Tektronix 7603 mainframe repair

Last week I was given a big heap of surplus test gear. It included a Tek 7603 oscilloscope mainframe. I already have a R7603, so it was nice to get another one, but sadly it was sick. On applying power, it just sat there, dead in the water, with wisps of smoke coming from the regulator board.

First of all I downloaded the service manual from bama.edebris.com.

Then I noticed half of the power rails were missing, as was the 130V rail fuse on the regulator board. Someone had obviously been at it before me. I replaced this fuse, and it blew immediately with a sizeable flash and pop.

It turned out that several transistors on the regulator board had failed, some short, some open. I replaced the TO92 ones with 2N5551 (NPN) and 2N5401 (PNP) and the larger metal can ones with 2N2219s, except for one that looked like it needed to stand a higher voltage, so I got a MJE340 and jammed it into the socket. The smoke had been coming from a crispy-looking 1.2k resistor which I replaced too, even though it still measured 1.19k.

After doing this, all rails were correct, and the unit powered up. However ALT mode wouldn’t work. I replaced a 7474 IC on the logic board (with a 74LS74 as I had no originals) and that fixed it.

Then, the readout wouldn’t work. On closer inspection, there was no readout board: I suppose it must have been optional.

The focus was still a bit blurry with the focus knob cranked all the way, but adjusting the focus preset in the HV box cured that.

Last of all, the graticule lights wouldn’t work. Usually it’s because all the bulbs are blown, but this time it was because the cable assembly that drives them was missing, presumed lost by the last guy who tried fixing it. I found a similar 4-pin cable in a box of junk, and the lights came on.

Finally, just when I thought I was done, I noticed the channels were bleeding into each other in Chop mode. I tried replacing the other 7474, but it made no difference. In fact there was nothing wrong with the beam switching, the problem was that the Y amplifier wasn’t settling properly after each switch. Now a scope Y amplifier is a major piece of analog voodoo: it contains dozens of tweaks to compensate its own frequency response, and that of the delay line. My refusal to settle had a time constant of about 50us, though, and the slowest trim listed in the service manual was 50ns, not a lot of use. However the schematic also showed two networks for compensating slower “thermals”, and it turned out that the trimpot in one of these had gone open circuit. I replaced this and since there was no trim procedure in the manual, I just tweaked both networks for minimum bleed between channels. I got it better than my other 7603.

I love how you can take a 40 year old piece of Tektronix gear, and you can find the schematics and fix it in a morning with parts that are lying around the place. They just don’t make ’em like that any more. Now it’s time to have a go at the HP 141T spectrum analyser. 🙂