I have bought this thing a couple years ago at a hamfest for a few pounds. I wanted something on my desk that I can use for my vacuum tube experiments. It fits the bill, it tolerates high voltages well, and has high enough input resistance, so I took it home.
At hamfests, if you ask the seller if the equipment you laid your eyes on is working, they will always say yes, if any light comes on in the front panel when it is powered up. I bought a few old piece of gear for cheap, and every time I asked I had the same answer. How they actually worked in reality, was another story.
This one was in a fairly good shape, it actually powered up, all the tubes were lit inside, and it showed signs of life in all possible ranges. It was not accurate nor stable of course, but I did not expect that from a gear more than 60 years old. So I thought I will give it new caps and see how it goes.
First, I did some research to find the correct documentation. I have found a schematic for the variant B, which is apparently very similar, the only significant difference is that it lacks of a regulated power supply, and probably because of that, the internal voltages are slightly different.
The disassembly was fairly simple, after removing the right screws, it has fallen apart to three large pieces, and all the internals became accessible. I quickly made a list of the caps inside, and put my order online. So I thought. Usually, caps for the same spec manufactured these days are significantly smaller in size than the ones you can find in old equipment like this. Not my package which apparently had one cap inside with the size of a practicing hand grenade, making the whole parcel too big to fit through my mail slot. It seems that what supposed to be a 3000 μF/25V was actually a 3000μF/400V cap. I checked my order, and yes, it was my mistake. I thought that was for HV, but no, it is in the low voltage section. So now, I have a spare steam roller if I or anybody else ever need that capacity, for like a railgun or something. Just because I do not have enough unnecessary crap lying around already. 😀
Whatever, I replaced the caps and turned the thing on. In the meantime, I have managed to find the correct service manual online so I could check the power supply voltages. In this thing even the heating voltage is regulated! That part was actually fine, but the HV regulator was off and I could not set the right voltage. It is quite a textbook regulator with a cold cathode tube as a voltage reference. So I checked the voltages at the important places. Apparently, the regulator could not reach its working point, and the reason for that was the voltage coming from the rectifier was not high enough.
As it seems, they used old-fashioned selenium rectifiers in the power supply and they deteriorated. The dead giveaway is the smell they produce (which is actually the smell of toxic fumes so do not inhale it if you bump into something like those). So I replaced them with a pair of HV silicon diodes, but this time it produced too high voltages for the regulator.
In contrast to silicon diodes, which have approximately 0.6…0.7V voltage drop across their terminals when they are conducting, selenium rectifiers has a several tens of voltage drop, and I had to simulate this somehow, so I added a few hundred ohms of resistors in series to the diodes to reach the same. It has the additional advantage to protect the diodes from the inrush current when the buffer caps are charged the first time when the unit is powered up.
It worked out nicely, but I still could not set the regulated voltage. So I have checked all the tubes, and the resistors, and I found that one of the resistors were way out of tolerance. After I replaced it, the power supply suddenly started working as expected. Yay!
After setting the correct supply voltage, everything seemed ready to start re-calibrating the instrument. I mean…., set it as accurate as I could, lacking a proper, calibrated voltage source or voltage meter. Having only a crappy old DMM and a cheap Chinese panel meter built in to my HV power supply, I did not have many options. Actually these two agree on voltages pretty closely, so “statistically” they are accurate enough, I guess….
So I started to go through the calibration process detailed in the manual. At Marconi, they used these awful slider trimmer pots in this instrument, and all of them were glued down with some substance. I had to remove that, and I think I made a mistake when I did not wait long enough for the alcohol to dissolve the glue. Apparently, the carbon stripe cracked somewhere in the middle in one of those pots. I only realised it after I finished with the first half (DC) part of the process. I managed to find a replacement, and started the entire process again. And again, and twice more, because these things are touchy and I accidentally moved the ones I previously adjusted.
During the process, I discovered that the whole instrument was still unstable. Even after I set zero several times, the needle started wandering around a few marks. Something was crumbling in there, but what? To find out, I took my hot air gun, and started to blow narrow stream of hot air into different parts of the instrument. After a few trials, it turned out, that a resistor in the differential DC amplifier caused the problem. It pushed the amplifier out of balance every time the temperature changed. It was in so bad shape, it fell apart as I unsoldered it. I put in a new one, and for the sake of symmetry, I replaced the counterpart of it at the other tube. Now the instrument was much more stable.
As I went through all the calibration once again, and checked the different ranges, a couple things became apparent. First, probably some of the fixed high-precision resistors are out of their tolerance, but my DMM is incapable of measuring accurately enough to judge, or even measure it at all in that range, so I have to wait until I get a better multimeter. Second, some of the potentiometers setting the zero levels are scratchy, even after some nice bath of DeOxit, so they have to go as well. But I think I will wait until I have enough issues with this VM to sit down and take it apart again.
Despite all of that, the instrument became stable and accurate already enough to be useful and measure some basic things I wanted.
So here it is, with its current state, with all the bad components I replaced.
I hope you enjoyed this little adventure with me and hopefully I could provide you some useful information about this instrument or about the process.
Radio Ramblings 