开云体育

GE X44HR242

Jared,

Your manual lists S3 as 4P6T. Entering that part number in Digikey shows it
as 2P6T. Is there a typo somewhere?

Tom,

I measure 2.6 K ohms between the two non-wiper terminals, but the pot is still in circuit, so this is really measuring everything that bridges the +50 V and ground planes in parallel, so is this a meaningful measurement?

It seems like I've eliminated every other possible suspects, and I don't have any other explanation for the measurements I've gotten for the pot resistance (but what the heck do I know? Am I wrong about what I expect to measure on a 25 K ohm pot?).

But I've come this far, and this scope was never meant to be anything other than a parts mule, so taking it down to the studs seems like the logical next step.

-- Jeff Dutky

Note that Jared has updated the main PWB Gerber file and the Instruction
Manual file; if you downloaded those earlier, you need to get the new files.

Jared and I are discussing how best to order these sets of PWBs. He is
familiar with the ordering details specifying board material, plating
and edge chamfering so I welcome his support on this. I will order the
board sets and repackage and ship them to those who commit to the
purchase. The produced boards should look like those in his YouTube video.

Jared,

Is there a way to easily bring the EBC of each pass transistor to the front
panel?? These transistors are items that often fail and need to be tested,
especially on these old frames.? Not trying to throw a wrench into the works,
but those pass transistors are a major point of failure and they can cause all
sorts of damage to the plug ins when they are bad.? Even if you could add some
pads near the appropriate connections at the rear to allow those of us who
wanted to add such functionality at our option?

--
Michael Lynch
Dardanelle, AR

I have one of the originals, and it is almost certainly "Tek" built as
opposed to built from the plans. For one thing, it has a Tek serial number
label and silkscreened front panel.

Dave Casey

I believe the notch filter could be built in any form factor. IIRC it does
not use power from the mainframe, so there's no reason it has to be in a
TM-style enclosure.

Dave Casey

I lifted one end of C1374 and tested it with my DMM916. It acted much more like a capacitor than like a resistor: while measuring resistance I could see it charge up (the resistance increased over several seconds till it became open circuit) then I switch over to voltage measurement and watched it discharge from about 1.1 V down to 0 V over several seconds). Using the capacitance measure shows this is a 0.8 uF capacitor, which is in spec for a the 1 uF electrolytic shown in the schematic.

i checked CR1371 and CR1373 in circuit using the diode check mode of the 916 and they both measured 0.536 V forward voltage drop. They look good.

I measured the trim pot (R1375) while C1373, C1374, and VR1374 were lifted. The trim cap has a range of travel of about 280 degrees:

0 deg = 0 ohms
45 deg = 3.1 K ohms
90 deg = 4.8 K ohms
135 deg = 7.0 K ohms
180 deg = 6.75 ohms
225 deg = 4.3 K ohms
270 deg = 2.7 K ohms

The specified value of R1375 is 25 K ohms, and I expect that it should range smoothly from 0 ohms to 25 K ohms.

I also verified continuity between R1375 and both the ground and +50 V rails.

I checked C1373 (the cap in parallel with VR1374) and it measured as 96.5 nF (it's specced as 0.1 uF on the schematic), so that looks good.

VR1374 is not shorted (I didn't check that it's breakdown voltage was 82 V, as I sadly have no way to do that).

This really seems to point to the trim pot as the culprit, which means I need to get the main interface board out. Joy.

-- Jeff Dutky