Allen MDS 45 Great/Pedal not playing

[James P]

When I play my Allen MDS 45 organ, the Swell manual all plays great as it should, but...not any stop playing on Great or Pedals.
Checked all connections & found 1 Spkr wire came loose on Spkr #4. Reconnected it—still doesn't play.
Then checked ALL connections inside organ, including the Quad amp. Still nothing! I even opened the Quad & changed the 4 amp modules position. #1 & 2 (Sw playing amps) became Ch #3 & 4 (Gt/Pedal non playing amps/speakers). And #3 & 4 became #1 & 2.
No change in organ's playing. Sw plays, and Gt/Pedal still nothing. But the Ch 4/Spkr 4 “hum” remains present through any/all of these tests/changes.
Any suggestions/ideas as to what happened & how to get Gt & Pedal stops to play again—even with the persistent hum?

[jbird604]

To test the audio system function and continuity, disconnect the RCA plugs coming out of all four outputs on top of the cage. Now, since you already know that the swell works, plug each of the RCA plugs, one at a time, into jack #1. Play the swell manual with all the stops pulled, and see if you get sound using each of the plugs individually. If each of them produces clean sound, that proves that all four of your amps and speakers are fine, and that everything else in the audio path between the cage and the amps is fine.

If you do NOT get sound when plugging plugs #3 and #4 into jack #1, then make note of that. It does indicate that something is wrong downstream from the cage, and you'll have to find that eventually.

Next, to test each of the four cage outputs, you are going to use cable #1 as your test device. Turn on all the 8' and 4' stops on each manual. Now plug cable #1 into each of the four cage jacks, one at a time. Plug it into #1 and play a few keys on the swell. You should hear something. Move the plug to #2 jack and play on the swell again. You should hear something. Now plug the #1 cable into jack #3 and play on the great manual. You should hear sound. Then move the #1 cable to jack #4 and play the great. Again you should hear sound.

If you got sound from all four jacks, that proves that your cage is ok. If you did NOT get anything from jacks #3 and #4, that proves that something is wrong INSIDE THE CAGE.

Now, armed with this information, you can start looking for the culprit. Please report your findings here and we'll continue.

[James P]

John,
I just completed testing process of all as described above. I found just as you suspected, cage if fine (I assume from this test!) Since I got sound playing from ALL --both Swell & Great when I plugged Ch #1, 2, 3 & 4 into Ch 1 cage port. And again no sound from Ch #3 or 4 when plugged into Ch #1 port-- but did get sound with Ch #1 & 2 with individual stops playing on Swell thru cage port #1.
Next test please! Thanks!!

[jbird604]

Ok. So let me confirm your results -- You are saying that ALL FOUR CAGE OUTPUTS do in fact put out some sound. Using only the #1 CABLE as the test device, since we know that the #1 amp and speaker are good -- you can plug the #1 CABLE into any of the four JACKS on top of the cage, whether it's jack #1, #2, #3, or #4, and you WILL GET SOUND. Correct?

If so, then yes, you have just proved that the cage is all good. That is a good thing, since cage problems are much harder to solve that audio problems.

And you have pretty much already ruled out amplifier problems, since you say that you swapped amplifier modules #1 and #2 for #3 and #4 and got no change at all. That should prove that you do not have a bad amp module.

Now..... If the CAGE is good, and the AMP is good, you must now test for problems with whatever comes BETWEEN the cage and the amp.

I'm assuming that the USRM board (reverb mixer) is the only thing between the cage and the amps. Cables #1 through #4 from the cage plug into the jacks on one side of the USRM, the signal goes into that board where it gets mixed down, sent to the reverb (ADR-4), then the returning signal from the reverb is re-injected into the audio stream before being sent out on the four audio cables that run to the amplifier inputs.

Here's what you need to do to confirm everything so far. Please follow instructions carefully.

(1) Obtain FOUR new, known-to-be-working audio cables with an RCA plug on each end. This can be two stereo patch cords from Walmart or out of your junk box if you have one like I do. Or it can be four individual audio cables with RCA plugs on each end. The patch cords need to be about six feet long.

(2) Remove the four RCA cords plugged into the top of the cage. Plug one end of the four new patch cords into each of the four RCA jacks (#1 through #4) on top of the cage.

(3) Remove the four RCA cords plugged into the amplifier.

(4) Plug the other ends of the four new RCA cords into the four inputs on the amplifier.

(5) Set the four amp knobs at about "6" and then play the organ. Do you hear all the stops in all divisions playing? If so, then this proves that your problem is in the USRM board that you have now bypassed. You may also be surprised to discover that the hum that has plagued channel 4 is now gone.

Let me know what happens and I'll advise you on the next step.

[James P]

Will do!! Going to purchase 4 6' cables from Walmart now to do test.

[James P]

John,
Purchased 4 RCA plug 6' cables & did as you stated-- connecting directly from cage to Quad Amp. PRESTO!! All stops playing on both Great/Swell & Pedal (using Bass Coupler tab from Gt to Pedal-- without Pedal board connected). Unfortunately-- the same persistent "hum" is as before in Spkr #4 (with other 3 Spkrs quiet as should be).
So do I now need to purchase a USRM board (see attached pic) where Ch #3 & #4 plug in/out of? #904-5778 (noticed the board is dated "1984" -- probably time for it to go out! LOL).
Advise please--thanks!

James

[James P]

Forgot to attach pic-- herein!

[you795a]

What you could try is this. With the organ turned off, unplug the brown voltage plug with the gray, black and yellow wire and plug it back in. Do that several times as the connection may have some corrosion or dirt on it. If you have a meter, you can also check to see if there is any voltage on that plug and/or if it correct voltage. Just observe which way the plug is plugged onto the board. Just don't short anything out. If this would work it would be a lot cheaper than a replacement board. Also, try unplugging and re-plugging in the gray audio cables. Nothing ventured, nothing gained.

[James P]

Just checked all as suggested & voltages are all correct as spec'd —exactly or within 1, e g 15 is 14.
Is the 904-5778.2 (attached pic) the guilty defective part—where #3 & #4 channels plug in/out of?

[you795a]

I don't have much experience with this generation of Allen but that is a Bass Lift board. I don't think that board is the problem. According to John, he mentions the USRM board. I am not sure of the 904 number of the USRM board. It would be labeled either on the board or a white label like on the board that you have pictured.

[James P]

Sir I believe you're correct that the Bass Lift board (pic) isn't my problem.
Is the USRM board located next to it or where? And does it say USRM on the board anywhere?
Lastly what's the problem/difference in replacing defective USRM board—vs leaving organ with RCA plugs bypassing defective board as tested—direct connections w/RCA from Cage to Quad amp?
Complete organ plays & sounds great—as was before. Just still has low pitch hum in Spkr 4 as has had before!
If no difference, why spend $$$ for another USRM board?

[James P]

Upon closer view, I do see the larger board to its left states: USRM-3, Assy 904-5818-assume that's the guilty “culprit”. Correct?

[you795a]

The one that you just found,USRM-3, could be the culprit. I believe it is a Reverb mixer. I don't see any reason why it would not work direct from cage to amp. You would lose any reverb if I recall correctly. John would be better at answering this though.

[James P]

Thanks for your input "you795a"! And... if the only purpose/loss with the defective USRM-3 board is "no Reverb", I can certainly live without that. It's in my 16' X 25' Living Room -- & I don't need the Reverb to sound great! When I had it on (just by habit), never noticed much difference in sound anyway.
Again appreciate your help! And all is playing as was before with direct Walmart RCA cables (<$20) connecting all directly from Cage to Quad Amp.

[jbird604]

I agree. If the organ works with your signal going straight from the cage to the amps, then you can leave it that way. It is possible that the real culprit is in fact the "Bass Lift" board, as I see in the pic that channels 3 and 4 (the two that you first reported as dead) go through it on their way to the amp. But now you have bypassed that board too, which is actually just fine. The organ will play perfectly well and all functions will be normal without that board in the path.

The function of the Bass Lift board is to provide a slight bass boost that supposedly is needed to make the HC-15 speaker cabinet's output "flat" down to 32 Hz. But in my experience, it doesn't matter whether or not it's in the circuit. After all, if you think the bass is a bit thin on any channel you can always turn up the bass pots on the cage boards.

Now that you have confirmed that (1) your cage is working properly, and (2) your four amp channels are all working, you are left with only the HUM, which I think has been bugging you for months, as I recall. I have a thought on that too....

Since you said that you swapped the amp modules around inside the ADC amp chassis, but the hum remained on CHANNEL FOUR, and you said somewhere that the hum is still present when the RCA cable is pulled off from the channel 4 input on the amplifier chassis, that presents an interesting diagnosis...

NORMALLY, I find that hum is either in the cage (a faulty USAP board) or it's in the amplifier module. But now we know that it isn't, in your case. So that leaves only a few items that could be faulty, to wit: (1) the volume control pot for channel 4 in the ADC amplifier chassis. Perhaps the pot has an internal break in the resistive track, so that the ground is not present on the low end of the pot movement. That might cause a hum. Or (2) a break in the ground connection somewhere between the channel 4 RCA input jack on the amp chassis and the #4 amplifier module. Double check the pot for a loose or cold solder joint, check the connection where the RCA plug goes to the volume pot, check the small audio cable that leads from the volume knob to the amp module. Somewhere in one of those places the connection has probably been broken loose.

If you will be extremely careful, you might be able to poke around inside the amplifier chassis with the metal cover removed. THERE IS SHOCKING VOLTAGE IN THERE -- 120 volts coming in and going straight to the big transformer. After the transformer the voltages are lower, but still could give you a tingle. But with great care, you might try wiggling the wires and push-on connectors and such that are associated with the signal inputs and the volume controls, and the wires that lead to the amp modules. If you find that touching or wiggling somewhere stops the hum, then check that area for the broken connection.

[myorgan]

I'd agree with John. Other than having the compete set of boards in the organ, I've bypassed the Sub Bass Lift board on at least 2 of my 4 ADC organs. On the ADC-4300, I split the channels FROM 4-channels, TO 6-channels (de-combining Great & Pedal stops). I seem to remember on one other organ, I added a sub-bass crossover and felt it sounded OK with just voicing vs. requiring the Bass Lift card. YMMV.

Michael