Power Supply Extender Cables


Be careful when using the multi-wire flat rainbow color ribbon jumper cables with connectors already installed that you can buy on eBay for a few dollars to extend the 7000 power supply cables. I used them to extend the cables on my 7904 after hanging the power supply out the back end of the mainframe. I found 4 tantalum caps that were shorted on the rectifier board. After replacing the caps, things were working fine until about 5 minutes after power on when all hell would break loose in the low voltage regulator board power supplies and the trace would tear and go off screen. The cables worked well until I encountered the thermal problem - the low voltage regulator board's 54V power supply would have its ripple increase to about 6V after awhile, and all the other supplies would follow suit. TP1625 on the rectifier board would go from 40V to 125V. It should be adjusted to and stay at 40VDC (no plugins, etc) according to the Tek Power Supply spec.

I thought I had a thermal issue with another component but it turned out that the ribbon jumper that I used to extend one of the low voltage regulator cables (the cable that connects to the main board) was heating up to over 175 F according to my infra-red thermometer. After freezing the connector, the voltage went back down to 40VDC. Took me awhile to find it.
The Tektronix ribbon cables fit very tightly on the pins and the wire gauge is heavier - maybe 22ga or 24ga - whereby the eBay jumpers were 28ga and the connectors didn't fit anywhere as tightly on the pins - thus a high resistance connection.

I hope this helps someone.

John Justin

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