Maco 300 Amp Rebuild...

 This is a MaCo 300 from Majestic Communications, built around 1977. It was a 2nd generation amp, the 1st generation used 7ea 8950 compactron tubes (a more robust version of a 6LB6 with 12v fils) in a 2 driving 5 configuration. This 2nd generation unit used 5ea M-2057 compactron tubes in a 1 driving 4 configuration. These are monoband amps... most shipped on 15 meters, with a tap for 11 meters. The primary target was the booming CB market of the 70's, but these were easily bandswitched or moved to other bands. I personally owned one I put on 20m and used with a Heathkit HW-32. I've bandswitched a few for multiple bands, and have successfully moved a few to 6m!

This one was needing a lot of attention. After digging in a bit, I found the tubes to be in bad condition and decided to convert it to Russian 6P45C tubes. The more I did, the more items I discovered needed attention :-) In the end, this was a total overhaul. Follow along, I'll add info between some of the pics to explain. The photos may not be in order, but you'll get the idea. In the end, its a pretty nice amplifier, after the rebuild. Here we go...

Oxidation around the switches. That's an indication of having been stored somewhere damp, in the past. I cleaned and lubricated all of the switches.

The transformer outer paper layers were coming apart and/or torn. The transformer had a lot of surface rust. I take care of that, later...

The tubes were mostly shot. These tubes are almost unobtainium, and very expensive. I decided to update with a beefy Russian 6P45C. These may look like sweep tubes, but are designed for rf amplifiers and pretty tough!

The two pics above show preparations to remove the original sockets, and refit with the new correct replacements.

The mount for that fan is making me crazy, gotta change that, too. :-)

Cutting circuits loose to replace the tube sockets. Ugh...those HV filter caps are leaky and need replacement. OK, that's added to the list...

Drilling out the rivets from the old sockets...

Holes opened up, new rivet holes drilled at the correct spacing. Deburring in this photo...

Beginning to remove rust and work on the paper outer wraps on this beast...

New gold plated porcelain sockets to install...

Paper wraps repaired and rubber bands used to hold them secure while adhesive dries...

The front meter had fallen out and was taped back in. I couldn't let that go, so I removed the tape and scraped the old adhesive off...

The meter was tested and reinstalled with silicone adhesive for a clean install...

The 3 photos above are of the fabrication of a fan mounting bracket. This will tidy that up a bit...

The 2 pics above show the new sockets installed and the transformer cleaned up / repaired.

Here's the fan, reinstalled with the new mounting brackets.

The 2 pics above show my 1000w iron for bonding the new tube sockets flanges to the chassis for a good electrical path.

Upside down on the workbench, racked up with a new batch of tubes...

Beginning the rebuild. New bypass caps, rf chokes, rewiring the new bias supplies for -18 and +18 vdc.

New HV filter caps installed and ready for wiring. 

New bypass caps for the low voltage bias circuits.

I can't stand a bunch of extra unused driver tank coil... I'll get to that later.

This had those puny 1a 750v diodes...I couldn't help myself and rebuilt it with 3a 1000v diodes in the high voltage supply...

I drilled the cabinet, installed a new larger grommet, and wired a new 3-wire power cord.

Bleeder resistors installed on the new high voltage caps. MaCo didn't use those, and the old caps would still hurt you, days after being shut down. 

Installing the new high current hv diodes...

Testing in the low power setting... 448vdc...

Testing in the high power setting. 897 vdc!

OK, the next gremlin to raise its head was the tuned input. Dirt dauber wasps had built in the paper tube and destroyed it...

The tube for the tuned input was paper, and was destroyed by the wasps. I searched my stuff and found a candidate for transplant. I planed to cut and splice another tube onto the coil form...

There it is, the rebuilt tuned input,  reinstalled in the rear apron of the chassis. These have to be adjusted with a plastic alignment tool...

I had to add additional isolation on the -18vdc bias supply, to eliminate the cap from charge pumping on rectified audio peaks...

Working on tune-up and final tank optimization. 

Driver tank coil optimized, and Arco trimmers added to act as the driver load control.

All finished, and running strong. I had a big signal on 29.000mhz AM today, during the band opening. Lots of fun!!