The ALC circuit in the 781 is too slow to respond. This results in the following phenomenon: If you set the power level to 30W,

for example, there is a leading pulse that can be more than 200W, particularly if the drive control is advanced. This results in a

serious problem if you are using an amplifier. It makes it very difficult to limit drive to an amplifier properly.

This behavior is clearly seen on a scope and to some extent on a good peak-reading meter. However I suspect many hams are

unaware of the problem as it does not show up on RMS meters due to the short duration of the spike.

I have used three 781's over the years ( ser# 19??, 21?? and now 2589) and they all exhibit the same behavior. This behavior is

not exhibited by other Icom radios I have checked such as the IC751A, IC735 or IC765.

When I had my previous 781, the tubes in my amplifier, a Ten-Tec Titan using a pair of 3CX800A7's, developed shorted grids,

even though they were only a few years old and lightly used. I suspected the 781 caused the failure, but sold it before the

amplifier was repaired. I recently bought another used 781 from Burghardt radio, serial # 2589, and within 3 weeks of obtaining

the 781, my tubes (less than 2 years old) are shorted again. I do not believe this is a coincidence! I run the amp conservatively,

and I doubt the tubes have 50 hours transmit time on them.

I believe the problem is vaporization of the gold plating from the grid of the 3CX800 causing the tubes to become gassy, which

causes arcing.

I spoke to Technical Support at Icom USA and was informed that the 781's just work that way and nothing can be done. That is

a ridiculous answer! Of course it can be fixed, the attack time in the ALC circuit has to be shortened.

This turned out to be easy to accomplish....

Locate the following components connected to the base of Q34 in the ALC amp, R107, R108, C47 & C48. These determine the

attack and decay time of the ALC amp.

Values as per Service manual:

R107 1M

R108 100

C47 2.2uf

C48 .47uf

Values as per Production ser # 2589:

R107 1M

R108 100

C47 .47uf
 

C48 2.2uf

Note that C47 and C48 have been swapped... a step in the right direction.

Values that work:

R107 1M

R108 4.7K

C47 2.2uf

C48 .022uf - .1uf

.1 reduces the overshoot from 200%+ to 20%, .022 eliminates it almost completely. However, as the value of this capacitor is

reduced, I'm concerned that IM products in the audio may start to increase due to syllabic compression of the RF envelope. I had

no way to verify this.

Perhaps a project for a better equipped workshop! I would think .047 would be fine, though I'm using .022 without any

noticeable degradation of audio quality.

 By    Mel, VE2DC

  If you remove the internal speaker, you can reduce the internal heat generated therein by quite a measure!

The stock FL-96 (455 kHz SSB: 2,8 kHz wide @ -6 dB) can be replaced with an FL-44A (2,4 kHz wide @ -6 dB) to improve

adjacent-channel selectivity and sharpen Twin PBT operation. In addition, the stock FL-102 (9 MHz, AM) can be replaced with an

FL-223 (9 MHz SSB: 1,9 kHz wide @ -6 dB) to provide an SSB-NARROW bandwidth setting.

Here is the filter installation procedure:

(I would suggest that you purchase an IC-781 service manual from Icom Parts for guidance, if you do not already have one.

Icom Parts should also have the FL-44A).

1. Remove the top and bottom case covers.

2. Lay the radio upside down on a towel, with the front panel facing you.

3. Using a magnetic-tip manual screwdriver with a #2 Phillips tip having rounded blade edges, remove all eleven retaining

screws from the IF board (the board with all the filters).

4. Unplug the multi-conductor header cables from the board, to allow the board to be moved aside.

5. Grab the inner edge of the board, and swing the board away from the chassis. Do not stress coax cables on board. Rest

board on a stand (e.g. 2 phone books) next to radio.

6. Locate FL-96. Using a solder-sucker or wick, and a temperature-controlled pencil iron, desolder 4 filter pins.

7. Remove nuts & washers from filter mounting studs. (I think the nuts are M3 or M4.)

8. Remove FL-96, wiggling it gently to free pins. You may need to re-heat pins quickly to release filter. The board is G-10, and

can take reasonable heat.

9. Place FL-44A on board; tighten down studs with nuts & washers.

10. Re-solder pins of FL-44A; clean excess flux with nylon brush & isopropyl alcohol. Also ensure that there are no solder blobs

or slashes.

11. Re-install board and all cables unplugged in Step 4.

12. Pull FL-102; insert FL-223, and flip 2 dip switches (per user manual) from AM to SSB.

13. Test radio, then replace covers.

Good luck! The whole job should take about 30 minutes. Note that the FL-223 is selected in USB or LSB mode by pressing the

[WIDE] button.