Cybernet PTBM121D4X modification ? - UK Vintage Radio Repair and Restoration Discussion Forum

Steve G4WCS · 26th Nov 2019, 9:41 pm

Refitting a pair of dc switching transistors and noted that the track from IC3 pin9 to T4 appears to have been deliberately cut. I wonder if it was in an attempt to make the power output across the bands less peaky as T4 contains a tuned circuit, and it has had a pll track cut at one time to take it onto 10M. Anyone come across this . No mention in the export service manual that its a factory mod ?

Techman · 27th Nov 2019, 2:13 pm

I don't know, but here's one I scrapped earlier - well perhaps 35 years ago

Steve G4WCS · 27th Nov 2019, 2:21 pm

thank you- no track cut there. Ill reinstate it and see if it makes any difference to the output

Steve G4WCS · 27th Nov 2019, 4:22 pm

Got this reply on a cb forum. So not a factory mod but my hunch was correct

A common mod to get more TX tuning bandwidth on the 121 is to cut the trace to the centre tap on T4 and solder a jumper from the track to the outer unused pin, I just last night rebuilt a Hygain V from a scrap board and did the cut track to T4 and replaced R150 for a 3k3 resistor and power on SSB is much better balanced across the three bands.

26th Nov 2019, 9:41 pm	#1
Steve G4WCS Heptode Join Date: Sep 2004 Location: Preston, Lancashire, UK. Posts: 632	Cybernet PTBM121D4X modification ? Refitting a pair of dc switching transistors and noted that the track from IC3 pin9 to T4 appears to have been deliberately cut. I wonder if it was in an attempt to make the power output across the bands less peaky as T4 contains a tuned circuit, and it has had a pll track cut at one time to take it onto 10M. Anyone come across this . No mention in the export service manual that its a factory mod ? Attached Thumbnails

27th Nov 2019, 2:13 pm	#2
Techman Dekatron Join Date: Nov 2009 Location: Lincolnshire, UK. Posts: 5,000	Re: Cybernet PTBM121D4X modification ? I don't know, but here's one I scrapped earlier - well perhaps 35 years ago Attached Thumbnails

27th Nov 2019, 2:21 pm	#3
Steve G4WCS Heptode Join Date: Sep 2004 Location: Preston, Lancashire, UK. Posts: 632	Re: Cybernet PTBM121D4X modification ? thank you- no track cut there. Ill reinstate it and see if it makes any difference to the output

27th Nov 2019, 4:22 pm	#4
Steve G4WCS Heptode Join Date: Sep 2004 Location: Preston, Lancashire, UK. Posts: 632	Re: Cybernet PTBM121D4X modification ? Got this reply on a cb forum. So not a factory mod but my hunch was correct A common mod to get more TX tuning bandwidth on the 121 is to cut the trace to the centre tap on T4 and solder a jumper from the track to the outer unused pin, I just last night rebuilt a Hygain V from a scrap board and did the cut track to T4 and replaced R150 for a 3k3 resistor and power on SSB is much better balanced across the three bands.