Hameg HM604-2 no auto triggering at switch on

[retroteck]

I recently bought an HM604-2 scope which had a trigger fault.

Basically there was no triggering at all, which turned out to be a faulty TR344,
a BF256B fet on the timebase board. Replacing this restored manual triggering to normal but when the scope is first switched on the auto trigger dosen't work.

However after about a twenty minutes or so the auto trigger begins to work, firstly with large amplitude input signals and then with progressively smaller inputs, eventually ending up with normal auto trigger sensitivity.

With a scope probe applied to pin 3 of IC306 the NE529 comparator on the timebase board at switch on, there is a large positive dc offset on the trigger signal input of about 8 volts or so which gradually over the previously mentioned warm-up period reduces to nearly zero.

The auto triggering begins to work normally when the most negative part of the trigger input signal on pin 3 crosses the zero volt threshold between positive and negative supply rails to the chip and when the dc conditions fully settle down the input signal is reasonably, (but not quite), symetrically placed above and below the zero volt level, which presumably should occur immediately on switch-on.

There is a trigger dc offset adjust pot (VR306) on the non-inverting input of IC304, a TLO82 but this seems only to give a very limited range of adjustment of about 50 millivolts to the dc conditions on the NE529 pin 3 input.

I have replaced the NE529 and TLO82 chips on the timebase board but with no improvement.
The plus and minus 12 and 5 volt rails seem fine both at switch-on and after warm-up and i can't find any obvious dry joints on the Timebase board.

Has anyone any suggestions as to where to investigate next. Here is a link to the manual, German I'm afraid, only one I could find for the HM604-2 with circuits.

https://www.sky-messtechnik.de/image...-0604-02D0.pdf

By the way does anyone know what the difference is between the HM-604 and the HM604-2.

Thanks in anticipation.


Rod

[Terry_VK5TM]

In general terms I can think of three possibilities, a faulty cap, a leaky transistor or a leaky diode.

Looking at the schematic, pin 3 of IC306 is controlled by the DC offset adjust you have already mentioned plus T352 & T353.

You need to check the conditions around those transistors and see what is happening, particularly the base voltages.

As most of that circuitry is DC coupled, the fault could far from the obvious places, so it will be a case of following back from those two transistors.

Double check on any and all electrolytic caps associated with that circuit and it's supply rails (low value caps especially).

[greenkit]

Hi Rod,
I have seen this in three 604s previously, and as far as I remember it became a known fault in this model, allegedly not repairable (according to Hameg some years ago). The problem is that the PCB has become conductive through penetrative contamination around the Hi-Z section of the trigger circuit, giving buried leakage paths in the bulk board material. Surface cleaning with acetone or similar does not resolve this.

Check C3099 in particular: you may find it has leaked and the legs where they pass through the PCB will show corrosion. In my case I built air bridges around this part of the circuit to isolate unwanted current paths around pin 2 of IC304: I made notes at the time, if you want a copy PM me an email address. All 3 'scopes had normal trigger action restored after this treatment. Capacitor leakage is likely the main difficulty: I also suspect treating the nearby slide switches to doses of switch cleaner (or WD40, spare the thought!) may not have helped through splash-over & the effect of dust over time.

You'll have to do a fair bit of dismantling to gain appropriate access for this job, but it's worth doing.

Noel

[retroteck]

Hi Noel and Terry

Thanks both of you for your input.

The problem with this fault is that you don't get all that much time from a cold switch-on to troubleshoot voltages before the fault slowly corrects itself and then you have to wait ages after switching off before the fault is fully apparent again.

Terry:
As you suggest I will check the dc conditions of the relevant transistors in the trigger amplifier dc feedback loop and I intend to replace all the electrolytic capacitors around this area of circuitry.

Noel:
Very interesting that this may be a common fault on these units. I did use a proper switch cleaner on the timebase board slide switches the other day but the fault was present before I did this anyway.

I would be most grateful for a copy of the notes you made about repairing this fault and am sending you a pm, thanks.

Obviously I will have to take the timebase board out of the unit again to gain access to the print side of the pcb, I have had it out twice already! What a pain that is and having to remove the front panel as well in order to do it.


Rod

[greenkit]

Rod,
You should have an email with the note attached...
Best wishes.
Noel

[retroteck]

Hi Noel

Got your repair data, very professional. The symptoms you described in the text are vitually identical to the ones on my scope. I will remove the timebase board again and implement the changes after making some checks suggested by Terry.
Might be a few days before I can get round to this due to other commitments but will keep you all updated. Thanks.

Rod

[greenkit]

Ok Rod,
Hope this works out. As Terry noted, all critical electrolytics should be regarded with suspicion when you have the T/B board out: hopefully this will be the last time you'll need to do this. I'll be interested to hear how you get on.

Noel

[retroteck]

Success at last!

Have finally had time to complete the repair.

I replaced some electrolytics that I thought could cause problems in the DC feedback loop if they were leaky.

These were : C3126, C3128, C3108 and C3099.

I also cut back the tracks leading to C3099 and mounted its replacement on the print side of the board and replaced the 1M resistor R3280 standing it proud of the board to be sure to clear any possible conductive path on the pcb as suggested by Noel.

Triggering is now instantly available on switch on and the scope is now working perfectly!

One thing that I did notice however is that the output of the TL082 (pin 1), which according to the circuit should be 0V +/- 2v in normal trigger mode is actually 6V but the input to pin 3 of the NE529 is correct at 0V with the trigger signal riding above and below this.

I did find that when the fault condition was present that the output volage on pin 1 of the TL082 was about 5V on switch on dropping to 4v when triggering eventually became normal.
I have found that the absolute voltage at this point is somewhat dependent on the particular sample of the FET used for T344 (BF256) futher back in the dc loop, as initially during the repair I replaced this FET as it was faulty and also I replaced it again during troubleshooting by cutting the leads and soldering a new one on top and once I got the board out for the third time I replaced it again properly.

Anyway I am not concerned about this as the scope is working fine and am glad to finish the job.

There was no brown corrosion on the leads of C3099, a 47uF electrolytic, which Noel found on his scopes but there was a blue ring on the positive
lead right next to the base which does look like leakage, see attached poor quality photo's.

During the work I managed to snap a couple of those white offset plastic switch couplers but they have responded well to epoxy resin.
Does anyone know what those ribbon cable board connector sockets are called as I managed to crack one of the push-on covers and ideally I would like to replace it.
All this caused by having the board out three times!

Thanks Noel and Terry for all your input as it has helped me tremendously in repairing this fault.


Rod