Problem with Tektronix 475 "ADD" function

This has got to be a DC offset in the inverter stage itself, that is even though it night not be a seperate stage, whatever stage does it.

This offset is being compensated for by the front panel position control, so after to get that straightened out you'll have to reset it's range of course.

If the problem is indeed in a vertical amp IC, you probably can't get it, but you may be able to swap it with the channel that doesn't invert. You would actually be adjusting out a problem, but if it works it works.

There is a guy in this NG who can probably give you loads of more specific info named Jim Yanik. Watch the thread, if you get a reply from him it will probably be quite enlightening. You might also search the group for his name and find something useful.

Another piece of information that would be helpful is whether the 6 volt offset is at a certain scale, and reads 6 volts at different gain settings, or is it simply at a certain position on the screen, i.e., if you change the range does indicated offset or the trace position remain constant ? This will indicate whether it's before or after the attenuator. Also does the trace move when you use the variable control, that is bringing it out of the calibrated stop ?

If you can't solve this problem without parts and can't live with it there are surplus places that might have one with serious screen burn or a bad CRT or something. One such place is Electronic Surplus in Cleveland Ohio. If they were open 24/7 guys like us could spend days there looking at all their cool stuff. If you talk to the right person there they will probably know if they have what you need. They also used to have quite a selection of Tek plugins, but IIRC the 475 doesn't use them. If it did it would simply be a matter of sliding in another vertical amp. I don't have their number handy and at the moment I dunno if they got a website. You may be able to get some NLA parts via mail from them but it's nowhere near as much fun as being there. If you're near Ohio it might be worth a visit if you have the time. If near Florida you might want to check out a place called Skycraft in the Orlando area. I would definitely call first, but if they have what you need a visit would be interesting if it is feasible for you. In either place I suggest you take your gold card, you will find some goodies believe me.

JURB

snipped-for-privacy@aol.com wrote in news: snipped-for-privacy@z14g2000cwz.googlegroups.com:

That's my opinion.It could be a invert balance adjustment,or a BNC that is generating a DC offset(I've seen a few of these). I doubt it's in the channel switch itself,that's just a fancy diode gate,IIRC.

IIRC,the 475 vert preamp uses those TEK-made spider ICs,155-0078-xx,and their gain and current supply resistors do go out of spec,they can cause gain,offset,or compression problems.I'm working strictly from memory,though,I have no access to schematics anymore.

Thanks for the recognition! (He's emailed me already.)

When working on DC offsets,I would not use any input signals.

You might run into me there! I love that place. Their stock of TEK instruments has declined drastically lately,though. I wonder if the local test instrument refurb shop bought them all up?

Jim and JURB,

Ok, I've checked the value of the offset that appears when using the "ADD" function and it is a constant one division (5 subdivisions) regardless of the vertical scale deflection setting. (Of course, both channels are always set to the same vertical deflection). So does this suggest that the problem is in the channel preamps or a following stage?

Also, I poked around the vertical switching circuit on the input side to U370 (monolithic switch) and found all DC voltages to be consistent with the schematic, thus suggesting that there's nothing wrong with it.

Suggestions?

Thanks. This is very helpful.

Best,

-Dave

snipped-for-privacy@drumheller.org wrote in news: snipped-for-privacy@o13g2000cwo.googlegroups.com:

I'd say that the problem would be in the preamps. I can't recall if there's a spec on offset for the "add" function. The "invert" function for Ch2 is done by one of those spider IC's I mentioned,and IIRC,there's a DC balance adj.for the inverted Ch2.(I could be wrong;no schematics for reference)

It might be that the Ch2.invert-add function is not spec'd for DC offset. You would have to center the trace with no signal input,then make your measurements.

D; regarding your statement:

If current setting or limiting resistors go out of spec, rather then open, you may well see no change in voltage readings. Even if they go completely open you still might not see any significant change. What's more any change you see might well be less than the device to device tolerance. A voltmeter is not the solution to everything. Even it's drop from a power supply isn't an accurte indicator whether the porper current is flowing or not.

There is a definitive way to check this without ridiculous test equipment, but it involves math and alot of thinking. Thus I'll go into it another time. It's late and I gotta work.

JURB

snipped-for-privacy@aol.com wrote in news: snipped-for-privacy@g43g2000cwa.googlegroups.com:

I don't trust those voltages TEK supplied on their schematics,anyways. I often wrote down my own readings on my personal set of manuals,along with part numbers of some components. I often wish I had had storage space for them when I left TEK,but alas...