Icom IC-448
 

I have problem with my 70cm rig, an Icom IC-448. I have used
it for packet radio about ten years (running 24/7) and
suddenly the squelch have stopped working. I have no schematics
but know, from the manual, in what area the squelch circuitry
is and have looked at the voltages there. But I am not that
much wiser from doing so. Pressing the monitor button will
open the squelch after an initial period, acting as if a capacitor
is charging up. But normal squelch does not work at all.

Anyone who have had this problem and found the cure for it?
Anyone who have a schematic?

73 de Lars, sm6rpz

--
Lars E. Pettersson | Chalmers University of Technology
| Gothenburg, SWEDEN

On 28 Aug 2003, Lars Petterson wrote:

I have problem with my 70cm rig, an Icom IC-448. I have used
it for packet radio about ten years (running 24/7) and
suddenly the squelch have stopped working. I have no schematics
but know, from the manual, in what area the squelch circuitry
is and have looked at the voltages there. But I am not that
much wiser from doing so. Pressing the monitor button will
open the squelch after an initial period, acting as if a capacitor
is charging up. But normal squelch does not work at all.

Anyone who have had this problem and found the cure for it?
Anyone who have a schematic?

73 de Lars, sm6rpz


I had an IC-28A 2 meter rig do that. It was the squelch control itself
that had gone bad. And it is a real bugger to change due to the place it
is mounted. It can be done though, you will have to contact ICOM to see
if they are still available. I had to replace mine about 5 years ago
after 6 or 8 years of packet service 24/7.
Good luck finding the part.
73 de KE8TQ
Fred

--

On 4 Sep 2003, Lars Petterson wrote:

In article ,
Fred wrote:
...
I had an IC-28A 2 meter rig do that. It was the squelch control itself
that had gone bad. And it is a real bugger to change due to the place it
is mounted.

Thanks for the info Fred!

Is that the TK-10420 IC? A 16 pin chip. The IC-448 uses one of those,
a narrow band FM IF chip, that also includes a squelch function. I
have gotten a circuit diagram and have found some odd voltages around
the IC, but I am still not sure if it is the IC, or any other component,
capacitor etc., that has gone bad. Have to do some more testing,
reflow some solder etc...

73 de Lars, sm6rpz

NO, the squelch control that goes bad is the actual mechanical pot, a
little bitty one with, thankfully a small wire harness attached that
solders into the front board. That is in the front of the rig, once you
take the face plate off you will see it. The wiper inside goes open and
that causes the squelch to not work any more. That is what happened on
this end any way. That by the way is only the second problem that I have
had with that rig. The other problem was the audio chip went bad and it
was easy to replace.

Fred KE8TQ



--

On 4 Sep 2003, Lars Petterson wrote:

In article ,
Fred wrote:
...
I had an IC-28A 2 meter rig do that. It was the squelch control itself
that had gone bad. And it is a real bugger to change due to the place it
is mounted.

Thanks for the info Fred!

Is that the TK-10420 IC? A 16 pin chip. The IC-448 uses one of those,
a narrow band FM IF chip, that also includes a squelch function. I
have gotten a circuit diagram and have found some odd voltages around
the IC, but I am still not sure if it is the IC, or any other component,
capacitor etc., that has gone bad. Have to do some more testing,
reflow some solder etc...

73 de Lars, sm6rpz

NO, the squelch control that goes bad is the actual mechanical pot, a
little bitty one with, thankfully a small wire harness attached that
solders into the front board. That is in the front of the rig, once you
take the face plate off you will see it. The wiper inside goes open and
that causes the squelch to not work any more. That is what happened on
this end any way. That by the way is only the second problem that I have
had with that rig. The other problem was the audio chip went bad and it
was easy to replace.

Fred KE8TQ



--

In article ,
Fred wrote:
....
NO, the squelch control that goes bad is the actual mechanical pot, a
little bitty one with, thankfully a small wire harness attached that
solders into the front board.

Ah, I see, that one works perfectly here.

I think I have solved the problem. It was not a component problem.
Under one of the PC-boards they had soldered a grounding lug. To
prevent this from shorting things on the PC-board they had glued
a small piece of plastic on the PC-board. This glue had turned into
a brown, rather brittle, substance, covering part of the squelch circuitry,
and also the input to the IF-amplifier. After removing this I found out
two things. The voltage at the input to the TK-10420 IF-chip raised
from 1.5V to 2V (which is the voltage supposed to be there), the output
from the chip rose from 2 to 3V (supposed to be 3.5V), and the S-meter
started to show signal strength again, something it have not done
for years... The squelch also started function again!

Using the test leads on my multi-meter I could measure a resistance
of 1-5 Mohm on this substance.

Old glue seem to be a bad thing to have on PC-boards :-)

73 de Lars, sm6rpz
--
Lars E. Pettersson | Chalmers University of Technology
| Gothenburg, SWEDEN

In article ,
Fred wrote:
....
NO, the squelch control that goes bad is the actual mechanical pot, a
little bitty one with, thankfully a small wire harness attached that
solders into the front board.

Ah, I see, that one works perfectly here.

I think I have solved the problem. It was not a component problem.
Under one of the PC-boards they had soldered a grounding lug. To
prevent this from shorting things on the PC-board they had glued
a small piece of plastic on the PC-board. This glue had turned into
a brown, rather brittle, substance, covering part of the squelch circuitry,
and also the input to the IF-amplifier. After removing this I found out
two things. The voltage at the input to the TK-10420 IF-chip raised
from 1.5V to 2V (which is the voltage supposed to be there), the output
from the chip rose from 2 to 3V (supposed to be 3.5V), and the S-meter
started to show signal strength again, something it have not done
for years... The squelch also started function again!

Using the test leads on my multi-meter I could measure a resistance
of 1-5 Mohm on this substance.

Old glue seem to be a bad thing to have on PC-boards :-)

73 de Lars, sm6rpz
--
Lars E. Pettersson | Chalmers University of Technology
| Gothenburg, SWEDEN