[GreenKeys] OT: Need help to T-Shoot this Tek 485 scope
Bruce Gentry
ka2ivy at verizon.net
Thu May 11 08:22:08 EDT 2017
A suggestion to find out if the CRT is bad or not- Connect a hand held
ordinary analog VOM (set on it's highest range to reduce loading on the
circuitry) between the grid and cathode. Isolate it and it's wiring with
care, it may be quite "live" because some scopes put negative voltage on
the cathode as well as positve on the anode. Note the readings and
record the voltage swing from dark to dazzling, and then set the
intensity at a medium level and record the voltage. If the trace becomes
dazzling and the voltage has not changed much, then your suspicions of
the CRT are likely true. If the voltage changes substantially, look
elsewhere. Measure the voltages on the grid and cathode each to ground
and compare proper operation to dazzling bright. If the grid voltage
chances drastically, but the cathode does change much, investigate the
circuitry driving it. If the cathode voltage has a major change, there
might be an intermittent heater cathode short, or something might be
going awry causing an incorrect voltage on the heater, leading to an
intermittent breakdown between it and the cathode. Some designs let the
heater float, others tie it to a fixed DC voltage, some connect it to
the cathode. Intermittents are the nastiest troubles, but a 485 is a
great scope and worth the time.
Bruce KA2IVY
On 5/11/17 7:16 AM, drlegendre . wrote:
>
>>
>> -------- Original message --------
>> From: Raymond Cote <bluegrassdakine at hotmail.com>
>> Date: 5/10/17 1:40 PM (GMT-05:00)
>> To: Gil Smith <gil at baudot.net>
>> Cc: Green Keys <GreenKeys at mailman.qth.net>
>> Subject: Re: [GreenKeys] OT: Need help to T-Shoot this Tek
>> 485 scope
>>
>> Replies ON LIST would be preferred. I suspect we would all
>> prefer to learn by other technicians methods. Replying only
>> to the originator does not help the trst of us learn.
>> There have been many messages that i have been interested in
>> learning about only to see a message that states "thanks for
>> the help in solving my problem".
>>
>> Please, OM's. Consider sharing you methods of tracking and
>> applying your fixes with the rest if us. Many if us are
>> hoarders (collectors) and have a plethora if gear that
>> trouble shooting practices of one item would apply to other gear
>>
>> Thanks fir reading and joining this method if teaching us
>>
>> Bench technician for 50plus years and have a commercial radio
>> phone snd general ham ticket. Every time i open up something
>> to investigate its innards, i still learn something new.
>>
>> Ray Côté
>> KD9CCZ
>>
>> Sent using Jedi Mind Trick!!
>> By
>> Raymond Côté
>> KD9CCZ
>>
>> On May 9, 2017, at 00:58, Gil Smith <gil at baudot.net> wrote:
>>
>>> I found a wealth of info on tek scope repair on a yahoo group:
>>> https://groups.yahoo.com/neo/groups/TekScopes/info
>>>
>>> Yahoo groups is a pain, but that is where they are. I found
>>> lots of folks to help when I wanted to re-cap my trusty 300
>>> MHz 2465 and 150 Mhz 2445. I started with the 2445. I
>>> replaced all of the caps with new/better spec units. When I
>>> got it back together, the trace had some jittering that was
>>> not there originally. F-me. Also, the power switch was now
>>> stuck in the on position, so I need to plug the power cord
>>> in to turn it on. Very disappointing outcome for a couple
>>> of days of work, plus parts cost. I am really pretty
>>> careful/anal and not generally a hack, but I guess I missed
>>> something. I bought a replacement power switch for the
>>> 2445, but I really have no interest in spending hours to do
>>> the R&R.
>>>
>>> As for my prized 300 MHz 2465, I now have no interest in
>>> screwing with it, as it still works perfectly. It has great
>>> resolution for seeing things like undershoot, which my DSOs
>>> cannot resolve. If it craps out some day, I will re-visit
>>> the re-cap (I have the bag of parts), but my original plan
>>> to re-cap is on hold unless needed.
>>>
>>> fyi, gil
>>>
>>>
>>> gil smith, AF7EZ
>>> greenkeys moderator
>>> gil at baudot.net <mailto:gil at baudot.net>
>>>
>>>
>>>
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