ADC Amplifier Issues

[eaaron]

Hello all,

So I have been having issues with our ADC amps with pots being noisy or dropping out randomly. Happened on a swell channel today mid interlude, Hautbois just up and left. I have exercised and lubricated all of them, I think the conductive material is just at end of life.

I decided to start cycling through and replacing them. I pulled the amp that took a vacation today, and have the not yet used en-chamade/solo amp as a fill in while I replace the pots. I opened up the unit to get values of the pots and found, in addition to grindy pots, the resistor/capacitor pair linking the rectifier ground to the chassis are burnt up. Actually the resistor burnt up and the cap is a scorched casulity. However I cannot read either the resistor color code or any markings on the cap. Does anyone happen to know the values of those components?

Also while I am doing this overhaul are there any other failure from age components you have seen go regularly in these amps that I should also change out?

Thanks.

Erik

[myorgan]

Erik,

Read my last post to this thread: https://www.organforum.com/forums/sh...l=1#post445709. I had a similar issue with my ADC-5400, and discovered it wasn't the amplifiers after all. I hope that helps.

Meanwhile, I'll look around for the pictures I took of the inside of my ADC amplifiers.

Michael

[eaaron]

Michael,

That is a great thread and something to look out for, I love when issues hide somewhere odd, but I don't have standard antiphonal relays. It is definitely the pots. As soon as I touch the top of the pot not even turning it the channel crackles to a bit of life, and if I exercise the pot it will work for a while then randomly drop out again. It has been an intermittent issue once every month or so and is hopping around from channel to channel. This was the first time it happened during a service all other times it showed up at power up. And found out this morning that our other organists have had the problem, they described it as things being quieter than they remember but never told me until today. Need to have a permanent, reliable fix.

In case you are wondering I am using a set of audio grade line level relays to switch the signal heading to the amps instead of the speaker line. I prefer not running 150'+ of speaker line x 8 channels, plus we are lucky to have an amp channel for each cabinet. This also gives me level control over the antiphonal cabs separate from the mains. Of course the voicing is shared, but we can't have everything.

Thanks.

Erik

[myorgan]

It has been an intermittent issue once every month or so and is hopping around from channel to channel. This was the first time it happened during a service all other times it showed up at power up. And found out this morning that our other organists have had the problem, they described it as things being quieter than they remember but never told me until today. Need to have a permanent, reliable fix.

Don't you just hate channel hopping?!!!;-)

Fortunately, most of my issues have been solved. If the power becomes an issue after the 2 hours up, I'll try to replace boards on stage, but if that doesn't work, I'll move the Sub-Bass Crossover to the Symphony's organ, change a double to a triple amplifier, and use that organ. I'm just hoping to use mine because I've had more time with it voicing it, and am more familiar overall with my organ. We'll see.

Ah, the wonders of older organs!

Michael

[jbird604]

Erik,

According to the on-line schematic, that resistor is 1K @ 1/2 watt, and the capacitor is .022 mfd @ 200 volts. I wonder if the burned-up resistor might in fact be your root problem here, rather than the input level pots. I don't know exactly what that resistor does in this circuit, but I just wonder. I have known a few input pots to get scratchy on Allen ADC amps, but I've never had to replace one.

I squirt in a generous amount of WD-40, exercise the heck out of it, and squirt some more, until it's quiet and reliable. Of course your pots may be too far gone for mere cleaning, but that's been my experience.

And I wonder if the resistor somehow prevents a voltage from being present across the wiper and the input end of the pot. The absence of the grounding resistor might be causing a voltage potential to appear there, thus burning a microscopic dead spot on the resistive material.

The reason I'm wondering this -- a few years back I had to replace a whole pile of ADC amps (17 channels worth) in a big MDS organ because there was a wiring fault at the outlet where the amp rack was plugged in. Whatever they had done wrong, it caused a voltage to run through all the amps, burning up the same resistors in all the amps (the one that's burned up in yours). It also zapped some other components on some of the modules. At the time of this event, the errant voltage also ran back into the organ console through the audio coax lines, damaging the USRM boards (this was a big organ and it had two of them) and the ADR-4 reverb units that were attached. Fortunately, the damage stopped there and the surge did not enter the cages.

So, I'm going "hmmmm" maybe there is a connection there that I didn't see before...

[eaaron]

John,

Thanks for the values. From looking at the amp the resistor/cap combo is just filtering the rectifier to chassis ground resistor to keep the chassis at the same potential, the cap to bleed HF noise from affecting the audio. It is interesting that the audio line grounds are floating from chassis, and it does appear that the only ground link is through the amp cards. Very similar to old school concert audio where grounds were only ground on one end of interconnects, i.e. a compressor insert would have its send ground attached at the console-lifted at the comp, and the return grounded at the comp and lifted at the console. Then someone realized putting ground filter caps on the inserts in the console solved the problem better.

I don't think the volume on these amps had moved since they were originally installed in their prior home. There was mummified masking tape over each pot in the respective positions. Plus I had to replace one pot when I was testing the organ before install as the wiper broke instantly the first time I tried to turn it.

That MDS sounds like the hot ended up on the ground leg, with the above said it would certainly blow the resistor in less then a second and feedback on everything connected to the ground in the system. I would also guess it probably damaged the DC supply connected to the USRM. Luckily for the cages they had a higher potential to ground then the way it went. I can't imagine that the potential difference would be enough to cook a spot in the conductive layer and not damage something on the mixer, but it is possible. Plus I have had this issue on all 12 amp chassis at different times so it could be age or the same resistor. Either way at a couple bucks a pot and a few cents for some caps and resistors <$10 per amp cage, I will probably shotgun all those components in each amp.

I do plan on pulling this entire amp apart to make sure there are no anomalies hidden that might be other symptoms or causes of an issue before moving on though.

Erik

[eaaron]

Ok so the amp I used to replace the issue amp had a really noisy channel so I ended pulling that one today, and found the same rectifier resistor blown in addition all 3 amp boards have a cooked to various degree resistor. The noisy channel, shown, has the worst of the blowouts. However this amp was not in use before today, been sitting not even connected to power or speakers or audio channels. The only connection on the amp was the mute switch circuit. Also the burn out did not happen today. No tell tail smell of magic smoke looks like it may pre-date install. Since the unit is powering up all channels I am guessing that it is finding ground reference through the speaker terminal negative side. Seems I might have a bit of sleuthing to do.

On a side note, are all the amp modules in ADC amplifiers the same? I have some amps that have 2 channels and some with 3. I am assuming that the ones with 3 are the same, you just don't want to load any channel as much as the 2 channel version as the transformer in the cage is probably the same size and it just allows more channels control on lighter loads.

Erik

[AllenAnalog]

Wow, that's a dramatic burnout. I'd certainly be looking for a systemic problem with that many amp problems. Something out of the ordinary was causing a large amount of current to flow through a small wattage resistor. Ground loops and broken grounds or common wires can wreak havoc with the intended return path for voltages and then it finds the next available path, often through components not intended to handle the current.

The ADC amps have a common power supply and 1, 2, 3, or 4 amplifier modules - all the same. Allen does not recommend using 3 or 4 channel amps when one of the channels has 32-foot pedal stops because of the sharing of the power supply with all amplifier modules inside the enclosure.

Another issue to consider - Allen specifies two wires going from each amp to each speaker (or speakers, if they are doubled). Connecting a "common" between the negative or positive outputs of multiple amps is forbidden. The outputs do float.

[jbird604]

Well, this gets curiouser and curiouser... When I looked inside some of the ADC amp units that I removed from the surge-damaged MDS amp rack, I saw much the same thing. The resistor and capacitor between the rectifier plate the chassis were burned up, and a small component or two was burned on some, but not all, the modules. I assumed that all this damage happened at the same time, when the custodian or somebody came along and plugged the amp rack into a different outlet and fire flew out of the amp cages, according to the report I got.

Maybe there's more to the story though. Maybe there was a problem with the ADC design from the start that caused these resistors to burn up without a catastrophic event. Best I can recall, the big MDS organ was working and sounding just fine before the outlet disaster, but there could have been problems developing that nobody had noticed before then.

If you figure anything out, please do post about it.

[myorgan]

I just thought I'd add photos of the ADC amplifier modules for reference on this thread. They are below.

The first two photos show the reverse side of the connections on the ADC amplifier (potentiometers, etc.), the next two show the individual modules as mounted inside the amplifier chassis, and the last photo shows an amplifier module removed from the chassis.

Michael

[myorgan]

Here's a second set of photos for reference. I hope someone finds them useful.

The first photo shows the logic card separated from the steel mount, the 2nd shows the capacitors' connections (& values--022 MFD, 240 VAC--I don't have a photo of the capacitors), and the third photo shows the 4 resistors which ended up going bad on at least 3 of the modules in 2 separate amplifiers (Motorola?, MJ15003, MEX, 8550 & 8538).

Michael

[eaaron]

So I have gone over two entire amp modules (after clearing out from the snow). Other than the burnt ground resistors everything else checks ok. Those burnt resistors were the one in my previous post that occurred on each card (that one was the most spectacular) and the rectifier reference. Shown here:(Michael I borrowed your photo for reference, saved me from having to take one:->)


What is scary with the way the circuit is designed is while the input grounds(shields) are isolated from the chassis, they are linked together at the connectors. These grounds are isolated from the chassis by resistors on each amp board, and also through the rectifier ground reference via the +/-45v power rails to the amp card. So a failure on one board or to one input will cascade failure to the rest. It would only take 9+ volt potential on an input (signal, if the mute is muted, or ground) for about 30 seconds(shorter the higher the voltage) to start the failures.

What's more is that even with all ground isolation resistors open the amp will power up as an isolated system, but will be trying to reference ground via what ever is on the other end of the input cable. Not knowing the circuitry of the ADC mixer or boost/cut board, I'm not sure how that would affect the console long or short term.

It looks as though the design of these was assuming they would be installed inside the console, not remote, as there would be no worry of voltage potential in a enclosed designed system. Mounting the amps away from the console means there is a potential for potential issues.

John - This would explain your amp-splosion. If the outlet of the console and the outlet of the amp rack are not on the same ground plane it is possible to have rather large ac voltage potential between the grounds, sometimes as much as 90 volts. This means that both devices must terminate to the same physical electrical panel, and ideally have an isolated home-run ground to said panel. Standard commercial grounding using conduit uses the conduit itself as the ground not home runs. This means that if a piece of building steel that one box or conduit is clamped to has a lower potential to ground the other outlet will try an reference that ground causing voltage build up, and the 1K resistors cannot handle that and will go up in a poof of smoke.

Michael - Were you referencing these in your previous post? Those 4 components are the output transistors not resistors. The 2 MJ15003s are NPNs in parallel producing the positive side of the wave, the MJ15004 are PNPs in parallel producing the negative side of the wave. Each is rated at 140v/250watt, so with a +/-45v rail the max potential output of the unit would be about 320 watts but that would be into 0 ohms, it's too late to do the math for 8 or 4 ohms.



Erik

[myorgan]

Michael - Were you referencing these in your previous post? Those 4 components are the output transistors not resistors. The 2 MJ15003s are NPNs in parallel producing the positive side of the wave, the MJ15004 are PNPs in parallel producing the negative side of the wave. Each is rated at 140v/250watt, so with a +/-45v rail the max potential output of the unit would be about 320 watts but that would be into 0 ohms, it's too late to do the math for 8 or 4 ohms.
[ATTACH=CONFIG]25607[/ATTACH]

Yup, they are--oops.:embarrassed: This is my day for mistakes. However, you've given me great information for future reference on function of the various amp components.

Michael

[jbird604]

Erik,

I believe you have sleuthed out the cause of our local MDS organ disaster. This MDS-85 is installed in a large older church built in the 60's, which probably does have steel conduits that serve as grounds. No doubt the electrical system is somewhat cobbled together, and there may well be multiple meters and main input panels. The console of course sits in the sanctuary, but the amp rack was in an equipment room with air handlers and such. I wouldn't be surprised to find that new electrical lines have been run to that equipment room long after the original church was built. When the organ was installed (not by me), I believe there was a long extension cord from the console location to the amp rack. I'm guessing that someone decided to eliminate that cord and plug the amp rack into an outlet in the equipment room, while the console was plugged into an outlet in the sanctuary. Very possibly on different electrical meters or different phases, certainly on different ground runs. And that is when the fire flew out of the amp cages.

There is a cautionary tale here. I believe Allen used to instruct us to be sure both console and amp rack were on the same circuit, preferably a dedicated circuit. That is a rule to take very seriously. This church was very lucky that the damage was confined to the amp rack and the USRM boards in the console. I had to replace all the amps, of course, and removed the USRM boards along with the ADR-4 units from the console, which were not being used anyway, since the church did not need any artificial reverb. The 15 volt power supplies that drove the USRM and reverb boards were also damaged, but were not used for any other purpose.

Next time it will probably take out the cages. I hope there isn't a next time! I made sure they understood that the extension cord had to remain in place and that they could NOT plug any part of the organ into any other outlet. This is one of the hazards of installing organs in older church buildings, and something that we can easily overlook.

[eaaron]

I'm somewhat surprised that Allen didn't put isolation transformers on the amp racks they built for all the ADC and MDS remote racks. This would have stopped the ground potential in it's tracks.

They could have board mounted the entire array, similar to their sub crossovers, pre-wired to the amps so you would just land the audio lines their instead. For a rack that has say 16 channels it install 2 8 channel boards, for good transformers and some board connectors it would be <$100 in parts each board (probably much less with Allen's bulk purchasing), so for a few hundred dollars on an already $$$$ organ it would be a small protective investment. And they could have had a Balanced up-charge option for the longer cable runs. Just need a transformer at each end, and they could charge an extra $500/rack.

Part of me may do this on our antiphonal rack as that is possible to have someone move the cord, the organ and all racks were brand new power runs dedicated for the organ, but the antiphonal rack has an old outlet about 3 feet away. The only issue we have had was the other way, someone plugged all the Christmas tree lights to into the amp-rack outlet, and the first service Christmas Eve, during a prelude it got very quiet and dark when the breaker over-loaded.

Erik

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Ok, in the home stretch and almost to a test on the first 3 channel amp. Does anyone know the test point location, dummy load and the proper bias voltage for the amp card? I can probably get close but the proper values would be better.

Erik