I already have a curve tracer, a Fairchild 6200B. Why did I buy this one? I couldn't resist the challenge of buying a broken one (for fairly cheap, although it was still $75 plus shipping) and fixing it. Besides I had a sneaking suspicion that the 575 is a better engineered and manufactured box. After working on it I have confirmed that opinion. For example the the switches in the Fairchild are low cost while I don't think the TEK switches are capable of being worn out. (Slight hyperbole here.) The PC board mounting in the Fairchild is just short of a mechanical nightmare, the boards have no card guides or cage, just a clamp plate that holds the top of all the boards, (10 or so). Its a little tricky to line them up. As for the PC boards in the TEK, well.... (there aren't any). For a technical compare and contrast see FAIRCHILD 6200B CURVE TRACER, CLOSE UP AND PERSONAL.

When I received the tracer I immediately realized I had made a slight tactical mistake. I told the seller (an EBAY auction) that I didn't want him to go hog wild on packing costs. He followed my direction well. It was packed with what I would consider the minimum acceptable protection for some old piece of equipment that maybe would never work again. The result was that in shipping the front took a good whack and the front panel is slightly dished in. The transistor fixture took most of it and was clearly bent. Later I found that the phenolic isolation wafers in the polarity and range switches for the collector sweep were broken from being pushed in. Also (but not from shipping) the CRT clamp and rotating mechanism was broken and not holding the rear of the CRT properly. So, right from the start it had several mechanical problems, one of the most common types of problems in old gear. Inside it was dirty, but not excessively so.

Fortunately the chassis of the transistor fixture is very strong, being cast, and only the top and bottom plates were bent. It was possible to take it apart and straighten everything out. I haven't properly fixed the CRT clamp and rotator yet, my first attempt failed. The phenolic isolators were easily replaced with a nylon washer from the hardware store and 2-56 hardware instead of rivets.

On to using it, right? Well, not exactly. It had been described as "blows fuse" and sure enough the fuse was blown. I replaced it, and on power up and wait I was rewarded with a fuzzy ball instead of a dot. Increasing the collector voltage had little effect, although the collector sweep voltage seemed to behave. TEK really did me a favor by placing test points for the power supplies because I was about to break a rule (check the power supplies first!), but with them right there and labeled I decided to check them. As the +300 was only 98 or so I realized that this was a high priority problem. Looking around more carefully I found a tube flashing purple, and it was even in the 300 V supply! The 6BW4 rectifier was no good. Replacing it (I have a stock of tubes) fixed the supply and gave me a nice sharp dot and  line for the collector voltage. Putting a resistor across (C-E) gave me a diagonal line, with very close to the correct slope. Wow I thought, what an easy fix. However before putting a transistor in it and being disappointed I decided to check the base drive first. There was no proper base drive, just +15V or so. Later I found out that 15V is the supply voltage for the transistor based current output drive, and the base drive was not stepping. Well the base step circuit has about 9 tubes in it, and can be hard to figure without the manual, so I arranged to rent one and put the box on the shelf. Meanwhile I got in touch with some knowledgeable folks at Tektronix Resource Site  (especially Alan Douglas - thanks) who gave some pointers on the base stepper and how to improve it for tube grid drive.

When the manual came in I was gratified that the block diagram of the circuit was almost exactly what I thought it would be. A full wave rectifier drives a pulse generator that drives an integrator. That produces a stair case wave form. The integrator is dumped when it reaches a certain, adjustable voltage. The resultant set of voltages can be used to drive a current source, and viola, base drive. To get voltage drive a resistor is placed across the B-E terminals. So while the block diagram was as expected, which tube was which (except for the diodes) wasn't so easy to figure out. The manual was clear on that matter however. Poking around in the circuit I discovered that the integrator voltage was way too high, and that the Schmidt trigger comparator that was supposed to decide to dump the integrator was never tripping. It turns out that the 575 was a victim of "unauthorized access" - that is the min steps and max steps internal adjustment pots were tweaked too far off of where they were supposed to be.

Adjusting the max steps pot gave me a proper feedback voltage and the dump started happening. However the step generator was far from correct. Each step wasn't flat but sagged back against the step. This caused many more steps that the proper max of 12 to be generated. As it warmed up it eventually got so bad that the staircase turned into a saw tooth. Something was bleeding charge off of the integrator capacitor. I tried replacing the capacitor, dump diodes and the cathode follower with no success. Replacing the 6AU6 integrator tube rewarded me with a proper staircase. Later when trying to recreate the problem for picture taking the old 6AU6 worked better. It still sagged and the first few steps were very non linear, but it never went completely out. Tapping the tube can cause the steps to get flatter or sloppier, and the number to change. My guess is that somebody tried to compensate for the dying 6AU6 by changing the adjustments. Maybe I'll put on my Fairchild  and see what's happening.... Now the 575 basically works. It probably needs to be calibrated, and the transistor sockets are definitely dirty. There may be more minor problems, such as base step phase, but it gives a very good impression of working.