tektronix 2246 intensity stuck high

[TekMan]

Hi folks,

this time I am looking for your your help:

I have a tektronix 2246 model A (the mil. version of the 2246).
Everything is working fine, except brightness control: Both (A and b
intensity) are stuck a highest brightness.

I already checked the intensity pots (going smoothly -2.5 to
+2.5Volts),
all voltages from the LV power supply (-7+ 7.5 -/+ 15V, 130 Vdc,
130Vac ): all fine, low riple.

High voltage seems ok, since I can focus the beam very well and
readout is clearly visible.

So, has anybody some voltages of the HV supply in mind or on schematic
-- then I can compare it with my readings.

A partly scanned schematic of the HV/crt part would be welcomed too.
and_de AT gmx.de for scans.

Has anyone an idea where to continue with fault search? Jim, any known
problems in this area to you?

Thanks for any help and ideas in advance!

Andreas

 

[Jim Yanik]

[email protected] (TekMan) wrote in

Hi folks,

this time I am looking for your your help:

I have a tektronix 2246 model A (the mil. version of the 2246).
Everything is working fine, except brightness control: Both (A and b
intensity) are stuck a highest brightness.

I already checked the intensity pots (going smoothly -2.5 to
+2.5Volts),
all voltages from the LV power supply (-7+ 7.5 -/+ 15V, 130 Vdc,
130Vac ): all fine, low riple.

High voltage seems ok, since I can focus the beam very well and
readout is clearly visible.

So, has anybody some voltages of the HV supply in mind or on schematic
-- then I can compare it with my readings.

A partly scanned schematic of the HV/crt part would be welcomed too.
and_de AT gmx.de for scans.

Has anyone an idea where to continue with fault search? Jim, any known
problems in this area to you?

Thanks for any help and ideas in advance!

Andreas

First thing I'd do is replace the DC restorer diodes(shifts the Z-axis
waveform to HV levels).TEK scope DC restorer diodes often would break down
under HV,yet test OK with a DMM.(CRT arcs could cause this)
The only way to be sure is to test with a curve tracer.
It could also be a bad HV cap.(same problem;HV breakdown)

 

[TekMan]

[email protected] (TekMan) wrote in
....snipped..

First thing I'd do is replace the DC restorer diodes(shifts the Z-axis
waveform to HV levels).TEK scope DC restorer diodes often would break down
under HV,yet test OK with a DMM.(CRT arcs could cause this)
The only way to be sure is to test with a curve tracer.
It could also be a bad HV cap.(same problem;HV breakdown)

thanks Jim, terribly sorry to tell that the HV diodes are working
fine. I tested all of them with a tek577, all good. And replaced with
known new ones. Both led to no change.
Caps tested and changed too, still have the same problem.

It seems that the signals from the (microprozessor intensity control
?) Measuring shows that intensity pot-information are not "routed" to
the CRT:. Even before the DC restorer diodes, I can't measure a change
in voltage(with another scope) when turning the intensity.

Grid bias pot helps to reduce the overall intensty, so to avoid
burning parts of the screen phosphor. Thias pot is located direct
nearthe restorer diodes/hv caps.

So it seems that there are no bad parts in the near-CRT area.

Are there any known faults in the circuit-pathes before the DC
restorer parts?
If yes, where to check next?

regards
Andreas

 

[Jim Yanik]

[email protected] (TekMan) wrote in

thanks Jim, terribly sorry to tell that the HV diodes are working
fine. I tested all of them with a tek577, all good. And replaced with
known new ones. Both led to no change.
Caps tested and changed too, still have the same problem.

It seems that the signals from the (microprozessor intensity control
?) Measuring shows that intensity pot-information are not "routed" to
the CRT:. Even before the DC restorer diodes, I can't measure a change
in voltage(with another scope) when turning the intensity.

Grid bias pot helps to reduce the overall intensty, so to avoid
burning parts of the screen phosphor. Thias pot is located direct
nearthe restorer diodes/hv caps.

So it seems that there are no bad parts in the near-CRT area.

Are there any known faults in the circuit-pathes before the DC
restorer parts?
If yes, where to check next?

regards
Andreas

Well,I don't remember enough about 2245/6 series to help you here.I guess
you will have to dig out the manual,read the circuit description to find
out how the intensity control works,and then TS down to where you lose the
signal,or control switching.That's what I would be doing,if I were working
on it.

 

[Asimov]

"TekMan" bravely wrote to "All" (27 Apr 04 04:18:56)
--- on the heady topic of "Re: tektronix 2246 intensity stuck high"

Te> From: [email protected] (TekMan)

Te> thanks Jim, terribly sorry to tell that the HV diodes are working
Te> fine. I tested all of them with a tek577, all good. And replaced with
Te> known new ones. Both led to no change.
Te> Caps tested and changed too, still have the same problem.

Te> It seems that the signals from the (microprozessor intensity control
Te> ?) Measuring shows that intensity pot-information are not "routed" to
Te> the CRT:. Even before the DC restorer diodes, I can't measure a change
Te> in voltage(with another scope) when turning the intensity.

Te> Grid bias pot helps to reduce the overall intensty, so to avoid
Te> burning parts of the screen phosphor. Thias pot is located direct
Te> nearthe restorer diodes/hv caps.

Te> So it seems that there are no bad parts in the near-CRT area.

Te> Are there any known faults in the circuit-pathes before the DC
Te> restorer parts?
Te> If yes, where to check next?

Te> regards
Te> Andreas

Sorry to jump in but if the intensity voltage doesn't change perhaps
there is a missing ground path. Perhaps a bad transistor switch used
to boost brightness for photo purposes? Check the beam intensity
modulation input circuit (on rear?). Auto intensity circuitry?

.... Letterman of Borg - "Ok, Top 10 reasons why resistance is futile:"

 

[TekMan]

"TekMan" bravely wrote to "All" (27 Apr 04 04:18:56)
--- on the heady topic of "Re: tektronix 2246 intensity stuck high"

Te> From: [email protected] (TekMan)
....snippety...

Te> Are there any known faults in the circuit-pathes before the DC
Te> restorer parts?
Te> If yes, where to check next?

Te> regards
Te> Andreas

Sorry to jump in but if the intensity voltage doesn't change perhaps
there is a missing ground path. Perhaps a bad transistor switch used
to boost brightness for photo purposes? Check the beam intensity
modulation input circuit (on rear?). Auto intensity circuitry?

... Letterman of Borg - "Ok, Top 10 reasons why resistance is futile:"

Hi Tektronix wizards,

we were all very close to solve the problem, but no cigar

All components show "good" in the curve tracer. No change in intensity
wiht the rear input for "Z-axis".

BUT: The reason why the intensity is stuck was so stupid simple (once
discovered).
By accident I noticed with magnifying glass a small crack in one of
the several wire jumpers (Jim probably knows what I want to explain:
The small wires on the mainboard, in the middle of each wire is a
white plastic bead. There are used frequently on the main board to
route the signals).

I don't know is someone else ( it is not my personal 2246) cut the
wire. Through the magnifying glass,the crack looked more like
"material fatique".

I scanned the whole mainboard for more suspect wires, and found a
secound one.

Okay, I soldered two new wires in, switched on: intensity now
perfectly adjustable. scope works like new ;-)

Thanks for all the help in the group, was fruitful discussion. anyway
If someone has a scan of the 2245/6 schematics, I'd be still
interested.

Jim, your opinion please: Did you see any of these/similar problems
in the 2245/5/7 boards?


Andreas

 

[Jim Yanik]

[email protected] (TekMan) wrote in

Hi Tektronix wizards,

we were all very close to solve the problem, but no cigar

All components show "good" in the curve tracer. No change in intensity
wiht the rear input for "Z-axis".

BUT: The reason why the intensity is stuck was so stupid simple (once
discovered).
By accident I noticed with magnifying glass a small crack in one of
the several wire jumpers (Jim probably knows what I want to explain:
The small wires on the mainboard, in the middle of each wire is a
white plastic bead. There are used frequently on the main board to
route the signals).

I don't know is someone else ( it is not my personal 2246) cut the
wire. Through the magnifying glass,the crack looked more like
"material fatique".

I scanned the whole mainboard for more suspect wires, and found a
secound one.

Okay, I soldered two new wires in, switched on: intensity now
perfectly adjustable. scope works like new ;-)

Thanks for all the help in the group, was fruitful discussion. anyway
If someone has a scan of the 2245/6 schematics, I'd be still
interested.

Jim, your opinion please: Did you see any of these/similar problems
in the 2245/5/7 boards?


Andreas

No,I think it's very rare for a jumper to crack,and there should not be any
mechanical fatigue possible,unless those jumpers are between two PCBs.
You need movement for mech. fatigue.
OTOH,It could have been damaged by the insertion machine,and maybe
vibration/temp cycling finished the job.

Most of the failures I recall in this series were TEK-made ICs.(155 and 234
prefixes)