Tektronix R7603 Story

Muckelroy

Member
It's been a very long time since I made a new thread here. This isn't so much about analog recording gear, but about a piece of instrumentation you may need to use to service analog recording gear. I obtained a Tektronix R7603 oscilloscope recently. It has a NASA asset tag on it, so I am not sure if it was quietly retired, or thrown in a NASA dumpster at some point.

It seems to be a '77 or '78 model. It is very clean inside and out. No bulging or leaky capacitors were visible. So, I took a gamble, and after setting the power supply jumper correctly, powered it on.

Not surprisingly, some smoke came from near the CRT tube after about 30 seconds. I powered it off. After letting it cool and checking all voltages in the area were safe, I started to triage it.

So far, I have not found why it smoked, or where the smoke came from exactly. The only sign of charring is the Anode cable, which is apparently perfectly normal. This cable carries 12KV, so it's very typical that atmospheric soot gathers on it.

I decided to make my photos publicly available here: iCloud Photo Sharing

So far, I've disconnected everything from the power supply except the regulator board. I'm trying to do some research to understand the power supply, and what is normal, and not normal. Thankfully, I have a good PDF of the service manual for use here.

This also is not my first oscilloscope. A friend of mine has a BK scope that works fine, and we share it. Sometimes I use it, sometimes he uses it. But, it would be super great if I could get this one to work as my own. I'll update this thread as major progress develops.
 
BVWS - British Vintage Wireless Society

We have the above. I bet there are some Good Ole Boys with a 'garAge' somewhere in Texas that have similar interests?

Tektronix was THE 'scope to have when I was in servicing. Never even SAW one let alone used one, had to slum it with Cossors and Telequipment!

Dave.
 
Thanks. Yes I saw that too. It's not burned. It's a 0.045 ohm 3 watt resistor. It is likely metal oxide construction, and is painted black. I lifted one leg and that resistor tests fine. The transistor it is attached to also tested fine.

I reattached the P901 ribbon cable (the one that goes to the logic board, and carries the "voltage sense" lines to the regulator boards.) This made the readouts of the voltage rails slightly better, but not perfect. I'm sure I can't test the power supply accurately until more modules are connected. None of the power supply rails have any appreciable ripple. The most AC ripple is the 130V rail which currently stands at 177VDC, and carries 0.146 VAC ripple.

The higher-than-normal rails should go down when a load is on the power supply. I have obtained a Fluke 80K-40 divider meter probe to measure the cathode voltage and will proceed with extreme caution.
 
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Ok, fine, that resistor just jumped out at me!

Now I 'would' be very interested to ask you for schematics and have a think but, I am helping another guy with an amp project at the moment and this 72yr old brain gets knackered!

However, grannies and eggs aside, when working with power supplies I used to make up resistive loads to simulate as well as possible the rest of the circuits so you don't bugger those in the process.

Since you said "regulated" supplies I doubt they will be THAT fussy about the exact load but many regulator designs can go a bit ape***t with no load at all.

Dave.
 
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