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Proton D940

Stereo AM/FM Receiver

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Proton D940


The Proton D940 stereo AM/FM receiver has been carefully developed with complete emphasis on the quality of sound.

Whether adding to an existing audio or video system or as part of an all new system, you are certain to notice several performance characteristics which make Proton products among the very best regardless of price.

One of the important characteristics of the Proton D940 is the exceptionally noise free FM performance made possible by the inclusion of our Schotz noise reduction system in an extremely sensitive audio tuner.

Also, our exclusive Dynamic Power on Demand (DPD) amplifier produces unsurpassed levels of dynamic power.


Tuning range: FM, MW

Power output: 40 watts per channel into 8Ω (stereo)

Frequency response: 20Hz to 20kHz

Total harmonic distortion: 0.02%

Damping factor: 90

Input sensitivity: 0.2mV (MC), 2.5mV (MM), 150mV (line)

Signal to noise ratio: 75dB (MC), 92dB (MM), 105dB (line)

Speaker load impedance: 2Ω to 8Ω

Dimensions: 420 x 96 x 288mm

Weight: 8.5kg



re: D940

I picked up a D940 this weekend at a local flea market - very dusty and a bit battered but the seller swore it worked.

It does work, but had what seems to be a common problem with this model - TUNED blinks even on strong signals, and it wouldn't find stations reliably in FM auto-search mode.

I ran some measurements and it looks as if the problem may just be misalignment. The 7.2 MHz synthesizer crystal is 100 PPM low (not great but not critical, I think), but when properly tuned, the quadrature-detector error voltage at TP1 was up at 5.2 volts (should have been under 50 millivolts!). Way off!

I got my nylon adjustment tools and gave the upper IF transformer slug a gentle turn downwards - about one turn brought the TP1 voltage down to within a few millivolts of ground. LOCKED stopped flashing, stays on when properly tuned, and the auto-tune feature now works.

Either the IF had drifted quite a lot in the past few decades, or this unit was never properly aligned in the first place. Considering how many reports there are of this problem, I wonder if a batch got out of the factory without being properly aligned?

Gotta do a more thorough alignment this weekend with a signal generator and a distortion analyzer, I guess.

Does anybody know the dimensions of the AM loop antenna, and the number of turns of wire in it, for this model?

re: D940

After a few years of ownership, I've concluded that there seems to be something fundamentally a bit wrong with the IF circuit in my unit. The upper (rear) slug in the IF transformer needs to be readjusted every year ot two - always in the same direction. I've had to do this at least three times - today I found the TP1 voltage was back up to 2.1 volts.

My guess is that the slug is creeping upwards "by itself" as time passes... maybe due to thermal cycling, maybe due to something odd about whatever sort of friction sheet is used to hold the slug in the transformer core. I tried lightly tacking it in place with a small dab of electronics-grade RTV, the last time I adjusted it - this doesn't seem to have done any good.

There's probably no long-term fix for this other than replacing or rebuilding the IF transformer - a tricky thing since the original type is probably unobtainable now and a rebuild would risk breaking it. I'll just readjust when the LOCKED light starts its flashing behavior again.

Still like this receiver very much - it's my "bedroom system" now, driving a pair of modified Minimus 77 speakers with a DefTech subwoofer.

re: D940

Re flashing LOCKED light: Could you kindly help me identify this IF transformer. What is the part number, location, color etc. Based on your description I am assuming that this is a transformer with two slugs, and that I am to turn the one closest to the rear of the D940? I do see a transformer labeled IFT403 which has a red slug towards the rear and a blue slug. I also see IFT101 which has a green case and it has two black slugs. Are either of these the correct transformer? Many thanks in advance for your help. (I do have a copy of the service manual.)

re: D940

Sweet, I have a really nice Proton D940 which has had the same FM locking / search problem since I got it four years ago. I just twiddled the IF transformer thing, and now it's working fine.

re: D940

If I needed to buy capacitors C901 to C908, where can I find them?
I also need one bass, treble front knob; where can I buy one?

Proton D940 Left Channel Dead until you turn the volume way up

I am pretty sure its not a volume control problem.

All the controls were cleaned with cleaner and the unit was working properly for a while.

Also, if I turn on the unit and advance the volume control half way up the dead left channel will come alive.

Please note when I turn up the volume the right channel increase normally, the left channel is silent and all of a sudden you get full volume.

There is no problem with the right channel.

If I power down the unit and power up the left channel will come up dead again until I turn up the volume control.

Not so good for my left speaker since it jumps to a loud volume instantaneously.

Any Ideas?

Proton D940 Left Channel Dead Until you turn up the Volume

Hi Everyone:

I think I found the problem.

Bad speaker relay on the left channel of the A speaker output.

I found a work around.

The Proton D940 has the capability to have an A and B set of speakers.

I moved my speakers to the B speaker output and it works perfectly.

The B speaker relay contacts are not usually used since most people would only have one set of speakers.

Has anyone found a replacement for the D940 speaker relay?


unstable volume

It sounds as if the left hand control needs replacing, but if you can forward me the circuit, I will have a look and try to help where I can


Proton D940 left channel dead until you turn up volume control


The service manual has been up loaded by jheur02. It's in the download section.

I looked at it but it not very easy to read.


ellsworth audio company's picture

Proton D940--Distorted FM, No FM, Hum on AM/FM.

FM comes in distorted when variac is increased to 130/140 vac. Low 15V supply to FM @Q909, zeners OK..... Leaky C923 (mis-labeld in manual on board view but not schematic) replace C923 with 1000@25vdc.

D940 FM

I just receive a D940 in used but good condition.

The sound is very good just like the NAD (since both units are manufactured by the same electronics company).

Well worth having if you can get one in good working condition.

Has anyone encountered this?

The "LOCKED" light on the front display constantly blinks. Any Idea's?
The Stereo light is on solid and I get 4 bars on the signal strenght.

It sound ok but I do get a little little noise on the FM so I will try replacing the C923 capacitor as suggested.

Also, I searched all over the Internet for a Service Manual without success.

Does anyone have one?


Proton D940 Service Manual/Schematic?

I have a Proton D940 that is very slow to wake up after power turn-on but works fine thereafter. I need some service documentation in order to proceed with a repair. There is some service data on site for the Protron 950 which simular and helpful but the but D940 is enough different that service data specifically for the D940 would be much better. Where can I get a copy of D940 service data? Many thanks---

D940 Service Manual

Look for it shortly. I finally got my SM scanned and will be trying to upload it for processing in the next day or so when I can get to a faster internet connection.


This website is not affiliated with or sponsored by Proton. To purchase D940 spares or accessories, please contact the company via their website or visit an authorised retailer.