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I had been dying to get a curve tracer, I really wanted one that I could test tubes with. The only tracer that I know of designed to test tubes was a Tektronix 570, and they are fairly scarce. I haven't been able to find one, at least at a price I was willing to pay. Then I got wind of a curve tracer made by Fairchild, model # 6200B. I saw one for sale somewhere, probably EBAY, but passed it up as I had no idea what it's capabilities were, and the person selling it didn't know much about it.
One day I was perusing my friends Systron Donner catalog when I nearly fell out of my chair with astonishment. There, in the back of the catalog with the system test stuff was a curve tracer that was the spitting image of a Fairchild 6200B, the Systron Donner 6200B! Well I don't know who bought who, I suspect that Systron Donner bought the Fairchild line, as the box is chock full of Fairchild transistors. It was a great find for me, because the catalog listed all of the specifications. I found out that it was good for 1KV collector voltage, could do voltage steps as well as current steps (for FETS) and the base polarity setting and collector polarity were independent. Here is was! A lesser known, sleeper sort of curve tracer that had most of the capability to test tubes. (No heater supply and no screen supply, but that is easily taken care of)
I started looking on EBAY again, and before long one came up, and as I had to have it, I won it. (~$250 as I recall) Well, when it came in I certainly didn't waste much time trying to get a transistor family curve on it. Much to my dismay, while I was able to get something similar to the correct display the box was very flaky. Some times I couldn't get any trace, other times the vertical sense would jump around. There were other things going on as well that were to transient to figure out.
It's been a long road, I have probably spent a solid 10 hours seriously trouble shooting the guts, and another 10 just exploring in order to get it to it's present serviceable but not perfect state. Along the way I spent another $50 for a manual (indispensable). It took awhile to discover that there was a leaky high voltage capacitor in the CRT plate supply. (See it in action!)
The main problem was with relays in the base circuitry. Neither voltage steps, (for FETS and tubes) or as I found out later base V on horizontal was working. Both problems were due to open relay windings. The box uses allot of reed relays, as it was designed to be remote controlled (optional), and therefore most of the switches control relays instead of switching signals. Many of the relays are very of what appears to be a custom design. They are large reeds with a huge winding that only requires about 1 or 2 mA to switch the reed. I think they did this because they use diode - coil winding logic so that the switches could be made simpler. They needed to keep the coil winding resistance up so they could series or parallel them and always guarantee that the relays would switch correctly. Any how I had a problem as those relays were un findable, and I can't find any relay manufacturer that makes stock parts for distribution that come close to the special parts in the tracer. So what did I do? On possibility is to rewind the coils, and I may try that. In the mean time I built my own sensitive relay with a congenital 5V relay and a transistor. The base circuit looks like the coil of a sensitive relay, and the 5V relay, in series with a resistor in the collector runs off the same supply that operated the original relay.
(See the fix)
On first power up the tracer would function, after awhile the trace would go bad, but the short in the high voltage supply was fooling me, I thought I still had intermittent connections or relays. Eventually I got around to checking the high voltage supply and the problem became obvious. So once again the first rule holds true! CHECK THE POWER SUPPLIES! (THAT MEANS ALL OF THEM) (But don't get killed or hurt) Now the only known problem is that the 20 mA range is dead. This is due to an open switch contact, not a relay, and although the switches are simple compared to what they would be without the relays they are still special wafers and not very likely to be replaced. I haven't gone so far as to remove the switch and attempt to repair it myself.
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