yaesu: FRG-7700; Failure this past spring (2011)

While watching the evening news on TV this past spring, I realized the distinct smell of burning electronics. I traced it to my den where the FRG-7700 was letting out all of it's "Magic Smoke".  A quick pull of the power cord and out the door we went. The whole radio was really HOT, and completely dead.

Last month I finally open the radio to attempt a repair and found the full wave bridge rectifier and associated space on the 12V DC Power Supply Circuit board burnt to charcoal/carbon. I also found that the mains wiring to the transformer primary were burnt, and shorted together.  What caused this catastrophe is still unknown, I suspect a power surge on the mains. It could just be a simple component failure due to old age, but for whatever reason, I found the radio functioned fine with an external 12V DC supply.

I had to replace the multi-input voltage PS transformer, build a new rectifier/filter/regulator circuit board, and install in the space of the old components. My fix has destroyed the antique value of the radio, but I also have it back in service, used daily, for just the cost of a new transformer.  All other parts were found in my junk box. 

I also added an additional fuse between the PS, and the radio circuit. I know it is just overkill, the primary fuse should protect everything, but it makes me feel better.

Mike.

 

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I was listening to my FT101ZD MkII and suddenly a loud hum and it started to smoke!

I unplugged it.

The fault was caused by a PSU PCB wire link on the -160V (Or something like that) corroding and dropping off. The pair of 6146B then tried to short the HT PSU. (One was damaged).

I poured some polyurethane into the transformer while it was still hot. After repair of wire link and replace one 6146B (fortunately a close enough match) everything was OK, the transformer was perfectly cool on receive with heaters off, so no shorted turns. I tested all the HT rectifiers and they were OK. The main fuse between the line cord and the transformer was a piece of nail wrapped in aluminium foil. If I had not swiftly unplugged the radio the irreplacable mains transformer would have been destroyed.

The Moral?

I now check all the fuses when I get a "fresh" rig.

The FT101ZD was fine the next few years. I sold it simply to get a MkIII with the extra "WARC" bands (I didn't have the luxury to keep it and buy another radio).

Obviously the original FRG7700 PSU either was lacking a fuse to protect the transformer or some fault not evisaged by designer. Anyhow, a working old radio, no matter how repaired, might be a more valuable  "vintage" item than a non-functional charred wreck.

There is always a tension of course between functional repair as if the radio was only a few years old and "archaeological" style preservation. I would tend to leave faulty parts in place and wire round them or remove them if they are "in the way" of a new part. I don't much see the value of "stuffing" failed parts with a new one. But other people prefer appearance to authenticity.

My experience is more with design of new systems and repair than with restoration. In the 1970s we would not have replaced two year old germanium Audio outputs in a UK radio or Rigonda TV Germanium parts with Germanium, but used Silicon parts and adjusted the bias. We would always have replaced paper caps with "met poly" or ceramic depending on value. Composition resistors with Carbon Film or Metal Oxide.

In the 1950s there were articles how to convert Radio sets with DK91, DF91, DAF91 and DL92 to DK96, DF96, DAF96 and DL96 to halve the LT battery running costs and about 30% or more on HT costs.

I think sometimes "Restoration" vs "Repair" can be tricky!

 

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