T500 - dead 16' Pedals

[Jordanm]

Hi All,

I've been having this intermittent problem with the foot pedals on my T500 for about the last 12 months. They will intermittently stop working or sometimes go dead after a second of playing. There's usually a pulse sound when played if the tone isn't there. The problem seems to worsen with cold weather or when not played for a while.

However, I've recently discovered this only seems to be a problem when the rocker switch is set to 16'. Switching to 8' seems to restore functionality, but I prefer the 16' tones.

I'm guessing that given the 8' tones work, this must eliminate at least some components/areas to check?

Any hints anyone?

[tucsondave]

It could be as simple as a connection. The T's used some push-on connectors on individual wires.
Check connection point #11 on the pedal divider board. That board is on the main amplifier/power supply chassis under the Leslie speaker. It's the board in the center. Pin #11 should be marked on the board on the left hand side next to the circuit pack. That's the 16' output.

td

[Brendon Wright]

Once again I wish I wrote notes during all of my tinkering....
...I've had this one but can't remember what it was!

First thing to check is under the left hand cheek block which has the pedal tabs. Make sure there's not a frayed wire or short. Check for the same on the pedal amp which is on the chassis next to the power amp... this time check for frayed wires, shorts (but not trousers, stocking or other discarded garments) disconnected pins AND pins and components that might have been knocked about a bit, the solder can develop cracks.

I'm trying to recall if the pedals go separately to the Auto accompaniment board or if they go together, but the plug on that or cracked joints could be causing a problem.

- - - Updated - - -

Here's all the possible paths the pedal wires follow:


That doesn't give you an answer, but my sixth sense is dang sure we've narrowed it down (to something inside the T500)!

[Jordanm]

Thanks for the replies.

I did check all the wires on the pedal board last week, and the switches look in good health though I wasn't paying full attention to the pedal tone rocker switch when I was under there (was there on a separate issue). I will revisit both. I've also given the auto chord board a good going over and that looks okay too. Looking at the wiring diagram, it gets quite complex between the auto chord board, switches and pedal divider board, so I reckon I'll find the problem there. Just hope I'm competent enough to trace it!

[Jordanm]

Switch looks good, auto chord board looks good, divider board is filthy, but looks good. Wiggled all wires and connections to no avail with the exception of the auto chord connector. It seems to make good contact, but I noticed when applying pressure at a certain point, I would get the 8' sound when 16' was selected and that at the same time, the tone would be lost on the 8' if I switched back, as if reversing the switching!

How do I bypass that board as a process of elimination?

Brendon, I'm sure you've said before that there are some pins you can jumper?

[Jordanm]

I've tested for voltage at pins 11 & 12 of the pedal divider board when a pedal is being played, and found consistent voltage at both. This signal also reaches the auto chord board at pins 22 (for 8') and 18 (for 16') - so my guess is that the divider is fine and the auto chord board is the culprit.

Unfortunately I don't know where to check from thereon in!?

There must be some points I can check for voltage on that board to narrow down components?

[Jordanm]

Still got nowhere with this problem. I think the problem is somewhere on the auto chord board, but there is nothing obvious visually. I have the schematics, but I can't properly understand the circuitry and therefore I don't know what else to check. the edge connector looks to make good contact and I've bent the pins up a little to make sure! I think it must be one of the components.

Is anyone electronically minded able to tell me where else on the board I could chef to eliminate component(s)

thanks

[tucsondave]

Pins 18 and 21 on the autochord board should go to approx. 8 volts when a pedal is pressed and in non-auto mode. Pin 21 would be 8 v with the 8/16 tab in the 8' position while both 18 and 21 would be 8 v in the 8/16 position.
Since you said they are present at the divider board then you are getting the key voltage from the autochord board.
Those voltages turn on the pedal signal using diodes on the pedal divider board D701 and D702. You could swap the wires on nails 11 and 12 to see if the problem stays the same.
If so then that would mean either the 16' divider or the keying diode D701 (1N4148) is the problem. If not then check R61 (47k) on the autochord board.

td

[Jordanm]

Pins 18 and 21 on the autochord board should go to approx. 8 volts when a pedal is pressed and in non-auto mode. Pin 21 would be 8 v with the 8/16 tab in the 8' position while both 18 and 21 would be 8 v in the 8/16 position.
Since you said they are present at the divider board then you are getting the key voltage from the autochord board.
Those voltages turn on the pedal signal using diodes on the pedal divider board D701 and D702. You could swap the wires on nails 11 and 12 to see if the problem stays the same.
If so then that would mean either the 16' divider or the keying diode D701 (1N4148) is the problem. If not then check R61 (47k) on the autochord board.

td

Hi Dave, and thanks for that info. The voltages I'm getting at those pins when a pedal is pressed is about 5v AC AND about 2.7v DC according to my multimeter. - this is true for both 8' and 16' when selected.

Are you telling me that those voltages/signal comes FROM the auto chord board? - I had previously been under the impression that the signals (i.e. the tones) came from the divider board at pins 11 & 12, to the auto chord, and eventually the preamp. Should I be looking at it like the auto chord board is simply sending a signal to the divider to tell it to send the tones to the preamp and therefore the problem is likely at the divider rather than the auto chord?

Sorry about this, my understanding of electronics is very basic!

[tucsondave]

Q14 and Q19 on the autochord board send 8 volts DC to the pedal divider board. Those voltages forward bias the diodes D701 and D702 which allows them to pass the 16 ' and 8' pedal signals back to the autochord board pins 18 and 21, through the resistors R60 and R61, Q20, then out pin 22.
Disconnect the wires from nails 11 and 12 on the divider board and measure the DC voltage on the wires when a pedal is pressed.

td

[Jordanm]

Thanks again,

That makes it a little clearer. I'll do those voltage tests as soon as I get chance, tonight hopefully. There's every chance I haven't measured the voltages correctly due to my inexperience. Also, this problem has been intermittent and still is (although the 18' is now almost completely absent). Does this indicate anything, i.e are any types of components more prone to intermittent problems?

[tucsondave]

If the 16' keying voltage is present at nail 11 of the pedal divider board then I would suspect the diode. If you have any resistors or capacitor lying about we can bypass the keyer diode.
You can also check the 16' divider directly using the organ's amplifier. We can go into detail about that later.

Intermittent? Anything can be, solder connections, resistors, semiconductors, etc.

td

[Jordanm]

If the 16' keying voltage is present at nail 11 of the pedal divider board then I would suspect the diode. If you have any resistors or capacitor lying about we can bypass the keyer diode.
You can also check the 16' divider directly using the organ's amplifier. We can go into detail about that later.

Intermittent? Anything can be, solder connections, resistors, semiconductors, etc.

td

So I've done everything you suggested. First swapping the pins of 11 & 12 at the divider. The problem stayed the same, so I suspect the divider. I tried various combinations with the pins, swapping, leaving one off, then the other, then connecting 11 to 12 only and vice versa. All the time, I just hear the 8' tone unless I disconnect pin 12

Anyway, while I was there I thought I'd check all the voltages when 18' is selected and a pedal played. These were the findings (All in DC);

Autochord Board

Pin 18 - 4.1v
Pin 21 - 2.6v
R60 - 2.6v at one side and 0.4v at the other
R61 - 4.1v at one side and 0.4 at the other

Q14 - B=6.3v, C=14.7v, E=5.9v
Q19 - B=6.4v, C=14.6v, E=0.9v
Q20 - B=1.4v, C=8.2v, E=0.9v

Pin 22 = 8.2v


Divider board;

Pin 11 - 4.1v
Pin 12 - 2.6v
D701 - 4.1v at unmarked side and 12.1v at marked side
D702 - 2.6v at unmarked side and 6.4v at marked side

I had the black probe attached to the chassis, hope that is right?
Anyway - don't know if that gives any clues as to what the problem might be or where to check next!

Hope you can help,

J

[tucsondave]

I really suspect D701. You could replace that.
Or:
You can make sure the 16' divider is working by disconnecting pins 11 and 12.
Wedge down a pedal.
Find an audio input. That would be a shielded cable on the amplifier board. Looks like pin 8. Touch it with a finger to see if you get hum.
If you do then touch that point with a finger and touch the marked end of D701 ( The junction of D701 and Q709 on the divider board).
You should hear the 16' pedal. If you have a small capacitor (1-5 mfd) and some wire you could use that instead of your body, but make sure the volume is turned down.
That will bypass all circuits between the divider and the amplifier.

td

[Jordanm]

I didn't have a spare diode to hand, and I didn't fancy using my body as a capacitor, but fortunately I had a 1mfd cap to hand. By connecting pin 8 of the power amp board directly to the marked side of D701 I heard nothing (apart from a pop when connecting!) I did the same with D702 for the 8' and I could hear a tone. I guess this means a problem somewhere before the diode on the 16' divider circuit?

Question is where to check next!