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Hickok 800K (or is it 600A)

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¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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I have a Hickok 800K that I'm getting back into working condition.

PXL_20210202_083715265_compress33.jpg

It has a sticker nothing that it's "factory wired - 600A circuits".

The paper roll is removed in this picture as I was cleaning the contacts on the switches and didn't want to damage it.

The power cord was in awful condition

PXL_20210202_084636343_compress96.jpg

It looked even worse on the inside...

PXL_20210202_080520431_compress32.jpg

I had desoldered the hot wire before I took this. Heaps of the insulation fell off! The white insulation initially looked burnt to me, but I think it was just pay of the deterioration.

A bit of prodding around revealed a broken wire connecting (or not connecting) to the transformer.

PXL_20210202_072137674_compress36.jpg

That's an easy fix

PXL_20210202_075741101_compress60.jpg

Pretty sure this was a hidden fault, the gap was nowhere as large as shown in the first image.

Only a couple of capacitors. One across the meter

PXL_20210202_072145359_compress87.jpg

That's a polarised 50uF 6V capacitor.

PXL_20210202_074111125_compress6.jpg

Replaced with a 47uF 63V capacitor. The original measured well over 100uF, most likely due to the leakage resistance in the order of a couple of MΩ.

And another one

PXL_20210202_074217533_compress38.jpg

This one is a 0.05uF 400V capacitor.

PXL_20210202_075554346_compress25.jpg

Replaced with a 47nF 630V capacitor.

There are some other caps, but they already to be mica, so they can remain for the moment.

I thought these were capacitors at first

PXL_20210202_072152236_compress15.jpg

But these are resistors. Precision resistors? Anyway, they measure pretty damn close to their markings.

PXL_20210202_082649874_compress38.jpg

This wire was moved up and away from the wire under it, and the transformer terminal below it. It looks like the wire under it was almost shorted (the insulation seems melted) and there was no visible has between it and the terminal. This may have been the second possible fault in the ddevice

The transformer primary reads about 15Ω, which is about what I expect. The 2 tubes look good, but they're only rectifiers, so I could replace them if necessary.

Next step is to grab my auto transformer and dim bulb tester to see what's happens...
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Oh, here's the back of the main panel

PXL_20210202_072127581_compress42.jpg
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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This device came with a be spare resistor

PXL_20210202_102656601_compress70.jpg

15kΩ measured as 17.3kΩ

I'm not sure where it goes, or even if it goes somewhere.

First test will be without the valves. Just to test the transformer for shorted turns, or other reasons for smoke that don't rely on any of the rectified rails.
 

(*steve*)

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Looking at the schematic for the 800K and the 600A, there is a 15k resistor and thankfully they are in the same place. There seems to be quite a few differences in the wiring between these two units.

Fortunately, the obvious place to put it is across a switch, and that may explain the shape of the leads.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Not wanting to damage the valves, I decided to make a couple of solid state replacements for the 83 and 5Y3 tubes.

PXL_20210203_035012537_compress40.jpg

Luckily I have the appropriate bases on hand.

PXL_20210203_035026712_compress16.jpg

The 4 pin socket had large and small pins (not totally obvious in the previous short.

PXL_20210203_035041848_compress17.jpg

And the innards are decidedly not original equipment.

Both of these tubes are directly heated, so the filament is replaced by a pair of 22Ω resistors in series. This draws far less from the 5V winding than the original valves (each had well in excess of 1A heater current)

The two plate connections connect to the AC inputs of a 1000V bridge rectifier via a pair of fuseable resistors that are technically rated for 250mA continuous current, and fail almost immediately at 1A. I hope they provide protection for the transformer if the bridge rectifier fails. The -ve output is clipped off close to the body as it's not used.

So far everything is the same for both, the difference comes in how the rectifier is connected to the "heater".

The type 83 tube is a mercury rectifier and exhibits an almost constant voltage drop. As advised by a number of web sites, the +ve output of the rectifier is connected to the middle point of the "heater" with a 5.6V 1W zener.

The 5Y3 does not exhibit a constant voltage characteristic, so the connection is made using a pair of 1kΩ resistors in parallel.

Now the next problem... (or problems)

Firstly, I can't figure out how to connect the 15kΩ resistor because it's a bit busy in there. That's just a mechanical thing though, I'll find a way.

The second and more serious issue is that the current drawn by this device increases dramatically once the input voltage hits 120VAC. In fact, the voltage is held at 120V by the series lamp.

Just to make things more fun, I get the same behavior with the tubes (in this case their replacements) removed.

The AC voltage on various secondary windings still increases as the primary current rises. Unfortunately, the analog meter on my variac has a fsd of 3A and I'm not seeing that deflect. So the current is still small, and the non-linear nature of the series 80W bulb may be fooling me.

Nevertheless, I'm looking at some capacitors that were not replaced (they look like mica) and I'm wondering if there's something up with them -- I know that there were paper capacitors mage in a very similar form factor. Could it be that?
 

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If you're interested in the tubes themselves:

PXL_20210203_043116670_compress56.jpg

5Y3 to the left, type 83 to the right
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Mains current drawn at different voltages:

AC VoltagemA (with lamp)mA (without lamp)
107.58.5
201314
301718
402121
502626
603131
704140
805553
906974
1009596
110128127
120n/a187


The current is not significantly different -- differences are probably due to reading the AC voltage on an analog meter and not taking a lot of care :)

Also noted a 2A primary mains fuse.

I'm going to check for shorted turns (by looking for very low secondary voltages) if that cap isn't the problem.
 
Last edited:

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So it turns out the bulb I was using was 60W, not 80W. When I bump it up to 120W it doesn't even glow and allows me to wind the voltage up to at least 130V.

The problem now is that with the more lightly loaded transformer, the input voltage adjustment does almost nothing to set the meter to the line test position. And weirdly, it shows a value that is too low...

To get 6.3vac on the heater off an installed tube, the mains voltage needs to be reduced to about 95V, but it's also clear that this will vary under load.

I think I need to find out where that missing resistor connects before I do anything more.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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I've found another very fragile wire that broke when I looked at it a bit hard. I suspect there are more, and they may be hiding in hard to reach places.

I have also noted some obvious wiring changes that appears to be old and most likely associated with the factory rewiring to 600A.

Alongside these, there are several nice shiny solder joints that appear to indicate someone has been working on this more recently. The quality of soldering varies, and I suspect that more than one attempt has been made on this.

I have been reluctant to touch any of the trimpots, assuming their settings are "correct". But it's also possible that these have been played with too.

I'm a bit reluctant to put the original valves in for testing because (for one thing) they dissipate 25W into a fully enclosed chassis. Unfortunately it's also rather inconvenient to operate in a manner where you have access to both sides of the front panel (everything is mounted on the front panel) and would be even more inconvenient with a pair of fragile glass envelopes fitted.

As soon as I have access to a laser cutter again, I'm going to make a jig to allow safer access to both sides and some much needed ventilation
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Just for completeness, the type 83 has a 3A at 5V heater, and the 5Y3 draws 2A from a separate 5V winding for its heater.
 
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