Micronta 21-522a Repairs / Mods

 This is one of the Radio Shack wattmeters that was very popular in the 1970's and 1980's. It is an average reading meter, and fairly accurate on 160-10 meters. This was a barn find, was pretty dirty, and had a bad modulation meter. I had another meter of the same dimensions, so I swapped the meter face and subbed it into the housing. The mods shown on the marked up schematic convert this to a passive peak reading meter, similar to the Daiwa, Palomar, earlier MFJ, PDC, etc. No battery is required, and it really enhances it's use when tuning amps and transmitters for peak power...

Ramn 500 HF Amp Repairs...

 This is a Ramn 500 monoband hf amplifier I repaired. It was picked up in a trade, and needed a good bit of work.

This was built somewhere around 1975, and no schematics exist on the web, that I could find. I reverse engineered the circuit as best as I could, correcting for hacks and changes over the last 50 years. This uses 2ea 8950 compactron tubes, driving 4ea 8950 compaction tubes. It runs about 1100 vdc on the plates, 550 vdc on the screens of the driver tubes, and a variable -20 to -50 vdc on the control grids. It has an inductive adjustable tuned input for input matching. 

As seen in the 1st video, the driver tubes were red-plating in about 10 seconds of key up with no rf drive. The final tune control had lost chassis ground, the negative bias supply was not functional (due to a mis-wired transformer winding), the capacitor bank was mis-wired (too many caps in the 1100vdc circuit, and not enough in the 550vdc circuit), the transformer HV center tap lacked a diode at the connection point of the 550 vdc cap stack, the final tank circuit was wired DIRECTLY to the antenna output connector (!!!) instead of being routed through the changeover relay, the rf input circuit to the driver tubes was broken and blowing in the wind (and missing a coupling cap), RF and DC grounds were lugged under the bolts that secure plastic feet and were ALL loose, the PA output coax was melted and mis-wired, etc. I added an additional ground for the transformer and main control board, the ventilation slots in the upper cover were mostly blocked and had an AC fan with dangerous wiring. I removed the shroud,  changed it to a DC whisper fan with reduced speed/sound, and installed a stainless steel fan cover.

Here's some pics with info, and videos...

All finished with a fresh coat of paint and better cooling...

The original setup had this top shroud that blocked much of the ventilation...

Whew.. what a mess. The silicone caulking kinda wields me out, but the main issue here was it being tapped and wired incorrectly.

This wire was disconnected, with no obvious location to connect...

Plate tune had been arcing...

Output coax was melted and hardened...

The opposite end was misconnected...

????

This black w/yellow stripe transformer lead was supposed to be grounded, but was not...

Orange transformer center tap connected to the cap bank with no diode!!!!

Paper residue from previously blown capacitors (no surprise!) was all over and needed cleaning out...

Lots of scraps from blown caps...

Input drive silver mica cap was roasted...

I removed main control board and relay to repair burned traces...

Part of the RF tank area tidied up and excess removed...

Tested all 8950 vacuum tubes...

DC and safety grounds attached to the same bolt that secured the bottom plastic feet, and were ALL loose...

Resoldered diode splice...

Punched another hole in chassis...

...and installed a new chassis ground lug for transformer and main circuit board...

Reverse engineered the gist of the tube amp section and associated voltages...

Before and after drawing of the filter capacitor block...

Rectifier added to the transformer center tap at the 550vdc filter cap string...

RF drive lead from excited connection. Totally disconnected and no coupling cap... SMH. :-)

Installing speed control resistor for new DC cooling fan...

Stainless steel fan cover installed...