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Old 16th Jun 2019, 4:54 pm   #121
OldTechFan96
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Default Re: Hameg HM307-3 Cleaning and Repair

I'm always happy to take more measurements! Its part of the fun.

Pin 3 (AT off, Hor Ext OUT, no sig, intensity max): -873V

Pin 4, depends on the position of the focus potentiometer, measured min/max:

Service manual: -590V --680V.

Fluke 25: -615V--716V

Avo8 MK1: -610--695V

Those voltages are too high compared to the SM. The resistors in the focus control circuit are all good, if I recall.
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Old 16th Jun 2019, 5:51 pm   #122
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Default Re: Hameg HM307-3 Cleaning and Repair

BTW, can I also confirm that your understanding of the AT control is as follows:

- switch with arrow pointing at AT and clicked into the bottom left position - AT is ON
- switch with arrow pointing away from the AT and in any other postion - AT is OFF

When AT is ON (switch clicked into the AT position) and Ext Hor is OUT you should get a running trace across the CRT.

When AT is OFF, (switch loose and away from the AT position) then the oscilloscope is in manual triggering mode. In this mode, when Ext Hor is OUT, then the oscilloscope is running from the internal timebase. You should not get a running trace until the trigger level is appropriately set by trial and error - but you are getting a spot.
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Old 16th Jun 2019, 6:55 pm   #123
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Default Re: Hameg HM307-3 Cleaning and Repair

AT on: potentiometer clicked into AT position. Fully anticlockwise.

AT off: potentiometer moved clockwise. Past the click/detent.

The AT is a switched pot; it works like a radio on/off volume pot.

Basically, you are correct Wavey Dipole.

With AT on and the hor. ext. out I do get a (rubbish) trace. See photograph 1.

Turning off AT causes trace to snap into the leftward extremities of the CRT. See photograph 2.

The dot can be moved into the middle of the CRT with the X position control. See photograph 3
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Old 16th Jun 2019, 8:55 pm   #124
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Default Re: Hameg HM307-3 Cleaning and Repair

Thank you for your patience. I wanted to be as sure as I could before posting the below. Your description in #123 of the operating conditions and the images of the traces that result are now clear.

Ok so to summarise what we know:
  • CRT display is low intensity
  • blanking voltage output on bright circuit output is switching between the two expected voltage levels
  • the cathode has been confirmed as isolated and not shorted to the -900v heater voltage
  • a running trace is displayed across the CRT in auto trigger mode
  • an (off centre) spot is displayed on the CRT when auto triggering is off (manual triggering mode) and trace is not being triggered regardless of the position of the Hor Ext control

Key pin functions for intensity, focus and blanking:

Pin 1: Heater (-900v)
Pin 2: Grid 1 (intensity)
Pin 3: Cathode
Pin 4: Grid 2 (focus)

Summary of voltages read:

CRT voltages (AT off, hor. ext. in, no sig, intensity minimum):
1: -907V
2: -1005V
3: -904V
4: -615V

Revised CRT voltages (AT off, hor. ext. in, no sig, intensity maximum):
1: -905V
2: -904V
3: -904V
4: -716V

Pin 3 (AT off, Hor Ext OUT, no sig, intensity max): -873V
Pin 4, depends on the position of the focus potentiometer, measured min/max:
Fluke 25: -615V--716V
Service manual: -590V --680V.

From the above we see:

Pin 1: Heater is confirmed to be at -900v
Pin 2: Grid 1: Intensity control varies voltage between -1kv and -900v as expected
Pin 3: Bright circuit confirmed as correctly switching between blanked (-870v) and unblanked (-900v) states
Pin 4: Focus grid voltage varies within reasonable limits as the focus control position is changed

How this should work is, that when Hor Ext is pushed IN, the timebase is disabled and the oscilloscope X amp can then be driven from an external signal (so called X-Y mode). In this mode, without any signals connected, the CRT will display a dot in the centre and the trace can be driven by a combination of the X and Y signal inputs once applied. (This mode is used to produce the familiar Lissajous patterns and is also used by the "octopus" component tester.) However, when Hor Ext is OUT, the oscilloscope runs using the internal timebase producing the usual left to right trace across the width of the CRT. In this mode, when triggering is off, the control has to be adjusted manually ('tuned') to get a stable triggered trace. When the trigger level is 'de-tuned', the trace disappears because there is no trigger pulse(s) being generated to trigger the start of the trace sweep so the display stays blank. If single trigger pulse where to be received, then a sweep would be initiated at which point the display would unblank and a trace of the sweep is displayed across the CRT display. Successive trigger pulses keep the trace running so that it is displayed constantly.

We had assumed at first, perhaps not unreasonably, that blanking was 'stuck on' because of the low intensity of the trace which seemed to be confirmed by the -870v output from the bright circuit, a voltage level that the circuit diagram indicates as the 'blanked' state. Various other measurements and transistor substitutions also seemed to confirm this. After the opto-isolator was changed it then became possible to detect both blanked and unblanked state voltages at the output, so the bright circuit was now fixed. However, surprisingly the trace intensity was not affected. Conclusion: blanking voltage does not appear to be affecting/reaching the cathode inside the CRT. However series resistor R366 and connections at the CRT were checked and were confirmed as being fine. Further head-scratching required...

On review of the posts it comes to light that the spot/trace appears to remains visible under conditions that it shouldn't be. Further confirmation indicates that we still have a condition where a spot/trace is displayed in the absence of a trigger while the timebase is running but when the display ought to be blanked. Since we know that the bright circuit now works, this turns things on their head. Clearly there is no blanking and evidently there never was. There has only been a consistent low emission trace. The bright circuit is now fixed yet cathode voltage changes are evidently ineffective. Presumably even if the cathode were low emission we would still see some kind of difference in intensity however miniscule and regardless of the level of intensity, the beam should still turn off in response to the blanking voltage being applied. This seems to again suggest that the blanking voltage is not reaching the cathode. Perhaps it has somehow become disconnected inside the CRT tube or perhaps there is an open circuit somewhere that we haven't yet checked.

However, a trace can be displayed on the CRT, so we still have emission, low brightness notwithstanding. Since it seems that the negative voltage from the bright circuit is not affecting the cathode, then from what source comes the emission? Well the heater is connected to a negative potential of -900v. Could the heater, then, be the source of emission and acting as a cathode? With the actual cathode not being connected and 'floating' in very close proximity to the heater, might it become negatively charged from the heater or simply have no influence on the beam at all? With no available blanking control signal at the actual cathode, the trace intensity would then stay the same regardless of how the oscilloscope controls are set.

However this is only a theory that possibly seems to fit the facts. I do not know for sure how the CRT tube would behave if the cathode were disconnected. I also don't know whether there is another failure mode of the cathode or the CRT in general that would cause it to become unresponsive to the change in negative voltage from the bright circuit, or whether other external conditions could cause it to do so.

Before we write off the CRT, there is a short piece of track on the PCB between the end of R366 and the wire 'trp' to the cathode on the CRT. It might be worth checking that this short piece of track is definitely OK (continuity from resistor leg to the joint where the CRT wire is attached), that the joint between CRT wire and the PCB itself is solid, and that there is continuity along that piece of wire from the PCB joint to the CRT socket.

I am hoping that this may turn out to be a fault in the CRT wire or track. However, if all that turns out to be OK, then I think conclusion has to be that unfortunately the cathode has become disconnected inside the tube. It might be difficult to see, maybe a visual inspection of the inner side of pin 3 through the neck of the tube (perhaps with the help of a magnifying glass?) might reveal something?

Last edited by WaveyDipole; 16th Jun 2019 at 9:22 pm.
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Old 16th Jun 2019, 9:54 pm   #125
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Default Re: Hameg HM307-3 Cleaning and Repair

Thanks for that detailed write up, Wavey Dipole. Very informative.

I removed the cathode wire and hooked it up to my continuity tester and it is good. No breaks present.

Likewise with the small solder pad.

I looked at the cathode with a magnifying glass and I could not see anything amiss.

When gently manipulating the CRT I sometimes hear slight 'ting' noise. I can't see anything broken. Maybe just the internal elements moving slightly? I kind of assumed that this was normal.
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Old 17th Jun 2019, 11:55 am   #126
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Default Re: Hameg HM307-3 Cleaning and Repair

One final thing that I can think of is to check the resistance between pins 2 and 3 of the CRT with the socket removed. Just a remote possibility that the cathode might be shorted to grid 1, in which case it would be pegged to between -900v and -1kv depending on the Intensity setting. The effect, I imagine, would be similar to being shorted to the heater.

As a matter of curiosity, what happens if you run the scope with the cathode wire disconnected (keep the end well insulated)?
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Old 17th Jun 2019, 4:45 pm   #127
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Default Re: Hameg HM307-3 Cleaning and Repair

The resistance between pins 1 and 2 of the CRT: meter reads O/L.

With the cathode wire removed the scope seems to behave the same.

I think we are at a point where swapping in a known good CRT is going to be the next step.

What do we all think?
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Old 17th Jun 2019, 7:07 pm   #128
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Default Re: Hameg HM307-3 Cleaning and Repair

Is there anything to be gained by checking the cathode current?
No, I don't know what it should be
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Old 17th Jun 2019, 8:54 pm   #129
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by OldTechFan96 View Post
The resistance between pins 1 and 2 of the CRT: meter reads O/L.

With the cathode wire removed the scope seems to behave the same.

I think we are at a point where swapping in a known good CRT is going to be the next step.

What do we all think?
Ok, so it isn't shorted to grid 1 then. I didn't think that was likely, but now ruled out. Part of me was hoping that by running with the cathode wire disconnected there would be some change which might point to some other explanation. As buggies says, with the cathode wire reconnected you could try measuring the voltage difference across R366 which might tell us whether there is any current flowing and how much, but it does seem pretty conclusive that fault is with the CRT.

I'm not sure whether under these circumstances rejuvenating techniques will be of any help, but here are a couple of links on the subject for future reference:

http://www.thevalvepage.com/teletech...ej/crt_rej.htm
http://www.ke5fx.com/crt.html

If you have access to another identical or similar CRT tube, then swapping with a known good one should put the matter beyond any doubt. Unfortunately, they don't seem to be particularly cheap on eBay, however there is one that has cropped up today as an auction starting at £10:

https://www.ebay.co.uk/itm/3-inch-Ca...r/273891919048

Otherwise it might actually be cheaper to buy a complete scope as a parts donor, or use the existing one as the donor:

https://www.ebay.co.uk/itm/HAMEG-OSC...A/153468529171

This is not particularly cheap, but you could perhaps make him an offer?
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Old 17th Jun 2019, 9:12 pm   #130
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Default Re: Hameg HM307-3 Cleaning and Repair

Thanks for the auction link. I'll keep an eye on it.

I will post and ad in the wanted section of the forum. I might get lucky!

I'll get back to you with the voltage across R366.
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Old 19th Jun 2019, 9:55 am   #131
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Default Re: Hameg HM307-3 Cleaning and Repair

I tried to measure the voltage across R366. My DMM hovered around 0V. At one point it looked to measure 0.6V to 0.8V.

I made sure my probes were making good contact but the measurements were inconclusive.
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Old 19th Jun 2019, 12:24 pm   #132
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Default Re: Hameg HM307-3 Cleaning and Repair

Hameg 307.
Good work by Wavey Dipole.
1. I am still by no means sure that the change in voltage from bright-up (measured at the collector of T363) is getting to the actual cathode pin 3 on the CRT through R366 as you move the ExtX in-out. We have had rather contradictory results quoted.
2.There will be no current through R366, and no voltage. It is only a buffer at 51ohms. I would suspect the printed circuit board hair line cracks or badly soldered (dry) joints, rather than mess about with that resistor.
3. The key information now seems to be that the CRT grid to cathode voltage is not varying from 5V to 40V with the ExtX control.
This implies that an internal connection in the tube have failed.

4. You have spotted the offer of a replacement 3RP1 tube. I'd suggest try for it.
This tube is short, at 238mm and the alternatives are few: Mullard D7-190, Telefunken D7-210.
The GEC-MOV 3WP1/ Mullard DG7-36 also offered is a very nice tube, but too long at 295mm, unless you can modify the oscilloscope case.
Most older UK source tubes are 2.75"/70mm diameter, whereas you want 3"/75mm screen diameter.
The operating voltages of all these tube are comparable. The A in the US type number is of no significance.
I have data sheets and other information on most Instrument CRTs. Otherwise refer to Frank Philipse Tube Data site. I have given him many CRT data sheets.

5. Glad you now report sensible voltages for the grid pin 2 and variation with the Intensity control. I won't say I suggested that be checked a long time ago.
6. Voltages around the tube. The acceptable variation is say 20%, so the CRT accelerator anode and focus anode operating voltages which you now quote are good enough.

7. The voltages quoted for all parts of the circuit on the diagram will be as measured on a typical machine, and serve to indicate what to expect, within 20%. So don't be concerned if what you measure seems a bit different.
8. The only voltages which must be close is the emitter- base on each transistor, which should be 0.6V, emitter positive for NPN, or a diode. If 0v or 5V, then you have a failed transistor. And reverse resistance should be very high.
9. I am sure you are aware that the positive lead on ohms setting is the negative terminal on an Avo, and the positive terminal on a DVM.

10. Buggies point about measuring the cathode current. Not much help I'm afraid. It will be between 5 and 20 micro amp, depending upon the beam intensity. The main current is taken by the focus resistor chain across the EHT rail.

10. I really do suggest that you do not worry about whether you get a line trace and settings of the AutoTrigger control. The only controls of importance for testing the CRT are Intensity at max and ExtX in and ExtX out.
ExtX out implies no trace/spot as the instrument is waiting for a signal and trigger. If you wind the Intensity up to max, you may see a spot on the extreme left hand side of the screen, use XShift if necessary.
ExtX in will produce a central spot. This was your test of the Bright-up circuit.

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Old 23rd Jun 2019, 10:17 pm   #133
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Default Re: Hameg HM307-3 Cleaning and Repair

I won those CRTs! Hopefully they arrive in one piece!
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Old 24th Jun 2019, 9:24 am   #134
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Default Re: Hameg HM307-3 Cleaning and Repair

That's good to hear and only by a small amount over the original £10 asking price. One never knows how things will turn out with eBay auctions!
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Old 28th Jun 2019, 6:27 pm   #135
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Default Re: Hameg HM307-3 Cleaning and Repair

The CRTs from eBay arrived today. The only problem is that they are way too big! The two CRTs are taller than the 3RP1A by about 6cm.

One CRT is marked '3RP1A' in marker pen but I don't think that it is a 3RP1A. Datasheets quote their hight as 9 1/8''. The CRT in question is 11 1/2'' high. The base is also loose.

The other CRT is marked 'CV3946' (DG7/36, 3WP1) and made by GEC.

I reckon that the CRT marked 3RP1A is in fact another CV3946 with a missing label.

I really should of double checked this with the seller before I bid. My fault!

The search for a good 3RP1A continues!

In the meantime I will use the 'new' CRTs to continue troubleshooting the scope.

I'll test the new CRTs and come back with the results. I will also rack my brains about what is still wrong with the scope.
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Old 28th Jun 2019, 9:18 pm   #136
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Default Re: Hameg HM307-3 Cleaning and Repair

That's a real shame. Its possible, I suppose, that the seller based their description purely on that marking without actually knowing what it was? I seem to recall that there was only one photo with a front view, so it would have been difficult to gauge their relative depth from that.

It might be another CV3946 or perhaps an equivalent from another manufacturer? There are differences in appearance, including the glasswork, colour of the opaque coating on the inside and electrode ends.

It is entirely your decision, of course, to use them to experiment with, but you do have the option of returning them for refund on the basis that the item is not as described.
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Old 28th Jun 2019, 10:30 pm   #137
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Default Re: Hameg HM307-3 Cleaning and Repair

Come to think of it, I am pretty disappointed that I did not get what I hoped for. I will try and return the items as they are covered by eBay's money back guarantee.
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Old 30th Jun 2019, 9:21 pm   #138
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Default Re: Hameg HM307-3 Cleaning and Repair

I have decided to not return the CRTs. I will use them to assist the repair of the Hameg scope. When I find a suitable replacement 3RP1A I can just pop it in.

I quickly tested the CV3946 and the trace was nice and bright. I did no further testing.

I'll have a think and start remembering what is still wrong with the scope.
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Old 2nd Jul 2019, 7:56 pm   #139
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Default Re: Hameg HM307-3 Cleaning and Repair

In the absence of a 3RP1A I will be using a CV3946 in its place. When switched on the raster is sharp and bright. A massive improvement over the old CRT. The brightness preset does need setting but I'll do that later.

The voltages in the CRT section are good.

Ongoing concerns:

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 trace looks to be full of ripple (see photo). I tested the capacitors in the PSU a while ago and they were good.

Should the CRT be blank when the hor. ext. button is pressed in? I get a weird dot when I push it in.

The next thing I will focus on will be matching a pair of BF256 FETs for the Y preamp.
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Old 3rd Jul 2019, 3:03 pm   #140
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by OldTechFan96 View Post
The trace looks to be full of ripple (see photo). I tested the capacitors in the PSU a while ago and they were good.
What method are you using to test the capacitors?

Quote:
Originally Posted by OldTechFan96 View Post
Should the CRT be blank when the hor. ext. button is pressed in? I get a weird dot when I push it in.
You should get a dot. When the 'Hor. Ext' button is pushed in, the horizontal position of the beam is no longer being driven by the timebase so you don't get a horizontal trace. The beam is now controlled by the the input to the 'Inp. Ext' terminal. With nothing connected, the beam just remains static. You should be able to move it though with the horizontal and vertical position controls. Its usually a good idea to turn the intensity right down when the beam is static to avoid burnout to the phosphor at that point. If you apply a small forward (positive) voltage to Inp. Ext the spot should move right, for a reverse (negative) voltage it should move to the left. This is the normal behaviour of the so-called X-Y mode.
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