[Kenwood] TL922A help

Mon Apr 27 08:47:01 EDT 2015

Greetings to those who are much smarter than me. I recently picked up a 
nice clean, supposedly working TL922A. Plugged in on the bench to check 
it out with no radio attached. Yes, I should have put it on the Variac 
first and yes I declined the offer to test drive it at the sellers home 
b4 I bought it.
The amp was in standby and set for CW. Hit the power switch and was 
greeted by SNAP, CRACKLE, POP, SIZZLE and HUM. Killed the power within 
1-2 seconds. I know now why it was humming, it's because it didn't know 
the words to the song it was trying to sing.
Opened it up. Reconnected it this time on a Variac. Watched one tube 
filament get nice and bright, the other dim. Smelled something getting 
warm this was at about 50vac input. Grabbed an IR thermometer, found the 
filament trans getting very hot 105f+. Shut it down again. Did more 
digging. Looked closely at the tubes the dim one grid folds looked 
discolored, possibly from overheating. These appear to be the original 
tubes from what I can determine from the date codes and paper work that 
came with the amp.
Found C3 shorted, replaced it, bias/alc/relay voltage ok now just around 
100vdc with little a/c ripple on it.
Looked further and found L7 470uh choke blown out. Figured now it had a 
grid to filament short, took out C3, D1 checks ok. R22 checked a little 
low 3.9-4.0k. Located local source for the choke.
With each step forward a new issue develops. Now with the hv safety 
defeated and no load/tubes installed. Started bringing up the ac again. 
At about 50vac, the panel meter shows about 100vdc, and a humming emits 
from probably the hv trans. Grabbed my IR thermometer and started 
probing the cap bank. Found R16 & R17 hitting 100f+ all the others 65F 
room temp. Ah, bad cap. Pulled the cap bank out, clipped one side of 
each resistor, all check at 47k as marked. Checked the caps on VOM, all 
slowly charge. Checked them all on my old Eico cap tester, all open the 
eye at rated voltage. No I don't have an ESR meter.
Now that junction is where the CW/SSB tap of the HV trans connects. Any 
ideas of where to look next. I know I need at least one tube, the RF 
choke, I was about to order a cap bank from Harbach, but now put that on 
hold.
Sorry for being long winded here, but wanted to get as much info out as 
possible. Reply to me off the reflector is ok too.
Thanks for any ideas.
Ray
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