By Clement Alberts KM6OKZ
I recently wrote a little article about my adventures with the Tek 577 curve tracer that Robert Shank KM6RSS was kind enough to sell me. Recall for a moment that the 577 was designed to be used with semiconductor devices.
In my case, I wanted to look at the four 7838 pentode power output tubes on the Fisher 400 stereo amplifier my father gave me back in 1968 (that being an entire project in itself). The 577 is capable of supplying control sufficient grid and plate voltages to power the 7868-vacuum tube. It cannot however supply a separate voltage to power either the screen grid or the filaments. The filaments were simply powered by DC supply. The screen grid required my obtaining a HP 6515A power supply.
This adjustable little joker can output almost 2KV. The screen grid only required 350V. The only thing I didn’t like about the 6515A is that I had to put in current limiting resistor in series. No resistor and you run the risk of popping a tube. After the fact, a HP609B with CC/CV capability for real cheap came up for auction on eBay. Real cheap is the Seller code for it doesn’t work. Working isn’t cheap. Working they go for anywhere between $400 and $800. So, the Seller implied it was dead by asking $75. So, I bought it. On to the repair… HP instrumentation documentation is readily available online. Moreover, HP always did a great job of supplying repair information. The documentation for the 6209B is a super excellent example. In fact, it’s better than most I’ve seen. I believe this to be so because the 6209B was first sold in 1967. They did things better back then. The PC board indicates each component. Moreover, there are a slew of numbered test points on the board that correlate to voltage test points provided on the schematic. In addition, there’s an entire section of the documentation that specifically deals with trouble shooting. It was, in short, indispensable information that permitted yours truly to affect such repairs as were necessary to breathe life back into a decayed corpse. One unobtanium SCR, three transistors, four glass diodes plus two electrolytic capacitors later…. and we have a winner. To me, the worst part of working on this kind of thing is removing components to test them. Unless they’re cans, capacitors aren’t so bad; just lift one end.
Tiny TO transistors aren’t so easy. I’ve made life easier by not scrimping on the tools I need to get at components. Examples would be the Hakko FR-301 desoldering gun. Way overpriced but trust me; worth its weight in gold. So now you know what to say when you’re asked what you want for Christmas. Another such example is the Peak Atlas suite of test devices. Highly recommended. I must have every model they ever made. As to the Peak “DCA-75”; Hook up a transistor in any order you like and bingo; you know its pin out, its condition, and its specs! To give you an idea of how wonderfully advantageous this is; one of the aforementioned bad transistors was one of a complementary NPN pair. So, I yanked out the sister, got its Hfe (gain) off the DCA-75 readout, matched it with what I had in stock and replaced both (matched to maintain symmetry). Simple. Especially so when you consider that HP transistors bear HP proprietary part numbers. One other example is the Sencore LC53. This box does a nice job of testing electrolytic capacitors. There are readily available testers that measure capacitance, and there are testers that just look at ESR. But they don’t look at leakage. Leakage measurement must be performed at a cap’s working voltage. This can be as high as 500V and thus potentially lethal if you not careful. I replaced the two I mentioned because they reasonably passed rating and ESR, but failed leakage.
A subtle but very important difference. One final note on electrolytic capacitors. It’s my experience that they do not necessarily need to be replaced just because they’re “old”. The three 60-year-old chassis mounted can electrolytics in 6209B all tested fine for all three parameters. Some would counter that they could fail tomorrow. I would counter by saying that, in light of their having lasted for 60 years, there is no reason to suspect that they won’t last another century. That’s what I think. Your mileage may vary.
In closing… I now have another high voltage power supply that’s good for about 350V and that is constant voltage/constant current capable. On the other hand, I acquired it for a specific purpose which is now behind me. So, I guess it’s another shelf queen. Until it’s needed again… or unless I forget I have it. 73, Clement KM6OKZ
