here that you got a spot with Hor Ext both in the in position and out position? With AT off and no signal, with Hor Ext out there should be no spot. The display should be blank until there is a signal input to trigger on, at which point it should produce a running trace across the CRT screen.

Can you confirm that you still get a spot in both switch positions?

Quote:
Originally Posted by WME_bill View Post
And the brilliance still remains very low and does not brighten up with the ExtX in/out?
It appears that the voltage between cathode p3 and grid p2 is 100v, (grid negative), so I am not surprised that the tube is dim, as that will have turned the brilliance down to virtually nil. For a bright trace, expect about 10v as I have already suggested.
Looks like it has been set to "intensity minimum" for these measurements...
Along with a confirmation of the above question, I would like a confirmation of the first 4 readings (pins 1 - 4) with intensity maximum and Hor Ext out please.

Quote:
Originally Posted by OldTechFan96 View Post
CRT voltages (AT off, hor. ext. in, no sig, intensity minimum):
Bill, in theory, if there were no charge at the cathode, would the heater, with it being pegged at -900v, then act as a cathode? Or would the 'floating' cathode shield emission?

Last edited by WaveyDipole; 14th Jun 2019 at 10:52 pm.
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Old 14th Jun 2019, 11:27 pm   #117
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Default Re: Hameg HM307-3 Cleaning and Repair

My mistake. For some reason I had the impression that there needed to be 100V difference between pin 2 and 3. I should not have messed with the brightness min/max pots this afternoon. Before these adjustments the difference was 31V.

I'll perform the min/max brightness adjustment procedure again and try to answer all of your questions.
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Old 15th Jun 2019, 12:30 am   #118
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Default Re: Hameg HM307-3 Cleaning and Repair

Min/max presets are fine and are set to provide the widest possible range on the Intensity control. All you need to do is rotate the Intensity control to the maximum possible brightness level before checking the spot and taking the measurements.
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Old 16th Jun 2019, 1:52 pm   #119
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Default Re: Hameg HM307-3 Cleaning and Repair

Revised CRT voltages (AT off, hor. ext. in, no sig, intensity maximum):

1: -905V
2: -904V
3: -904V
4: -716V
5: N/A
6: +95V
7: +88V
8: +99V
9: +108V
10: +109V
11: N/A
12: -901V

Answers to Questions

WME_bill

1. There is no change in brightness when the hor. ext. button is pressed in or out.

2. The only change when the hor. ext. button is pressed is the cathode voltage. -900V to -867V. They look to be the blanking/unblanking voltages.

3. Agreed. Pushing the hor. ext. button makes the centered dot move from the middle of the CRT to the right by 15mm.

4. H/K resistance rechecked: 3.27MΩ. H/chassis :1.9MΩ

5. Cathode: -906V. Grid: -903V/-867V.

6. Anode +100V. Pin4: pot min: -615V. Pot max: -718V. The spot does change from fuzzy to sharp.

7. Yes, no change.

8. Cathode voltage does change depending on the position of the hor. ext. button. Everything has been measured with AT off, hor. ext. in, no sig, intensity maximum.

Wavey Dipole:

With AT on there is a spot present when the hor. ext is pushed in and out (a spot regardless of hor. ext. position).

The same thing happens with the AT off, a spot regardless of the hor. ext. position.

I hope these answers are satisfactory.

I have attached some examples of the dot and trace I'm getting.

EDIT: Point noted about increasing the heater voltage. I only have a variable DC PSU and no AC supply. Unless DC can be used for the heater?
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Last edited by OldTechFan96; 16th Jun 2019 at 2:01 pm.
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Old 16th Jun 2019, 4:21 pm   #120
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Default Re: Hameg HM307-3 Cleaning and Repair

Thanks for your kind patience it taking these readings. I note these were done with Hor Ext in. I dare not ask you for any more! I have a theory, however, if you can see your way to taking the trouble, just for completeness, it would help to have pin 3 and 4 with AT off, Hor Ext OUT, no sig, intensity max please? You don't need to bother with any others.

BTW,, there was a large jump in the focus voltage (-615 vs -716) which is interesting since I would have thought that this should have stayed the same more-or-less unless the focus was changed dramatically.

Last edited by WaveyDipole; 16th Jun 2019 at 4:34 pm.
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