Hameg HM 604

[twizzle]

Hello

If there are any current or previous owners of this particular model with experience of the particular fault I have that could throw me some clues...please do.

Unit is totally dead and will not power on. Component Tester fuse (Fast 50mA) located on internal PCB is blown and any replacement fuse blows immediately on pressing power on switch. I had thought that the scope should still work with this fuse blown but the Component Tester function would just not work and that's all. Clearly this is not the case...or it should usually be, were it not for something serious and problematic going on that I cant get to the bottom of.

This ring any bells with anyone? The same or similar?

I've started trawling through the schematics for this model but without any idea of what slice of cake the sixpence is in and the difficulty in getting to a lot of the internals is making progress slow.

Thanks in advance

Kevin

[singapol]

Basic troubleshooting...are the power supply voltages listed in schematic present? Next...understand
how an analogue scope works, that should give clues. Is there a trace?

[twizzle]

Hi

Thanks for the response.

I have another analogue scope (Gould Advance) that I've used for years so I know my way around these things. I can read schematics have some decent test gear and repair to component level on a lot of different types of board, so that's where I'm at skill wise.

However, I'm missing something on this problem. The PCB fuse blows (for the component tester function) every time on power on and then (as far as I can determine) there is nothing from the power transformer into the PSU section(?) which obviously makes little sense. No power LED ever comes on or even on for an instant...but the fuse blows. As I've always understood it that fuse is purely to protect the CT section from misuse so why it blows on power on when no components are being tested yet leaving the scope totally dead is the issue.

My scope is at my work on the bench so I plan to have another more thorough look Monday time permitting and the hope was that I might continue my diagnosis with a pointer from someone familiar with this scope that might have seen this problem before.

Many thanks

[bktemp]

According to the schematic the output of the 50mA fuse at the component tester goes only to two 1M resistors for measuring the signal and to the component tester output. Also there are current limiting resistors, limiting the current of the component tester. So even when one component is shorted, it shouldn't blow the fuse.
Measuring all voltage of the mains transformer is the first step. Also check the wiring if something there is a wrong connection, a cable plugged in backwards or similar problems.

[twizzle]

Thanks for that, I see the 1M's. I think I need to flip out that 741 just to see also.

The scope has never been opened before or if it ever had been by the previous owner then this is the first time it's been opened since the problem suddenly happened. That way I'm fairly certain I can discount any connections on the wrong way or anything as fundamental as that. Fairly tough to get anything off and back the wrong way around inside this scope the fit is that tight :-)

If I can get the power button pressed and the fuse not blowing that's progress.

Is there anything you see in that circuit that would suggest to you total non operation of the scope IF that part of the circuit containing the CT fuse were at fault?

Thanks and regards

[oldway]

I have 3 HM605, I use one of them daily to repair amplifiers and receivers because the component tester is very usefull to troubleshoot without need to power on the defective amplifier.
I also repaired a lot of analog oscilloscopes.

You wrote: "Unit is totally dead and will not power on".
The first thing to do is to use a 100W incandescent bulb in serie with the AC power supply to protect your HM 604.
HM604 or HM604-2 ?
Check the net input fuse ...is it allright ?
Without input voltage, switch your oscilloscope on ....measure the resistance of the input plugs  ...How much do you read ?
If you measure between 35 and 100 R, that's allrigh. (NB: I don't know the exact value)
If you measure infinity, you are in trouble.....

[twizzle]

Hi

It's a standard 604

Managed to spend some time on it today and made some progress. Transformer all checked out fine so no issues there. Cause of the blowing 50mA component tester fuse and more importantly the 315mA fuse at the rear (which I hadn't noticed) was a shorted 4.7mf 100v electrolytic cap in the power supply (C2012 to be precise) Thinking that was all there was to it I had one in my spares box and replaced it but more problems lay in wait. Scope powers up now but with a single dot on the phosphor. Clearly has a lot to do with the fact that I've no +140v and +68v (+5v and +27v only) coming out of that section of the power supply going to the Z board but dammed if I can find the reason! Made so difficult as access to components is ridiculously limited.

[oldway]

Power supply voltages of Hameg HM604 are:
+5V
-5V
+12V
-12V
+68V
+140V
I did not seen +27V ?
If you don't have +68V, that's normal that you don't have 140V too because they are in serie.

[WaveyDipole]

I would check whether the indicated AC voltages are present on the output of the transformer and the indicated DC voltages are present on the output of the rectifier D2001. If not, then check C2001 and C2002 and the rectifier.

If the above is OK, then also check whether R2001 (3ohm) is open. If its open, then before replacing it check T203 (BD651) as the overload caused by C2012 might have caused it to break down. Finally on the 140v line, check C2014 for a short and whether R2016 (14R3) is open.

I'm not sure whether the current indicated above those two resistors means that they are fusible types, but if so, then they are expected to fail if the current exceeds the stated level.

That should be enough to be going on with for now!


 

[twizzle]

Brilliant and right on the money WaveyDipole.

I hadn't seen your post but in the mean time while picking away at this thing discovered R2001 (3ohm) was nearer 400 ohm than 3ohm. Replaced that and everything sprang into life...for about 4 minutes then the 140v and 68v lines dropped dramatically and it all went pear shaped, turning it on and off then I get about 2 minutes of action until the voltages drop again. Will replace T203 as you suggest as I reckon that's where the problem is now. The only saving grace to all this is that everything I need to de-solder and solder is available through a tiny space above the PCB although the skill of a gynaecologist is called for. Were it not for that I'd be seriously snookered. Should have the replacement transistors next week so let you know.

Best regards and thanks for your comment.

[WaveyDipole]

If the line came up OK once the resistor replaced, then T203 may actually be OK. I'm not fully familiar with the failure mode of power Darlington transistors, but usually when transistors fail they leak current or short and the failure is permanent. I suggested checking it with a multimeter because if R2001 is replaced with an ordinary resistor, it would not fail at the expected current level and further damage could occur.

This now has the hallmark of a capacitor shorting internally on warm up. C2010 is a possibility since that shorting would deprive the T204 emitter of current. C2001 and C2002 are also possibilities. What voltages do you read across C2001 and C2002 when the line goes down? If you lift one end of R2001, do the line voltages across those two caps still drop after a couple of minutes?

[twizzle]

The voltage across C2001 and C2002 has always been absolutely as per specification right from the start of this problem and has never altered. I actually replaced C2010 previously as a precaution and when checking the cap that came out it was spot on. Unless I've now replaced a good capacitor with a dud... It was only when I replaced R2001 that everything seemed to work and indeed does but just for a couple of minutes at a time. If I monitor the 140v line it stays there for the first minute or so then suddenly drops to about 100v and then drops again to 25v after a short while. Scope off and back on again and the same thing happens although a bit quicker each time.

The quoted voltage of -15.5v at the emitter of T203 is correct at start up when the +140v and +68v rails are up. As soon as those voltages drop then that -15.5v jumps positive and ends up around +25v I seem to recall. Any clues there?

Don't forget, all this kicked off with a totally shorted C2012 which meant I couldn't turn the scope on at all. Replacing that got me up but with ridiculously low +140 and +68 lines and then R2001 getting replaced seem to be the answer. Whether that cap I replaced unnecessarily as a precaution with a possible dud has buggered up preceedings then I'm not sure.

I'll have another crack at it all Monday :-)

[bktemp]

Did you check the current on the +68V and +140V rails?
The current indicated near R2001 and R2016 are the typical currents, so you should see ~0.43V across R2001 and ~0.39 across R2016.

[WaveyDipole]

The quoted voltage of -15.5v at the emitter of T203 is correct at start up when the +140v and +68v rails are up. As soon as those voltages drop then that -15.5v jumps positive and ends up around +25v I seem to recall. Any clues there?

This is interesting as with only a 3ohm resistor between that point and negative end of the rectifier bridge, and  since you say that the voltage across C2001/C2002 remains constant, it should not be possible to get +25v at that point. This would suggest maybe a cracked track or joint that is opening up and losing contact as the PCB warms up.

I mean no disrespect, but I would be inclined to just re-check the solder joints on R2001 and also whether there is any crack or damage to the track near that point and from there back to the rectifier itself. If you have replaced C2010 with a new cap then it will probably be OK.

bktemp, thanks for clarifying that the indication is typical current. I wasn't sure what the indication meant.

[twizzle]

Thanks to both of you for your input, gratefully received.

Will look at your comments and recommendations on Monday and report back as necessary.It's proving to be a rather perplexing problem that I think in the end will be very simple but I've just not nailed it quickly enough.

Cheers

[twizzle]

Well...the scope is now working perfectly.

I'd checked and rechecked everything I could (after replacing the shorted electrolytic and the 300 ohm 3 ohm resistor) and everything really did seem OK. However, I'd ordered some replacement transistors for T201 and T203 as they were just pence. Today they arrived so I tossed a coin and replaced T201 first. Bingo. I can't find anything at all wrong with the one I took out using my component tester and multi meter BUT I had a slight suspicion about this transistor as I felt sure I could get a bit longer operation (more than a minute or so) by using freeze spray on it. Hard to tell really. Anyway, that's it done. Never personally had a transistor that has behaved like this but there you go.

Thanks to all of you who chipped in and offered useful advice, much appreciated.

Cheers

[WaveyDipole]

Well done! I'm glad to hear you have got it working.