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Old 3rd Jul 2019, 4:19 pm   #141
OldTechFan96
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Default Re: Hameg HM307-3 Cleaning and Repair

I tested the PSU capacitors using my Hunts leakage tester and a component tester. Non of them were leaky and all had low ESR. I do have replacement capacitors available.

Do you reckon the ripple is coming from the PSU? Or could it be coming from anywhere in the circuit? Outside of mains smoothing, there is only a handful of capacitors in this scope.

Thanks for clarifying the hot. ext. situation.
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Old 3rd Jul 2019, 4:45 pm   #142
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Default Re: Hameg HM307-3 Cleaning and Repair

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Originally Posted by OldTechFan96 View Post
Hot resistors: R197 and R198 in the final Y amp. Could be due to the lack of FETs in the Y preamp.

R304 in the PSU 140V rail.

All of the above resistors are cool when the EY connector is removed from the Y preamp. This makes me think that the lack of installed FETs is causing this.

To a lesser extent R181 and R182 in the X final amp feel hotter than they should be.
The Y plate final amp transistors and associated series resistors do run pretty hot anyway so it is just possible that this is their normal operating temperature. So long as they are not releasing any smoke and you can centre the trace (which from the sound of it you can) with the Y-Pos control more-or-less centred and not having to be at or near either extreme of its travel, then the Y amp will probably be OK.

Having said that, if you don't have FETs in place then you can try temporarily grounding the inputs of R020 and R021 to prevent them from floating towards -12v. This should keep the output voltage at both EY1 and EY2 at around +15V until you get some FETs in place. I wouldn't have thought that should be necessary though.

Quote:
Originally Posted by OldTechFan96 View Post
I tested the PSU capacitors using my Hunts leakage tester and a component tester. Non of them were leaky and all had low ESR. I do have replacement capacitors available.

Do you reckon the ripple is coming from the PSU? Or could it be coming from anywhere in the circuit? Outside of mains smoothing, there is only a handful of capacitors in this scope.
It is possible that with the covers removed, the instrument is picking up noise from another source such as the hash generated by nearby workbench lights (CFL and LED) as well as other equipment.
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Old 4th Jul 2019, 4:32 pm   #143
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Default Re: Hameg HM307-3 Cleaning and Repair

The worst resistors are R197, R198 and R304 when it comes to how hot they are. You cannot touch them for long as they will burn your finger! Especially R304. The reason why I am concerned is the strong 'hot' smell that they give off.

The Y position potentiometer does not move the trace. The potentiometer did test good out of circuit a few weeks ago. This might be related to the lack of FETs.

BF256B JFET Matching:

Using the method described on this website I have measured the IDSS of 8 BF256B FETs for the front end of the Y preamp.

FETs 2 and 8 are the closest match so I will try them. I'll also need to make sure that the input/output voltages on IC001 are correct.
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Old 4th Jul 2019, 6:50 pm   #144
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Default Re: Hameg HM307-3 Cleaning and Repair

Further progress!

I installed two closely matched FETs and made sure that the pinout was correct.

A tweak of the Y position control brought the trace to where I could see it. The scope is now responding to signals applied to the Y input. I connected the square wave generator to the Y input and can see a (sort of) square wave.

The amount of ripple is so high that I can't tell if the waveform has triggered correctly.

Assorted voltage measurements:

IC001:

Input:

Pin 1: 1.998V

Pin 14: 1.998V

Output:

Pin 7: 3.66V

Pin 8: 3.54V

The IC is slightly warm to the touch. I will keep an eye on this.

EY1: 15.7V

EY2: 14.7V

Collectors of Y amp transistors:

Pin 6: 97.5V

Pin 7: 98.1V

These voltages are not quite right and slightly out of balance. The collector voltages could be off due to one of the resistors being slightly high.
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Old 5th Jul 2019, 5:41 pm   #145
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Default Re: Hameg HM307-3 Cleaning and Repair

Well it seems you have found a pair of FETs that are well matched and everything seems to be in the right ball park. The slight differences may be attributed to the need to a little fine tuning, which will be part of the calibration process. Well done!

When I first saw that picture, especially with it being in the dark I imagined that the multiple trace images were down to camera shake? The blurring of the blob of light suggests that too. On the other hand, the lettering in the background and the edge of the tube do look quite sharp. Are you actually seeing multiple trace images as shown?

The curved ends on the horizontal lines can be compensated for by adjusting the probe.

Regarding triggering, is the trace static, or is it still moving across the display? A properly triggered trace should be absolutely static with level at to AT. With AT off you have to find the "sweet spot" by adjusting the level, but the trace should remain static when you do. Alternating the position of the +/- control determines whether you are triggering on the upward or the downward slope of the signal and should visually invert the trace and return it back to its normal orientation. The trace should remain static at both settings.
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Old 5th Jul 2019, 6:39 pm   #146
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Default Re: Hameg HM307-3 Cleaning and Repair

I did have my phone flat on the bench to take that photograph so I think what you are seeing is the ripple in the trace which it bouncing it around. I'm just seeing the one trace. The ripple makes taking a sensible photograph impossible.

If I remember correctly the square wave did look to be creeping rightward. I don't think manipulating the controls did anything. The amount of ripple makes things difficult.

The next step will be to find the source of the ripple. I have been probing the PSU outputs with my big Hameg scope. There looks to be AC present on the 140V and 180V rails. This is with the scope cranked up all the way. This is the first time I've measured ripple with a scope so I think the ripple on these rails is small enough to get away with.

There looked to be some ripple on the +12V rail which I'll investigate shortly.
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Old 5th Jul 2019, 11:08 pm   #147
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by OldTechFan96 View Post
I did have my phone flat on the bench to take that photograph so I think what you are seeing is the ripple in the trace which it bouncing it around. I'm just seeing the one trace. The ripple makes taking a sensible photograph impossible.

If I remember correctly the square wave did look to be creeping rightward. I don't think manipulating the controls did anything. The amount of ripple makes things difficult.
I suspected that the imaging device was quite static when the photo was taken as the rim of the CRT and the lettering is quite sharp, but I wasn't sure and just wanted to confirm. So there is one trace visible but it is bouncing around and creeping forward.

From the photo is seems pretty clear that the trace is bouncing in a vertical (Y) direction, but perhaps rather less clear whether the horizontal direction is affected as well. I would suggest having a look at the +24v and the -12V rail as they are both used to power the Y pre-amp and there is also a feed from the 24V line into the Y-plate stabilisation circuit (T121) in the final amp.

I think it may have been mentioned in the early part of this thread, but the PSU circuit diagram indicates the maximum acceptable Vpp ripple on each rail both before and after the regulator. You could use your second scope to examine the amount of ripple at those points on the lower voltage rails. If the ripple is excessive before the regulator, then the problem is likely the smoothing cap or possibly the rectifier. If it is fine before the regulator but excessive after it, then the problem could be the cap that follows the regulator, or even the regulator itself.

With the Trig. Ext. button in the out (off) position, the oscilloscope should use the signal at the single Y input as a trigger source to synchronise the trace so the controls should be having an effect. When the Trig. Ext. button is pushed in (on position), without a trigger signal the input the controls will do nothing.

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Old 7th Jul 2019, 9:30 pm   #148
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Default Re: Hameg HM307-3 Cleaning and Repair

I've been looking at the voltage regulators and all of the voltages are correct.

The input pin of both regulators do have about 0.1V of ripple. The outputs show pretty much no ripple with the Y amp cranked all the way up.

The only rails with ripple (Y amp set to max) are the 140V and 180V rails. I'm wondering if this small amount of ripple is being amplified and is what we are seeing on the CRT?

I'll replace the 3 47uf 250V smoothing capacitors. One must be leaking AC.
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Old 4th Jul 2020, 10:32 pm   #149
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Default Re: Hameg HM307-3 Cleaning and Repair

Thread reopened at OP's request.
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Old 7th Jul 2020, 11:58 pm   #150
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Default Re: Hameg HM307-3 Cleaning and Repair

I have decided to have another a look at the Hameg 307. Unfortunately I have not found a replacement 3RP1A CRT. I'm using one of the longer replacements I bought.

The issue I'm having at the moment is that the trace is wobbling up and down.

Manipulating the time-base and Y amp controls produce the desired result and are doing what they should.

I fed a square wave into the Y input and probed the outputs of the Y amplifier with my main 'scope. I had a nice square wave at EY1 and EY2.

Next I probed pins 1 and 2 of the CRT socket, which is where the output of the final Y amplifier is fed to. There was a clean square on both pins.

The same is the case with pins 9 and 10 of the CRT a nice clean saw-tooth waveform.

Since only clean waveforms have been found thus far, I have eliminated the possibility excessive ripple on the +24V, -12V, 140V and 180V rails.

This only leaves the -1000V rail. I scoped it with my probes set to X10 and a 6KV capacitor in series. 20mV ripple X10 200mV ripple on the -1000V rail?

C308 and C309 (0.47uf, 630V) were tested at 500V and were not leaky.

D301 - D302, D304 - D305 (EM513) were also tested at 500V and were not leaky. They are rated for 1600V so the 500V test was not sufficient. They could be breaking down at there operating voltage.

I will change these and see if anything changes. I pulled apart an old PC PSU which could be handy for some parts. I have some 1N4007 if they are suitable.

It's nice to look back at this after a year!
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Last edited by OldTechFan96; 8th Jul 2020 at 12:00 am. Reason: More info.
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Old 8th Jul 2020, 7:49 am   #151
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Default Re: Hameg HM307-3 Cleaning and Repair

Could this be the magnetic field from the mains transformer affecting the CRT. I've had a similar problem when the isolating transformer under my work bench is on and the scope is placed above it or a power supply is on next to the scope.

Try triggering the scope from the mains, line setting on the trigger selection. If the trace is stationary then it is almost certainly an issue with the stray magnetic field from the transformer.

Keith
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Old 8th Jul 2020, 11:47 am   #152
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Default Re: Hameg HM307-3 Cleaning and Repair

Indeed and with covers being off and everything open on the bench shielding is very much compromised so E.M.F. is likely to be more of a problem. My fluorescent magnifier lamp on the bench sometimes causes such problems as well.

Sorry you haven't been able to find that 3RP1A CRT tube for a reasonable price yet. The cheapest at the moment on eBay is from Germany at nearly £40 including postage which is quite a bit, but I see Mullard tubes going for about the same.
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Old 8th Jul 2020, 12:23 pm   #153
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by KeithsTV View Post
Could this be the magnetic field from the mains transformer affecting the CRT. I've had a similar problem when the isolating transformer under my work bench is on and the scope is placed above it or a power supply is on next to the scope.

Try triggering the scope from the mains, line setting on the trigger selection. If the trace is stationary then it is almost certainly an issue with the stray magnetic field from the transformer.

Keith
Spot on! That's sorted it! I put the CRT shield back on and things are stable.

I've seen a few replacement CRTs but they have been more expensive than the 'scope.

There is still a few niggles to be ironed out. Is there an 'Adjusting Plan' available for the 307-4?

I still have the 3RP1A and I wonder what exactly is wrong with it. I have a 500V battery Megger and will check for shorts ect.
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Old 9th Aug 2020, 4:20 pm   #154
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Default Re: Hameg HM307-3 Cleaning and Repair

Just spotted a HM-307 on eBay with a starting price of 99p.
https://www.ebay.co.uk/itm/203071324317
Might be useful for a repair or parts donor.
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