Replace a Crystal Diode

[Winky Dink]

I pulled out a Heathkit T-4 Signal Tracer which was almost-restored six years ago. I did all of the electronic restoration except for the signal probe which I found confusing at that time. The T-4 detects RF and audio, has a noise-locating function, and can be used as a substitute speaker. This is the first time I've had any practical use for it.




I went over every step in the assembly manual and found no problems until I got to the probe. Here is the electronics in the probe. Sometimes the simplest things confuse me and I need to make a physical real-world drawing to understand the schematic. Here I just modified a Heathkit image of the probe's terminal plate. Yes, it was miswired and the yellow arrow shows where that conduction should go. A should go to C instead of B.



Link to Flickr photo: https://flic.kr/p/2o3Z5ZG

Incidentally, when I desoldered and untwisted the leads it was apparent that it had originally wired correctly. So, the leads from the capacitor and crystal diode have been twisted, soldered, desoldered, untwisted, soldered, desoldered, untwisted, and need to be soldered and twisted one more time. I want to replace them because of the possibility of having an undetected break in the leads.

The .001 disk cap is not a problem, but I don't kknow anything about diodes. (Except that they can be really bad if you put them in backwards.) So, I have diodes 1N401, -4002, -4004, -4005, -4007, 1N5399, and NTE125.

QUESTION: Can I use one of those to replace the crystal diode?

Thanks, Henry.

[jr_tech]

Originally Posted by Winky Dink

The .001 disk cap is not a problem, but I don't kknow anything about diodes. (Except that they can be really bad if you put them in backwards.) So, I have diodes 1N401, -4002, -4004, -4005, -4007, 1N5399, and NTE125.

QUESTION: Can I use one of those to replace the crystal diode?

Thanks, Henry.

Those all appear to be silicon power rectification diodes... I suspect that Heathkit used a sensitive germanium diode such as a 1N34 or 1N60 in the probe.
If the original diode checks ok, I would use it.

jr

[Winky Dink]

Thank you, kind sir. I'll preserve the crystal diode and finish the tracer tonight.

[Electronic M]

Originally Posted by Winky Dink

Thank you, kind sir. I'll preserve the crystal diode and finish the tracer tonight.

If it happens to test bad a low voltage drop signal Schottky diode would be a good replacement.

[Tube TV]

The original heathkit was a 1N34. Any detector diode in a am radio would work.

Yes it looks to me like it should go to C. It's bypassing the .001 capacitor and just going (probetip - diode - line to sig tracer) and there is no direct setting when the switch is set to direct mode.

[Winky Dink]

I rewired the probe and put the whole gadget back together. Powered it up and the eye tube didn't glow. Took the gadget apart again and found that of three tubes in parallel, only the first one was working--no filament voltage to the other two. The original solder connections were "stick the wire through the hole, then glue them together with solder." I redid a lot of the connections from scratch, re-melted and added solder to most of the others, and so far it seems to work well.

One more thing. I know Electronic M wants to see this. It seems that the electrons were confused about which was to go when they got into the probe. So, to ensure optimal function, I added a reproduction of the original Heathkit decal. (And cleaned the aluminum, renewed the plastic, and polished the screwheads--all for a tool!)



Thanks for all your help. Now I can try to locate the noise in my TV, which is why I started this whole thing.

[Tube TV]

If the TV is a transformerless set be sure to us an isolation transformer.

[Popester]

I would add that regardless of whether or not the tv is transformer less or not. ALWAYS USE AN ISOLATION TRANSFORMER WHEN SERVICING ANY TV/RADIO.