Need help repairing vintage receiver

[Mark]

Hi

I recently got hold of an old Hitachi SR-703 receiver that's been in
an attic for 15 years.

I hooked it up and it appeared to work, the FM signal strength meter
was high, the stereo beacon was on and it sounded good. Then after
about 1 minute the stereo beacon went out and I lost the sound. The
signal strength meter was still high, the FM centre meter was still in
the centre, just no sound and no stereo beacon. I can tune in all my
local FM stations with high signal strength but still no sound and no
stereo beacon.

AM works fine, phono, and aux inputs work fine, power supplies appear
OK. The FM muting button is switched off.

I've checked the entire tuner board for dry joints and found none.
I've replaced the stereo decoder chip (HA1196), I've replaced the FM
IF amplifier chip (HA1137). The only other chips on the tuner board
are an HA1151 which I believe is an AM decoder (so I assume it's OK)
and an HA1142 which I believe is an input preamplifier (also assumed
OK).

I don't have a circuit diagram so I'm lost for ideas. The fact that it
worked perfectly for 1 minute after 15 years storage suggests
something like a dried out capacitor, but I would have expected this
kind of fault to affect other areas of operation.

Any ideas greatly appreciated, it would be a shame to scrap this well
built and once well respected piece of kit.

Thanks
Mark

 

[Aries Quitex]

Hi

I recently got hold of an old Hitachi SR-703 receiver that's been in
an attic for 15 years.

Fine unit, as far as I know.

I hooked it up and it appeared to work, the FM signal strength meter
was high, the stereo beacon was on and it sounded good. Then after
about 1 minute the stereo beacon went out and I lost the sound. The
signal strength meter was still high, the FM centre meter was still in
the centre, just no sound and no stereo beacon. I can tune in all my
local FM stations with high signal strength but still no sound and no
stereo beacon.

AM works fine, phono, and aux inputs work fine, power supplies appear
OK. The FM muting button is switched off.

I've checked the entire tuner board for dry joints and found none.

Resolder, anyway. There is a 10% chance that there are dry joints, but you
just don't see them.

I've replaced the stereo decoder chip (HA1196), I've replaced the FM
IF amplifier chip (HA1137). The only other chips on the tuner board

Maybe there is a crystal resonator that aged under the extreme climatic
situations on the attic? Sometimes they are in an orange or blue
plastic-case about the size of a 6-pin DIL, but stand upright on 2 pins.
Also there are units in small ceramic 3-pin housing.

are an HA1151 which I believe is an AM decoder (so I assume it's OK)
and an HA1142 which I believe is an input preamplifier (also assumed
OK).

I don't have a circuit diagram so I'm lost for ideas. The fact that it
worked perfectly for 1 minute after 15 years storage suggests
something like a dried out capacitor, but I would have expected this
kind of fault to affect other areas of operation.

Lesson one from first year of R&TV-Tech apprenticeship:
Always junk the 100µF capacitors against new ones.

Due to whatever reason they have the highes fault rate of al ElCaps.

When you have an oscilloscope handy (Lucky one, if) check if there's any
audiofrequency (AF)-signal coming out of the FM-Board (assuming that a
1985-model still used separate boards for amp, fm and am)

 

[Asimov]

"Mark" bravely wrote to "All" (08 Jan 04 02:48:52)
--- on the heady topic of "Need help repairing vintage receiver"

Check the ESR of the electrolytic caps around the stereo decoder IC,
quadrature coil or detector, and tuner board supply filters, often at
one edge. The tuning voltage comes from the tuner front end but the
audio comes from the 10.7MHz quadrature coil (or dectector) ahead of
the stereo decoder. You should find a mpx signal at the output of the
dectector using an audio signal tracer, if not then it could be the
detector itself or its driver. What IC is used for stereo decoder?

Ma> From: [email protected] (Mark)

Ma> Hi

Ma> I recently got hold of an old Hitachi SR-703 receiver that's been in
Ma> an attic for 15 years.

Ma> I hooked it up and it appeared to work, the FM signal strength meter
Ma> was high, the stereo beacon was on and it sounded good. Then after
Ma> about 1 minute the stereo beacon went out and I lost the sound. The
Ma> signal strength meter was still high, the FM centre meter was still in
Ma> the centre, just no sound and no stereo beacon. I can tune in all my
Ma> local FM stations with high signal strength but still no sound and no
Ma> stereo beacon.

Ma> AM works fine, phono, and aux inputs work fine, power supplies appear
Ma> OK. The FM muting button is switched off.

Ma> I've checked the entire tuner board for dry joints and found none.
Ma> I've replaced the stereo decoder chip (HA1196), I've replaced the FM
Ma> IF amplifier chip (HA1137). The only other chips on the tuner board
Ma> are an HA1151 which I believe is an AM decoder (so I assume it's OK)
Ma> and an HA1142 which I believe is an input preamplifier (also assumed
Ma> OK).

Ma> I don't have a circuit diagram so I'm lost for ideas. The fact that it
Ma> worked perfectly for 1 minute after 15 years storage suggests
Ma> something like a dried out capacitor, but I would have expected this
Ma> kind of fault to affect other areas of operation.

Ma> Any ideas greatly appreciated, it would be a shame to scrap this well
Ma> built and once well respected piece of kit.

Ma> Thanks
Ma> Mark

.... Which sparks some mnemonic circuitry.

 

[Asimov]

"Aries Quitex" bravely wrote to "All" (08 Jan 04 12:16:29)
--- on the heady topic of "Re: Need help repairing vintage receiver"

AQ> From: Aries Quitex <[email protected]>
AQ> Lesson one from first year of R&TV-Tech apprenticeship:
AQ> Always junk the 1005F capacitors against new ones.

AQ> Due to whatever reason they have the highes fault rate of al ElCaps.

I thought it was 1uF 50wv electros...

.... KPLA: Klingon radio: All glory, all the time!

 

[Jerry G.]

The easiest way to fix this, is to get a 100 mHz scope, and trace the signal
path starting from the tuner section through. You will find where the signal
is stopping. I suspect an IF, detector, or AGC problem. It may be as
simple as the signal not passing through the selector switch, or there may
also be a muting problem.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================


Hi

I recently got hold of an old Hitachi SR-703 receiver that's been in
an attic for 15 years.

I hooked it up and it appeared to work, the FM signal strength meter
was high, the stereo beacon was on and it sounded good. Then after
about 1 minute the stereo beacon went out and I lost the sound. The
signal strength meter was still high, the FM centre meter was still in
the centre, just no sound and no stereo beacon. I can tune in all my
local FM stations with high signal strength but still no sound and no
stereo beacon.

AM works fine, phono, and aux inputs work fine, power supplies appear
OK. The FM muting button is switched off.

I've checked the entire tuner board for dry joints and found none.
I've replaced the stereo decoder chip (HA1196), I've replaced the FM
IF amplifier chip (HA1137). The only other chips on the tuner board
are an HA1151 which I believe is an AM decoder (so I assume it's OK)
and an HA1142 which I believe is an input preamplifier (also assumed
OK).

I don't have a circuit diagram so I'm lost for ideas. The fact that it
worked perfectly for 1 minute after 15 years storage suggests
something like a dried out capacitor, but I would have expected this
kind of fault to affect other areas of operation.

Any ideas greatly appreciated, it would be a shame to scrap this well
built and once well respected piece of kit.

Thanks
Mark

 

[Sofie]

...... gee.... I always thought it was the 330uf and 1000uf @ 6.3 V and 10
V.

 

[Mark]

"Mark" bravely wrote to "All" (08 Jan 04 02:48:52)
--- on the heady topic of "Need help repairing vintage receiver"

Check the ESR of the electrolytic caps around the stereo decoder IC,
quadrature coil or detector, and tuner board supply filters, often at
one edge. The tuning voltage comes from the tuner front end but the
audio comes from the 10.7MHz quadrature coil (or dectector) ahead of
the stereo decoder. You should find a mpx signal at the output of the
dectector using an audio signal tracer, if not then it could be the
detector itself or its driver. What IC is used for stereo decoder?

Stereo decoder chip is Hitachi HA1196, I replaced it already.

 

[Sunny]

Hi

I recently got hold of an old Hitachi SR-703 receiver that's been in
an attic for 15 years.

I hooked it up and it appeared to work, the FM signal strength meter
was high, the stereo beacon was on and it sounded good. Then after
about 1 minute the stereo beacon went out and I lost the sound. The
signal strength meter was still high, the FM centre meter was still in
the centre, just no sound and no stereo beacon. I can tune in all my
local FM stations with high signal strength but still no sound and no
stereo beacon.

AM works fine, phono, and aux inputs work fine, power supplies appear
OK. The FM muting button is switched off.

My SR-503L (purchased new in 1979) is still *in* the attic because it
developed the exact same problem shortly before the last time we moved
house, and I haven't gotten around to looking at it. It's very similar
to the SR-703 - just has lower output power IIRC.

Not sure if this could be a hint as to the cause, but for quite some
time turning the FM mute on for a few seconds, then back off would
restore sound on mine - this trick no longer worked last time I had it out.

The owners manual includes a block schematic, but not a full circuit
diagram. If this would be of any assistance I'd be happy to scan and
mail to you.

Needless to say I'd appreciate your posting the fix if found.