Another PTek. Another questionable combiner. This one doesn’t even make any damn sense. I’m scared to open up the top of the transmitter to find out why it’s wired the way it is. The resistors are sitting on top of that hand cut piece of random PTFE and will cause a fire if they ever dissipate any significant energy. This is inside an FM2500PS transmitter.
This is a two port Wilkinson combiner that combines together the output of the left two pallets and the right two pallets. Why it’s floating on the thick PTFE slab, I cannot understand— these resistors appear to have the terminal configuration in which one lead of the resistor is the heatsinking base, and the other is the solder tab which just passes right through otherwise. WHY IS THIS BOTH INSULATED FROM AND ELECTRICALLY CONNECTED TO THE HEATSINK??!! Basically, what WILL cause this combiner to blow chunks would be any imbalance between the left and right sides of the transmitter – a single module failure will roast the entire rig. Catastrophically. See video below.
The lower line from each side goes to the start of the harmonic filter network, where they are just unceremoniously smashed together. This is… about the caliber of a badly built CB amp.
Dare I open the top and look around or have I suffered enough torture already??
(edit: yes… sadly I did!!!)
Page spam cut— click to continue. If you dare. I warned you, and Alex Hartman always warns ME not to open these transmitters and look around. But I do anyway. Then my brain hurts. ARGH
Here’s the other end of that combiner network, basically. The coaxial cables going to the TNC connectors are the quarter wave transformers of a Wilkinson combiner. The isolation resistors are found between each pallet and bolted straight to the big heatsink. For once, that kind of makes sense. So… how can this transmitter detect and protect against meltdown in the event of a module failure?
See those stringy little white wires? That’s the only connection between the CPU and the RF amplifier stages. It goes onto that PIC board at the back at a terminal labelled “AGC”. On PTek’s older amplifiers, this board had a different function and appeared to actually control a buck converter that could set the collector voltage to the PA’s. On this one it’s just glue logic and serves as a bus expander or something.
The “AGC” wire ends at the gate of each PA transistor package via this voltage clamp network (that appears to be a few common diodes in series to limit the voltage, then a 0 ohm resistor to ground) and an R/C filter. The brass tubes pass through the “binocular” core of the transformer and have DC continuity through. Most other transmitters I’ve worked on have had far far better biasing networks than this. No other transmitters I’ve worked on have been THIS SHITTY though.
This is just a typical PTek PA module. The copper pads are.. not… they’re two wings made of cut up copper flashing material. On a similar note, the capacitors on that harmonic filter above are just metal plates clamped above pieces of PTFE above striplines on the board, but I found the screws loose so—–
before I even get into that let me point out the following
I’m not touching this shit to remove it and look at the other side but what you’re looking at is where 2500 watts of RF exits the transmitter by flowing through a 1/4″ wide trace on this board then coming out via a piece of coax braid soldered between that and the ass end of an EIA flange connector. See, you can see where those isolation slots in the metering coupler board indicate the confines of where it has to live. I’d expect to find this thing inside a 100 watt transmitter… not a 2500!!! WTF?!
You can just see some kinda thick-ish PTFE film under the board.
The “AGC” wire carefully dressed to sharp edges on the chassis and the, uh, godawful power divider between the PA inputs. It’s only mildly better than the one I previously had catch fire on me, and I say that only because it isn’t entirely made out of cut up copper flashing.
Here’s what happened to that other combiner, set to the wonderful music of Sisters of Mercy.