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 Post subject: Weston 798 Type 3 tube tester problem
PostPosted: Jul Tue 27, 2021 10:13 pm 
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Location: Mesa, AZ 85206
Hi folks

I decided to start a different thread here, as I have spent the better part of the last three days trying to figure out a problem with the unit, a Weston 798 Type 3 tube tester.

I got it knowing it would take some work.

What I have discovered in the restoration process is that when the tester is set to test a 6V6, the cathode (pin 8) and the plate (pin 3) have continuity (22Ω or so...)

I have completely gone through tracing pins 3 and 8 in the tester, and am now observing that the two meet up i shorts/tube tester switch (which of course makes sense). I can so far see nothing out of the ordinary, but the 3b tester switch looks like it can allow that to occur!

The only schematic I have is for the Type 5b...and that schematic for the shorts/test tube switch is very different from my observed unit...the 5b schematic clearly shows that the cathode and plate are disconnectes in the test tube position.

So my ask is two fold: does anyone have a schematic for the type 3? And, if you have a type 3, could you please use the test settings recommended (switch B at 1, switch C at 2) and see if there is continuity between pins 3 and 8 at the regular octal socket?

Thanks!

Steve


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 Post subject: Re: Weston 798 Type 3 tube tester problem
PostPosted: Sep Tue 21, 2021 4:16 pm 
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Location: Mesa, AZ 85206
Hi.

I just wanted to bump this up as I am still looking for a schematic!

I am currently just stuck in trying to figure out why the plate and cathode are tied together when the switch is in tube test position...when I compare the circuit between my type 5b and this unit it appears that the shorts/tube test switch is wired incorrectly.

Thanks again if anyone has this schematic!

Steve


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 Post subject: Re: Weston 798 Type 3 tube tester problem
PostPosted: Sep Tue 21, 2021 9:42 pm 
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Location: Sunbury, Ohio 43074
I suppose the missing question here is .... does it test the 6V6 correctly? If so, chalk it up to a design change between the models and assume its supposed to be that way :)

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 Post subject: Re: Weston 798 Type 3 tube tester problem
PostPosted: Sep Tue 21, 2021 9:53 pm 
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Joined: Mar Sun 20, 2016 11:04 pm
Posts: 1141
Location: Mesa, AZ 85206
Thanks! But since the plate is tied to the cathode, there is no plate voltage differential... So, no go...

edited:

Went back and looked at my notes...I had set this aside a few months back...

All Gm readings were 1/3 of what they should have been...that started my looking at plate voltages...that's when I found the ties to cathode...

Steve


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 Post subject: Re: Weston 798 Type 3 tube tester problem
PostPosted: Sep Fri 24, 2021 12:51 am 
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Joined: Mar Sun 20, 2016 11:04 pm
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Location: Mesa, AZ 85206
Well, I have figured out the cathode-plate issue, and am now trying to figure out why the Gm readings are about 30% low across the board for amplifier tubes for a given setting of the D control, but with accurate diode/rectifier readings.

There is a 600Ω calibration pot on the signal path, but the wave form looks like my other, and the differences with the pot either wide open or fully closed do not bring the unit into calibration.

I am again against the issue of a schematic...the type 3 I have in front of me has significantly different "D" and "A" controls from the type 5...the type 3 has dual pots, the type 5 has a single pot...and without a schematic I can't even be certain of the correct values of the D pot.

Almost all of the diodes either test at full CCW or full CW of the D control (meter sensitivity) so I can't check mid range of the D control of the diodes.

So, again, I am hoping that somewhere, sometime, someone has a schematic for some other version of the Weston that is not a type 5...I know that folks here would be swimming in as dark a lake as the one I am in without one.

Steve


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 Post subject: Re: Weston 798 Type 3 tube tester problem
PostPosted: Sep Wed 29, 2021 3:11 am 
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Location: Mesa, AZ 85206
Fixed now...interesting in its final solution.

The capacitor (circled in red) is a mica cap...it lies underneath a small standoff board, so the value of the mica cap cannot be seen unless unsoldered and removed from the "reactor"/inductor.

When measured it with one end off, I got 98.9nF, or about .098µF...the type 5 schematic showed it as being 0.02µF, but given the closeness of measurement to 0.1µF, I figured it was likely just a change in production.

Well, today I asked myself "when was the last time I saw a mica cap that large"...unsoldered and pulled out the mica cap, and it was labelled as 0.02µF, but measuring about 0.1µF...

Replaced it with a 0.02µF cap, and now the Gm values are reading as they should...

edited:

This cap, at the measured value of about .1µF yields about 310Ω reactance to the 5kHz; at the correct value of .02µF the reactance is about 1600Ω....so, I was losing more of the signal to the Gm switch with the out of tolerance cap.

I should have been more suspicious earlier, but put too much trust in mica caps....

Steve


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