If anybody has seen this problem............I picked up a couple of these
radios from the local airport. Two of them work ok, but the other two are
running open loop with their gain. In other words, no AGC. Any ideas on what
could be causing this?
I do have the service manual, and I have replaced the three 10uF
electrolytic caps on the main board. Still, the problem persists. Thanks in
advance, for any help!

Pete

Pete KE9OA wrote:
If anybody has seen this problem............I picked up a couple of these
radios from the local airport. Two of them work ok, but the other two are
running open loop with their gain. In other words, no AGC. Any ideas on what
could be causing this?

Either the front end is not getting proper AGC voltage, or the detector
stage isn't producing it. What do you measure on the AGC line, and does
it change when you tune stations in and out?
--scott

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

I am not sure where the AGC line is in this radio...............I ordered
the service manual for this radio from ESSO last Friday.
The S-Meter is functioning, but still, I realize that this doesn't tell the
whole story. I am expecting to see a shorted AGC timing capacitor.
Once I get the schematic, it will be a little bit easier to muddle through
the circuit. For the time being, I am stuck.

Pete

"Scott Dorsey" wrote in message
...
Pete KE9OA wrote:
If anybody has seen this problem............I picked up a couple of these
radios from the local airport. Two of them work ok, but the other two are
running open loop with their gain. In other words, no AGC. Any ideas on
what
could be causing this?

Either the front end is not getting proper AGC voltage, or the detector
stage isn't producing it. What do you measure on the AGC line, and does
it change when you tune stations in and out?
--scott

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

Pete KE9OA wrote:
I am not sure where the AGC line is in this radio...............I ordered
the service manual for this radio from ESSO last Friday.

That is the key. If you don't have it, you can work back from the S-meter
and trace it, but the manual will make your life much better.

The S-Meter is functioning, but still, I realize that this doesn't tell the
whole story. I am expecting to see a shorted AGC timing capacitor.

Maybe, but again the S-meter should be after the time constants of the
AGC circuit. But at least you know something good is coming out of the
detector.

Once I get the schematic, it will be a little bit easier to muddle through
the circuit. For the time being, I am stuck.

Put it on the shelf until the manual arrives. If you fix it now, when
you get the manual you'll hit yourself on the head and wish you had waited
for the manual to arrive because it would have been so much easier.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Thanks Scott,

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. These older
designs used high Q transformers in the I.F. strip. The newer KR-85 units
have what seems to be lower Q Toko units.Still, the KR-85 is a pretty good
receiver. I do like these T12 series much better, though.
It looks like there is a 2.2uF electrolytic cap that acts as a low-pass
filter after the AGC rectifier. I think that if I figure the
T-12C radio out, I will be able to repair the T-12B. If I had something
like this years back, I would have never bothered to learn how to design
receivers......................maybe that was a good thing!

Pete

"Scott Dorsey" wrote in message
...
Pete KE9OA wrote:
I am not sure where the AGC line is in this radio...............I ordered
the service manual for this radio from ESSO last Friday.

That is the key. If you don't have it, you can work back from the S-meter
and trace it, but the manual will make your life much better.

The S-Meter is functioning, but still, I realize that this doesn't tell
the
whole story. I am expecting to see a shorted AGC timing capacitor.

Maybe, but again the S-meter should be after the time constants of the
AGC circuit. But at least you know something good is coming out of the
detector.

Once I get the schematic, it will be a little bit easier to muddle
through
the circuit. For the time being, I am stuck.

Put it on the shelf until the manual arrives. If you fix it now, when
you get the manual you'll hit yourself on the head and wish you had waited
for the manual to arrive because it would have been so much easier.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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."