Hammond M-3 Very Quiet Vibrato

[Whitneydrew35]

I’m new to Hammond organs. I’m 16 and I’ve been playing them a few months but when it comes to working on them I’m very new to it. I picked up an M3 on offer up for free because the guy knew nothing about it and had it listed as a piano hahaha. I turn it on and everything works fairly good besides that when the upper or lower vibrato switches are on it is very quiet. Like extremely quiet. There is sound, and the vibrato does work, and the vibrato options work too it is just very faint to hear. Can anyone give me a tip or clue on how to solve this problem? I realize it could just be a bad tube but i want to know all possiblities so I can trouble shoot :) thanks!
-Drew

[Jaim]

Welcome
Its helpful to know the signal flow.
Non Vib. From matching transformer to Terminal A on O29 amp. From there it goes to V2 (6AU6) to (terminal D ) intermediate amp consisting of 2 halves of 12X7 (V4). From there to 1/2 of V3 (12AU7) which drives the 2 6V6 power tubes.

Vib. From matching transformer to Terminal B on O29 amp. From there it goes to V1 (6AU6) to 1/2 of V3 (12AU7) out terminal C which drives the vibrato line box. The output of the line box run to the scanner. (round item on LH end of the tone wheel generator). The scanner output (shielded cable) runs to terminal D.

When you have a problem like yours the first step is to swap the 2 6AU6 tubes.

The M3 is a great little fun organ. 16 - wow. I have socks older than that:-)

Jim

[Whitneydrew35]

Thankyou very much! I will try that first thing tomorrow :)

[Whitneydrew35]

Okay so I swapped out the 6AU6 tubes with the ones on my nice M3 and nothing changed. Still have a very quite vibrato channel on both manuals. :(

Does anyone have any other suggestions on fixing this problem?
And like I said I am new to working on Hammond organs so try and explain things so they are easy for me to understand. Please help me ! :(
Thanks again !
-Drew

[geoffbrown]

If swapping tubes hasn't resolved the problem but you can still hear a faint vibrato sound, then it's reasonable to assume that the amplifying circuitry of the vibrato section, is not working.

The vibrato signal can bypass the two amplifying tubes via the feedback circuits giving a faint sound when vibrato is selected.

I would download the M3 service manual and identify the R10 resistor, value 1meg, this must be tested out of circuit, but of course it could be other resistors or capacitors in the vibrato circuits, it would be a good idea to check all voltages on the vibrato section.

[Jaim]

As Geoff indicated. If you have a meter you can compare the DC voltages on the 2 6AU6's Remove the tubes to check. No need to go under the chassis. Observe best safety practices.
The screen grid resistor failure is common in the AO29 and AO28 amps.

Jim