A100 AO-28 problem, connections and test procedure request

[Bob-i]

I have an A100 that I believe is a 64, found at the estate sale of a church organist. It was completely functional for years then suddenly developed a motorboating that seemed to indicate bad filter caps. I narrowed it down to the AO-28 by listening to the input of the power amp.

After replacing all the electrolytics in the preamp I reinstalled it, but I don’t get any output. I can hear hum from the poweramps, but no signal is passing.

I want to continue troubleshooting, but I can’t locate a diagram that shows what all the connections on the preamp are. I also have a few resistors on these connections that I don’t see on any Hammond schematic.

My usual troubleshooting routine for guitar amps is to check the input signal, and out signal. If there is input but no output I’ll then trace through the circuit and see where the signal is lost.

I believe I have everything connected as it was, please take a look and advise.

Thx

[Tim_at_Jonas]

Double-check your work. And check all of the DC voltages.

Connect a signal to the RCA jack on the Expression box and see if you get any output. If so, you have eliminated a good deal of the circuitry as a possible issue. If not, clean the tone pot and see if that does it.

On the schematic I have (a non-Hammond drawn), the inputs are labeled as they are on the chassis:

A is the Non-Vibrato input from the matching transformer.

B is the Vibrato input from the matching transformer.

C is the output to the vibrato line.

D is the input from the scanner.

E and F are the connections to the Volume Tablet (it just shorts a couple of resistors IIRC).

P is where the channel balance resistor is located.

H is the Percussion channel input (a transformer).

[Bob-i]

That’s very helpful, thx. I’m still confused by the extra resistors that are not on the schem. They may remain a mystery.

I’ll try the RCA next. I wonder if possibly a tube or socket was flaky and went bad when I pulled the tubes.

[Bob-i]

No sound from the RCA, but I get signal when I input on the E and F. That seems odd because it appears that the RCA connects through a cap to E.

I’m done for tonight, any other thoughts are appreciated.

[Tonewheel]

You may already have this for the A:



Dave

[David Anderson]

Tip: Before disassembling something with lots of wires, take a digital photo.

In terms of extra resistors, are you talking about the 47k resistor on the percussion? That's on the AO28 schematic, but Hammond installed it on the outside.

[Bob-i]

Tip: Before disassembling something with lots of wires, take a digital photo.

In terms of extra resistors, are you talking about the 47k resistor on the percussion? That's on the AO28 schematic, but Hammond installed it on the outside.

Of course, service 101. My problem is I’m colorblind so many of the faded colors look alike.

Yes, there’s a resistor on the percussion, but also 2 resistors on the G terminals. I haven't measured them (can’t read colorcodes) but I suspect that these were added when the original owner had a line out added. That said, I don’t think that’s the problem. Since I get output when I connect to the volume tab connections, there’s a problem somewhere else in the preamp.

This is more challenging than a stand-alone amp because it needs power from the power amp, not to mention pulling that thing out of the organ to begin with.

[enor]

Of course, service 101. My problem is I’m colorblind so many of the faded colors look alike.

Yes, there’s a resistor on the percussion, but also 2 resistors on the G terminals. I haven't measured them (can’t read colorcodes) but I suspect that these were added when the original owner had a line out added. That said, I don’t think that’s the problem. Since I get output when I connect to the volume tab connections, there’s a problem somewhere else in the preamp.

This is more challenging than a stand-alone amp because it needs power from the power amp, not to mention pulling that thing out of the organ to begin with.

Hm. No it does not need power from the power amp - the AO-28 is completely self contained

[Jyvoipabo]

Hello,

The procedure to follow is very simple.

1 Find the schematics at the end of :

theatreorgans.com/hammond/faq/a-100/a-100.html

2 Put the amp on the table and check all DC voltages (no signal).

JP

[muckelroy]

If you have no signal at the Phono RCA in, but you do have signal at either of the volume tab terminals, this means V4 (the 12AX7) is probably not functioning. It's common for that tube socket to go loose as it's a wafer type socket.

I usually have to tighten those tube pins from the underside when servicing these preamps. You can usually jiggle that tube slightly and see if signal returns. It may take several seconds, if the tube heater is not lit, and upon jiggling it then lights up and warms up the tube.

As for motorboating (your original problem) this is typically caused by the vibrato scanner / line circuit developing some type of short to ground. Desolder the black wire at the "D" input and if the motorboating goes away, you know that it was coming from the vibrato system somewhere.

[Bob-i]

Hm. No it does not need power from the power amp - the AO-28 is completely self contained

Thx, I checked the schematic and it does receive AC from the poweramp, but not DC as I was thinking.

- - - Updated - - -

If you have no signal at the Phono RCA in, but you do have signal at either of the volume tab terminals, this means V4 (the 12AX7) is probably not functioning. It's common for that tube socket to go loose as it's a wafer type socket.

I usually have to tighten those tube pins from the underside when servicing these preamps. You can usually jiggle that tube slightly and see if signal returns. It may take several seconds, if the tube heater is not lit, and upon jiggling it then lights up and warms up the tube.

As for motorboating (your original problem) this is typically caused by the vibrato scanner / line circuit developing some type of short to ground. Desolder the black wire at the "D" input and if the motorboating goes away, you know that it was coming from the vibrato system somewhere.

I did clean all the tube sockets while I had the tubes out, but I didn’t re-tension. I already suspected the V4.

As for the motorboating, I searched the forum and there were several threads talking about the filter caps. With these caps being 50 years old it seemed like a good idea anyway. Thx for the info on the scanner.

[muckelroy]

If the organ was working fine before (was it?) and it wasn't working fine afterwards, then it's most likely either a wiring problem, or a tube socket/connection problem.

If the filter caps were working fine for 50 years, it's highly unlikely that they would suddenly decide to fail. Yes, a good idea to replace them, but not likely the sole cause of this problem.

The wire that exits the vibrato scanner is shielded, and if this shield touches any of the terminals on the upper side of the vibrato line box, that can induce motor boating. Also, the scanner itself can develop internal shorts that cause motorboating.

[David Anderson]

I usually have to tighten those tube pins from the underside when servicing these preamps. You can usually jiggle that tube slightly and see if signal returns. It may take several seconds, if the tube heater is not lit, and upon jiggling it then lights up and warms up the tube.

However, if you tighten tube socket contacts on wafer-type sockets, be very, very gentle. This includes 122/147 amp sockets. Just a tiny bit too much pressure, and those contact flanges will snap off, at which point you have to replace the socket.

[Bob-i]

However, if you tighten tube socket contacts on wafer-type sockets, be very, very gentle. This includes 122/147 amp sockets. Just a tiny bit too much pressure, and those contact flanges will snap off, at which point you have to replace the socket.

Been there, and honestly replacing the socket isn’t that big of a deal, sometimes it’s worth it.

[Bob-i]

After a few more hours troubleshooting I think I’m moving in the right direction. This isn’t a Fender or Marshall amp for sure, so it takes time to determine the components placement.

I pulled the AO-28 and fired it up. The first thing I noticed was that all heaters are lit. Next I checked the B+ on all the tubes and lo and behold, V1 has none. I checked continuity through the B+ rail and all seems ok, however there’s no voltage at pin 5 of V1, nothing on Rp and nothing on the B+ node, C60. I’m starting to think I received a bad cap, but that’s unlikely.

My brain hurts right now so I’m packing it in for the night. My next step will be to double check continuity and if I don’t see anything there, I’ll put a discrete cap in at C60.

Thx again everyone for the support, documents, suggestions and all.