Hameg HM 107 scope CRT burning out

[darko31]

Hi guys,

I've came across Hameg HM 107 which wasn't working. I've opened it up, and started looking for obvious stuff, and sure enough the fuse on the primary side was blown. I've looked at it, and it said 0,375A. I only had 0.5A fuse, so I've whacked it in thinking it isn't that much higher than original, it won't cause problems. Boy, I was wrong.

When I turned it on first there was loud buzz from the main transformer. That lasted for a couple of seconds and then it stopped. Then the line appeared on the CRT screen, I've played with it a little, it kinda seemed it was working. Then at the center of the screen started appearing blank, like the phosphorous was burning out, and scope started to smell really funny. I turned it off immediately. Now when I turn it on, that "hole" remained on the screen, and started expanding.

Anybody have any idea what the hell happened?

Here's schematic  http://www.akh.se/hameg/hmosc/hm107-diag.gif

[Andy Watson]

Whatever fault caused the 0.375A fuse to blow, is still present - probably one or more of the power supply capacitors (any/all of the five electrolitics in the power supply section).
Look up "capacitor reforming".  Ancient valve equipment may need to be resuscitated slowly after a period of inactivity - to give the capacitors chance to reform. However, it appears that you are approaching the next stage of the demise - the excess current caused by the failed capacitors will kill the rectifier valves - rapidly!

[N2IXK]

It sounds like your CRT was gassy. This would result in excessive current draw (humming transformer and blown fuse) , with the possibility of an "ion burn" to the phosphor in the center of the screen.

Do you see a blue/purple glow inside the neck of the CRT? If so, the tube is toast...

[darko31]

I can see only yellow/orange glow in the neck, that should glow like that if I'm not mistaken?

I've looked better at the fuse and it actually said 0.315A, on the schematic it's 0.3A, which makes stuff a bit worse.

If the tube is gone, oh well  .

[N2IXK]

The orange glow is normal. That's the heater that warms up the cathode.

What voltages do you measure on the +320, +285, +180, and -475V rails?

[darko31]

Sorry for the delay, I was on vacation. I haven't measured rails because I've desoldered electrolytics and when I measured their capacity, multimeter showed ~27uF on both caps. The caps come in odd packaging, here's a pic

https://www.dropbox.com/s/9ekjof3vaztgmhi/2014-08-03%2021.08.25.jpg

it seems that there are two caps connected in parallel in one package, it says on them 50+50uF, 350/385V-. Both caps showed far less capacitance, so I presume they are the main problem. I can't recognise the logo, three headed dragon/snake, maybe Targaryens .

Just to check before I order, would regular 100uF/400V like these be good replacment?

http://www.digikey.com/product-detail/en/400TXW100MEFC18X30/1189-1750-ND/3564724

I sure hope that tube valves are okay, although they aren't so expensive to replace.

Thanks everyone.

[amyk]

Caps like those are usually used for a reason... you may want to check the connections carefully. If they are indeed parallel then a 100uF/400V should be OK.

The brand is Hydra.

[sync]

it seems that there are two caps connected in parallel in one package, it says on them 50+50uF, 350/385V-.

The caps are not parallel. The can is the negative terminal (written on the cap: "Minuspol am Gehäuse"). The other two terminals are connected to the positive sides of the two caps.

If you measure the capacitance between the positive terminals then 27uF sound good (two 50uF in series).

[darko31]

The brand is Hydra.

Well, hail Hydra then.

@sync

Yep, you're right, I've checked the capacitance with the can as negative terminal and it indeed shows ~50uF. So the caps should be fine. I've started checking voltages, the 6.3V for the CRT heater was 5.44V which is quite low. But the transformer was very warm to the touch, and not the core but the windings. I haven't yet checked other voltages, but they'll probably be lower than nominal. So I suppose that transformer is toast, probably windings are shorted?   

[tautech]

Download the pdf in the first post:
https://www.eevblog.com/forum/repair/repair-documents-and-links-sticky-me-please-mods/msg477909/#msg477909
Let it be your guide for scope repair.