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neonhomer
Dearborn 5-2750
Premium Member
join:2004-01-27
Edgewater, FL

neonhomer to SmokChsr

Premium Member

to SmokChsr

Re: Tektronix 485 - no power up

Thing is, it only did that when the U comb was removed.

I didn't notice any continuity between those two pins.

Regardless, I guess I will pull the xfmr board back out and try to clean that area of the board up. I dunno what I can get in there with, maybe a Dremel and a wire wheel on low speed, or a small screwdriver to scrape away the carbon...

SmokChsr
Who let the magic smoke out?
Premium Member
join:2006-03-17
Saint Augustine, FL

SmokChsr

Premium Member

said by neonhomer:

I didn't notice any continuity between those two pins.

Not uncommon, think of it this way, the few hundred or so millivolts that your ohm meter uses, don't always find the same current paths that a 3,000 volt power supply will.

iknow
@optonline.net

iknow to neonhomer

Anon

to neonhomer
said by neonhomer:

Thing is, it only did that when the U comb was removed.

I didn't notice any continuity between those two pins.

Regardless, I guess I will pull the xfmr board back out and try to clean that area of the board up. I dunno what I can get in there with, maybe a Dremel and a wire wheel on low speed, or a small screwdriver to scrape away the carbon...

the manual says to pull the u comb, bend pin 3 up, then PUT THE U COMB BACK IN. there's a difference from leaving it OUT!. you ARE getting HV!. but the multiplier may be shorted, disconnect that, and see if the clicking stops.

Mdoc
Ehh... munch munch... what's up, Doc?
Premium Member
join:2007-03-27
Sterling, VA

1 edit

Mdoc to neonhomer

Premium Member

to neonhomer
said by neonhomer:

Thing is, it only did that when the U comb was removed.

Ah, so it was because the U comb was removed that you saw the flash. OK.

In the last schematic pic I posted, there's a blue dashed arrow. It's pointing at the resistor that connects to the x6 multiplier. Remove it and test for no clicks. (With the U comb in)

neonhomer
Dearborn 5-2750
Premium Member
join:2004-01-27
Edgewater, FL

neonhomer

Premium Member

Okay.. will do and post results later....
neonhomer

neonhomer

Premium Member

Just a brief update - Still haven't done anything else with the scope. Just haven't felt like dragging it out.
neonhomer

neonhomer

Premium Member

Okay.... R1602.... problem is this resistor is on the transformer board. So I have to pull the xfmr board, remove this resistor, and then put the board back and then power up the unit.

Is there a way to do this w/o pulling the xfmr board?

drjim
MVM
join:2000-06-13
Long Beach, CA

drjim

MVM

How long are the leads on it? You could always clip one lead, lift it, and then solder back to the stub in the board.

neonhomer
Dearborn 5-2750
Premium Member
join:2004-01-27
Edgewater, FL

neonhomer

Premium Member

No. The resistor is located on the xfmr board, and it is under a cover. So it looks like pulling the xfmr board is the only way. Ugh...

I kinda wish I could have this thing to someone and have them fix it. Unfortunately, I don't have that kind of money...

Mdoc
Ehh... munch munch... what's up, Doc?
Premium Member
join:2007-03-27
Sterling, VA

3 edits

Mdoc to neonhomer

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to neonhomer
Click for full size
said by neonhomer:

Okay.... R1602.... problem is this resistor is on the transformer board. So I have to pull the xfmr board, remove this resistor, and then put the board back and then power up the unit.

Is there a way to do this w/o pulling the xfmr board?

Yes, you can do another way, but this will isolate what I don't want isolated: the circuitry for the CRT. You can just remove pin 3 of U comb and put the comb back in, and then test for no clicks. After this, re-insert pin 3.

Remove P1960 (diagram 15). The result should hopefully be that no clicks are heard. If still clicking, the problem is localized to before the P1960. We'll get to that if we get to this bridge.

Re-insert P1960 and do either one of these 2 approaches:
1) Remove everything in the circle (right side of pic above) from the A13 power board and test for no clicks. If no clicks, then re-attach each power line until you hear clicking. When that happens, you've narrowed down the culprit considerably, and that's where you should look much closely for problems. Or

2) Remove power lines singly and test as you go. The approach is similar to above, but inverted.

What you're looking for is a condition where there are no clicks. That's a condition where that part of the circuitry can be ruled out.

I hope the removal of P1960 demonstrates my thinking: that this stops the clicking. If it does not stop, either I'm barking up the wrong tree or my approach is wrong.

drjim
MVM
join:2000-06-13
Long Beach, CA

drjim

MVM

You guys keep mentioning "combs". Is that another term for one of the ribbon-type cables that has large pins crimped on the ends of it?

neonhomer
Dearborn 5-2750
Premium Member
join:2004-01-27
Edgewater, FL

neonhomer

Premium Member

No. The combs almost remind me of a resistor pack. It's basically jumper pins in brick of five that interconnect the boards. When I get a chance to tear this thing apart again, I will take a picture of one of the combs...

drjim
MVM
join:2000-06-13
Long Beach, CA

drjim

MVM

I've probably seen them, but can't visualize what they look like.
Don't you have to be pretty careful bending the pins out of the way? I'd think they'd crack after a few bends!

neonhomer
Dearborn 5-2750
Premium Member
join:2004-01-27
Edgewater, FL

neonhomer

Premium Member

Yeah. I've only bent it out of the way once. So if I have to do it again, I am going to swap it with a different one So I don't stress the same one.

Mdoc
Ehh... munch munch... what's up, Doc?
Premium Member
join:2007-03-27
Sterling, VA

1 edit

Mdoc to drjim

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to drjim
said by drjim:

I've probably seen them, but can't visualize what they look like.

It's not one of these!

neonhomer
Dearborn 5-2750
Premium Member
join:2004-01-27
Edgewater, FL

neonhomer

Premium Member

Similar...... but only has 5 pins in it, and I don't believe they are electrically conductive to each other.

I am beginning to think the problem is with the transformer board, more to the point, the diode box thingy...

Well.... I can't find parts for this thing... so do I part it out, or just hold on to it and see if I can luck across another one for parts or the parts I need...

iknow
@optonline.net

iknow

Anon

said by neonhomer:

Similar...... but only has 5 pins in it, and I don't believe they are electrically conductive to each other.

I am beginning to think the problem is with the transformer board, more to the point, the diode box thingy...

Well.... I can't find parts for this thing... so do I part it out, or just hold on to it and see if I can luck across another one for parts or the parts I need...

is it one of these? »www.sphere.bc.ca/test/te ··· ts5.html you don't need the exact one, just a compatible one. (same voltage and multiplication) even one from a T.V. that would have the same specs would work. or one could be made with HV diodes and caps..

Mdoc
Ehh... munch munch... what's up, Doc?
Premium Member
join:2007-03-27
Sterling, VA

Mdoc to neonhomer

Premium Member

to neonhomer
said by neonhomer:

I am beginning to think the problem is with the transformer board, more to the point, the diode box thingy...

That's why I wanted you to isolate it by removing R1602 to confirm that. If you can find an easier way to break the line through R1602 to the multiplier, do that if you can.

neonhomer
Dearborn 5-2750
Premium Member
join:2004-01-27
Edgewater, FL

neonhomer

Premium Member

IIRC, the only way to break that connection is to remove the xfmr board again, and to unsolder it.

I just haven't had the motivation to tear the thing apart again to do it.

If I do isolate that line, what should happen?

Mdoc
Ehh... munch munch... what's up, Doc?
Premium Member
join:2007-03-27
Sterling, VA

Mdoc

Premium Member

said by neonhomer:

If I do isolate that line, what should happen?

Theoretically, that it should stop clicking.
If it does, you've successfully isolated the problem.