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