What dummy load to use for Tektronix 2465 power supply?

[slburris]

I'm trying to bring a Tektronix 2465 (no A or B) back to life.  I've recapped the power supply boards and I get some front panel lights, but it's hung otherwise.

The +5v digital is 4.3v and many of the other related voltages read low.  This would be easier to work on outside of the scope.  Does anyone know what the minimum dummy load should be?  Is it sufficient to load just the +5 digital since that's part of the feedback loop?

Scott

[David Hess]

The dummy load specifications are in the service manual.

[Bryan]

I would suggest joining the yahoo TekScopes group. Loads of information on recapping the 2456x scopes. As Dave suggests it's all in the service manual, but what I used below. A user recently had a issue similar to yours and it was because one of the caps he replaced was reversed. There is a  mistake in the schematic. I believe it's C1034. Also look for shorts on your recaps.


A 2 ohm 25 watt resistor on +5vd . 100 ohm 10 watt resistors on the +5,+15,-15, -5 v, -8v,

On the 87v I used a 2k resistor. In the theory of operation in the service manual. 87v rail should not exceed 100ma. -8v rail should not exceed 480ma.. +42v rail should not exceed 400ma, so I used a 150 ohm. Basically size your resistors accordingly so not to overload and have the current limiting kicking in.

I did not find it necessary to load the +10v reference.

Hope this helps

[slburris]

Ah, it would help if my printed copy of the service manual was actually complete :-(

Looking at a PDF, I see the +5v digital test load, need to update my binder.

I am subscribed to the various Tek groups on Yahoo, although I've ignored them for awhile
since Yahoo keeps screwing around with groups.  I found some discussion about C1034
but it's not clear *why* it was put in backwards.  Was the silkscreen on the board wrong?

I'll put out a note to the Tek list if I can't find anything about low +5 digital.  I'm hoping that
there's a well-known part to check.  If not, I'll build the dummy load, pull out the rubber gloves,
get out the portable multimeter and handheld scope, and wade into the line side of the power supply.
Not my favorite task as I'm much more of a digital guy than an analog guy :-)

Scott

[Bryan]

I'll put out a note to the Tek list if I can't find anything about low +5 digital.  I'm hoping that
there's a well-known part to check.  If not, I'll build the dummy load, pull out the rubber gloves,
get out the portable multimeter and handheld scope, and wade into the line side of the power supply.
Not my favorite task as I'm much more of a digital guy than an analog guy :-)

Scott

I had a issue with the opto-isolator failing after a recap. The issue was discussed in a rather lengthy thread on recapping my 2465b. Will see if i can find the Yahoo forum thread Would be a strange coincidence, but who knows.

[slburris]

I took this over to the tekscopes group and the issue turned out to be the feedback voltage was not getting through to the optoisolator via the pins between the two boards of the power supply.  Once that was fixed, the power supply came into regulation.

Scott

[Bryan]

Wow, same issue as mine. Wouldn't have been able to fix it without the fine folk on the Tekscope group. Appears a recap must stress the opto's past their failure point as they age.