Sunday, October 18, 2009

KW Atlanta Transceiver

This cute little radio followed me home from last weekend's De Anza Swapmeet. It's an "Atlanta" transceiver manufactured by KW Electronics, Ltd., of Dartford, England.

KW Electronics manufactured radio equipment for the British market. I believe the Atlanta transceiver was their one foray into the American ham marketplace and was sold here back in the early 70's (although I must admit that I never heard of the company at that time).

As you can see from the photo, the radio isn't in "original" condition. There's a toggle-switch just to the right of the band-switch (discussed below). There's also an additional DC "accessory" connector in the upper left-hand corner of the Power Supply front panel, and I'm not sure if the meter is original, or not (I suspect it's not).

The radio uses two 6LQ6 sweep tubes in its PA (also compatible: 6JE6 tubes). PA final voltage is spec'd at 800 volts, but mine measures 700 volts in receive, and 650 when loaded in transmit mode (I don't know if the power-transformer is original, or if it has been replaced).

So, given the lower PA voltage, the output power isn't quite as high as I would have expected it to be, but it suffices.

On the receive side there can be "popping" on the starting edge of loud signals. The AGC is audio-derived (rather than being derived prior to the detector) and it has a rather slow attack time, so some amount of leading-edge popping is to be expected.

But, despite these small problems, overall it's a nice package.

Here are some of the issues I ran into getting it on the air...

1. No Power Switch -- Wired Permanently ON. There was a "goof plug" on the front panel, just to the right of the band-switch. I opened up the radio and discovered that, at one time, there had been a power switch mounted to the back of the AF Gain potentiometer, but it had been removed by someone. Apparently they then drilled a hole in the front panel for a toggle switch, but by the time the radio got into my hands the toggle switch had been removed and the radio wired to be permanently On. I removed the "goof plug" and put a toggle switch into the existing hole.

2. Very low speaker volume -- someone had added a cap in series with the speaker and also ran a separate ground for the speaker from the radio to the power-supply unit (which they left unconnected). Why, I don't know, but I suspect they made these two mods to reduce speaker hum. But the cap was much too small, only 1 uF or so (thus presenting a very large series-impedance at AF frequencies). I removed it and wired the seperate ground to the speaker, and it works fine now.

3. Intermittent receive signal strength. Receive signals would sometimes be loud, and other times weak. If I touched the rear panel, I could make them fluctuate: clearly an indication of a bad connection somewhere. After hunting around, I finally found a lose connection at the bottom of L2 (inductor in the ouput pi-network). A very awkward location. Luckily, I had a very narrow soldering iron, so I could get to the bottom of the coil and repair it!

4. Low Power Output -- only 50 watts or so. I brought the power up by peaking transformer T1 per, I thought, the instructions in the manual (they're a bit ambiguous). However, carrier suppression was now terrible. I was able to get better carrier suppression, but I had to change T1's alignment procedure. Here's what I did:
  • T1 has two cores. Move the core closest to the chassis all the way to the chassis-side of T1. Let's call this the "bottom" core.
  • Transmit and insert some carrier, then adjust the top core for max TX signal.
  • Stop transmitting, and then adjust the bottom core for max RX signal.
  • Repeat, if necessary.
5. Poor Carrier Suppression. First, I adjusted the frequencies of the two carrier-oscillator crystals so that the bandwidth in both modes (when operation, say, on 80 meters) was close to identical (BW about 300 - 3100 Hz, measured using a white-noise source fed into the mic inputand a spectrum analyzer on the output). This put the carrier level for both sidebands at about the same level (when viewed on a spectrum analyzer). Then, with the Carrier Balance knob pointing straight up, I adjusted C114 for minimum carrier in both sideband modes. Then use the front-panel pot for fine-adjusting. You might need to do this several times before getting a good null. (Note, I'd tried to null carrier by nulling with the pot first and then adjusting C114. I could never get good suppression this way.)

Ongoing Problems...

1. Carrier does not remain suppressed. Don't yet know why...
(Update, 25 October 09: While poking around, I discovered that the two 100K resistors in the balanced modulator (7360) bias-network circuit had drifted an enormous amount -- one measured 232K, the other measured 335K! I ran a quick calculation (using the measurements of voltages that I'd made) and discovered that, if they had been 100K resistors, they each would have been dissipating more than 0.5 watts. Not good (given that they're 0.5 watt resistors), and perhaps the cause of the enormous change in resistance value. I've replaced them with 100K ohm, 1 watt resistors. Brief testing shows promise -- the carrier suppression seems to be a bit more in line, now.)
2. Noticeable distortion on very loud receive signals. Surely AGC related, but I don't yet have a fix...


1. Voltage Chart Errors. Take the voltages listed in the manual's "Voltage Chart" with a huge grain of salt. Some are clearly wrong, such as the voltage on V16 pin 6. There is no way it can be 210 volts -- it comes, via a resistor, from the 150 volt regulator!

Similarly, some of the "positive" voltages are actually negative (unless I am really screwing up my measurements!).


Yahoo Group: KW-Radios This is a great resource for schematics, manuals, etc.

KW Atlanta Photo: Atlanta

No comments: