[Milsurplus] AN-762 amp with the RT-662

[Patrick Jankowiak]

John,

Having some experience with repairing 'GRC-106 basket cases' and 
using the GRC-106 and GRC-106A, I can tell you that the RT-662 
and RT-824 (the model with 0.1KHz frequency setting resolution) 
uses a signal received back from the AM-3349 to control the RF 
output level from the RT unit.

You will need to concoct a system to 'fool' the RT into thinking 
there is an AM-3349 attached and providing a certain amount of RF 
gain.

If this is not done, you may find that SSB TX 'gain' is 
excessive, and that the AM mode carrier will saturate to the 
maximum output level of the RT unit, leaving no room for modulation.

This is taken care of in the normal situation by an ALC signal 
from the AM-3349 feeding into the "transmitter audio and IF 
module 1A5".

There is also an ALC signal which comes internally from the RT 
unit's output section and feeds the same module in order to 
control the RT unit's RF output to the nominal 0.1W level.

(reference TM 11-5820-520-34 page 1-12)

The large interconnection cable between the two units has two ALC 
lines. Both signals are rectified or detected signals, so they 
appear as DC-coupled signals which vary in voltage according to 
the instantaneous point of the audio waveform of modulation (or 
simply a DC level if CW is used)

Pin "B" is only for tuning and senses the DC level created by 
rectifying a portion of the final amplifier grid drive, and in 
normal operation is grounded through a 820 ohm resistor in 
parallel with a 1800 ohm resistor. Practically, only the 820 ohm 
resistor need be used, in order to keep a diode reverse biased.

(reference TM 11-5820-520-34 Figure FO-2)

Pin "C" is the actual PPC (peak power control) ALC signal from 
the output of the AM-3349. The PPC is set with PC control A1R15 
and feeds transistors A5A3Q4 and A5A3Q5 in the transmitter audio 
and IF module 1A5.

The APC (automatic power control) adjustment, affecting AM 
carrier, CW, and FSK levels, is set by pot A1R14. The signal 
feeds A5A3Q1 in the transmitter audio and IF module 1A5.

both A1R15 and A1R14 are on the TR-662 front panel and chassis 
assembly 1A1. I don't recall right now exactly where, but all are 
marked clearly.

The transmitter audio and IF module 1A5 has a D-connector J1. The 
ALC signals are processed on board A3 (1A5A3). The PPC signal 
enters through pin 3 and goes to Q4's base. Q4 drives Q5. The APC 
signal enters through pin 7 and goes to Q1's Base.

There are test points on the top of module 1A5 for PPC and APC 
signals.

The test points indicate 0 to 3 VDC during normal operation. 
Remember that APC is power control (carrier level for AM) and PPC 
is the PEP level control.

That is about what I know of it. You will have to do some 
experimenting.

It is also possible that an adjustment of the internal ALC level 
which is fed to pin 5 or 6 (can't tell which from the schematic, 
but it goes to transmitter audio and IF module 1A5 board A3 
(1A5A3) terminal E4) will, in the absence of signals from the 
AM-3349, provide some ALC, but it is my experience that this only 
limits the maximum output, and therefore the situation is a 
strong carrier with no modulation on AM, and too much gain on 
SSB. There does not appear to be a pot to adjust this, but I 
could be wrong.

(reference TM 11-5820-520-34 Figure FO-18)

This is more detail than required. The user can supply an ALC 
voltage to the 'big jack' pin "C". The specification of the 
signal is positive, zero to whatever small voltage is required to 
give 0-3VDC at the PPC and APC test points. The exact voltage to 
be expected there depends on PEP and carrier level-related DC 
voltages 'seen' by the RT from the amplifier.

Be careful with having too low an impedance from your test 
voltage source for these experiments, or you will be replacing Q4 
and Q1 in the 1A5A3 board.

I hope that others who love the GRC-106 will offer their opinions 
on these matters as well.

Patrick

> From: jwatkins <jwatkins at omantel.net.om>
> Subject: [Milsurplus] AN-762 amp with the RT-662
> 
> Hi All,
> 
> I will be getting the AN 762 kit in a few days (understanding wife is
> bringing it with her from the States) and will be trying it with my RT-662.
> The 662 has 100mw output, and it should be interesting to see what all the
> AN 762 (140w Motorola design) does with that amount of drive.  I plan to
> make a small QRP amp also to eventually drive the 762.  Has anyone tried
> this yet?  If you have, any info would be appreciated.
> 
> Further developments will include putting the all of this into an AM-3349
> shell that I have (haven't actually seen it, but it is in the garage).  That
> will give me one real GRC-106A and the sort of GRC-106 with the solid state
> AN 762 setup.
> 
> John  WD5ENU/A45XV