help with deaf SX-190
 

I am trying to fix my recently and suddenly deaf SX-190 receiver and
can't seem to find the problem. I have a DMM, an uncalibrated sig
gen, and a 100 MHz scope available. Of course, I have schematics,
etc.

The radio is almost completely deaf on all bands, with no indications
on the s-meter. I can hear the calibrator on all bands at a very low
level. I have verified that the HFO, VFO, power supply, and audio amp
are all working. I have injected a very small signal into the antenna
input and can see amplitude increasing at each stage through the 1st
455 kHz IF amp. I see 6Vpp at the collector of Q13. Looking at the
collector of the next stage(Q14) the signal is down about 30 dB.
Between the two stages, there is a mechanical (ceramic?) filter and a
transformer. DC bias on the stage looks fine.

It looks as though either the transformer or mechanical filter have
croaked, but I can't be sure. Any suggestions for how to test a
mechanical filter? Do those things die?

Incidently, I have checked the solder connections and they look OK.

Thanks,

MR

Mark Rehorst wrote:

It looks as though either the transformer or mechanical filter have
croaked, but I can't be sure. Any suggestions for how to test a
mechanical filter? Do those things die?

I don't know if it applies to this radio but in that era there were some
mechanical filters that had something like foam rubber inside that turns
to a mushy goo over time.
I have heard of people repairing them simply by cracking it open and
cleaning with a solvent.
I seem to recall Kokusai as being one of the affected brands but my
memory is foggy on this.

Good luck,
Bill

Mark Rehorst wrote:

It looks as though either the transformer or mechanical filter have
croaked, but I can't be sure. Any suggestions for how to test a
mechanical filter? Do those things die?

I don't know if it applies to this radio but in that era there were some
mechanical filters that had something like foam rubber inside that turns
to a mushy goo over time.
I have heard of people repairing them simply by cracking it open and
cleaning with a solvent.
I seem to recall Kokusai as being one of the affected brands but my
memory is foggy on this.

Good luck,
Bill

Thanks for the input. I'll see if I can isolate the problem to
anything else.. Failing that, I guess I'll pull the filter off the
board and see what its insides look like. Time to start looking for a
parts set, I guess!

MR

Thanks for the input. I'll see if I can isolate the problem to
anything else.. Failing that, I guess I'll pull the filter off the
board and see what its insides look like. Time to start looking for a
parts set, I guess!

MR

This could be your lucky day...............I have one of those assemblies on
hand. It is a yellow core Toko transformer, with the markings MFH 51 T. If
this fills the bill, shout me down, and I will mail it out to you.

Pete

Mark Rehorst wrote in message
...
Thanks for the input. I'll see if I can isolate the problem to
anything else.. Failing that, I guess I'll pull the filter off the
board and see what its insides look like. Time to start looking for a
parts set, I guess!

MR

This could be your lucky day...............I have one of those assemblies on
hand. It is a yellow core Toko transformer, with the markings MFH 51 T. If
this fills the bill, shout me down, and I will mail it out to you.

Pete

Mark Rehorst wrote in message
...
Thanks for the input. I'll see if I can isolate the problem to
anything else.. Failing that, I guess I'll pull the filter off the
board and see what its insides look like. Time to start looking for a
parts set, I guess!

MR

Thanks for all the input! I pulled the filter off the PCB so I could
bypass it with a cap and then realized the first IF transistor gets
biased via the transformer inside the filter can. I put it back on
the board and the radio came back to life! It was a bad solder joint
the whole time! I sure wish they had cleaned the flux off the boards
when they built these radios. I might have been able to spot the
problem without all this fuss...

Now where did I put that can of flux remover?????

MR

Thanks for all the input! I pulled the filter off the PCB so I could
bypass it with a cap and then realized the first IF transistor gets
biased via the transformer inside the filter can. I put it back on
the board and the radio came back to life! It was a bad solder joint
the whole time! I sure wish they had cleaned the flux off the boards
when they built these radios. I might have been able to spot the
problem without all this fuss...

Now where did I put that can of flux remover?????

MR

Mark Rehorst wrote:

Thanks for all the input! I pulled the filter off the PCB so I could
bypass it with a cap and then realized the first IF transistor gets
biased via the transformer inside the filter can. I put it back on
the board and the radio came back to life! It was a bad solder joint
the whole time! I sure wish they had cleaned the flux off the boards
when they built these radios. I might have been able to spot the
problem without all this fuss...

Now where did I put that can of flux remover?????

MR

Its also possible that you have a bad solder joint or broken wire
inside the part, and the removal and resoldering has made a temporary
connection inside the can. I have seen it a few times on bad IF
transformers and other parts. Just keep it in the back of your mind, in
case it quits working again.
--


Michael A. Terrell
Central Florida

Mark Rehorst wrote:

Thanks for all the input! I pulled the filter off the PCB so I could
bypass it with a cap and then realized the first IF transistor gets
biased via the transformer inside the filter can. I put it back on
the board and the radio came back to life! It was a bad solder joint
the whole time! I sure wish they had cleaned the flux off the boards
when they built these radios. I might have been able to spot the
problem without all this fuss...

Now where did I put that can of flux remover?????

MR

Its also possible that you have a bad solder joint or broken wire
inside the part, and the removal and resoldering has made a temporary
connection inside the can. I have seen it a few times on bad IF
transformers and other parts. Just keep it in the back of your mind, in
case it quits working again.
--


Michael A. Terrell
Central Florida

Also check to be sure there are no cracked cores in the interstage
transformers. Those are hard to find sometimes. If all stages tune
correctly then they might be ok. When the cores crack they detune the
coil they are in.

Mark Rehorst wrote:

I am trying to fix my recently and suddenly deaf SX-190 receiver and
can't seem to find the problem. I have a DMM, an uncalibrated sig
gen, and a 100 MHz scope available. Of course, I have schematics,
etc.

MR

Also check to be sure there are no cracked cores in the interstage
transformers. Those are hard to find sometimes. If all stages tune
correctly then they might be ok. When the cores crack they detune the
coil they are in.

Mark Rehorst wrote:

I am trying to fix my recently and suddenly deaf SX-190 receiver and
can't seem to find the problem. I have a DMM, an uncalibrated sig
gen, and a 100 MHz scope available. Of course, I have schematics,
etc.

MR

On Fri, 31 Oct 2003 11:22:03 -0500, jim breeeyar
wrote:

Also check to be sure there are no cracked cores in the interstage
transformers. Those are hard to find sometimes. If all stages tune
correctly then they might be ok. When the cores crack they detune the
coil they are in.

I'll keep it in mind for future reference. My radio has been fixed-
it turned out the second mechanical filter had a bad solder joint.

Thanks,

MR

On Fri, 31 Oct 2003 11:22:03 -0500, jim breeeyar
wrote:

Also check to be sure there are no cracked cores in the interstage
transformers. Those are hard to find sometimes. If all stages tune
correctly then they might be ok. When the cores crack they detune the
coil they are in.

I'll keep it in mind for future reference. My radio has been fixed-
it turned out the second mechanical filter had a bad solder joint.

Thanks,

MR