Marconi 2955, repair

I was playing with my new GPIB interface when I noticed magic smoke coming out from my test equipment. I switched it off and took it apart. I was unable to locate where the problem was. After reading a few comments in different forums, I decided to first look at the 10MHz oscillator (OCXO). I removed screws holding the HV board, then I removed the grey plastic case covering the oscillator, unsoldered the small board (6pins) and took it on the workbench.

I connected a 12 Vdc supply (current limited at 500 mA) at pin 4 and a 5 Vdc supply at pin 3, left the SYNC input floating and measured the frequency at pin 1 with my counter. GND is pin 2. The results didn't show anything wrong.

Frequency nominal: 10MHz
cold: drift +200Hz
warm (after 15 minutes): drift -500Hz

I resoldered everything back and decided to power up my 2955 with some precautions:

• external power supply set at 15Vdc and limited at 5A. You need that current to start this kind of equipment. After waiting a few seconds, I had to switch everything off. I "smelled" something around the HT coil...:-)

I measured a few diodes and identified at least one faulty one; diode D9 (BA159) on AC1 board.

I ordered them and added other components just in case like D14 (5.6 V Zener), TR3 (BU806 NPN transistor), D11 (MR854 diode), TR4 (2N2369 NPN transistor).

A few days later, the items were in the mail box.

I replaced D9, switched the external power supply on et voilà. Job done. CRT is back and I can proceed with my GPIB interface and some scripts...