Hi all,

I got hold of this cheap ($100) Tek 485 that is working fine except for
three things: The variable volts/div pot on channel B doesn't pop up
smoothly, and when it does it doesn't turn at all; the lights on the
100M/50R selection on channel B don't work (the switch works though),
and the variable timing pot doesn't turn.

Never having done anything on a 485 I opened up the case, turned it
upside down, and I can see the two aluminum "boxes" for the channel
attenuators. I removed the knobs from the B channel and a few screws,
and I can "pop" the attenuator back, but not enough for me to actually
pull it up and get to it -- it meets the power supply board too soon
(there's also a little aluminum "T" connecting the A and B boxes).

Is there any way for me to get to the attenuator circuitry without more
serious dismantling? I downloaded a copy of the service manual but it
didn't help much.

Best,

- Cesar

wrote in
oups.com:

Hi all,

I got hold of this cheap ($100) Tek 485 that is working fine except for
three things: The variable volts/div pot on channel B doesn't pop up
smoothly, and when it does it doesn't turn at all; the lights on the
100M/50R selection on channel B don't work (the switch works though),
and the variable timing pot doesn't turn.

Never having done anything on a 485 I opened up the case, turned it
upside down, and I can see the two aluminum "boxes" for the channel
attenuators. I removed the knobs from the B channel and a few screws,
and I can "pop" the attenuator back, but not enough for me to actually
pull it up and get to it -- it meets the power supply board too soon
(there's also a little aluminum "T" connecting the A and B boxes).

Is there any way for me to get to the attenuator circuitry without more
serious dismantling? I downloaded a copy of the service manual but it
didn't help much.

Best,

- Cesar


You pull out the entire atten. module,not just the top PCB.

After removing the atten knob,there's a nut on a bushing that must be
removed. Then there's a screw on the rear side to be removed,and a hex-
nut/spacer in between the two atten assemblies,on that "T" bracket. Then
you have to pull the Peltola cables out from their jacks,then the harmonica
connector.Then the atten module tilts out.

The serivce manual has the procedure in it.Maybe your 'copy' omitted it.


--
Jim Yanik
jyanik
at
kua.net

Jim,
At one time, we would remove the attenuator and send it to the factory
in Beaverton, if we couldn't clean them and fix the problem. One of
the best scopes ever made...
Jim Yanik wrote:
wrote in
oups.com:

Hi all,

I got hold of this cheap ($100) Tek 485 that is working fine except for
three things: The variable volts/div pot on channel B doesn't pop up
smoothly, and when it does it doesn't turn at all; the lights on the
100M/50R selection on channel B don't work (the switch works though),
and the variable timing pot doesn't turn.

Never having done anything on a 485 I opened up the case, turned it
upside down, and I can see the two aluminum "boxes" for the channel
attenuators. I removed the knobs from the B channel and a few screws,
and I can "pop" the attenuator back, but not enough for me to actually
pull it up and get to it -- it meets the power supply board too soon
(there's also a little aluminum "T" connecting the A and B boxes).

Is there any way for me to get to the attenuator circuitry without more
serious dismantling? I downloaded a copy of the service manual but it
didn't help much.

Best,

- Cesar


You pull out the entire atten. module,not just the top PCB.

After removing the atten knob,there's a nut on a bushing that must be
removed. Then there's a screw on the rear side to be removed,and a hex-
nut/spacer in between the two atten assemblies,on that "T" bracket. Then
you have to pull the Peltola cables out from their jacks,then the harmonica
connector.Then the atten module tilts out.

The serivce manual has the procedure in it.Maybe your 'copy' omitted it.


--
Jim Yanik
jyanik
at
kua.net

Is there any way for me to get to the attenuator circuitry without more
serious dismantling? I downloaded a copy of the service manual but it
didn't help much.

Best,

- Cesar


You pull out the entire atten. module,not just the top PCB.

After removing the atten knob,there's a nut on a bushing that must be
removed. Then there's a screw on the rear side to be removed,and a hex-
nut/spacer in between the two atten assemblies,on that "T" bracket. Then
you have to pull the Peltola cables out from their jacks,then the harmonica
connector.Then the atten module tilts out.

The serivce manual has the procedure in it.Maybe your 'copy' omitted it.

Thanks! I'll try that tonight. The service manual I got from the
downloadable manuals at

http://bama.sbc.edu/tektroni.htm

Maybe it's missing something...

Best,

- Cesar

"carneyke" wrote in
oups.com:

Jim,
At one time, we would remove the attenuator and send it to the factory
in Beaverton, if we couldn't clean them and fix the problem. One of
the best scopes ever made...

Well,I gotta disagree with that.
The power supplies (LV and HV)were very hard to troubleshoot and repair,and
that's the most failure prone part of the 485.
They buried the HV in the *middle of the scope* under everything else,and
there were those comb connectors tying the circuit boards together...

And those attenuators were a PITA.they were always getting flaky.


IMO,the 2465 series was the best of TEK.
Another fav of mine was the lowly 422(non-battery version);15Mhz,but a
sealed cabinet and few problems,just repaint the case,replace the worn
rubber feet,and cal.The insides of the 422's were always nice and clean.
It was a pleasure to take a grubby,scraped-up 422 and make it look new
again.customers were always amazed when they got them back.

--
Jim Yanik
jyanik
at
kua.net

Thanks, I got the module off. Now to a new question:

It seems the 5k pot is stuck somehow. I removed the big shaft and the
shaft/pot connector and it turns out it is the pot that is not turning.
Well, it turns but it is *very* hard to do it. First, what would be the
reason for such a behavior? I never had a potentiometer doing that.

Also, is there anything special about this potentiometer, or is it just
an old 5k carbon film linear potentiometer? I know that the volts/div
switch is all full of special things about it and is irreplaceable. I
just want to be sure that's not the case with that pot (the schematic
seems to point to a normal potentiometer).

Best,

- Cesar

Looks like the pot needs to be replaced or at least to be re-lubed.
Since t will be difficult to get a irect replacement, try this: get teh
pot out, drill a very small hole in the pot housing and apply 1 -3
drops of contact cleaner/lube "Kontakt60" into the housing. Turn the
the pots shaft for a couple of times. Repaeat in necessary. Let dry at
least for half an hour .

I used this procedere a dozen times with success, the 485s I repaired
still work smoothly.

The pwr supply is really difficult, as Jim already mentioned. Always a
challenge to repair. Great job for a rainy weekend

The 2465 is a class of its own. Superb scope, always a good invest to
do.
The 422 is nice too, but the 454A is what I would collect: A little bit
difficult to repair, but rock-solid made.

Well, just a collectors view...


hth,
Andreas

No, looks quite complete to me, no missing pages.

I use it too.


hth,
Andreas

Jim,
I wasn't refering to working on them. The 485 has a very bright / fine
focused CRT. The 1 Megohm / 50 Ohm attenuator with 350 MHZ bandwidth
really made the scope and it's small size was very nice. The low
voltage / high voltage section is a dog but we didn't have many fail. I
did a lot of calibration / performance checks on them. The attenuators
were a little tricky to calibrate considering it had the 50 Ohm / 1
Megohm built into one. I have a 2465B (400 MHz) on my bench and use it
a few times per week. I do like that more than a 485. Before getting
the 2465B (12 years ago) my favorite was a 475A with the DM package. I
really dislike digital scopes as most of my work is troubleshooting,
but the cursors make critical measurements easier. Take Care, Kevin


Jim Yanik wrote:
"carneyke" wrote in
oups.com:

Jim,
At one time, we would remove the attenuator and send it to the factory
in Beaverton, if we couldn't clean them and fix the problem. One of
the best scopes ever made...

Well,I gotta disagree with that.
The power supplies (LV and HV)were very hard to troubleshoot and repair,and
that's the most failure prone part of the 485.
They buried the HV in the *middle of the scope* under everything else,and
there were those comb connectors tying the circuit boards together...

And those attenuators were a PITA.they were always getting flaky.


IMO,the 2465 series was the best of TEK.
Another fav of mine was the lowly 422(non-battery version);15Mhz,but a
sealed cabinet and few problems,just repaint the case,replace the worn
rubber feet,and cal.The insides of the 422's were always nice and clean.
It was a pleasure to take a grubby,scraped-up 422 and make it look new
again.customers were always amazed when they got them back.

--
Jim Yanik
jyanik
at
kua.net

wrote in
ups.com:

Andreas Tekman wrote:
Looks like the pot needs to be replaced or at least to be re-lubed.
Since t will be difficult to get a irect replacement, try this: get teh
pot out, drill a very small hole in the pot housing and apply 1 -3
drops of contact cleaner/lube "Kontakt60" into the housing. Turn the
the pots shaft for a couple of times. Repaeat in necessary. Let dry at
least for half an hour .

I used this procedere a dozen times with success, the 485s I repaired
still work smoothly.

Thanks! I can't find Kontakt60, but I can probably find something else.

The only remaining thing is the 1M/50R bulb. I wonder if it could be
replaced by a LED (with the appropriate resistor).

That is what I would do.The incandescent bulb draws IIRC,at least 50ma,so
the IC will easily switch a LED at 5-10ma. You even have enough V to run a
bright-white LED,but I would use a 3mm LED instead of the usual 5mm.
I installed LEDs in other TEK test gear on my bench because I tired of
dealing with hard to access lamps.(and also like to mod things!) 8-)

The schematic shows
this bulb connected to +5V on one side and to an IC (through a 10R
resistor) on the other. I'd guess the IC pin goes from +5V to GND when
it wants the bulb to light up, so I could put an LED plus a 270R
resistor in place of the bulb. Has anybody done this? If not I will
probably remove the A channel and check what's supposed to happen.

Best,

- Cesar





--
Jim Yanik
jyanik
at
kua.net

Andreas Tekman wrote:
Looks like the pot needs to be replaced or at least to be re-lubed.
Since t will be difficult to get a irect replacement, try this: get teh
pot out, drill a very small hole in the pot housing and apply 1 -3
drops of contact cleaner/lube "Kontakt60" into the housing. Turn the
the pots shaft for a couple of times. Repaeat in necessary. Let dry at
least for half an hour .

Done. I found that my local Fry's carries Deoxit Faderlube (the spray
version). I didn't even need to remove the pots (thankfully, it would
be a pain for the timing one). Amazing stuff that is. Both pots now
turn very smoothly.

Best,

- Cesar