Repairing vintage Philips 7871 receiver

[N_Cook]

Hey all,

I have a small (but major) problem with my Philips 7871 receiver. The
treble pot only controls the right channel. The left channel is stuck
on 0 dB: when i turn the pot, there is no change in the sound. The pot
works fine with the right channel and the bass pot works fine with both
channels. I have a feeling I need to buy a multimeter, but its been
many years since I used one and I've completely forgotten how to use it.
I also am looking for a service/repair manual for this, but I can't find
one. I took off the casing and looked around for any visual clues, but
everything looks fine inside. A few questions:

Is there anything else I should look for?

Anyone here have experience repairing this particular piece of
equipment?

If I bought a multimeter, how would I go about troubleshooting this
problem?

How much would it cost to repair this problem if I took it to a tech

Any idea where I could find a service/repair manual?

Please help!!!

Thanks
Rick

A crystal earpiece is about the only piece of test equipment you need for
this sort or repair, in all probability. To check the central pins of the
treble control pot to start with.

 

[sickrick]

Hey all,

I have a small (but major) problem with my Philips 7871 receiver. The
treble pot only controls the right channel. The left channel is stuck
on 0 dB: when i turn the pot, there is no change in the sound. The pot
works fine with the right channel and the bass pot works fine with both
channels. I have a feeling I need to buy a multimeter, but its been
many years since I used one and I've completely forgotten how to use it.
I also am looking for a service/repair manual for this, but I can't find
one. I took off the casing and looked around for any visual clues, but
everything looks fine inside. A few questions:

Is there anything else I should look for?

Anyone here have experience repairing this particular piece of
equipment?

If I bought a multimeter, how would I go about troubleshooting this
problem?

How much would it cost to repair this problem if I took it to a tech

Any idea where I could find a service/repair manual?

Please help!!!

Thanks
Rick

 

[sickrick]

A crystal earpiece is about the only piece of test equipment you need for
this sort or repair, in all probability. To check the central pins of the
treble control pot to start with.

Ok, I'll pick one up...but I have never used one before, can you point
me to some info on how to use one? I am assuming that i would not use
the jack plug, but would instead cut it off and ground the wire while
probing the treble control pins with the other wire?

Ugh I hope I don't have to replace the treble control pot, that would be
a pain with the way its layed out...

 

[sickrick]

Is there a tone control defeat switch (sometimes called
flat) on this unit? If so push it on and off a few times and
see if the tone function is restored. You should also try
turning the affected control back and forth several times.

David

Is this switch usually on the inside of the device? I'm looking for it,
but I can't find anything like that...its just a tone control bypass
switch? I don't think this unit has one...

I tried dialing the treble pot back and forth, but it does
nothing...this problem is consistent and there is no scratchy sound or
cutting in and out

thanks

 

[[email protected]]

Hey all,

I have a small (but major) problem with my Philips 7871 receiver.  The
treble pot only controls the right channel.  The left channel is stuck
on 0 dB: when i turn the pot, there is no change in the sound.  The pot
works fine with the right channel and the bass pot works fine with both
channels.  I have a feeling I need to buy a multimeter, but its been
many years since I used one and I've completely forgotten how to use it.  
I also am looking for a service/repair manual for this, but I can't find
one.  I took off the casing and looked around for any visual clues, but
everything looks fine inside.  A few questions:

Is there anything else I should look for?  

Anyone here have experience repairing this particular piece of
equipment?  

If I bought a multimeter, how would I go about troubleshooting this
problem?

How much would it cost to repair this problem if I took it to a tech

Any idea where I could find a service/repair manual?

Please help!!!

Thanks
Rick

Sounds like the wiper has lost contact with the slip ring inside due
to either oxidation or dried up grease. Just get ahold of some Caig
Deoxit spray. Shoot it into the opening where the solder lugs/pc pins
enter cover. It may be difficult to get to depending on the
orientation of the pot in relation to everything around it. I've made
extensions for the spray tube out of creativly manipulated shrink
tubing.....saves alot of tear down when it works.

 

[David Nebenzahl]

Ok, I'll pick one up...but I have never used one before, can you point
me to some info on how to use one? I am assuming that i would not use
the jack plug, but would instead cut it off and ground the wire while
probing the treble control pins with the other wire?

Ugh I hope I don't have to replace the treble control pot, that would be
a pain with the way its layed out...

Forget the "crystal earpiece". It's an obsolete piece of hardware you'll
find very difficult to find, and not what's needed here anyhow.

You'll have to excuse "N_Cook", who is an inveterate bodger and tinker.

You need a multimeter (aka VOM); any kind will do, digital or analog.
(The digital ones sell for as little as $3 in the US.) Put it on one of
the resistance measuring scales and measure the resistance between the
wiper (center terminal) and one of the outer terminals as you turn the
control; the resistance should change smoothly between (X ohms) and 0
ohms. If it doesn't, the control is shot and needs to be replaced.

Surprised nobody has suggested the first thing to do, which is to squirt
some electronic cleaner into the pot and try to clean it. Often this is
all that's needed to fix a scratchy or non-working control. Be sure to
use something made for cleaning electronic components, not something
like WD-40.


--
Made From Pears: Pretty good chance that the product is at least
mostly pears.
Made With Pears: Pretty good chance that pears will be detectable in
the product.
Contains Pears: One pear seed per multiple tons of product.

(with apologies to Dorothy L. Sayers)

 

[sickrick]

Unfortunately, I suspect that you may very well need to. The symptoms
lead me to suspect that the pot has gone open... either the wiper has
broken, or the conductive track has been worn through. Since pots are
mechanical contrivances, they're subject to mechanical wear-out.

I'd guess that the three most common age-related problems in vintage
receivers of this sort are open/noisy switches, open/noisy pots, and
bad (leaky or high-ESR) electrolytic capacitors.

yeah but the same pot works fine for the right channel, its not noisy at
all..very smooth in fact, which leads me to believe its a problem with
something else since only the left channel is not being affected by the
treble pot

 

[sickrick]

Forget the "crystal earpiece". It's an obsolete piece of hardware you'll
find very difficult to find, and not what's needed here anyhow.

You'll have to excuse "N_Cook", who is an inveterate bodger and tinker.

You need a multimeter (aka VOM); any kind will do, digital or analog.
(The digital ones sell for as little as $3 in the US.) Put it on one of
the resistance measuring scales and measure the resistance between the
wiper (center terminal) and one of the outer terminals as you turn the
control; the resistance should change smoothly between (X ohms) and 0
ohms. If it doesn't, the control is shot and needs to be replaced.

Surprised nobody has suggested the first thing to do, which is to squirt
some electronic cleaner into the pot and try to clean it. Often this is
all that's needed to fix a scratchy or non-working control. Be sure to
use something made for cleaning electronic components, not something
like WD-40.

i think the pot itself is fine: the same pot controls the right channel
which sounds fine...are you guys saying the pot itself is stereo? there
are 6 pins on the pot

 

[David Nebenzahl]

i think the pot itself is fine: the same pot controls the right channel
which sounds fine...are you guys saying the pot itself is stereo? there
are 6 pins on the pot

"The pot" is actually 2 pots ganged together mechanically, one for each
channel. Each pot has 3 connections; the outer connections to the
resistance, and the center "wiper" that gives you the variable
resistance. It's possible that one of the 2 pots is bad.


--
Made From Pears: Pretty good chance that the product is at least
mostly pears.
Made With Pears: Pretty good chance that pears will be detectable in
the product.
Contains Pears: One pear seed per multiple tons of product.

(with apologies to Dorothy L. Sayers)

 

[hr(bob) [email protected]]

"The pot" is actually 2 pots ganged together mechanically, one for each
channel. Each pot has 3 connections; the outer connections to the
resistance, and the center "wiper" that gives you the variable
resistance. It's possible that one of the 2 pots is bad.

--
Made From Pears: Pretty good chance that the product is at least
mostly pears.
Made With Pears: Pretty good chance that pears will be detectable in
the product.
Contains Pears:  One pear seed per multiple tons of product.

(with apologies to Dorothy L. Sayers)

I agree with David, spray some tuner cleaner, or even Old Spice
aftershave or WD-40 into the two sections of the pot and see what
happens, you don't have anything to lose, and it has fixed many an old
pot for me over the last 40+ years.

Bob Hofmann

 

[Ross Herbert]

:Hey all,
:
:I have a small (but major) problem with my Philips 7871 receiver. The
:treble pot only controls the right channel. The left channel is stuck
n 0 dB: when i turn the pot, there is no change in the sound. The pot
:works fine with the right channel and the bass pot works fine with both
:channels. I have a feeling I need to buy a multimeter, but its been
:many years since I used one and I've completely forgotten how to use it.
:I also am looking for a service/repair manual for this, but I can't find
ne. I took off the casing and looked around for any visual clues, but
:everything looks fine inside. A few questions:
:
:Is there anything else I should look for?
:
:Anyone here have experience repairing this particular piece of
:equipment?
:
:If I bought a multimeter, how would I go about troubleshooting this
roblem?
:
:How much would it cost to repair this problem if I took it to a tech
:
:Any idea where I could find a service/repair manual?
:
lease help!!!
:
:Thanks
:Rick

If you subscribe to this forum
http://www.audiokarma.org/forums/showthread.php?t=51608
you might be able to contact Brian. He seems to have a manual.

 

[JB]

I agree with David, spray some tuner cleaner, or even Old Spice
aftershave or WD-40 into the two sections of the pot and see what
happens, you don't have anything to lose, and it has fixed many an old
pot for me over the last 40+ years.

Bob Hofmann

Yuck! why not just swish it around in gasoline, spit in the back and give
it big WHUMP!

Seriously, tuner wash was good stuff but tuners don't have switches anymore
so it's gone! Isopropyl 90% is a cheap and safe wash (70% has too much
water) followed by a drop of very light mineral oil (baby oil). I suspect
there is a lot of Isopropyl in Old Spice. WD-40 leaves a crusty residue as
a corrosion inhibitor. Around here a spray can of Isopropyl costs $15 a can
at the electronic emporium vs. $2 a bottle at the drug store. I can make
the can last 5-10 years.

 

[Bob Larter]

I tried dialing the treble pot back and forth, but it does
nothing...this problem is consistent and there is no scratchy sound or
cutting in and out

Probably a simple dry joint on one of the center pins of the treble pot.
Should be very easy to fix by cleaning & resoldering.

 

[David Farber]

Hey all,

I have a small (but major) problem with my Philips 7871 receiver. The
treble pot only controls the right channel. The left channel is stuck
on 0 dB: when i turn the pot, there is no change in the sound. The
pot works fine with the right channel and the bass pot works fine
with both channels. I have a feeling I need to buy a multimeter, but
its been many years since I used one and I've completely forgotten
how to use it. I also am looking for a service/repair manual for
this, but I can't find one. I took off the casing and looked around
for any visual clues, but everything looks fine inside. A few
questions:

Is there anything else I should look for?

Anyone here have experience repairing this particular piece of
equipment?

If I bought a multimeter, how would I go about troubleshooting this
problem?

How much would it cost to repair this problem if I took it to a tech

Any idea where I could find a service/repair manual?

Please help!!!

Thanks
Rick

I found the schematic for this set. There is a high cut filter switch which
if failing, could be affecting the high end. Also, there is a small 1uF
capacitor, C233, that may be affecting the frequency response.

It should be fairly simple for an experienced technician to compare the left
channel to the right and figure out where the problem is.

Good luck.