Tek 465 repair hints, svc manual? (fix story)


E K
 

Got the 465 horizontal sweep fixed a couple of weeks ago, thanks to David DiGiacomo for a service manual. This didn't take very long, but had some interesting twists and turns.
1. Quickly verified that the over-current protection of the +110V power supply was kicking in (easily done by checking whether the smaller transistor is on).
2. Found the power supply isolation procedure thoughtfully included in the manual and disconnected the horizontal amplifier by desoldering four components (two feed resistors into the final output stage, a resistor for the X10 MAG neon light (!), and a 6.7V zener also connected to the output stage.
3. The power supply went up somewhat, to ~50V. Followed the second half of the isolation procedure, disconnecting the Z axis amplifier / CRT circuit. The supply cheerfully returned to 110V. Ok, it's not the horizontal amplifier after all!
4. Spent a some time chasing around the intensity amplifier, checking caps, removing some of the transistors, and trying to find the presumed short. This didn't lead anywhere.
5. Decided to reconnect the horizontal amplifier (with Z axis amp still disconnected) and the power supply dropped back down to below half of nominal. After a little bit of confusion, noticed the mistake -- the Z axis circuitry is coupled to the horizontal amp via R1475, so it was the latter after all.
6. Checking the various semiconductors and capacitors yields nothing, somewhat surprisingly. No obviously shorted caps, no shorted transistors or diodes. Turns out the supply does not come back even with all the transistors removed!
7. There are very few possible paths between +110 and ground with the output transistors out of circuit, found a drop of several volts across R1283 (a 47Ohm resistor). Aha! It's connected to ground via C1283, a 0.01 uF 150V ceramic cap that previously escaped suspicion. Sure enough, removing it restores the supply. Once it's removed, I of course notice a small charred spot next to one of the leads, and feel like an idiot -- should've noticed it a long time ago. Try to make myself feel better saying that the charring probably appeared over time as I was debugging. ;-)
8. Replacing the 0.01 uF cap and putting everything back together brings back the supply and the horizontal sweep. An astigmatism adjustment and the scope is ready for use, the only remaining problem being a flaky CH1 vertical position pot, to be fixed some other time.
9. You may have heard about the tantalum caps in these scopes, I know I did. But the ceramic caps must have also come from a bad batch or weren't really rated 150V.
All told, an investment in a service manual, a one cent capacitor and 8 total hours or so of fun yield a working 465 scope.
E K <dataclue@yahoo.com> wrote:
Working on a Tek 465 with a horizontal deflection problem. Namely, no horizontal deflection under normal operation and no response to horizontal position adjustment. Limited horizontal deflection if "beam find" pressed, however. It seems that there is a sweep signal being generated, so expecting the problem to be further down the line. This is somewhat buttressed by the fact that a +110 testpoint reads about 40V, other easily checked voltages are normal (e.g., +15, -8).



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Mark Anton
 

In many types of equipment, I have found shorted disc and mylar caps just often enough to be aware that it happens. Not really frequent at all, but often enough to know that they never go bad.
Actually have found shorted discs where the voltage should not have been a problem at all. Like a 3 volt p-p signal riding on 6 volts of DC with a .01 50 volt disc. Where did my signal go? Took the cap out and measures 4 ohms. No stress there but the cap failed. So it dont suprise me either, but its not the first thing I go looking for either.

Mark.
----Original Message Follows----
From: E K <dataclue@yahoo.com>
To: TekScopes@yahoogroups.com
CC: E K <dataclue@yahoo.com>
Subject: [TekScopes] Re: Tek 465 repair hints, svc manual? (fix story)
Date: Mon, 17 Feb 2003 18:58:13

-- the Z axis circuitry is coupled to the horizontal amp via R1475, so it was the latter after all.
Turns out the supply does not come back even with all the transistors removed!
7. There are very few possible paths between +110 and ground with the output transistors out of circuit, found a drop of several volts across R1283 (a 47Ohm resistor). Aha! It's connected to ground via C1283, a 0.01 uF 150V ceramic cap enough, removing it restores the supply. Once it's removed, I of course notice a small charred spot next to one of the leads, 8. Replacing the 0.01 uF cap brings back the supply and the horizontal sweep. You may have heard about the tantalum caps in these scopes, I know I did. But the ceramic caps must have also come from a bad batch or weren't really rated 150V.
.
E K <





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Miroslav Pokorni
 

The mylars are supposed to be self-healing type, if there is enough energy
in the circuit to burn the foil, otherwise short remains. Mylar insulation
exhibits high resistance, so ESD, probably repeated, might have a part in
initial failure. In old times, ESD was not taken seriously; I believe even
explosives people did not take notice of ESD until around mid 80s.

As for ceramics, a physical crack opens all sorts of ways to build up a
short, primarily through whisker growth (metal used for plates is the
culprit). And again, if there is not enough energy in circuit to clear up
initial short or clearing does not happen while short is still nascent, the
whiskers can build up over time to successfully short even a power supply.
The crack forms primarily through physical or thermal abuse of cap. Those
are micro-cracks, certainly not visible to a naked eye and quite possibly
not visible under a microscope, either. You would have use a penetrant to
detect them.

Back in early 80s, when high capacity ceramic caps, at the time that was
0.010 and 0.050 microFarads, there was a series consideration that such caps
could not be reliable because of high failure rate. I do not think that
silver ink was used at that time, because that was just around time of Hunt
brother's silver gig, so silver was still expensive. Over time, cap industry
fixed the problem, but if you want to retain the fait in quality of caps,
never, ever take a tour of a cap factory. To paraphrase worn out saying:
'about sausages, laws and ceramic caps it is best not to know how they are
made'.

Regards

Miroslav Pokorni

----- Original Message -----
From: "Mark Anton" <heightstv@hotmail.com>
To: <TekScopes@yahoogroups.com>
Sent: Monday, February 17, 2003 11:13 PM
Subject: Re: [TekScopes] Re: Tek 465 repair hints, svc manual? (fix story)








In many types of equipment, I have found shorted disc and mylar caps just
often enough to be aware that it happens. Not really frequent at all, but
often enough to know that they never go bad.
Actually have found shorted discs where the voltage should not have been a
problem at all. Like a 3 volt p-p signal riding on 6 volts of DC with a
.01
50 volt disc. Where did my signal go? Took the cap out and measures 4
ohms. No stress there but the cap failed. So it dont suprise me either,
but its not the first thing I go looking for either.

Mark.
----Original Message Follows----
From: E K <dataclue@yahoo.com>
To: TekScopes@yahoogroups.com
CC: E K <dataclue@yahoo.com>
Subject: [TekScopes] Re: Tek 465 repair hints, svc manual? (fix story)
Date: Mon, 17 Feb 2003 18:58:13

-- the Z axis circuitry is coupled to the horizontal amp via R1475, so it
was the latter after all.
Turns out the supply does not come back even with all the transistors
removed!
7. There are very few possible paths between +110 and ground with the
output
transistors out of circuit, found a drop of several volts across R1283 (a
47Ohm resistor). Aha! It's connected to ground via C1283, a 0.01 uF 150V
ceramic cap enough, removing it restores the supply. Once it's removed, I
of
course notice a small charred spot next to one of the leads, 8. Replacing
the 0.01 uF cap brings back the supply and the horizontal sweep. You may
have heard about the tantalum caps in these scopes, I know I did. But the
ceramic caps must have also come from a bad batch or weren't really rated
150V.
.
E K <





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E K
 

50 volt disc. Where did my signal go? Took the cap out and measures 4
ohms. No stress there but the cap failed. So it dont suprise me
The interesting thing is that this cap wasn't really "shorted". Part of what threw me off for a while during this whole process is that the 110V supply is really low-current compared to what I'm used to, so a tens of milliamps count.

As for ceramics, a physical crack opens all sorts of ways to build up a
short, primarily through whisker growth (metal used for plates is the
culprit). And again, if there is not enough energy in circuit to clear
Fascinating, the bit about whisker-growth.

Personally, I have never had a good visual show. Had some stinkers
The 465's cap didn't have enough to be noticeable. However, I have seen some really exciting things -- like balls of white fire from exploding IGBT's powered by a few amps at 300 VDC. Never anything really exciting from a capacitor, however, most of the unfortunate electrolytic experiences were much more sedate than usually described.









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Craig Sawyers <c.sawyers@...>
 

Never
anything really exciting from a capacitor, however, most of the
unfortunate electrolytic experiences were much more sedate than
usually described.
Well, I had an electrolytic blow up in my face once. The pressure relief
was facing me, and POW! Fortunately I wear spectacles, and that stopped the
goo going in my eyes. I can't quite remember exactly what abuse that
capacitor had had - I suspect that I was expecting more of it than it was
prepared to give (probably exceeding ripple current rating by an order of
magnitude or some such in a quick-and-dirty lash up).

Craig


Miroslav Pokorni
 

Electrolytics can make a spectacular and quite dangerous 'exit from the
scene'. Personally, I have seen number of tantalums which failed and not
every time the cap was mounted wrong way. Frequently, there is not enough
pieces left to reconstruct polarity, but often enough tantalum slug remains
in place to verify that cause was nastiness of tantalum rather than backward
mounting.

I have also seen a PC motherboard that caught on fire. Company that I worked
for at the time, was making SIMMs and the particular one had only ceramic
caps, all surface mounted components. The cap shorted, most likely one of
those micro-cracks due to high rate of heating the cap. The subsequent
washing supplied moisture, which was never dried out properly. The product
seating on the shelf for months permitted enough silver migration to build
up to a solid short and when module was plugged in, the short heated up FR4
to point of fire, what transferred to connector body and eventually to
motherboard. Company had to buy new motherboard for customer.

However, the most dangerous electrolytic failure that I heard about was a 16
V 'computer grade' (2" diameter X 3" length), wired backwards and facing
front end of bench. That happened at my first employer, few weeks before I
started work with them. The slug, aluminum foil winding with electrolyte,
flew between heads of two guys who were testing prototype and hit the wall
on the other side of lab. I was shown the 'point of impact', there was a
dent �" deep and that was mortar covered brick wall.

Regards

Miroslav Pokorni

----- Original Message -----
From: "Craig Sawyers" <c.sawyers@tech-enterprise.com>
To: <TekScopes@yahoogroups.com>
Sent: Wednesday, February 19, 2003 12:42 PM
Subject: RE: [TekScopes] Re: Re: Tek 465 repair hints, svc manual? (fix
story)


Never
anything really exciting from a capacitor, however, most of the
unfortunate electrolytic experiences were much more sedate than
usually described.
Well, I had an electrolytic blow up in my face once. The pressure relief
was facing me, and POW! Fortunately I wear spectacles, and that stopped
the
goo going in my eyes. I can't quite remember exactly what abuse that
capacitor had had - I suspect that I was expecting more of it than it was
prepared to give (probably exceeding ripple current rating by an order of
magnitude or some such in a quick-and-dirty lash up).

Craig


Richard L. Hess <lists@...>
 

Ok, off-topic, but fun.

Test fixture with seven cards fresh out of card-stuffing/wave solder.

Owner of firm brings the bankers by and explains my "cool" automated test setup.

I'm on a deadline, so I start testing the new cards.

Power up the test frame, smoke all over the place! All seven cards had both rails' tantalums in backwards.

Owner and bankers exit very promptly.

I take the boards, still smoking, back to the module fabrication group. The boards were not the only source of smoke. I think some was coming out of my ears.

Cheers,

Richard