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Jackson 648A - A couple of issues need resolving.

P

Peavey78

New Member
I have had this tube tester for like 25 years now. Just did a calibration based on Bob Putnak's webpage about the 648 series. Did the solid state conversion first, then ran through the various test procedures. All went well! I still have 2 issues with the tester. First, the Life Line switch is malfunctioning. For those that are not familiar with it, it is a momentary toggle (SPDT) which drops the filament voltage 10% when the switch is thrown. It had worked in the past, but now it just goes open when I hit the toggle - heater voltage drops to zero. If anyone can point me to a proper replacement, please do. The other issue was the leakage test. Bob said use the shorts test setup, but instead use a 1 Meg resistor in place of a short. Should see the shorts light dimly on at the 1 Meg setting, and get brighter as you advance to 2 Meg. But I don't see the glow at 1 Meg, and it's barely there at 2 Meg. There is a 220K resistor in series with that shorts potentiomer and I was wondering if dropping it to 180K or even 150K would alleviate this issue? Bob did say to look at the pot and resistor if there were any out of the ordinary issues, but the pot does measure 1 Meg per print, and the 220k measures 223K - not far off. How to proceed? All else is well with this tester - plate, screen and grid voltages all line up as per testing.

-Bob-
 
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Have you disconnected and tested the life switch? It's just a standard monetary single pole switch. As far as the short indicator I suspect that the neon bulb needs replaced or someone replaced it with the wrong one. It has to be a NE51. Neon bulbs can have different characteristics. I had to replace mine. I have two videos on the Jackson 648R if they can be of help. The first is repair, and the second is an in depth circuit analysis of how it works.

Ron S.
 
Oh and I forgot to mention capacitor C. It can cause issues with the leakage test. It may need replacement if you haven't.
 
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Yes Ron, I did desolder 2 wires from the switch to test it, it is definitely open when you momentarily toggle it. Sometimes I have to hit it again to get the other side to work again. The bayonet bulb has NE51 printed on the face, however I have a Mouser order coming in a couple days with a new bulb, as well as that capacitor... and other rebuild parts for my other tube tester. The second video was very informative in how this machine does what it does, thank you for that. I should also have mentioned the pots and switches all have had the DeOxit treatment, however I have not tested or replaced any of the resistors. When I do I will use 1% types.
When all is complete I have upwards of 400 tubes to test (most inherited from granddad years ago) and weed out the defective ones, then put the rest through my uTracer curve tracer and get accurate pictures of what is really going on inside. Thank you again.

-Bob-
 
Can you open the switch up to access the contacts? Might need to be polished mechanically if they aren't just smoked.

If the switch isn't available, but similar ones from the same brand are I've had some luck with buying a new one for parts to rebuild old ones before but i know nothing about this specific switch
 
Life-Line_Switch.jpg

Gadget, I may be able to grind off thos 2 rivets with a Dremel or small file. Probably will, as it's no longer functioning at all. For now, I just joined the proper wires together to get the right filament voltage per the selector setting. Going to also ask some radio forums to see if anyone has this. Hard to see from this pic, but it is a sandwich of 7 layers of what looks like bakelite. The common terminal is behind the right side. In this position, continuity goes to the left terminal. When pressed, continuity goes to the visible right terminal. Playing with the toggle, I can occasionally hear a small rattle inside. It will be interesting to open it up - never did a spring-loaded one before.
Oh my Mouser order came 2 days early, so I replaced the capacitor, and it had no effect on the leakage test. I will keep looking for a suitable switch replacememnt.

-Bob-
 
No, not the cheapest thing - but is sure IS the thing. Data sheet says same hole size as the original, same thread as well. Good find, and thank you! It will be in my next order - as I mentioned earlier, I never swapped out the resistors yet so those will be ordered as well. It's a really fine tester and I just want it to perform as it did new.

-Bob-
 
Peavey78 said:
View attachment 3728439

Gadget, I may be able to grind off thos 2 rivets with a Dremel or small file. Probably will, as it's no longer functioning at all. For now, I just joined the proper wires together to get the right filament voltage per the selector setting. Going to also ask some radio forums to see if anyone has this. Hard to see from this pic, but it is a sandwich of 7 layers of what looks like bakelite. The common terminal is behind the right side. In this position, continuity goes to the left terminal. When pressed, continuity goes to the visible right terminal. Playing with the toggle, I can occasionally hear a small rattle inside. It will be interesting to open it up - never did a spring-loaded one before.
Oh my Mouser order came 2 days early, so I replaced the capacitor, and it had no effect on the leakage test. I will keep looking for a suitable switch replacememnt.

-Bob-
Click to expand...
I have a good collection of old switches, pots & other odd bits & am 99.75432% that I can go right to it.
Before you try drilling rivets(then how to reassemble?), pm me later today, I'll look when I get back home. If I score, its yours, no charge.
 
So, I finally did my replacement of the Life Test switch (Thanks again to Cat for the offer, but I did purchase one), and replaced all the resistors as well. Some of those were way off base. Resistor R was within 1% tolerance, but some outliers were R10 (3.9K, measured at 4.75K), and R7 (510 ohms, measured at 693 ohms). All are now at least 1%, most closer to .1% or better. As the top cap wire was broken off, I added a banana jack to the unit as well, and soldered a plug and new wire to the clip. The tester works just fine now. I just wish I had done a before-and-after test of a few tubes prior to the resistor changes, to see how far off it was. The leakage test works as designed, as well.

Jackson_Bottomside.jpgJackson_Topside.jpg
 
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