MRA3-L6 amp capacitor question

[mrb]

Starting to wrap up the amp work on my Seeburg M100B. The unit currently has a MRA3-L6 amp installed in it. I assume it was running at some point prior to someone fooling with it without a manual. I found few things that did not jive like my my c22 .06 mfd capacitor was missing. That problem was solved. The item I have a question about is c25 250mmf capacitor. At some point someone replaced it with a .001 mfd capacitor similar to c12,c13 and c14. Is this a common theme on these amps? The manual states a 250mmf mica capacitor I just want to make sure there were no updates to the manual I have and missed. The amp is a post 14,000 serial amp and the capacitor runs from tube socket v6 pin 1 to socket ground.  Attached a couple links to photos. Currently testing my resistors to see if they are out of spec.

Brian
Seeburg M100B 
WRA5-L6 selector
MRA3-L6 amplifier


www.amazon.com/photos/shared/PJurixQkQGSlK9ObSE2Zig.x4qForDHh_RwFaT6_qGkpM


www.amazon.com/photos/shared/wR_6JDv5QueNcF4Gm_23jQ.OUmSfjdJxfE5DV4bLDPZqN

[jukeboxmarty]

Neither of your links are working.

The capacitor c25 should be a 250mmf(pf) as stated in the parts list, earlier versions used a 50mmf. Substituting a .001mfd would affect the tone characteristics.

As far as the .06mfd capacitor being omitted, this is a common way of disabling the AVC.

-Marty

[mrb]

Ok  duly advised on the avic.  W
as not aware of that method. I will probably have to remove it then. I will pop that 250mmf in then. 

Thank you sir.

[jukeboxmarty]

I am an advocate for leaving the AVC intact, but if you leave the .06mfd capacitor in, you can always disable the AVC by removing the 6SK7 tube.

[mrb]

I would actually prefer to have the avic operational if I could... Think I saw something about grounding one pin  and that would eliminate the need for the switch on the mechanism that I don't believe I have and the 6SK7 tube could remain,  I might have misread that though.

When I purchased the machine there was no tube in the 6SK7 socket. The machine was a bit of a mess and I am trying to go through it and make it right. Someone damaged the 27pin cinch Jones socket so it got replaced and one of the other plugs the broke the alignment key so it was refreshed. Working on changing out a few resistors that drifted currently. Some were still in spec but are right on the border so figure best to get them now while I am in here. It's getting near time to dive into the pinbank and the other contacts.

Thanks for your help.

[jukeboxmarty]

Hi Brian,

The "MS" contact on the 'cam switch' controls the squelch circuit of the AVC. This switch is required for AVC operation. Grounding it does not disable the AVC.

If you desire AVC operation you will need to keep the .06mfd capacitor (C22) installed, as well as install a 6SK7 tube.

[mrb]

Looks like I lack the MS contact on my cam switch grouping. Looking through my manual I found page 2172 shows and discusses the MS contact switch and adjustment. A few pages later 2180 it shows a wiring diagram but lacks the wiring for the MS switch... Is there an updated schematic or addendum with that information in it.  Would be nice to upgrade it to take advantage of the avic I am sure somewhere a MS switch could be located.

[jukeboxmarty]

Being that we were discussing an MRA3-L6, I lost sight of the fact that you were working on an M100B.

The MRA3 amp was originally installed in the model M100C, which was equipped with the AVC function.

The Model M100B originally had an MRA1 amplifier, which did not have AVC circuitry... So no 'MS' contact.

You could add in the 'MS' switch but the problem would be adding the extra wire into the harness.

Now it makes sense why they disabled the AVC in your amplifier!

[mrb]

Well then I will remove that .06 mfd cap and not run the 6KS7 tube.  That would be the easiest solution for the time being. Perhaps in the future I will find a m100c to add to the collection. 

[mrb]

Still working on this had some other issues that I had to work through. Mechanism was all gummed up and had to rebuild the pinbank. I need to make sure signal is going through to the rca plug before doing any further work on the amp.  

Appreciate your help and insight Marty. I hope to have this Seeburg filling the house with music soon.

[mrb]

Update: I got the amp up and running. I had been doing some research on here and ran across an older thread from Ron Rich. That stated grounding Pin 3 on the mute plug affectively does what the ms switch would do in operation. So ended up doing that and leaving the .06 mfd cap and the 6KS7 tube even though neither of them need be used with my setup. The amp sounds great!