Need Transistor Recommendation for Conar 212 Transistorized VOM

[BrandonD]

This is a great meter when it works. The transistors are for the meter circuit of the Conar 212 TVOM designed in the 1970s.

I need an accurate recommendation for low power N-channel JFET’s, that are available to buy. I have the manual but it does not specify.
The meter was working before. 9 volts hit it (my goof), not an overloaded test mistake. I came to the conclusion that it is a transistor because now when I turn it on, the needle hits the right pin and stays there, regardless of settings.

The movement is 200 microamps, and the numbers are linear on the dial (except for ohms), so I assume the transistors need to be linear in the range used.

On the transistors it says TS20 and has a Motorola “M”.

Circuit diagram follows.

Please don’t guess if you are not expert!

Please do tell us why you made the recommendation you did.

Thank you,

Brandon

[bob91343]

Try J112.  They are cheap and pretty good, used by HP and others.  I don't think it's a critical component, especially if you replace both of them.

[fzabkar]

Here are Motorola's old databooks:

https://mirrorservice.org/sites/www.bitsavers.org/components/motorola/_dataBooks/


Edit:

You can see what FETs were used in other VOMs:

https://elektrotanya.com/showresult?what=vom&kategoria=All&kat2=All

[BrandonD]

Thanks for the good advice so fast!

Looks like I can resurrect it. 

Brandon

[octillion]

If one of the JFETs is bad, you can replace it with a JFET with an IDSS of a few mA (like the J112 suggested above).  Both transistors are just source followers.  They should be similar just for the purposes of temperature compensation (as the temperature changes, you'll have to twiddle the "zero" pot less if temperature characteristics are similar).  In the DC and Ohms settings, Q1 drives the meter, while Q2 is statically biased.  In the AC setting, Q1 buffers the AC signal, which gets converted to a negative DC voltage in the doubler above, and applied to the gate of Q2, and Q2 "drives" the meter (it conducts less, and its source resistor is really doing the driving).

You may want to check the JFETs before replacing, just to confirm one of them is the problem.  If either the 6.8 kOhm source resistor of Q1, or the 51 kOhm resistor above Q2 were open, you would have the same symptom.