Thursday, November 8, 2012

Can you hear me now!? Kenwood TL-922A

For some time I have had my eye on a few linears, and when this one showed up on eBay recently I could not resist. The Kenwood TL-922A unit was for sale and a weekend trip from Ottawa to Manchester New Hampshire made a safe and enjoyable way to get the unit home.

In the picture below, the amp is shown along side of my father's (VE1JNA - SK) Kenwood TS-850SAT, and these do make a nice pair.

Kenwood TS-850SAT and TL-922A
The TL-922A is a heavy brute coming in at 31kg. Flipping this beast over, when working on it will leave marks on your desk if you are not careful!

The HV supply has two modes which change the plate voltage from 2.2kV to 3.1kV for CW and SSB modes. In SSB mode this amp is specified to generate 1kW out for approximately 80W in. (This is what the curves at the back of the service manual say!)

Call me old fashioned, but if you are like me, then you too believe radios must have plenty of knobs and switches to be interesting. I should add that the metering on the TL-922A also meets my expectations too.  I know analogue meters miss the peaks but there is a satisfaction in seeing the physical needle move as you tune up the amp.

TL-922A Metering

Many folks have commented on the physical build quality of this design, and I will add my comments to the pile-up. I did not confirm when this unit was built, but being of the late 1970's, I can say this 30 year old unit will be here for many years to come because it does not show its age. I was very lucky because the previous owners, and I do not know how many there were, have taken good care of this amp.


Kenwood TL-922A

I have picked over much of this amp from outside and in, and in terms of modifications, there are only a few.
  1. The Tubes are a pair of 3-500Z's from the good folks at RF Parts.
  2. The suppressors are the original Kenwood items and have not been changed out.
  3. The 10M band addition was done as described here
  4. The HV caps have been replaced with Cornell-Dubilier 210uF 450VDC
  5. The SS-100 Universal Soft-Start from Harbach was installed
Thee are two signs of undesired operation, but first I must say that the unit was very clean otherwise. The 1000pf 15kV plate cap, in the center of the image below, has a odd "scar". You can see it in the picture below. The discoloration is highly localised on the edge of the cap, and is "baked on" in that it does not rub off. I am unsure what would have caused this.

Burnt Plate Cap?
The bandswitch was ruined and I had half expected this to be the case. I was able to get a new switch from Multi Tech Industries . Although it took me a bit longer to get it in, it was a easy job none the less. You do have to be careful to make sure every thing gets connected back to where it started.

Installed Multi Tech Band switch
Interestingly, the amp was able to run on all but 20m. A close inspection of the 20m input filtering led me to a pair of mica caps that had been pushed against each other. This caused their leads to cross and touch, shorting out the inductor. Simply fixing this, made the input SWR drop from over 100:1 to 1:1 and 20m was now in business too.

Shorted input filter for 20m

As an example, operating on 40m, the setup below shows 55W in and 650W out. The plate and grid currents are 450mA and 150mA.

55W in and 650W out

Having only had the amp for a few weeks, I know this is going to be a good addition to the shack. If you are in the market for a this piece of Kenwood history, I hope you are able to find an Amp as well cared for as I was able to.

MHz       1.8    3.5     7     14     21    28
Band      160m   80m    40m    20m    15m   10m

Plate     3.5    7      7      14     21    >21   
Load      0      1.8    2.2    4      4.2   4.8
Ip (dip)  Yes    Yes    Yes    Yes    Yes   Yes
Ip        480    500    500    500    500   350   (mA)
Ig        200    150    180    175    150   150   (mA)

HV       1900V  1900V  1900V  1900V  1900V  1900V  at load

Pout      550    600    700    800    625    435   (W)
Pin        65     55     75     70     55     40   (W)
SWRin    1.5:1   1:1   1.3:1   1:1   1.3:1   2:1