By Glen Millen VK2FC
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By Glen Millen VK2FC |
I've decided to update this page to show the most recent TL-922 upgrades/modifications I have completed on my own Kenwood TL-922. Sorry if it takes a while to load this page. More pages of alternate modifications will be added as time permits.
On close inspection of the power supply filter caps I found that 6 of them had vented through the small raised section on the ruber end of the capacitor so I decided to replace them. I could only purchase some 470uF/450 Volt types manufactured by Jamicon. The replacements were the same diameter but only half the length of the originals so I had to add a spacer to stop the caps sliding down the white plastic carrier. The photo above shows the originals after removal from the carrier.
Because inrush current at switch-on is always a concern I have added a slow-start power up circuit. This utilises a Motorola TDA1085 motor speed controller and a TRIAC to ramp up the A.C. mains supply to full potential (240VAC in my case) over a 2 second period. I removed the original screw terminal A.C. voltage selection components to provide room for this addition which is mounted on a flat piece of aluminium plate. This will help to make the new filter caps last longer as well as the filaments in the two Eimac 3-500Z that were manufactured in 1997. I have one small problem now after fitting the slow start PCB. As the A.C. supply ramps up slowly, the fan control relays chatter for a couple of seconds until full terminal voltage is reached. I will have to think of a 'cure' for this!
This is an overall view of the TL-922 rear panel. Note that I have added a fan guard and changed the entry grommet for the power cord. The original was cracked and allowed the 3 core A.C. power cable to twist inside the amplifier.
Whilst the filter capacitor bank was removed from its normal location I added a 1 Amp fast blow fuse to the 3kV supply. The fuse carrier is supported by a small piece of aluminium bent at 90 degrees. To get the filter caps out you need to remove the angled piece of steel that runs from the front panel to the rear panel. The assembly that contains the safety interlock switch is also unscrewed from its mounting location which allows enough space to fit the two back-to-back diodes (D14 & D15) to the terminals of the plate current meter.
The photo above shows the new bias diode assembly comprising diodes D3 to D5 (top 5). The bottom 2 back-to-back diodes are the grid current meter protection. Two CL60 inrush current limiters (RF Parts) can be seen here too. These are wired in series with the primary of the filament transformer. This is a bit of overkill since the addition of the slow-start board but ideal when you use the step-start relay as opposed to the slow-start
I also added a 2000pF high voltage choke bypass capacitor in parallel with the original to improve decoupling on the lower bands. I did this while the filter cap assembly was out. I now get an extra 200 Watts on 160M after completing the modifications above and below!
Here I have tied the three grid pins of each tube together with some brass strap. I also added added 3 x 680pF Silvered Mica grid bypass capacitors to each tube. A 1 Ohm resistor connects each grid to ground. These resistors are intended to provide a level of protection to the grids if or when an internal arc in a tube. occurs.
This is the solid state relay I use for the transmit/receive bias control function. It is glued to the small metal bracket that retained the original mechanical relay and located near the inner tube socket. Discrete components will achieve the same outcome but I like this compact solution! Here is the Data Sheet on the relay.
Once the original antenna relay has been removed there is enough space to add two vacuum relays. I used Jennings Model RJ-1A in this conversion. Note the relays have been 'shock' mounted through rubber grommets to reduce the mechanical coupling to the chassis. This aids in making the amplifier keying very quiet.
Here is another type of parasitic suppressor. A 100mm piece of 1.8mm copper wire is bent into a 'U' shape and two 100 Ohm resistors placed in parallel.
This little circuit built on the piece of Vero board is the PTT switching circuit comprising transistors Q1, Q2, Q3 & Q4 that controls the vacuum relays and the two lamps on the front panel that indicate 'standby' and 'transmit'. The benefit of this is that a relay is not required to do the switching and the only mechanical noises come from the rubber mounted vacuum relays. The original lamps were replaced with two high intensity white LED. They are powered by DC from the 8 Volt winding on the filament transformer. I will make a PCB for this circuit when time permits. Last updated on Tuesday, 09 February 2010 |
Amateur Radio VK2FC |
