Pete KE9OA wrote:

I did receive the manual today. This receiver uses an AGC diode in each of
the I.F. amplifiers, similar to the way that it was done in the Collins
651S-1. No wonder this rig has such a good dynamic range.

And if any one of those stages fails, the system will still work with only
slightly degraded AGC, unless somehow one of those diodes fails into a short
and pulls down the AGC line. This is something fairly easy to try.

It looks like there is a 2.2uF electrolytic cap that acts as a low-pass
filter after the AGC rectifier.

Where is the meter there? If the cap fails into a short, will the meter
still work? If so, it's not that cap.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

The meter comes after the AGC transistor. The AGC circuit consists of a
germanium detector diode with a low-pass filter that feeds a DC amplifier.
The amplified DC voltage controls the gain of the 3 I.F. stages and the RF
amplifier. It is almost impossible to get at the RF amplifier and the mixer.
These devices are embedded in the bandswitch assembly, along with two of the
AGC diodes. Now, if one of those diodes is cracked, that could also cause a
problem.
One of these days, I will dig into the radio. I've got a good T-12B and
T-12C, so at least I have a couple of reference standards.

Pete

"Scott Dorsey" wrote in message
...
Pete KE9OA wrote:

I did receive the manual today. This receiver uses an AGC diode in each of
the I.F. amplifiers, similar to the way that it was done in the Collins
651S-1. No wonder this rig has such a good dynamic range.

And if any one of those stages fails, the system will still work with only
slightly degraded AGC, unless somehow one of those diodes fails into a
short
and pulls down the AGC line. This is something fairly easy to try.

It looks like there is a 2.2uF electrolytic cap that acts as a low-pass
filter after the AGC rectifier.

Where is the meter there? If the cap fails into a short, will the meter
still work? If so, it's not that cap.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Pete KE9OA wrote:
The meter comes after the AGC transistor. The AGC circuit consists of a
germanium detector diode with a low-pass filter that feeds a DC amplifier.
The amplified DC voltage controls the gain of the 3 I.F. stages and the RF
amplifier. It is almost impossible to get at the RF amplifier and the mixer.
These devices are embedded in the bandswitch assembly, along with two of the
AGC diodes. Now, if one of those diodes is cracked, that could also cause a
problem.

Can you get into the switch assembly well enough to use a DMM with
a diode test function? An open germanium diode is definitely a prime
suspect. If you find an open one, you can just solder a replacement one
in parallel, since a little extra shunt capacitance won't hurt in this
case and it will prevent having to do further disassembly.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

I wish I could...........it is very hard to see into the depths of the
bandswitch assembly. Picture the RF section of a BC-348 in miniature. I am
going to give it another go later on this evening. I would like to get those
other two units working.

Pete

"Scott Dorsey" wrote in message
...
Pete KE9OA wrote:
The meter comes after the AGC transistor. The AGC circuit consists of a
germanium detector diode with a low-pass filter that feeds a DC amplifier.
The amplified DC voltage controls the gain of the 3 I.F. stages and the RF
amplifier. It is almost impossible to get at the RF amplifier and the
mixer.
These devices are embedded in the bandswitch assembly, along with two of
the
AGC diodes. Now, if one of those diodes is cracked, that could also cause
a
problem.

Can you get into the switch assembly well enough to use a DMM with
a diode test function? An open germanium diode is definitely a prime
suspect. If you find an open one, you can just solder a replacement one
in parallel, since a little extra shunt capacitance won't hurt in this
case and it will prevent having to do further disassembly.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

I finally figured this one out............it was the AGC amp transistor.
Unfortunately, it is an NPN germanium device, but just as an experiment I
tried a silicon device. No more overloading, but I will try to find some
germanium replacements.

Pete

"Pete KE9OA" wrote in message
. ..
I wish I could...........it is very hard to see into the depths of the
bandswitch assembly. Picture the RF section of a BC-348 in miniature. I am
going to give it another go later on this evening. I would like to get
those other two units working.

Pete

"Scott Dorsey" wrote in message
...
Pete KE9OA wrote:
The meter comes after the AGC transistor. The AGC circuit consists of a
germanium detector diode with a low-pass filter that feeds a DC
amplifier.
The amplified DC voltage controls the gain of the 3 I.F. stages and the
RF
amplifier. It is almost impossible to get at the RF amplifier and the
mixer.
These devices are embedded in the bandswitch assembly, along with two of
the
AGC diodes. Now, if one of those diodes is cracked, that could also cause
a
problem.

Can you get into the switch assembly well enough to use a DMM with
a diode test function? An open germanium diode is definitely a prime
suspect. If you find an open one, you can just solder a replacement one
in parallel, since a little extra shunt capacitance won't hurt in this
case and it will prevent having to do further disassembly.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Pete KE9OA wrote:
I finally figured this one out............it was the AGC amp transistor.
Unfortunately, it is an NPN germanium device, but just as an experiment I
tried a silicon device. No more overloading, but I will try to find some
germanium replacements.

Give me a number and I'll see what we have in the junk box.

I personally have no qualms about using silicon replacements if you
rebias, but I know people want to keep things stock.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Thanks Scott.........it is a 2N1304.

Pete

"Scott Dorsey" wrote in message
...
Pete KE9OA wrote:
I finally figured this one out............it was the AGC amp transistor.
Unfortunately, it is an NPN germanium device, but just as an experiment I
tried a silicon device. No more overloading, but I will try to find some
germanium replacements.

Give me a number and I'll see what we have in the junk box.

I personally have no qualms about using silicon replacements if you
rebias, but I know people want to keep things stock.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."