Author Topic: Tek 7704 pops and clicks.. Also: TEARDOWN!  (Read 4686 times)

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Offline JPortici

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Tek 7704 pops and clicks.. Also: TEARDOWN!
« on: May 12, 2016, 06:04:20 am »
Hi eevblog
I recently acquired a 7704 through ebay which according to the listing it had just been recapped and calibrated.
It arrived, i tried it briefly without any plug ins to see if the crt was working and it was, so then i switched it off and installed a 7a26 and a 7b80 which were bought from another listing.
It works!
However, in the first minutes i can hear small pops coming from inside the scope that then go silent,
however, this happens randomly: sometimes every 10-15 minutes up to a couple of times per minute, a louder pop and i see the image on the crt compressing then going back to normal, like if i was screwing around with the on-off button

This of course worries me, anybody have any idea on where to start to look at? I'd rule out the plugins because when this happens i see no difference in the signal, just the image shrinking, but no distortion in the horizontal or vertical axes
« Last Edit: May 21, 2016, 10:47:29 am by JPortici »
 

Offline tautech

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Re: Tek 7704 pops and clicks, image compressing for a moment
« Reply #1 on: May 12, 2016, 07:36:38 am »
I'd imagine it's HV tracking but from where.  :-\
You'd want to trace it reasonably quickly as each subsequent event is leaving a carbon track that will make further tracking more likely.

Possibilities:
Any part of the HV divider circuitry......dirty dusty HV PCB, HV caps, failing resistor/s but hopefully not the HV transformer.  :scared:
PDA leakage......cracked PDA HV lead insulation..... leakage to chassis.
Or the PDA cable grommet on the tube is damaged.....these can be a bitch to fix but entirely possible with silicone sealers.

Note
PDA voltages are dangerous even with the scope off and unplugged.
Slide a thin screwdriver (grounded) under the grommet and discharge the multiplier. Do this a least a couple of times.
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Offline McBryce

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Re: Tek 7704 pops and clicks, image compressing for a moment
« Reply #2 on: May 12, 2016, 08:14:47 am »
When I fix old CRTs I sometimes just leave them running with the case removed in a darkened room. The arc is quite often easy to spot.

McBryce.
 

Offline TerraHertz

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Re: Tek 7704 pops and clicks, image compressing for a moment
« Reply #3 on: May 12, 2016, 08:56:50 am »
As suggested, it's most likely intermittent arcing of the CRT anode voltage. Or one of the focusing electrode voltages.
It happens in old CRT units because areas with high voltage attract dust particles from the air. Those become charged by corona ionization of the air, then are attracted to the nearest surface and stick there.
The gungy film that builds up tends to be hygroscopic. As moisture builds up the film becomes more conductive. Eventually a surface discharge occurs along the conductive surface. That drops the EHT voltage briefly, hence the screen deflection factor change. The arc evaporates moisture on the path that formed, so it doesn't happen again for awhile. It may also settle down and stop happening as the equipment warms up.

One way to approach the problem is to simply clean everything EHT-related. Anywhere you can see a dirt buildup. Particularly HV cables and the CRT glass around the anode cap. If the cap is removable, take it off and clean both it and the glass. Turps and tissues, with tissue in tweezers for hard to reach awkward spots. Once clean, wipe with dry cloth/tissue to remove the turps too. CRT anode caps that weren't glued on should be greased with a silicone grease before replacing, to seal and keep moisture away from the contacts.

Do you know how to unclip CRT anode caps, and do it without zapping yourself? CRTs can hold the several KV of charge for a very long time; hours to even days.
« Last Edit: May 12, 2016, 08:58:29 am by TerraHertz »
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Offline JPortici

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Re: Tek 7704 pops and clicks, image compressing for a moment
« Reply #4 on: May 12, 2016, 10:59:29 am »
Nope, never worked with anything over 220 (and that was mainly in school), i always work with electronics under 50 volts
I am aware that i have to discharge the CRT before event thinking of looking at it the wrong way but that's it.

The seller did say he recapped and recalibrated the unit before the listing and i would expect he had the internals cleaned as well (which it was from what i could see when i briefly inspected the unit last night.. but i didn't remove the cover so i haven't looked at the HV section)

If you have any tips to share it would be much appreciated. i'd have a closer look during the weekend... in the meantime: is it dangerous to operate the scope in this state?
 

Offline Gyro

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Re: Tek 7704 pops and clicks, image compressing for a moment
« Reply #5 on: May 12, 2016, 11:25:38 am »
When I fix old CRTs I sometimes just leave them running with the case removed in a darkened room. The arc is quite often easy to spot.

McBryce.

Seconded, it's the easiest way by far.  :-+

Edit: Unless the EHT connection is close enough to be flashing over to the inside of the cover, inspect it carefully.
« Last Edit: May 12, 2016, 11:31:08 am by Gyro »
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Offline TerraHertz

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Re: Tek 7704 pops and clicks, image compressing for a moment
« Reply #6 on: May 13, 2016, 12:50:36 pm »
If you have any tips to share it would be much appreciated. i'd have a closer look during the weekend... in the meantime: is it dangerous to operate the scope in this state?

OK. The EHT cap on the CRT will be a big rubber or silicone disk on the side of the tube, like a suction cap with the thick-insulation EHT wire coming out of it. Inside in the center it has a metal thing that clips into the EHT button set in the glass. The button is usually a cylindrical hole with an inwards-facing flange level with the glass surface.
The disk connector can be either a kind of snap-in spring (that can be pulled off) or more usually a metal U-shaped spring with outward hooks on the ends of the U. These grip under that flange, so these type do NOT just pull off.

The CRT anode consists of a conductive layer deposited on the inside of the glass over most of the fat end of the tube. With the grounded conductive paint (black stuff) on the outside of the tube, this makes a big capacitor. Which is what you have to discharge to be safe to work on. You have to short the inside of the tube (via the EHT connector) to the outside of the tube. Which will always be electrically connected to the instrument chassis.

How to discharge it: Screwdriver with a thin, longish shaft (flat head best) and a very well insulated handle, plus a fairly short and solid alligator clip lead. Clip one alligator to the screwdriver shaft, the other end to the closest metal part that is *definitely* well attached to the conductive outside of the CRT. On TVs this is obvious - the wire braid straps held by springs to the outside of the tube. On scopes it's sometimes a bit less obvious.
Anyway, once you are sure the screwdriver can't become +20KV hot relative to anything, slide the blade end in under the EHT cap, towards the center contact. If there's any charge in the CRT you'll hear a SNAP as it arcs to the screwdriver before it even touches the contact. Not to worry, it's normal.
But don't stop there. Keep pushing it in till it definitely touches the center contact metal. Then leave it there, at least 15 seconds. Because there can be charge on various internals of the tube, that can leak gradually back to the EHT conductive lining so even if you briefly shorted the EHT contact, it can become somewhat charged again later.

Once you are really sure all the charge is gone (paranoia is appropriate) then you can pull the EHT cap off the tube.
If it doesn't come off with a firm pull (like a pop-stud) then it's probably the hooks type. For these, try pushing the cap center inwards plus sideways, to get one side of the 'U' to unhook. Sometimes you can peel the rubber disk edge up enough to see it to figure out how to release it. Or try feeling for the clip with the screwdriver.  Just never, ever poke anything conductive under that cap unless it's firmly wired to the case ground.

With some scopes the rubber EHT cap is glued onto the CRT. Leave those alone; there will be a connector of some kind in the EHT cable. Same routine - you have to get at the conductor with a grounded lead. Don't do what I did with one scope, which was unplug that connector (intending to short the pin to chassis) but then fumble the connector so it dropped past a PCB. That scope (a Tek 7104) is still dead.

Oh, one other caution. The metal contact sealed in the CRT glass has a thin bottom end, with vacuum on the other side. Don't be poking it with pointy objects. (Though it's fun to do deliberately with big old TV CRTs, as one way to make them safe for disposal.)

As for 'safe to operate like that' - depends what you mean by safe, and where the arc is occurring. If you mean safe for you, sure. Safe for the equipment - if the arc is in dust on the CRT glass, no problem. If it's along a surface that can burn and carbonize, forming permanent conductive tracks, then no.
« Last Edit: May 13, 2016, 12:54:49 pm by TerraHertz »
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Offline JPortici

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Re: Tek 7704 pops and clicks, image compressing for a moment
« Reply #7 on: May 13, 2016, 06:35:37 pm »
Got this from the seller
Quote
the pop that you hear is a little lightning in the high voltage cascode.
This original device was damaged, and I build a new one, because no replacement is receiveable at the moment.
This is build up with 4 kV C's and 6 kV Diodes ( 3kV minimum). I isolated the circuit with silicone grease. But this is, so it seams, not sufficient enough.
Buy the way this is the mistake I described in the description, but only in german, not in english...
The effect depends also on air humidity. So this part should be isolated with special urethane or polyethylene. Maybe one sharp solder tip at the output
of the cascode (+21 kV) is also a weak spot.
But the diodes are strong enough (500 mA) so no failure sould occure.
Best regards
 

Offline tautech

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Re: Tek 7704 pops and clicks, image compressing for a moment
« Reply #8 on: May 13, 2016, 08:14:42 pm »
Shame the seller did a half arsed job.  :--

Some links for you to consider:
http://www.sphere.bc.ca/test/tek-parts/tekparts5.html

Member Grapsus made his own as shown in this thread:
https://www.eevblog.com/forum/repair/tektronix-465/
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Offline JPortici

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Re: Tek 7704 pops and clicks, image compressing for a moment
« Reply #9 on: May 14, 2016, 09:37:28 am »
*still haven't opened the scope*
So, the 21kV multiplier (U4214)?
First result for the part number (152-0493-00) points to an interesting thread ;)
https://www.eevblog.com/forum/testgear/tektronix-7704a-voltage-multiplier-diy-replacement/
 

Offline tautech

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Re: Tek 7704 pops and clicks, image compressing for a moment
« Reply #10 on: May 14, 2016, 12:26:00 pm »
*still haven't opened the scope*
So, the 21kV multiplier (U4214)?
First result for the part number (152-0493-00) points to an interesting thread ;)
https://www.eevblog.com/forum/testgear/tektronix-7704a-voltage-multiplier-diy-replacement/
Nice.  :-+

Link to pdf with schematic and PCB layouts:
http://rfscientific.eu/sites/default/files/articleextra/hv_tek_multiplier.pdf

Be sure to pot it with a quality brew suitable for HV, member Grapsus from France used this stuff:
http://fr.farnell.com/pro-power/ppc124/potting-resin-pu-250g/dp/1776512

Pics are in this post:
https://www.eevblog.com/forum/repair/tektronix-465/msg672083/#msg672083


Keep us posted on your results.
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Offline JPortici

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Re: Tek 7704 pops and clicks, image compressing for a moment
« Reply #11 on: May 14, 2016, 03:29:31 pm »
Yes, seems like the seller did already built it but the insulation is propbably not good enough. I think i'll uncover and switch it on in the dark to see where the spark is originated (as suggested)

Concerning safety, both for me and the scope of course. I don't want to be zapped or to zap the rigol that i think i'll put on top of the tek.. and i don't want to have to change the crt.
 

Offline JPortici

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Re: Tek 7704 pops and clicks, image compressing for a moment
« Reply #12 on: May 21, 2016, 10:44:01 am »
Okay.
A free saturday morning.
A sunny day.

Teardown time!

isn't it beautiful?


one can easily remove the sides so you can access and inspect all the boards without disassembling. Nice.

And here's the culprit

well this is scary enough. Going to what could be my last meal!
more to come
« Last Edit: May 21, 2016, 11:31:28 am by JPortici »
 

Offline JPortici

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Re: Tek 7704 pops and clicks.. Also: TEARDOWN!
« Reply #13 on: May 21, 2016, 01:04:16 pm »
There was some darkening in the protective case in the area near the HV power supply (duh! the multiplier failed and was replaced)
I discharged it and..

the reconstructed unit.
but the cable...


see how it is hanging almost touching the intensiti/readout pots?
hmm..
also this

however i could not go into more detail as i don't seem to have the tool to unscrew the knobs, so i could not remove the front panel completely, remove the handle and look at the actual connection of the HV cable to the crt.
i'd say that the cable need more investigating.
 

Offline tautech

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Re: Tek 7704 pops and clicks.. Also: TEARDOWN!
« Reply #14 on: May 21, 2016, 01:09:18 pm »
Time to fire her up in a darkened room and watch for the fireworks.  :popcorn:
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Offline JPortici

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Re: Tek 7704 pops and clicks.. Also: TEARDOWN!
« Reply #15 on: May 21, 2016, 01:10:31 pm »
the readout board says 1985.
this is at least from 1985 or younger! 31 years old
all the ICs have 1984 datecodes though, week 22 to week 39

A landscape


all the other photos are here
Part1:http://imgur.com/a/6HO8U
Part2:http://imgur.com/a/flfdZ
 

Offline med6753

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Re: Tek 7704 pops and clicks.. Also: TEARDOWN!
« Reply #16 on: May 22, 2016, 02:40:43 pm »
Why is the metal chassis all buggered up where the anode lead comes out of the HV supply? Those sharp points are an open invitation to corona discharge.

I agree with others. Dark room. Lower the intensity. See if you can find out where it's arcing over.
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Offline Gyro

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Re: Tek 7704 pops and clicks.. Also: TEARDOWN!
« Reply #17 on: May 22, 2016, 02:52:47 pm »
It looks as if part of that ribbed white plastic was supposed to lock into the panel as a through=panel insulator (hence the elongated hole). Unfortunately it won't do that any more because of the 'replacement' EHT tripler gets in the way.

I'd do the darkened room test before you disturb the scene of the crime any further, otherwise you may not find the actual point of flashover as easily.

Edit: it would also explain why the EHT cable seems a bit long.
« Last Edit: May 22, 2016, 02:56:19 pm by Gyro »
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Offline TerraHertz

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Re: Tek 7704 pops and clicks.. Also: TEARDOWN!
« Reply #18 on: May 24, 2016, 01:18:47 am »
AFTER you try the dark room test, if the area around the EHT connection cap to the CRT is still suspect, you may find it easier to remove that sheet metal plate down the center of the case, rather than removing the front panel.

Another possibility is to remove the whole CRT and mu-metal shield assembly, to which all the connectors are clip-ons. Just be careful when removing the deflection drive wires from the pins in the side of the CRT. Do not bend the pins (or you risk cracking the glass seal.)

If you find yourself having to disconnect wires, best to take a lot of photos before you do. It's possible to figure out wiring afterwards from the manual, but rather tedious.
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