Hameg HM404-2 focus problem

[john_s]

Hi all,

I have the above oscilloscope that goes a bit out of focus after maybe 1/5 hour of operation. It works perfectly otherwise.

Hameg is no longer in Australia, maybe there is someone still in Germany but the shipping costs would be prohibitive.

The circuit diagram is available and I have been poking around a bit, someone suggested bad power supply caps but it would be a dog of a job to work on the power supply, and then without guaranteed success.

I have changed a resistor coming from the focus pot to a slightly higher value and it helps a bit but not much.

Anyone with any hints? Or do you know a company in Sydney that could fix it for a reasonable price?  I'm just around the corner from David if he hasn't moved from Norwest. 

[BreakingOhmsLaw]

Hello John,

is the unit consistently shows this error 10 minutes into operation, you have a thermal error.
The usual way to find this is to use ice spray on possible components.
Keep in my not to spray on anything that carries the high voltage and keep in mind the condensation might short circuit something. What I do is to make a small funnel from Capton tape on the part and flood that with spray.

If the screen goes out of focus, you have one of these problems:

 - Focus voltage failing
 - Anode voltage dropping.

To dermine this: If the image only goes out of focus, but does not change size, the focus voltage is bad. This can have many reasons, but with the service manual is available and this should be fairly easy to track down.
https://elektrotanya.com/hameg_hm404-2_oscilloscope_sch.pdf/download.html

However, ask yourself this: Is it worth your time and money to repair a 40MHz CRO? These can be had in working condition for under €80 on Ebay.
Even the cheapest Siglent / UNI-T / OWON / Rigol entry level digital scope will give you a more possibilities than a CRO.  And: To repair a scope, you (mostly) need a scope. So you might as well dish out the 300 bucks for a cheap digital one.

[john_s]

I'm semi retired so I no longer need fancy new tools  the old one will do if I can get it to work a bit better.

I'll try to borrow a meter that will work with the 2KV to check around the focus circuit, mine will only do about 1KV.

Using a computer fan over the case's holes near the neck of the tube helps a little and there seems to be a thermistor on the base of focus control transistor, maybe that has gone bad.

And by the way it's hard for a hoader to let go of things......

[Greenkit]

Hi John,
I have sympathy with your focus problem. As per your previous response, a thermal difficulty is definitely possible: I'd look carefully at all those high value resistors in the focus network after you've established the power supplies are good. Also to note that older Hamegs can suffer with PCB tracking from electrolytic leakage / dust build up over the years. The conductive paths resulting can upset high voltage circuitry and other parts. I have a couple of '604 scopes that predate yours by about 10 years I think. Both have needed a rebuild of their high-Z trigger circuits due to radial capacitor leakage into the PCB, and one had a just-perceptible corona path across a high-voltage resistor in the focus section. This gave poor focus / screen flashes for the first 5 minutes or so after switch-on, after which everything settled down. The corona was visible only in a darkened room: a couple of nearby capacitors had give-away corrosion on their leads. Use acetone to clean up any surface tracking: if it's real bad I blow dry with a heat gun and follow up with a coat of transformer varnish. Your 'scope may have mostly SMD caps, but they're worth checking visually for bulges / white powder discharges just in case. Take care if working around the -2kV supply.

In general, Hamegs are very good: I also have an even earlier '412 (20 MHz) that still performs well. Good luck with sorting yours out!

[john_s]

So back on this, I had the service manual from electrotanya unfortunately the power supply layout and I guess the actual circuit is a bit different, I have searched for quite a while for another manual abut all links lead to the same manual, some people even want money for it!

I have rented a 6kv probe for the weekend and the -2kv measures as -1.89kv, a bit lower but not a great deal. The focus voltage measures at -1.5kv.

Will try and warm up the boards with a heat gun now, see what happens.

[shakalnokturn]

If the screen goes out of focus, you have one of these problems:

 - Focus voltage failing
 - Anode voltage dropping.

To dermine this: If the image only goes out of focus, but does not change size, the focus voltage is bad.

You'll also get a dimmer trace if the anode voltage is going low.
How is your scope reacting?

[john_s]

Brightness seems OK but could be better I guess. I have removed the power supply from the scope, as I mentioned above it is different from the type shown in the diagram.

It only has components on the top of the board, the one on the diagram has components on both sides. All the 1M resistors are OK, so not going high, at least the dozen or so I have tested.

One thing I noticed is some "corrosion"?/junk on the 2 connectors going to the tube (a 4 way with the -2kv and a 3 way), I have cleaned both of them with Servisol and a brush. (the covers are easily removed to get to the contacts)

Presently ingesting a double shot espresso and then continue a bit more.