Left channel doesn't work

[cgibsong002]

So I'm trying to help my friend fix his receiver. He has a proton D940. Sounds very, very good. Its a bit screwed up though. The right channel doesn't work on either A or B (right now I'm running it from A and B with both lefts and set to mono). I'm not sure how these receivers work, is there possibly a transformer for left side and one for right side? Maybe something got blown? Unusual that both lefts work fine but neither rights do. Also, there's a couple random buttons that, if you press, the thing starts massively cutting out and getting scratchy. I attribute this to him trying to clean the whole thing out with steel wool Hopefully that can be fixed with some contact cleaner and dust spray.

[dakatabg]

Can you post us pictures from the front, back and inside the receiver?

[cgibsong002]

unfortunately one of the screws is badly stripped, so I haven't gotten inside the thing yet.

[dakatabg]

Try to clean the pots and switches with contact cleaner and if that won't help it will be something internal or something blown on the left side. Open it and see if something is burned or has dark color!

[cgibsong002]

as soon as i can get to home depot i'll get some contact cleaner and duster, as well as some anti-lock up so we can try to get that screw un-stuck. hopefully it's as simple as a few burnt out resistors, or possibly even just some corroded or dirty contacts.i'll post some pics once i can get inside.

[JoeE SP9]

I wouldn't say it's unusual for one side to work and the other not.

There are no transformers that can cause the problem you're (not) hearing. There is only one transformer. It supplies power to both channels. If one channel is dead it's normal for the A and B speaker outs to work on the working channel.

Do you get any sound at all from the bad channel?
What do you mean "he used steel wool to clean it"?

[kelsci]

I found a report on this unit. I can only imagine the sound quality this unit gives.

To me it sounds like a burnt out channel with is usually some kind of power supply problem; not the power transformer per say, but the diodes that power that channel. I would start by looking at where the diodes are and as one other board member suggested see if there is anything that looks burnt. Gnerally anything to do with Nad is expensive to repair, but some on audioreview suggested taking it to a tv shop if you or someone else does not have the technical knowledge to fix at home. What is not known here is wheter there is a pre-amp or power amp section problem but this receiver has pre-outs and main-ins. If one has a power amp around, you can test the pre-outs. If they play, it is then a power amp problem. One would need a spare pre-amp of some kind to test the poweramp section although I believe this is a poweramp one channel problem. Once opened, you can definetely perform the maintenace that was suggested. YOu never know what will work. I once had a Nad 7220PE which was only rated 20 watts per channel so I know what this receiver is capable of doing.



Proton D940 Stereo Receiver

1986 Winner of AudioVideo International Magazine's Hi-Fi Grand Prix Award, this Proton Audiophile Level Stereo Receiver has remarkable transparency, depth & punch.
This receiver is rated by 15 reviewers on the highly respected "Audio Review" site as a 4.87 out of 5 stars. The reason is obvious when you here the quality of sound this receiver puts out...I have Audes 111.1 speakers, 80 audiophile WPCs. amd this receiver made them sing...They're bookshelf speakers, but the Proton brought our wall shivering bass...Without going overboard, suffice it to say that they'll drive just about anything with all the gusto you could imagine...

The D940 was designed by the highly respected British company New Acoustic Dimensions (NAD) and even assembled in the same facility from 1985 through 1991 with a retail price of $500.00.

It is conservatively rated at 40 watts per channel but with +6 dB of IHF dynamic headroom its dynamic power for musical transients exceeds 160 watts per channel at 8 ohms, 280 WPC at 4 ohms and 380 WPC at 2 ohms. The D940 has inputs for DAD (Digital Audio Disc, CD), VIDEO, PHONO, TAPE 1 & TAPE 2.

Features:

Bridgeable. Engage the "Bridging" button & the two power amplifier channels form a mono block with more than double the output power.
DPD (Dynamic Power on Demand) Circuit. This means it can deliver 40 watts continuously and handle much higher peaks with ease.
Separate Record and Playback selectors. With this, you can listen to any component while your tape deck or CD recorder records from another source. Very handy if you make a lot of recordings and don’t want to have to listen to what you’re recording.
Preamp-out/main in jacks.
A.C.C. Anti Clipping Circuit (switchable). Very similar to NAD's "Soft Clipping" feature which reduces distortion and safeguards against possible damage to your speakers.
Gold Plated Phono Input. Handles both MM and MC cartridges. The MC features Hi and low filters.
SNR: “Shotz Noise Reduction". Reduces back-ground hiss common in medium strength FM signals.
8 FM & AM presets.
Connections for two sets of speakers.
Bass EQ. This equalization circuit boosts deep bass levels by 12 dB providing the sort of authentic bass “feel” that might otherwise require a costly separate subwoofer system.
3 AC outlets in back (1 switched, 2 unswitched).
Dimensions W x H x D: 16 1/2" x 3 3/4" x 11 1/3"
Weight: 18.75 lbs

[cgibsong002]

Ok, got it open.





there's definitely plenty of dust and a bit of corrosion on the lower middle layer. definitely going to spray the whole thing down with some cleaner and let it soak (after spraying out the dust). BUT, i did get a good bit of info from my friend, which i did not know before. See how there are these connectors for the pre-to power?

He said that he used to be able to get the receiver to work if he shoved something in between those two to spread them apart. So I'm thinking 90% likely that there's either a bad solder joint, or corrosion on the contacts there. It's very hard to get to those solder points on the inside, so I'm banking on the fact that there's just a bit of corrosion on the contacts that's causing the issue.

[JoeE SP9]

as soon as i can get to home depot i'll get some contact cleaner and duster, as well as some anti-lock up so we can try to get that screw un-stuck. hopefully it's as simple as a few burnt out resistors, or possibly even just some corroded or dirty contacts.i'll post some pics once i can get inside.

When you get the cover off, post some nudies of the innards. If you have another power amp handy, check if both sides of the preamp section are working.

If you have blown diodes in the power supply replacing the diodes may not cure the problem. You need to look for something pulling excessive current throught the diodes.


Edit:
If forcing those RCA jacks apart made the amp work properly the chances are nearly 100% that you have a bad solder joint. Any corrosion that gets on an RCA jack will usually get removed by the action of plugging in a cable.

[cgibsong002]

only problem is I couldn't see an easy way to get to those to check the joint, or for that matter to fix them. I didn't have a real good look though, maybe there's an easy way.

[kelsci]

You can certainly try fooling with the pre-amp/power amp connections. True that most of these receivers etc for the average joe are hard to work with particulaly to find a cold solder joint. I wish they were built like Dynaco equipment which was easier to get at.

[pixelthis]

This is why they have dumpsters.
IF YOU ARE enjoying this "project" good for you. But I doubt you will be successful.
SOME STUFF is just old , and the only "fix" is a well placed Nato round.
HAVE FUN , ANYWAY.

[pixelthis]

So I'm trying to help my friend fix his receiver. He has a proton D940. Sounds very, very good. Its a bit screwed up though. The right channel doesn't work on either A or B (right now I'm running it from A and B with both lefts and set to mono). I'm not sure how these receivers work, is there possibly a transformer for left side and one for right side? Maybe something got blown? Unusual that both lefts work fine but neither rights do. Also, there's a couple random buttons that, if you press, the thing starts massively cutting out and getting scratchy. I attribute this to him trying to clean the whole thing out with steel wool Hopefully that can be fixed with some contact cleaner and dust spray.

BTW there are only two amps, A and B run off of the same amp, so you are running
two speakers off of one amp, basically.
Try cleaning the contacts, and a few tranquilizers might help, but my audio store used to sell
proton, and while quite well reviewed the return rate was rather high. Big problem is that if you replace one thing another will break soon after. STUFF just gets old.

[cgibsong002]

i'm cleaning the contacts and if that doesn't work, we're just leaving it in mono. it's not worth taking it apart to solder it. and it's quite possible that it just needs to be cleaned. i've had multiple amps and speaker cabs that didn't work until i cleaned the different jacks.