Rodgers 825 problem

[retrochad1]

We recently got a mid 1980's Rodgers 825 with a large amount of speakers. The problem with it is that occasionally the organ will be almost completely unresponsive to anything played on the keys, except sometimes the mixtures will work. While the problem is going on, the harp and carillon will play but transposed off of the correct note, and the expression pedals will not change the volume level of the mixtures. Then all of the sudden it will start working fine again.

If you view retrochad on youtube you can see the organ although this shows it working properly with 2 8-ohm speaker cabinets.

This is the first Rodgers I will have worked on (have experience with Hammond, Allen etc.) I was hoping this Rodgers would be a completely discrete-component, non-computer unit but I have come to find out through reading this forum that the keying/stop functions are handled by a CPU. When the problem is going on it seems like no data is getting sent to the organ stop keyers and wrong data is getting sent to the harp/carillon. The combination action does seem to work OK when the keying problem is going on.

Just to think of where to start...
Is the stop/keying CPU in the metal shield up near the keyboards on the left when looking into the back of the organ from the rear? It looks like it is kind of difficult to get into that area.
Is there a backup battery somewhere that could be malfunctioning/leaking?
Are circuit board edge connectors/chip pins a good place to start (oxidation?)

Any suggestions you can give would be great.

Also, what is the recommended impedance load of the built-in amp. We got 2 speaker panels with 6x 6x9 speakers and 4 tweeters; 1 speaker tower with 8X 6X9's and 4 tweeters; 8 mini-cabinets with 2X 6X9's and a tweeter, and a cabinet w/2 12" woofers.
What impedance are the mini-cabinets with 2 6X9's? Is the amp expecting to see a 4-ohm load...it seems like some kind of series-parallel setup will be needed on these mini-cabinets. We first tested with headphones and then one 8-ohm cabinet hooked to the pedal and one to the great principal/reed before we got all of the speakers (still need to try find a way to get some additional large cabinets that were too difficult to lift from the organ chambers). Thanks again!

[jbird604]

The speakers are fairly irrelevant. The built-in amp can drive 3 or 4 ohm loads but is happy with 8 ohms. The typical M10 cabinets (with six 6x9 plus four piezo tweeters) have an impedance of around 6 ohms because of the way the speakers are wired in series/parallel. the other cabinets you describe may or may not be real Rodgers cabinets, perhaps they are Walker cabinets of some variety. Unless you are installing the organ in a large church, each channel will be OK with a single cabinet as long as it has a driver or two for the middle range plus a tweeter of some kind.

So, to make the whole organ play, you only need some kind of decent full-range speaker on each of the channels except the Pedal channel, which must have a hefty subwoofer. The pedal speaker that would have been supplied with an 825 would be a P32, which has two 15" drivers in a ported box, rather huge and heavy. The one you got with two 12" woofers could be a P16, generally only used with organs that lack a 32' pedal stop, but maybe the dealer sold this one with it for some reason. It will work as a pedal speaker, but you mustn't play it very loud or the 32' Bourdon will rip those 12" cones right out!

The CPU is inside a metal enclosure and you will have to remove several screws to open it. The CPU board attaches by a card-edge connector to the "multi-function" board. That card-edge connector can have issues, so you should start by removing the CPU board, re-seating all the socketed chips on both the boards while you're in there. Clean the socket and the card edge carefully, apply a one-molecule thin layer of Vaseline, and re-insert. Check throughout the system for any loose connections. Most of the data lines are hard-wired, but it's always possible for there to be a bad solder joint.

All boards in the system are interconnected by cables carrying data, strobe, clock, +12, and ground, and you should verify that every important board in the organ is getting its supply of all these. The system is very straightforward. The CPU sends out a pulse train that is interpreted by all the 4094 chips throughout the organ as specific turn-on/turn-off signals, thus closing and opening gates everywhere, each 4094 "attuned" to its own spot in the pulse train. Any bad 4094 chips in the system can throw the whole thing out of kilter, but failures are rare and you can easily spot it if you follow the pulse train around the system.

This organ is not too complex to repair with standard parts, but you do need a service manual. Good luck!

[retrochad1]

Thanks John...there was indeed a huge Walker woofer cabinet in the church we got the organ from, with 2, 15" woofers in it. This organ was installed into chambers where there was once a pipe organ and we had to pass all the speaker cabinets down about a 16" square access hole. We took the woofer drivers out of the big cabinet in hopes we could maybe make our own subwoofer cabinet. There has been air conditioning lines installed over a place where the woofer cabinet may have been hoisted into the chamber so I don't know if we can get it out. There was also another cabinet with one 15" woofer plus maybe a 10 or 12", plus 6X9's and a tweeter. We will need to see if we can get help in hoisting it out.

For test purposes we have been using 2 Wharfedale speaker cabinets each with a 12" woofer and tweeter on just the pedal and great principal channels. We only yesterday were able to go back to the church to get most of the original speakers for the organ.

This is actually going into our living room at home in a fairly small house. We already have the console set up. We are both organ enthusiasts so we have no problem with attaching as many of those cabinets as possible to the walls to get the maximum out of the organ.

Pretty much all of my electronic expertise is with tube type equipment, with some transistor amplifiers. This will be my first time at digital logic troubleshooting. Will an oscilloscope work in looking at the pulse train?
Also do you know where we can get a service manual. Will the factory sell one to an individual? We do have a contact of a good Rodgers technician but we would like to do as much work as we can on our own in fixing the problem.

[PeteKnobloch]

Since the combination action is working when the problem occurs, I wouldn't suspect the CPU being the issue. I would start at the power supply. Make sure your transistors on the heat sink are soldered to to boards. The old socket connecters had issues with deteriorating plastic.

[jbird604]

Yes, do check the power supply first. I should have thought of that. But a thorough cleanup and maintenance will include servicing the CPU board connector, so you should do that too. Those power supply transistors on the heat sink could well be your issue, or some other irregularity in the DC voltages. The big filter caps go bad and allow excess ripple, which will take the system down too.

RE the transistors on the heat sink, the original idea of connecting them to the pc board via sockets was not all that bad a notion, and I usually get them to working again just by cleaning and polishing the legs and the sockets to re-establish a solid connection. But the official recommendation is to solder them to the board. That makes future servicing of the board much more difficult, so be sure to check all the caps and re-flow the solder on the big pins before you make the board basically non-removable.

The service literature is available as a pair of pdf downloads, so you might ask Rodgers to send it to you. You'll need the 870B/900B/890 service manual plus the supplement for the 825/925. The service manuals are very detailed but also a tad obscure if you're not well-versed in this serial-keyed technology. But issues with these organs have been (in my experience) quite easy to troubleshoot. Once you have the power supply putting out stable voltages and the connections within the system all snugged, it will probably work much better and you'll be down to tracing little nit-picky problems such as cross-linked notes or the occasional dead note. Serious problems involving large groups of dead notes will be traceable to a bad 4094, and audio problems normally point to a bad op-amp chip on the output board.

I got an 890 into the shop a few weeks ago and have been slowly fixing all its problems. After a quick once-over it worked almost perfectly, but I spent hours tracing tiny wires to find out why playing a certain A on the swell unit principal caused the adjoning A# to sound along with it and vice versa. It was a tiny wire whisker on one of the decoder boards! I'm still trying to get to the bottom of why a particular pitch plays on most of the stops but not on all (it's not one of the oscillators because some stops that draw from the same rank of oscillators are not missing the pitch in question). So you can look forward to some pretty hairy troubleshooting, but for the most part organs like this have very few serious problems.

An oscilloscope might come in handy in tracing those pulse trains, but I hardly ever resort to using one. After you get a handle on how the thing works, you can almost put your finger on the bad chip just by looking at the schematics.

For playing this organ in a house, you will surely find that a single quality speaker per channel is all you need. That enormous Walker sub you had to leave behind would have been awesome, but it is surely overkill for a house!

[retrochad1]

Thanks again for the suggestions. While playing the organ this afternoon it acted up again and I gave some gentle taps with the foot to the bottom panel area to the right of the expression pedals (where the power supply is) and it came right back so that really sounds like where the problem may be like you all had said. I am recovering from getting a tooth pulled today so don't have strength to disassemble it today and start working on the power supply but I will get into it as soon as a I can and post my results.