Marconi 2955A - Fault when using Directional Power Heads

[weejag]

I have a 2955A which I thought worked perfectly. All tests pass and the calibration is near perfect. 

I purchased the two variants (1-50MHz and 25-1000MHz) of the Directional Power Heads.
I initially thought that they worked as they both gave reasonable readings on the CW setting.
However, on switching to PEP there no power reading is obtained when applying RF to the Power Head.  In fact a forward power reading of 620mW, or 940mW, or sometimes 0mW is displayed on the forward power.  This reading does not alter when applying RF to the Power Head.
I also discovered that when turning up the volume control a tone was present on the PEP setting (about 1.05kHz).  This changes to a clicking or "motorboating" (about 2Hz) when switching to PEP.

I tried both Directional Power Heads on another 2955 and they worked perfectly.  They have intact seals and look unused (a military kit).

I checked the DIN connecting lead and it seems fine (again military).  I checked voltages:-
Pin 2 - 12v     Pin 7 - Not Connected 
Pin 4 (Forward Power) -   0v on CW, 0.67v on PEP
Pin 5 (Reverse Power) -   0v on CW, -0.67v on PEP
Control pins act as per the manual  (H = 5v  L = 0v) :-
Pin 6     Pin 1     Pin 3
  H          H           L          (CW)
  L          H           H           (PEP)

I have looked at internal connections on the accessory socket.  I have also tried to investigate board AB/1 where the F/R power lines terminate but I'm struggling to find the problem.
Having purchased the power heads I would like to have them working properly and would be grateful for any ideas as to how to proceed.  Has anyone come across this problem before?

Regards,
weejag 

 

[George Edmonds]

Hi

You are probably asking in the wrong place.

Try https://groups.io/g/Marconi-Test-Instruments.

This is a dedicated support group for Marconi Instruments.

G Edmonds

[weejag]

Thanks.
I'll try there too.

Regards,
weejag