Oscilloscope woes

[Tony Moore]

Hi Dr John,

Many thanks for your response. I'm glad that you've eliminated the tube as a possible cause. That was my main concern.

Yes, the fault is seen with both channels with both beams either in alt or chop.

On the 2nd photo I set the inputs to GND on both scopes, adjusted the beams to the centre horizontal crosshairs and then switched to DC on both. Each scope was set to 0.2v/div. Test signal is supposed to be 0 - 0.5v. (It looks at little less according to the top scope but that's not been calibrated either!)

I will try to source a service manual with cct diagrams.

Is it your opinion that it will be fixable? It doesn't seem to be much of a fault but I suppose it depends on being able to track it down successfully. I'm up for the challenge though, I get to learn something about the internal workings of scopes!

Thanks for your help.
Tony

[John Caswell]

Hi Tony,
The fact that you have a display and it looks fairly sensible means, in my opinion anyway, that there is not anything seriously wrong and it should be fixable.
The main problem with all modern scopes is that virtually everything is DC coupled so a manual is really necessary to set rail voltages correctly etc.

Regards

John

[Richard]

Tony,

The tube looks good to me. In a mad season of scope buying a few years ago I bought several from Radio Rallies. 3 needed actual repairs (resistors, transistors and an optocoupler) and all needed setting up. All worked fine after and none had broken tubes. Yours looks like it will be an easy problem once you get a circuit though some old transistors are getting scarce.

The lovely scope I use now was very cheap with a declared fault that the display was rotated and there wasn't enough adjustment in the range to set it horizontal. I started thinking ah, that'll be the resistors either side of the pot - then realised the tube clamp was loose and all it needed was rotating and tightening up he he

Clearly don't buy a basket case, and probably steer clear of valve ones now, but scopes can be cheap and easy fun to sort if you go to rallies.

Rich

[Richard]

Interesting thought BTW that some older transistors were plug-in like miniature valves.

Whether that was a hangover from valve mentality or because those ones were fragile or known to fail I don't know - perhaps John knows?

Rich

[Tony Moore]

Hi Richard,

Yes, I quite like the idea of repairing and setting it up, assuming the parts are still available.

It's 50Mhz which is a bit better for the CD digital stuff than my friends 15Mhz HP that I've been borrowing for ages. Perhaps not a 200Mhz storage scope but then I don't do enough to justify splashing out to that extent.

I haven't been to a radio rally for 20+ years! (I don't know why not)

Ahhhh, those were the days...

Tony

[Richard]

Tony,

Just realised it's the Hitachi, I was thinking it was the HP which I've never played with doh, I would have mentioned it before;

They are easily laid out and well built inside with lots of space but the switches are prone to muck and yours looks filthy :-) Open it up and spray them all copiously with Servisol. Work them well and you may be lucky.

Rich

[Tony Moore]

Hi Richard, John,

Service/operator manual on the way. Cost half what I paid for the scope but without it I'm lost.

Aparently it contains cct diags, calibration instructions, parts lists, etc.

I did pop the lid off and have a look inside, as you say Richard, very well laid out and quite clean. There is a small board at the side of the tube with two connections going onto the tube neck that look like they'd be for vertical deflection, each wire ends up next to a pair of power transistors so I guess those are the push-pull transistors you referred to John?

After I've received the manual I might need to take you up on your kind offer John to have a chat on the phone to point me in the right direction.

Many thanks,
Tony
ps. Richard, where can I find this Servisol that you mentioned? Would Maplin stock it?

[Richard]

Hi Tony

Yes,

It's a cleaner with a trace of lubricant, a common switch such as "mag" might be the fault,

http://www.maplin.co.uk/module.aspx?...o=4153&doy=7m1

Rich

[John Caswell]

Hi Tony,
Yes not a problem re contact, pm with landline phone on the way to you.
With a minor correction to Richard's comment don't spray any of the attenuator rotary switches copiously with Servisol as the lubricant is conductive to a degree and will seriously mess up the attenuator HF response. If you can access those switches easily then clean with Isopropanol and the smear a little Silicone grease on the rotor and rotate to spread on the stators.
Slide switches are fine and pots within reason.

Yes, quite correct re the small board, however you will probably find lots of transistor pairs in push pull dotted throughout the 'scope, as the tube drive is balanced, so if the trace shift pots are mechanically central then voltages on these transistors are normally identical, or equal and opposite depending how the cct voltages are derived. As the vertical shift is wrong, then it is possible that one of the voltages feeding the vertical amplifiers is out of range but the service manual will help there.

regards

John aka Dr John

[FoxJT]

Hi All,

I am not sure if it has been covered elsewhere, but has anyone investigated using a PC scope? There are a variety of options available when googled; external boxes, PCI cards plus software, etc.

Any recommendations / comparisons with "real" 'scopes, etc.?

Thanks,

jeremy.