OBSERVATIONS FROM SCOTLAND 20th May 2008 GM1SXX
Fun with Fox Tangos... or 'too much Sake'
On Sunday 18th May, I attended the Magnum Rally, I almost didn't because I arrived before the doors opened to find the main hall looking quite devoid of stalls, compared to previous rallies I've attended there. I changed my mind at the last minute and stayed, joining the queue to get in. Once I got in to the hall, I had a very quick recce of the stands. Nothing took my fancy, until I spotted a Yaesu Fox-Tango 102 for sale. I did the usual 'sniff test'... all seemed fine, so I bought the radio at the asking price of £110 after talking to the seller. He said he thought it had a bias fault, something that's exceedingly common in the FT102.
Now the FT102 is a brute of a radio. It's as far from modern small lightweight Japanese 'rice-boxes' as you can imagine. The seller very kindly agreed to keep the radio for me until I left for home, very good of him. At the rally, I didn't buy any more goodies, but did meet some old friends, in particular Azeem Din GM8KWQ, and Eddie Higgins 2E0EDY, both of whom I've known for a great many years. I demanded to see what Azeem had in his bag, so he took me aside and showed me his 'prize catch'... a very compact spy receiver. It was in excellent condition and quite a rare find.
I didn't stay a long time at the rally. I had hoped to buy a cable winch, but none were on sale. I collected the FT102... damn! they are heavy brutes, and proceeded to carry it from the harbourside to Irvine railway station. I swear that my right arm is now about three inches longer than my left. I was lucky with trains, there are only two an hour but one arrived after just a few minutes. At Paisley Gilmour Street, I got off the train and headed for the taxi rank. NO WAY was I about to carry that brute home! I arrived home to cries of.... oh 'no! 'not another b***dy radio!'. Nice when your 'other half' is so delighted to see you :-)
Because it was warm and sunny, I lugged the radio through to the back garden and collected screwdrivers and a couple of small containers to hold the screws, and proceeded to open it up for a look. The interior was very clean, indeed obviously it was owned by a non-smoker... a good thing in view of the number of un-sealed relays used. No signs of major butchery, so I checked that a 3 amp fuse was in place, pulled the four electric jam-jars, put them to one side and applied power. No smoke, and no blown fuse, always a good sign. I stuck a foot of scrap wire in the aerial socket, switched it to 20M and was rewarded by a couple of German stations having a rag-chew. The receiver works on all bands, very good so far. The transmitter uses a hybrid circuit, transistors in the exciter and valves in the driver and PA stages. The FT102 uses the ubiquitous 12BY7A (!)as a driver and no less than three 6146B(*) valves in the PA stage. I expected any major problems to be in the PA stage.
I took the partly disassembled radio upstairs to my shack/junkyard and turned my attention to the transmitter. With no valves in place, I proceeded to check the various voltages. FT102's are notorious for bias failures in the supply to the screen grids of the PA bottles. This one checked out fine, as did the 900V HT line. I turned my attention to the driver stage, replacing the 12BY7A valve. I keyed the mic but there was no faint blue glow inside the driver valve. For some reason, the 12BY7A glows in normal use, a sure sign that the HT is healthy and current is being drawn. This one was dead.... as the proverbial dodo bird! (Usually a blue glow indicates a 'gassy' valve... but every 12BY7A I've seen and thats a few, glows in the dark!)
I pulled the 12BY7A and checked with my DMM and although bias voltage was present, there was no voltage on the anode of the 12BY7A driver. I keyed the TX and listened out on my RACAL RA1772 receiver for the transmitter. I was rewarded by a very clean strong signal, so the TX strip worked as far as the transistors went! Using a DFM, I copied the transmitter signal at the control grid of the driver valve socket.
Consulting the service manual, I traced the orange 300V HT line to the power supply PCB. I could immediately see a problem. The two resistors in the 300V supply had been cremated! The 300V supply is derived from a half-wave rectifier, just a diode followed by a 39R 1W limiting resistor and a pair of paralleled 10uF at 450V caps that smooth the ripple out. From there a 390R 2W resistor feeds another pair of capacitors. It's very basic indeed.
The power supply card for 300V HT and bias. (300V section marked)
Part PSU schematic showing the 300V 'circuit'.
My problem is that the PSU board's leads are exceedingly short. The board sits in a steel frame and sits in slots at the bottom while two screws secure the top of the card in place. Around 20 thin wires go from the PCB through a narrow slot in the chassis and into the wiring loom. Removing the screws only allows the PCB to be leaned forwards a little, so it's not very accessible for repairs. To allow me to test the driver stage, I'll 'lash-up' a duplicate circuit on some VERO perfboard. I 'lifted' the wires feeding 250V AC in and 300V DC out and soldered on short extension wires for testing purposes. The parts needed, mainly the HV caps, are on their way to me from EBay. If everything works OK, I'll consider trying to replace the original PSU parts with new ones.
I believe what has happened is that the diode marked DO1 has failed short-circuit, feeding 250V AC into the smoothing and reservoir caps, destroying them and the resistors.
The PA compartment is clean as is the PCB the valve sockets are on. I have every belief that this sets previous owner(s) was/were kind to it. The PA valves look good, no internal staining or damage and the lettering can still be scraped off with a fingernail... a sure sign that they have not seen much service.
I'm looking forward to getting this radio on the air. The receiver is exceptionally good and it's fitted with a narrow CW filter as well as having an effective notch filter and passband tuning. The 102 is a monster of a radio. It has a die-cast front panel and a massive power transformer. I've only owned it for a couple of days but I like it a great deal already.
Too much Sake?
One interesting thing about the FT102 is that on a different PSU board, yes, there are more than one!, the 900V HT supply components share board space with the 6 volt supply to the VFO. I get the feeling that the Yaesu design team perhaps had a bit too much Sake on the day they designed that circuitry! Hell, who cares! I seems to work.
Just in case you think I'm joking.... here's a drawing of that scary PSU board.
Notes.
(!) The 12BY7A was originally designed for use in TV signal distribution amplifiers.
(*) The 6146 and its variants were designed as transmitting valves. This is in direct contrast to the 6JS6C and many other large glass-bodied valves used in amateur radio gear as PA's. Most of these were designed for use as line output stage amplifiers in early hybrid colour TV sets.
to be continued........when my parts arrive from EBay.
FT102 links...
http://www.observations.biz/MANUALS/FT-102_user.pdf
http://www.observations.biz/MANUALS/FT-102%20Svc%20Manual.pdf
http://foxtango.org/FT-102/FT-102%20Page.htm
http://www.arrl.org/news/features/2006/06/08/1/?nc=1
73 Al.
GM1SXX
