Power issue on Heathkit Condenser checker

[Alex_Baker]

Hey forum folks, here is my latest financial mistake. I picked up this Heathkit condenser checker for a few bucks, it is in poor shape but I have always wanted one, and who knows it may actually be useful!

I know this forum does not deal with tube much, but I hope we won't need any special tube knowledge for this repair. Pictures and a schematic are attached below this post.

The issue is that I cannot get any +150 volts, nothing at all. I actually don't have the right tube for the rectifier, which in this case is a diode connected triode, so in the picture you will see that I put a 1n4007 across the socket instead. I don't know if that could possibly the source of the problem or not. I get loads of negative voltage when I power this thing up, like in the neiborhood of 500V!

I guess what I need is for somebody to explain how this power supply is supposed to work, I have messed with a fair few power supplies but nothing like this one. For reference the tube socket in the chassis is for the rectifier and the socket floating in the air is for a cathode ray "eye" tube.

[TimFox]

What AC voltage do you measure across the 500 V secondary, measured carefully?
First, measure without anything connected at the rectifier-tube socket, then with your rectifier diode.
If you have negative voltage but no positive voltage, it suggests a short circuit across the +150 V, possibly the filter capacitor across the 47 k resistor.
(The original tubes seem to be WW II surplus military parts.)

[Alex_Baker]

About 410VAC with the diode unhooked, with the diode in circuit I cannot go full 120V on the input or the negative voltage exceeds the limit on my 450V capacitor.

[TimFox]

The ball-park 650 V output from the half-wave rectifier divides into positive and negative w.r.t. ground due to the ratio of the resistors across each end, plus the magic-eye tube current.
With a 1N4007 instead of the strapped 1626 triode, the total voltage should be somewhat higher than on the diagram.
Another test is to unplug the 1629 and see if the +150 re-appears.

[Andy Watson]

There's probably a break in the chain of 22k resistors along side the switch labelled as "rear section rear side". These form the return path for high voltage supply.

Also, this unit has obviously been worked-on - those are not the original resistors - you might want to check their voltage rating. From memory I believe they are only specified for about 250V.

[Alex_Baker]

Also, this unit has obviously been worked-on - those are not the original resistors -

That was me who worked on it, everything was pretty crusty so there are a bunch of new parts.

Another test is to unplug the 1629 and see if the +150 re-appears.

How would removing the 1629(the 1n4007) allow the +150 to appear? you wouldn't have a complete circuit at that point would you?

[TimFox]

The 1629 is the magic-eye tube, part of the load on the rectifier.
The 1626 diode-connected triode, replaced by a 1N4007, is the rectifier.
I said to try unplugging the 1629 to see if is shorted, etc.

[George Edmonds]

Hi

First may I say that you stand a good chance of  possibly killing yourself with this as it contains very lethal voltages and currents. With no protective ground to the metal chassis

The 1626 is a WWII RF transmitting tube.

You CANNOT use a 1N4007 in place of it for two reasons,

a) The 1N4007 has a much lower forward voltage drop and resistance which will give you some 700VDC total HT.  Not the 560VDC of the original using a 1626 tube
b)  At full mains supply voltage a 1N4007 will fail as with a 500VAC secondary the diode must have a  PIV of 500 X 2.828 which is 1,414V.  The 1N4007 PIV is only 1,000V

All replacement resistors should have a working voltage of some 500VDC, most modern one are 250VDC

Yes, I am old enough to have designed tube circuits in my younger days and know the pitfalls.

G Edmonds

[Alex_Baker]

The 1629 is the magic-eye tube, part of the load on the rectifier.
The 1626 diode-connected triode, replaced by a 1N4007, is the rectifier.
I said to try unplugging the 1629 to see if is shorted, etc.

I see that now, I apologize. I seem to have lost my eye tube in my workspace somewhere, it has not been installed while doing these tests which sounds like it may be a problem.

I with I had the 1626 when I got this device, instead somebody plopped in a VT-126b, which is a dual diode with the correct pinout, but a 6V tube.

[TimFox]

Normally, I liked Heathkit's drafting style in the old days, but they left off reference designators in this one (e.g., C23).
Anyway, I agree with the statements above that this box is dangerous.
Details:
How they got away with a 1626 for a rectifier is beyond me:  its maximum plate rating is 250 V DC (which implies 500 V peak) and the grid-cathode voltage rating is -150 V.
As noted above, you need at least 1500 V prv for a rectifier in this circuit.
With the 1629 removed, the positive and negative voltages with respect to circuit common ("ground") are determined by the resistors in the circuit splitting the 650 V or so total voltage across the capacitors.
Your absence of positive voltage suggests either a short on the positive side or an open on the negative side.
Resistors tend to fail open, while capacitors tend to fail shorted.
I suspect either an open resistor on the negative side, or a shorted electrolytic capacitor on the positive side.

[Alex_Baker]

I found a broken ground on negative side, so an open. Fixing that I get some voltages that make a little more sense.

Unfortunately it appears that the transformer has been replaced, the high voltage secondary is only 410V with 120V on the primary. With a 1n4007 installed and a 70k load resistor on the positive supply I get about +110VDC and -370VDC, neither of which are very close to what they are rated for.

This morning I spent the better part of an hour looking around the house for my eye tube, it seems to have vanished. Not having either of the proper tubes to put in here I don't feel very compelled to proceed with this mess, especially given the transformer situation.

[TimFox]

The 1629 (12.6 V heater, octal base, version of the commercial 6E5) will operate down to 100 V, but I understand why you might want to terminate this project.
https://frank.pocnet.net/sheets/127/1/1629.pdf

[Alex_Baker]

The 1629 (12.6 V heater, octal base, version of the commercial 6E5) will operate down to 100 V, but I understand why you might want to terminate this project.
https://frank.pocnet.net/sheets/127/1/1629.pdf

Yeah, I thought about buying one, but they are $25 to $40 online which it just not worth it for this thing. I will keep my eyes open for another one because I still want a condenser checker, but this specimen is worth more in parts.