Technical information

Low MIC Gain
The stock Tempo high output hand mic required the mic gain almost fully clockwise and some bands
would never indicate ALC action. I understand the original design was a bit conservative, low noise,
high linearity and all that. Since I couldn’t even use my D-104, I had to make changes, stone the crows.
The mic amp is on PC-080 - an OP AMP + gain pot + NPN transistor. I download the PC-080 Toshiba
Q505 TA7063P datasheet. The OP AMP datasheet shows a test circuit using a 22k feedback resistor
for a 45DB gain. The 2020 has a 6.8k (R551) feedback resistor – the conservative bit.
The fix/mod is easy. Unplug PC-080 card, replace R551 with a 15k resistor and batta bing, batta boom,
mic gain now at 12 o’clock drive ALC to a mid point indication with stock mic and many others. The
waveforms still look nice, no obvious distortion by increasing the OP AMP gain a wee bit.
Since have I no reference data, I can’t classify MOD or FIX at this time. Mind you, an old electrolytic
might also be in play. All I know is my 2020 is perfect now.
Low Power Output
Problem 1
I followed the Driver board calibration instructions and found that I no longer had 15 or 10 meter output.
The problem was the alignment instructions are flat wrong. The DRIVER card input coil L251 should be
peaked on 15 meters, not on 75 as shown in the table. It is so broad on 75 that you can easily set it at a
point where no 15 or 10 gazouta happens.
Problem 2
I found that power output would drop off from 100 watts to 88 on 14 Mhz after warm up. Replacing the
driver tube made no difference. I purchased 2 new, matched GE 6146W tubes from RF Parts and all is
well now.
Problem 3
Some bands have only 60 watts max out. New Driver and final tubes made no difference. The problem
was determined to be the band switch sprocket slippage combined with an intermittent relay contact for
PA bias switching. See the sprocket change section above and remove the plug-in relay and properly
clean and burnish the contacts. I was testing on the bench only in the Tune position so I wasn’t seeing
the PA bias intermittent behavior.
Problem 4
After sprocket and chain replacement, 80, 40 and 20 had fine outputs of 100 watts on all SEGMENTS.
15 and 10 still stuck at 50-60. The problem was the band pass filters, see below.
LO Oscillator Adjustment
I followed the alignment instructions precisely using TP-2. You will chase red herrings unless you use a
high quality RF VTVM to set all outputs to 1.5VRMS first then couple the counter/scope-counter to TP-2
through a 1-2pf cap to set the required frequencies for each band. If you don’t use the small coupling
cap, the frequency will read low on your counter.