I have been busy restoring radios for the business, so I haven't had much time to work on the Gates lately (or the article) as of late. I will return to it before the end of May (2012) and finish up the article. Stay tuned.
To answer some of the questions, this particular radio was manufactured in Sept. 1957. It appears that Gates made several SW transmitters intended for commercial SW use during this period. This was the smallest one they made. Some were what we normally expect from Gates Radio as far as size. The HF-20 was a 18kW output (carrier) transmitter that was 5 racks, 210" wide, 49" deep & 78" high at 16,000lbs. Typical Gates! Another was the 1 rack HF-1 1kW transmitter. However, these were primarily for broadcast use on SW, and didn't contain microphone inputs (line input only) and no PTT.
Then we have the smaller units. These were complete self-contained transmitters containing mic inputs and PTT functions. The largest was the CMW-1 which used pair of 100TH modulating a second pair of the same. It was very conservatively operated at 700W input, although it could be run as high as 1kW output with this tube compliment. The next was the CMG-1 250W output (carrier) using a pair of 810's modulating a a pair of 4/65A's. It had a built in mic input, PTT and a self contained Gates Audio Limiter. A very nice transmitter. Both of these were large floor standing racks.
Of the smallish types, in addition to the radio I have here, there was a 65w input M-5569/5570. This was a transmitter (5569) that vaguely resembles a Johnson Viking I in appearance, but also has a separate power supply/modulator of the same size (5570). Unlike the radio I have here, the two units can be rack mounted in a standard 19" rack, but do also come in their own metal Gates racks. This was a Pair of 6146's modulated by the same. The modulator had a built in compressor with a threshold setting & gain adjustment as well as metering for the amount of downward compression. Nice feature!
Many of the components used in the M-5078 are off the shelf Gates parts used in the exciters for its larger transmitters, so the duty cycle on this little transmitter is, well ... crazy high! I did some calculations and on some parts, the maximums on some parts are only at about 20% (the fixed capacitors on the loading, for instance.) However I do not think audio fidelity was of a primary concern for this market, unlike the broadcast products.
The modulator does not use any "clippers" or other sort of tricks to limit modulation peaks, which is good. However, the audio bandwidth is limited to about 300-3kHz, through the use of "filtering" in the audio section in the form of High-pass & Low-pass sections. It was common in this period of high traffic on the HF bands to limit this, not to mention tighter restrictions by the FCC then exists today. I will most likely broaden this slightly
by changing some of the values of coupling caps and possibly wrapping some negative feedback into the modulator to reduce distortion artifacts, which is probably at around 5% in the current circuit. I will also probably play around with the gain on the modulator to recover any loss incurred by using the feedback. But will first proceed with circuit as originally designed by Gates before making mods of this type. I should easily be able to achieve a flat response of about 100-5kHz with distortion at about 1-2% @ 100% modulation when all is said and done, which is more than sufficient.
This thing did have quite a bit of tinkering done to it, as it turns out. I have not mentioned this yet in the article, since I am only now at the point in the restoration of testing all the components. I'm finding numerous changed values on the components. While the radio worked fine, the changes made were ... well ... OK, but made the radio more like an amateur transmitter in performance, and certainly not CCS. The choices for duty cycle ratings were marginal at best, and were most likely "junkbox parts" to make the radio functional for the Hammy Hambone who did the work. The values selected were also "best guess" or whatever was on hand, like the microphone input using a higher than normal value for a low impedance mic. The value wasn't even a compromise to allow for a high impedance mic. It was just a high value, too high in fact. Also the RF choke was removed completely off the mic input.
The PTT function as added, while functional, was done poorly. I will correct all of this and make it more inline with the original Gates CCS design criteria. The PTT mod control wires ran all over the chassis, and were not incorporated into the existing harnesses. They caused quite a bit of trouble when the modulator/PSU chassis was taken apart. Each is a separate chassis, connected together via a terminal strip. The PPT wires could have been routed through the spare terminals available. Alas, they were just directly wired haphazardly and tied off to random things underneath. Also the wires were not high enough gauge for the current they are carrying, and were of a different type (teflon insulated solid wire.) While it is just a personal preference, I will change this to a higher gauge cloth type wiring like the original Gates wiring. The relay was also a junkbox component. I will also use a much smaller (and quieter) relay and eliminate the added unnecessary transformer for this application. The relay used has a low voltage coil (24V) so a transformer was needed. Again it was probably what was on hand. I will change it to a 120VAC relay and eliminate the transformer completely.
The decal thing is quite fun. I cannot tell you how nice a radio restoration looks when you have the decals redone as well. It just puts it over the top and makes the radio look brand new. The "icing on the cake", so to speak. The decals on this radio came out very well, and I can't wait to "brag" about it,
Seriously, I just really like doing these types of details and get very happy when it all works out. I have the images and promise to show the technique, as well as the finished product in the article very soon. My hope is to benefit everyone who would like to do this as that extra step that puts their own restoration project over the top.
As far as the paper oil caps, I am not a big fan of "Orange drops" so popular these days. Several reasons, very large and the leads are not true axial, which makes for sometimes difficult placement within the existing circuitry. Also I find a large variation in values, sometimes even beyond the rated tolerance. What I do use are axial lead tubular polypropylene (the little yellow ones) in most cases to replace oil/paper caps. I do test the components out of a large lot to find the nearest value as a QC thing. However, I do find the typical poly's made available are usually within tolerances, unlike the Orange Drops.
The etching primer is a great idea, but the most important thing to do is to make sure you stabilize the metal underneath. Even sanding away all the visible rust or corrosion doesn't mean it isn't still there on a molecular level. Converting this surface is essential to prevent rusting from starting over underneath the new primer & paint.
I have used TunerLube as well, however I do find it has a tendency to harden up over time as well since it does use some non-synthetic petroleum products. But it isn't as bad as some the other choices used by some.
Hope to have the article updates coming soon and thanks for the kudos folks.