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Old 24th May 2019, 11:28 pm   #21
WaveyDipole
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by OldTechFan96 View Post
More progress:

Those four BF199 have been replaced. This brought the voltages at EY1+2 to 15V. Right where they should be.

The voltages on TR123 and TR124 are correct, apart from the collector voltages. One sits at 87.4V and the other 92.8V.
Ok, that's looking better! Sounds like everything from the IC to the final stage is now as it should be. Regarding the collector voltages on TR123 and TR124, the diagram shows a +135v input, but comparing the connector layout on the "adjusting plan" diagram a few pages earlier with what looks like the same connector (CP301) on the PSU diagram suggests that this is derived from the 140v output from the PSU via a 1k2 resistor. Since this output is unregulated, it may be be running a little high. However, where this feeds into the Y amp final stage, there is a resistor chain R186, R187, R188 to earth. The base of T121 is fed from between R187 and 188. It might just be worth checking what voltage you have at TR121 base and all resistors in that chain for to make sure nothing is o/c or has a dry joint.

If everything looks OK, the TR123/TR124 collector voltages should now be adjusted using the preset R223 so that they are the same on both outputs, even if they are at slight variance from nominal.

Quote:
Originally Posted by OldTechFan96 View Post

Voltages at IC output:

7- 6V
8- 6V

Voltages at IC input:

1- -11.8V
14- -11.8V

The voltages are too high. Is the IC OK?

The above measurements were made without the FETs in place.
Well from what you write, 7 & 8 are high, 1 & 14 are low, but I'm being pedantic.....

This is why I asked about the FETs, because without them in place, the -11.8v is only to be expected. You will observe that there is a -12.0v feed to the potentiometer R042 and via the remaining resistors and preset to pins 1 and 14 of the IC, so we expect these to be pulled low to around -12.0v. Once the FETs are in place, some current will flow from the positive supply via R016 and the voltage at the inputs of the IC should rise to the expected nominal of +0.5v. For now, since we have an incorrect voltage at the input of the IC, we can also expect to have an incorrect voltage reading at the output. However, the fact that everything is balanced is a good sign and I have reason to hope that the IC is OK.

You might find that the voltage at the IC input (pins 1 & 14) may vary from the nominal +0.5v and this will vary depending on the characteristics the FET. Hopefully your BF256's will be OK. Make sure that the Y-Pos pot is dead centre when comparing these voltages which ideally they should then the same on both pins. So long as these are in the right ball park then fine adjustment can be subsequently made during the calibration procedure

Quote:
Originally Posted by OldTechFan96 View Post
Next I will work out the pinouts for FET sockets. I bought some BF256 for this.

The pinouts in the datasheet differ from what the component tester says. What should I trust?
In what way do they differ? I had a look at a datasheet from 3 separate sources and manufacturers and there do seem to be differences. The Motorola one has the diagram labelled incorrectly. The ON and Fairchild datasheets differ in pinout. In this case I might be inclined to follow the meter.

Since on a JFET, the drain and source are usually reversible, it is conceivable that the tester might show them the other way around. Bear in mind also that touching the pins on FETs with fingers is sufficient to build a charge and confuse the readings. Short all pins for a few seconds before inserting them into the tester.

EDIT: Your post #20 crossed over with mine. I note your comment regarding swapping the IC with same from another scope. As per the above I agree that the IC is probably OK.

Last edited by WaveyDipole; 24th May 2019 at 11:58 pm.
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Old 25th May 2019, 12:15 am   #22
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
In what way do they differ?
My component tester was telling me the pinout GDS while most of the data sheets were all GSD.

I bought ten BF256s so I can experiment.
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Old 25th May 2019, 12:16 am   #23
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by OldTechFan96 View Post
Looking through the service manual of my Attached is what I think is the pinout for the FET sockets.
That looks Ok to me.

Quote:
Originally Posted by OldTechFan96 View Post
My component tester was telling me the pinout GDS while most of the data sheets were all GSD.

I bought ten BF256s so I can experiment.
Well, it seems they both at least agree on the position of Gate. Since its a JFET, source and drain should work either way around. Technically there may be a minor difference depending on the doping of the junctions (I'm sure someone can provide a detailed technical explanation if required) but in practice should be unnoticeable. If you know the manufacturer of the part, then follow their datasheet, but if not, then follow the tester.

I wonder whether the fact that the tester "sees" it as GDS this means that the doping favours that configuration?

Last edited by WaveyDipole; 25th May 2019 at 12:31 am.
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Old 25th May 2019, 1:07 pm   #24
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Default Re: Hameg HM307-3 Cleaning and Repair

TR121 B voltage: 5.3V.

The resistors around TR121 and TR122 are all fine.

I connected up two meters to the C of TR123 and TR124. Adjusting R223 (100Ω) had no influence on the voltage reading.

I removed it and measured 140Ω across it. It worked fine when I tested it with my Avo8. I'll have a look at the resistors around R223.

EDIT: Resistors all OK. Better get the FETs sorted before I bother with adjustments.

Last edited by OldTechFan96; 25th May 2019 at 1:20 pm. Reason: More info
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Old 25th May 2019, 2:30 pm   #25
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Default Re: Hameg HM307-3 Cleaning and Repair

FET Installation:

I am sure that I have the FETs installed correctly. None of the junctions are touching.

Y position is set dead centre.

IC in:

Pin 1- 1.9V
Pin 14- 2V

IC out:

Pin 7- 2V
Pin 8- 4.9V

Now the EY voltages are unbalanced:

EY1- 8.1V
EY2- 21.9V

R198 is getting very hot and IC001 feels like it is getting warmer than it should.

Thoughts as to what is causing trouble:

FET has been installed incorrectly. Will try other permutations.
BF256 is not a suitable replacement for TIS69.
EY output balance potentiometer has been fiddled with in the past. I'll wait for further comment before I attempt adjusting this.
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Old 25th May 2019, 3:04 pm   #26
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Default Re: Hameg HM307-3 Cleaning and Repair

The fact that you are getting a voltage at the base of T121 suggests that the resistor chain is not o/c otherwise you would be getting zero.

At this point I have to make an apology. I made a mistake here. That 100Ω pot (R223) is the astigmatism adjustment, nothing to do with the Y amp balance. It can't possibly have an effect on the DC balance of the plate voltages from the collectors of T123 and T124. The section subtitled "Astigmatism Correction" makes this clear.

The subsequent section "Troubleshooting the instrument" gives a voltage range of 81-89v for the Y plates so there is a bit of leeway in terms of what the output voltage should be, however, ideally the voltages should be the same on both plates to within a fraction of a volt or so.

The fact that the emitter and collector voltages are both slightly different means that something is not quite balanced, but not that far off either. Any variance between R190 and R191, also R197 and R198 as well as the two coils L101 and L102 is going to throw the balance. You said you measured those resistors. You are confident that each of those two pairs measures exactly the same (to within a small fraction) on both sides of the circuit? Although it seems unlikely there would be a fault with the coils, I suppose it might be worth checking them as well. They also should have a near identical DC resistance. After that, we are left with just the transistors themselves and one of them may have been been stressed for a period of time. Easily proven or ruled out I suppose by temporarily substituting the pair.

Hope all goes well with sorting the FETs.
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Old 25th May 2019, 3:18 pm   #27
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Default Re: Hameg HM307-3 Cleaning and Repair

Just seen your post crossed over with mine.

Quote:
Originally Posted by OldTechFan96 View Post
FET Installation:

I am sure that I have the FETs installed correctly. None of the junctions are touching.

Y position is set dead centre.

IC in:

Pin 1- 1.9V
Pin 14- 2V

IC out:

Pin 7- 2V
Pin 8- 4.9V

Now the EY voltages are unbalanced:

EY1- 8.1V
EY2- 21.9V

R198 is getting very hot and IC001 feels like it is getting warmer than it should.

Thoughts as to what is causing trouble:

FET has been installed incorrectly. Will try other permutations.
BF256 is not a suitable replacement for TIS69.
EY output balance potentiometer has been fiddled with in the past. I'll wait for further comment before I attempt adjusting this.
Ok, so input voltages are pretty close to one another (only 0.1v variance) although noted that they are some 1.5v higher than expected. However, the output at pin 8 of the IC considerably higher than at pin 7, which is amplified at EY1 and EY2. Before we draw te conclusion that the chip is fried, what are the VCC voltages at pins 5 & 10? Pin 5 should be at -5v, and pin 10 at +6.8v. If not, then one of the corresponding zenner diodes (Z001 or Z002) might be at fault.

Last edited by WaveyDipole; 25th May 2019 at 3:24 pm.
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Old 25th May 2019, 5:11 pm   #28
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Default Re: Hameg HM307-3 Cleaning and Repair

Both R190 and R191 are both spot on at 150Ω.

R197 measures 2672Ω.

R198 measures 2865Ω, an increase of 6.1%. This will be replaced. It should bring the deflection voltages together.

Both of the above are 2700Ω, 4W and 5% tolerance. Measurements taken out of circuit.

IC001 VCC voltages:

Pin 5- -6.2V. Too high.

Pin 10- +6.9V. Correct.

I've checked both zener diodes out of circuit and they are both good.

Those servicing notes will be useful. Since there is nothing on the CRT I'll measure around the CRT circuit.
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Old 25th May 2019, 10:24 pm   #29
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Default Re: Hameg HM307-3 Cleaning and Repair

X final amplifier measurements (the acceptable range is 84.1V-90.1V):

C of TR113- 112V
C of TR114- 96V

Both are unbalanced, like the Y amp.

R181 and R182 measure spot on out of circuit. The voltages on TR111 and TR112 are correct.

CRT section investigation:

85V missing on E of T354. R368 was O/C. Replacing it restored E voltage.

-8.9V was found on pin 4 (-590V--680V) of the CRT. The focus potentiometer has a partly broken inner ring. It is still partly attached. I have glued it.

All of the other voltages are present. The are either a bit too high or a bit too low.

I'll see if the pot is salvageable in the morning.
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Old 26th May 2019, 8:42 am   #30
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by WaveyDipole View Post
Pin 5 should be at -5v, and pin 10 at +6.8v. If not, then one of the corresponding zenner diodes (Z001 or Z002) might be at fault.
Quote:
Originally Posted by OldTechFan96 View Post
IC001 VCC voltages:

Pin 5- -6.2V. Too high.

Pin 10- +6.9V. Correct.

I've checked both zener diodes out of circuit and they are both good.
Looking at the sheet at right angles I misread the voltage at pin 5. Using a different reader that allows me to rotate the page at 90deg so I can read it the right way up, I see that the voltage at pin 5 should, in fact, be -6v, so your reading is not that far out.

Quote:
Originally Posted by OldTechFan96 View Post
X final amplifier measurements (the acceptable range is 84.1V-90.1V):

C of TR113- 112V
C of TR114- 96V

Both are unbalanced, like the Y amp.

R181 and R182 measure spot on out of circuit. The voltages on TR111 and TR112 are correct.
The "Troubleshooting the instrument" gives the range with position control centred as 95-105v and the circuit diagram shows a nominal 100v so these readings are not that far out. The voltage will shift depending on the position of the X-Pos control and as long as you can get them to balance out with the X-Pos control approximately centred, and the balanced voltage is within the range, then this should be OK.

Just curious, but is the AC input set to 220v or 237v?

It occurs to me that one way to test the functioning of the uA733 IC is to lift the outer end of R020 and R021 and feed a 0.5vDC supply into both ends simultaneously. The problem might still be down to what we are getting at the input.

Last edited by WaveyDipole; 26th May 2019 at 9:07 am.
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Old 26th May 2019, 9:20 pm   #31
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Default Re: Hameg HM307-3 Cleaning and Repair

Thanks for the ongoing support WaveyDipole, it is much appreciated!

I've checked the mains transformer and it set for 237VAC.

I'll try some different BF256s. If I have no luck with them I'll look into another equivalent. I will try and feed some voltage into the input.

More CRT section investigation:

The -1000V rail was running a bit high at around -1050V. I brought it back into spec with R318.

Potentiometer fixed. I will source a new one in the future. Now I'm getting between -642V and -742V at pin 4 depending on where the focus pot is turned. Higher than it should be, but better than nothing.

I'm getting all of the voltages I should be in the CRT section and I'm still not getting a trace. I may have missed something, or is the CRT duff? It worked when I switched the scope on before it was stripped down. All I have done is reattach the base. Do the X and Y amps need to be working perfectly before a trace appears?

The square wave generator works. I checked it with my scope. A square wave can be found at pin one of the optocoupler.

The voltages on IC304, TR352 and TR353 look OK.

EDIT: I can't find C352 on the PCB layout. It is an electrolytic in the CRT section.
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Last edited by OldTechFan96; 26th May 2019 at 9:30 pm. Reason: More info
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Old 27th May 2019, 8:52 am   #32
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Default Re: Hameg HM307-3 Cleaning and Repair

Does it glow or flicker at any point during power up or switch off? What's the position of the Trigg. Ext and Hor. Ext buttons? Ensure that both in the off position, although with Hor. Ext depressed you should still be getting a dot. Also have the trigger level control set full left to AT (auto trigger) to ensure a free running trace.

BTW, what's the status of the Y-amp? I ask because if those plate voltages are way off then the trace might be off the bottom or top of the CRT display. If the Y-Pos adjustment doesn't help, try depressing the GD button to ground couple the input, or disconnect at EY1 and EY2 again.

Last edited by WaveyDipole; 27th May 2019 at 9:06 am.
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Old 27th May 2019, 2:36 pm   #33
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Default Re: Hameg HM307-3 Cleaning and Repair

I have just been messing about with the front panel controls. Ext. trigger and horizontal are off. Trigger level set to auto.

I turned the intensity control to max and fiddled with the focus pot. A faint trace appeared. Y position does noting and X position works.

The min/max brightness pots look to have been fiddled with. These being incorrectly set could explain the faint trace.

What is the correct procedure for setting the min/max brightness? The service manual is a bit vague, in my opinion. Is there a separate calibration manual?

The Y plate voltages are still imbalanced. R198 has drifted out of tolerance. This will be replaced.
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Old 27th May 2019, 4:02 pm   #34
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Default Re: Hameg HM307-3 Cleaning and Repair

Ok, so we know we have a working:

- timebase
- x control
- focus pot

Regarding manuals I could only find:

User and service manual:
https://cdn.rohde-schwarz.com/hameg-...07_english.pdf

Additional notes:
https://www.eserviceinfo.com/downloa...oadurlid=22721

Others were copies of the same. Strangely, I didn't find one with the diagram you posted in #31 so i have saved a copy of that pic!

Just a quick note regarding the astigmatism control, I see from the CRT diagram that you posted that R369 which is 100k, controls astigmatism, so R223 is something else.

Regarding the intensity control, it could be that the pots need cleaning so that is probably the first thing to do. regarding adjustment, it seems that R356 controls the upper (bright) limit and R357 controls the lower (dim) limit. To start with, have the timebase set to something like 1ms. I reckon that the Intensity pot (R358) should be first set to its lowest and R357 adjusted until you can just barely see a trace. Next set the Intensity control to its maximum and set R356 to a high brightness. Turn down the Intensity pot to dead centre and adjust R356 for a comfortable level of brightness. Finally check the range by rotating the Intensity pot. Make any minor tweaks as desired. The exact voltage present at pin 2 of the CRT is approximate and should be around -950v when the Intensity pot is dead centre, but the exact voltage will depend on the position of the presets as well as the position of the Intensity pot itself. It could theoretically range from -1kv and -900v but it just needs to be in the right ball park.

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Old 27th May 2019, 5:10 pm   #35
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Default Re: Hameg HM307-3 Cleaning and Repair

HM307.
Wavey Dipole seems to be talking sense, so I hesitate to join in.
Have you checked the static voltages around the tube. You must stop the timebase (by external Trigger) and no Y input so that you get steady voltages.
You don't say what voltmeter you are using. Make a high voltage probe from a string of >1Mohm resistors into your Avo or Digital, and calibrate from the 140v rail so that you know the ratio to multiply by. See postings previously on the Forum.
Do not measure the grid voltage of the tube directly, as you will spoil the tube.

X and Y Amplifier operation.
Check voltage between the Y plates. You should be able to make it zero by moving the Y shift control. And about +90v above chassis.
Do the same for X plates (about +100v).
The spot should be central on the screen when these voltages are zero, and move with the shift controls.
If not, then look at the Y Amplifier stage by stage, working from the output/ plates end. Do the same with X Amplifier.
This will show where the voltages are badly out andthat's where your trouble is. Voltages should be within 10% at this stage. You want to get it working enough to see what is happening. When that is achieved, then you can adjust more carefully and worry about calibration if necessary.

Lack of spot.
Using your high voltage probe, check the voltages at each point down the EHT chain R357 to R352. Work out the expected voltage at each resistor junction and compare. I wonder about the 33v zener.
Bright-up / unblanking often fails. Does the output to the cathode / collector T353 (care: a high voltage point) change by 30v or so when you change the input R363-C354 from 0 to +3v or so.
High voltage transistors seem to fail frequently, and also the diode of the opto-coupler. They may seem fine when tested out of circuit, but when under stress, faults show up.

The key point is stop the timebase and remove any input, so that you have static conditions and can measure stable voltages everywhere. Use a free running TB for the amplifier testing if you want to give you a line on the screen.
Removing components and testing them alone does not always find the trouble. In fact it can often create new ones as things do not go back quite right.
wme_bill.

Last edited by WME_bill; 27th May 2019 at 5:23 pm.
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Old 27th May 2019, 6:20 pm   #36
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Default Re: Hameg HM307-3 Cleaning and Repair

More trouble:

Just before I tried to adjust the min/max brightness pots I thought that I should check the -1000V to make sure that it is still in spec. It looks like it has drifted way out of spec (-1110V) despite it being adjusted only yesterday. The HV pot does not have any effect on the voltage of the -1000V rail. It works as it should out of circuit.

Looking a the doubler circuit, the negative output sits at -1167V (too high) and the positive output sits at +63V (too low).

I'll start checking around IC303 and TR301 to see if anything is out of order.

Thanks for that information WME_bill. I will follow it in due course. The DMM I am using is a Fluke 25. I also have an Avo8 MK1.
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Old 27th May 2019, 7:07 pm   #37
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by OldTechFan96 View Post
I'll start checking around IC303 and TR301 to see if anything is out of order.
Yup, I would probably be looking for dry joints and perhaps be suspicious of those two electrolytic caps.

WME_bill thank you for joining in and you are most welcome to do so. BTW, regarding the Y amp, I rather suspect that the new input FETs may not be suitable because the input voltage at the IC is too high (about +2v vs +0.5v). Without them installed the pre-amp was balanced once the faulty transistors were replaced, and the Y plate voltages were within something like 5-6v of each other. With the JFETs installed, there was a significant differential again at the output of the pre-amp stage and through to the plates. The op swapped the IC at one point, so this seems an unlikely culprit but I am unsure. I had a similar issue with one particular scope and had to find FETs with a particularly low drain-source zero bias (IDSS) current. Any old JFET would just not do. However this was a scope with a much higher bandwidth and I'm not sure whether this is so critical for a lower bandwidth scope.

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Old 27th May 2019, 8:56 pm   #38
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Default Re: Hameg HM307-3 Cleaning and Repair

Attached is a picture of what I've tested so far. Highlighted components have been tested and are good. Underlined components have not been tested. I will do some more voltage measurements.

The IC might also be present in my other Hameg scope. I'll sub it in if it is a match.

I'll keep an eye out for any dry joints.
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Old 28th May 2019, 9:43 am   #39
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Default Re: Hameg HM307-3 Cleaning and Repair

That doesn't leave much in terms of components. From the board layout the it looks like you might have have the DIL version of the uA741. If it is fitted into a socket then it may just need reseating. On the other hand, if it is soldered in, then unless your "other Hameg scope" is a parts donor, I'm not sure I would be willing to disturb a working scope. 741 op amps (uA741C/N, LM741C/N, CA741C/N) are still available to buy. The metal can version tends to be quite pricey but the plastic DIL version is quite cheap so this shouldn't be too much of a problem to replace if that's the problem.

With that kind of a voltage drop, T301 must be getting rather hot?

BTW, is anyone else surprised that C313 is rated to only 1kV? Isn't that pushing it to the wire a bit? Probably down to the price of capacitors in the day, but I would have thought a 2kV would make more sense here?

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Old 28th May 2019, 3:17 pm   #40
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Default Re: Hameg HM307-3 Cleaning and Repair

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. Unlikely to be capacitors as the voltage has gone up.
Regulating transistor and 741 IC. 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.
But drift could also be caused by the transistor failing in use as it heats up. Try a substitute, any medium power high voltage will do. Try raiding an old TV cathode drive panel. That will do for a quick test.
I always hesitate to unsolder components to test them and resoldering. I prefer voltage tests in situ.
Coming back to the FET input to the Y Amp. The current of the two FET's with zero gate-source voltage (Idss) has to be matched.
Put your Avo in series with the FET drain from 12v or so with a low resistor to limit current(100ohm) and gate+source together to ground.
Or see Wireless World Sept 1972 p443 as below. See also WW Dec 1971 p579 et seq for simple FET tester.
The FET match can be brought closer in circuit by R040 and then the balance control R042.
The importance with the FET is the matching for drain current with zero gate source voltage, Idss.
So don't waste time trying other types. Your BF256 will do as well as any. The circuit is quite uncritical as they are acting as source followers, provided they are matched and preferably low Idss. Get half a dozen of any FET and search closely similar Idss. It was difficult to manufacture FET with consistent characteristics, which is why they have such a wide range in data tables. The makers will match closely at a price.
For Wavey Dipole, I found a version of the HM307 manual on the web with the missing page of the CRT circuit. I think it was one described on Google as: HM307 Service user ID11526.
That page seems to be missing in Electrotanya, KO4BB, Rhode and Schwarz (amazing, yet they actually own Hameg and make the thing), Manuals.lib and All-about-circuits. Just shows how much these sites copy from each other.
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.
Oh yes, I suggest replace the focus pot 470k, rather than mess about repairing it. It lives at high voltage, and with a plastic shaft, I'd feel much happier using it. Value not critical 1/4M to 1M will do as it is shunted by the 180k. None of the values in that EHT resistor chain are critical, as long as the voltage ratio around the focus control is about right. Get your calculator out; the current is negligible down the chain.
wme_bill
Attached Files
File Type: pdf HM307 circuit4_CRT.pdf (498.6 KB, 11 views)
File Type: pdf SimpleFET tester.pdf (70.2 KB, 14 views)

Last edited by WME_bill; 28th May 2019 at 3:35 pm.
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