Re: Tektronix D755 oscilloscope excessively bright at switch on

"Ian French" <IanFrench@xxxxxxxxxxxxx> wrote in
news:43060c0c$0$19741$cc9e4d1f@xxxxxxxxxxxxxxxxxxx:

> David C. Partridge" <afb12xwy@xxxxxxxxxxxxxxxxx> wrote in message
> news:4305d224$0$372$cc9e4d1f@xxxxxxxxxxxxxxxxxxxxxx
>> Follow up to tell folks the current status and probable cause of the
> problem
>> (I think):
>>
>> I replaced the old 10nF ceramic smoothing caps in the grid bias
>> supply as they were getting distinctly warm after the scope had been
>> on suggesting that they were getting rather lossy (on a damp day they
>> would steam gently for a minute or two after turn on).
>>
>> Didn't solve the brightness problem but new polyesters do NOT get
>> warm, so
> I
>> feel better :-).
>>
>> Static grid voltage at output of grid cut-off adjustment trim-pot was
>> drifting up during warm up, while voltage on input side was pretty
>> much constant. Suggests it might be the trim pot drifting with temp.
>>
>> The trimpot (R315) is supposed to be a 2M2 linear Cermet 20% 1W
>> according
> to
>> the parts list, but looks to me like a standard Piher PTC15 1/2W
>> part. Measuring the resistance as the scope cools after power off
>> seems to
> confirm
>> the hypothesis (10% increase in value over 15 minutes, probably as
>> much as 15-20% by the time it reaches room temp).
>>
>> If I could source a replacement readily I'd just ram a new one in,
>> but I suspect that the parts list is right and the original WAS a 1W
>> part, and
> has
>> been replaced somewhere along the way with a 1/2W part. Finding a
>> 2.2M 0.5W trimmer of any sort (never mind one that will fit) seems a
>> bit hard, and as for a 1W part ...!
>>
>> Before I go totally insane hunting for a suitable part, I'd just like
>> to
> get
>> a sanity check on my diagnosis from folks more experienced with this
>> than
> I.
>>
>> Thanks
>> Dave
>>
>>
> Hi David,
>
> One posible fix (but a bit clunky) is to find out the values of the
> resistance is from the ends of the trimpot to the wiper, by measuring
> the existing pot, and replacing it with 2 fixed resistors of a
> suitable power rating .
> At least you could then tell if your diagnosis is correct.
>
> Bye for now,
>
> Ian.
>
>
>

I would sub a straight 2 Meg pot,with 100K resistors on each side of it,if
you could not get the original value in the wattage called for.

Chances are a 2Meg/1W pot would work fine by itself,too.

--
Jim Yanik
jyanik
at
kua.net
.

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