Re: Old GB7RL hardware
Just a stab in the dark, Andy - it could be something to do with vehicular use. Some automotive electrics were positive earth at one time I think. Was some Pye kit built that way too?
From: Andy Talbot andy.g4jnt@... [ukmicrowaves]
Sent: Wed, 12 Aug 2015 13:24
Subject: [ukmicrowaves] Old GB7RL hardware
One of the items donated to our stand at Hamfest was a bright orange rack mount unit. It came with the comment that it "had something to do with 23cms"
It didn't get sold so I bought the unit home to study and see what was inside. Turns out it is / was the GB7RL packet node, (according to a label on the front) which after a bit of Googling didn't appear to have ever be operational. Does anyone here know differently?
The hardware consists of a 1299MHz receiver / converter to 70MHz, a packet demodulator and remodulator and a PA delivering about 8 watts at 439MHz. That I think then feeds into a high power tripler for a 1300MHz output . There is a diplexer for the antenna. It is very sturdily built, making heavy reuse of a complete set of plug in modules from an old piece of Plessey kit. The vintage of the RF equipment looks like 1980's to perhaps just early 1990's. So far I've only looked only at the receiver and 439MHz power amps (the latter works, the former is not at all sensitive)
Now, the really odd bit and I wonder if anyone knows why it would have been done this way :----------------------
Every module runs off a NEGATIVE supply input. All the RF modules, in fact everything by the look of it, has its PCB ground lifted above chassis with a DC floating RF / signal ground. The hot -Ve input then goes to this isolated signal ground while the positive input goes to real chassis
WHY WHY WHY WHY WHY
Even the BGY22/BGY23 modules in the PA are bolted to an aluminium block, insulated from their heatsink by a large mica washer.