Repairing vintage Philips 7871 receiver

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.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook
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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

Reply to
David

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

Reply to
sickrick

If this unit has such a switch, it would be on the front panel. It looks like you need at least an ohmmeter to see if the pot if working correctly. If it is working properly by comparing readings from both channels, then post back.

David

Reply to
David

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.

--
Dave Platt                                    AE6EO
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Reply to
Dave Platt

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...

Reply to
sickrick

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

Reply to
sickrick

=A0

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.

Reply to
boardjunkie

On 4/3/2009 12:58 PM sickrick spake thus:

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.

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Reply to
David Nebenzahl

news:rec.antiques.radio+phono

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And another motherboard bites the dust!
Reply to
Michael A. Terrell

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

Reply to
sickrick

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

Reply to
sickrick

On 4/3/2009 3:45 PM sickrick spake thus:

"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.

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Reply to
David Nebenzahl

ere

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

Reply to
hrhofmann

: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

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you might be able to contact Brian. He seems to have a manual.

Reply to
Ross Herbert

Let's not ignore the possibility of a simple bad solder connection at the control itself...

Mark Z.

Reply to
Mark Zacharias

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.

Reply to
JB

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

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Reply to
Bob Larter

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.

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David Farber
David Farber\'s Service Center
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Reply to
David Farber

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