Hickok 533 Restoration - Inviting Feedback

[Geoff]

I'm getting ready to refurbish a Hickok 533 tube tester and am inviting any feedback from the forum. I will most likely replace the 83 rectifier with a solid state replacement since the original I have is bad. The 5Y3 tube I have is good so I will keep that.

I'd like to get some feedback from the more knowledgable forum members about what their experiences are refurbishing this particular model. What type of cleaner do you use on the rotary switches, push-buttons, etc.? Of particular interest I'd like to get some feedback on calibration. Any recommendations or feedback would be greatly appreciated.

- Geoff

[marty59]

It's been advised against replacing the tubes with SS because they provide the proper loading to the tube under test whereas a solid state component does not, besides increasing the voltages in the tester, and throwing off the value and calibration.

I'd also caution on spray cleaners/lubes due to the residues they can leave behind, and can conduct and soak into the the wafer material.

I have not done alot in this area of expertise myself but there are some facts out there and besides input from here some research should be in order.

[Geoff]

I tested the resistance on the meter. Schematic calls out 80 ohms. I got 64 ohms with wires still connected to the meter. Do I have a bad meter or does this seem normal?

Thanks,
Geoff

[Geoff]

I went through this tonight, checking resistors and replacing the 0.1 mfd tubular capacitor. Amazingly most of the resistors are within tolerance, even the carbon comps.

I disconnected the wires going to the meter and got 80 ohms so that appears to be fine.

But I did find something interesting. There is an 8,500 ohm resistor installed. There is no 8,500 ohm resistor in the parts list. Here it is:





Not sure what it goes to since it is not called out anywhere. The only thing I can think of is I've been able to trace out all of the resistors but cannot find R6 (6K resistor). Could this 8,500 ohm resistor be a substitute for the 6K? And what is the band around the 8,500 ohm resistor. It has no affect on resistance when moving it back and forth.

I also noticed that there is a 330K resistor installed where there should be a 500K (R10):



Both of these mystery resistors appear to be original. Anyone have any thoughts on these?

And one last thing. I want to make sure I am reading the two mica capacitor color codes correctly. I am using the color coding chart found here from Radio Daze:

http://www.radiohistoria.fi/yabbfile...2_edited-5.jpg

This one I believe is 470 pF (0.00047 uF). This is most likely C2 (0.0005 uF, 1000V) but the green dot indicates that it is only rated for 500V.



This one I believe is 240 pF (0.00024 uF). However, the schemamtic does not call out for this capacitor. It should be C3 (0.0027 uF, 1000V). Again, the green dot indicates it is only rated at 500V.



Any feedback on what I found would be greatly appreciated. The schematic is located here:

http://antiqueradios.com/gallery/v/t...matic.pdf.html

Click "Download document"

- Geoff

[stusnyder]

Might be some sort of factory revision that was undocumented.

[N2IXK]

The band around the resistor is used to set the resistance value. The resistance element is exposed on the opposite side of the resistor, and the band can be positioned to any point along the length.

Using an 8500 ohm adjustable to replace a defective 6000 ohm sounds like a reasonable explanation. What is the measured resistance of the resistor as installed? somewhere near 6000 ohms?

[earlyfilm]

Originally Posted by Geoff

Could this 8,500 ohm resistor be a substitute for the 6K? And what is the band around the 8,500 ohm resistor. It has no affect on resistance when moving it back and forth.
- Geoff

Geoff,

Yes, this probably was a substitute for a 6K and someone (probably factory) made a fixed-adjustable out of a wirewound. They used a wire-wound and scratched off the ceramic coating, so a dimple in the band could touch the resistance wire.

The band was probably was bumped at some time and the contact point on the inside of the band no longer touches the resistance wire. Check to see if this has happened, or it might simply be that the connection just corroded over time.

James

[Geoff]

I fully examined the 8500 ohm resistor and did not see anywhere that the outside was scratched off so it could make contact with the band, However, with the band removed I get a reading of 6,850 ohms.

Well, I think my Bias pot is trashed. It is R7 (3,000 ohm pot, schematic posted in response above).



I tested it before cleaning and in certain areas around settings 40 – 80 I get alternating spots that show open and spots where the resistance can jump as high as 15K.

I then fully disassembled it:



I then cleaned the restive element with non-residue contact cleaner. Here are the results:





I then put it back together and tested just the element. It was 2.98K:



I then tested it off-line and got all kinds of spurious readings. Disregard the settings as the dial scale was not set properly:











I then reinstalled it, set the dial scale properly, and hooked the wires back up.

Very strange results. When installed in the chassis and the chassis is in my makeshift chassis holder I seem to get better results but things still jump around a lot between settings of 40 – 80. However, when I rotate the chassis so that it is laying flat like it would in the case it shows open most of the time between settings of 30 – 80. In other words, it behaves much worse when laying flat.

So it looks like this pot is trashed. Not sure why as the wiper seems to be making good contact. What’s everyone else’s opinion? Would it behave better if I were applying power to it? If I need a replacement would a generic 3K ohm pot work? Anyone have any recommendations where I could get one?

Thanks,
Geoff

[Reece]

Not much to a pot. The resistance element is either continuous or partly open at some point(s). The wiper needs to bear on the element sufficiently. I'd examine the resistance under a strong light and with an ohmmeter along its length. A DVM has a tendency to jump around also: try it with an analog ohmmeter.

[bandersen]

Those pots have a weird taper to them so a generic replacement won't track properly. I'd do as Reece suggested and try to get it working.

[Zenith6S321]

I had the same problem with my Hickok 539B's bias pot. I found a couple of places where the resistance wire had broken. I fixed it by carefully locating where the two breaks were, and then applied conductive paint. I got the paint in a kit at an automotive store for use to repair a rear window defroster resistive grid. The paint has a much power resistance than the pot resistance wire, which I was not able to solder to. That was 1.5 yrs ago and it still works fine.

Dave

[Geoff]

The bushing the shaft goes through is connected to the center lug. The shaft has the wiper connected to it. So I set my tester to test continuity between the shaft and the bushing (lug). The results showed that is what was causing the intermittent readings. I was actually able to get an open reading if I pushed sideways on the shaft in certain areas. I'm confident that the wiper was still making good contact when pushing the shaft sideways. It doesn't happen in all positions but it is there.



Can anyone recommend a fix for this?

Thanks,
Geoff

[N2IXK]

How about greasing the inside of the bushing with an electrically conductive grease like these:

http://www.cool-amp.com/conducto_lube.html

http://www.mgchemicals.com/products/...e-grease-8463/

http://www.amazon.com/Chemtronics-CW.../dp/B00447S32W

[earlyfilm]

Originally Posted by Geoff

Well, I think my Bias pot is trashed. It is R7 (3,000 ohm pot, schematic posted in response above).



I then fully disassembled it:



I then cleaned the restive element with non-residue contact cleaner. ff

Geoff,

I don't have a 533, but checked this pot on slightly earlier and slightly later Hickoks.*

I looked at your picture and then at my Hickok 540 (which I don't plan to restore, as I'm not into pre-WWII radios) and my Hickok 600A (which is in line for a restoration by me, as soon as I get some of my other test equipment calibrated) and I don't see the contact slider plate in your second shot.

Is it there? It should be a brass plate with a right angle bend and have a hole the size of your 1/4" shaft on one end and the other end has a hole that the center slider screw fits through. This plate completes the circuit from the slider to the center terminal.

Have you considered the possibility that someone may have disassembled the pot before you and left something out, or put it together out-of-order?

* Hickok timeline:
540 1942-1945 military, civilian 1945 until stock ran out, and usually like mine, wired as a 560.
533 1949-1952
533A 1953-1959
600 1949-1952
600A 1953-1958


Warning on using any conductive grease! If any gets washed on to the resistance element support by cleaner, then you may have just ruined the pot.

James

[Geoff]

James,

Yes, that appears to be what the problem is. I was thinking that there must be something other than the just the shaft making contact with the bushing so I took it apart again today. The problem was with the way the wiper makes contact with the brass bushing. Here is the underside of the wiper:



The place where I am pointing to is what makes contact with the bushing. Here is the bushing it makes contact with:



If there is too much play in the up and down movement of the shaft then the area that I am pointing to does not have a consistent and reliable contact with the bushing.
The wiper is crimped on:



What I plan on doing is (carefully) removing the wiper and putting a bend on the circular section that I was pointing to so it will stand up more and make a better contact with the brass bushing. It may be a day or two before I have time to do this. If anyone has a better idea please feel free to offer it up.

- Geoff