Magnatone M10A on the bench

[TubeGeek]

I have a dead Magnatone M10 amp that came in.  I took it apart last night...this is quite the circuit.


Steve...I may be asking you some questions


The power transformer has been replaced previously.  The owner paid $600 for a new PT.   I found the filament CT was loose on a terminal strip. It was barely making connection.


The amp worked for one week and then has sat in a corner for 4 years.  It is my duty to get her fixed up as the amp has sentimental value to the owner. It belonged to his grandfather.

[sluckey]

Is it totally dead? No filaments lit?

If so, check the interlock switch on the end of the chassis. This interlock will remove mains voltage when open. The interlock switch will open when you remove the big screw on the end of the chassis to remove the chassis from the cab. You will have to put that screw back in the bracket to close that switch. That switch is problematic on my M10A.

[xm52]

The ceramic disk cap in the lower left looks burnt around the top.


If it has a death cap, I'd check that.

[sluckey]

There are actually two death caps in this amp. One is located very near the power switch (probably that white tubular cap) and the other is in the power supply chassis mounted in the bottom of the cab.

[TubeGeek]

The ceramic disk cap in the lower left looks burnt around the top.


If it has a death cap, I'd check that.

You are seeing a shadow there. The cap is ok.

[TubeGeek]

Thanks for the replies, I will be getting into this one soon.  I am wrapping up on a few other items on the bench first.  2x Urei 1176LN compressors, a Urei 1178 compressor, Garnet BTO....


I haven't powered the amp up yet.  I inspect them first before applying power.  The owner claimed it was "dead".

[TubeGeek]

Is it totally dead? No filaments lit?

If so, check the interlock switch on the end of the chassis. This interlock will remove mains voltage when open. The interlock switch will open when you remove the big screw on the end of the chassis to remove the chassis from the cab. You will have to put that screw back in the bracket to close that switch. That switch is problematic on my M10A.

I didn't know that switch was even there!


This one has been disconnected previously.  I'll have to look into that.


Here are a few more pics.

[terminalgs]

check the molex style connector.  clean and re-tension all the little tangs.  They usually do this on the filaments ( I chickened out on this one and ran the filaments via another plug).  the power switches are notorious for flaming out like this as well.

[TubeGeek]

Update:


I have finished rebuilding the power supply.  I followed in Sluckey's footsteps, thanks for the detailed notes on your Maggie Steve. It has been a huge help and time saver.


Before I began rebuilding the power supply, I did plug everything in and give it a try.  There was an intermittent connection somewhere in the 12 pin plug.  When I move the connector the signal would come in for a bit and go out.  I opened the preamp chassis and discovered a loose wire.  The wire from pin 8 of the 12 pin plug was broken off of the terminal strip where it connects to a 22k bias resistor.  This is more than likely the problem.  I figured it could also be a loose pin inside the connector too.  I fixed a few of those pins and re-tensioned them.

I will replace the electrolytic caps in the preamp chassis in the next day or two.  I see a couple 1uF 450V caps in the preamp, I assume 22uF in both positions is a better choice.  I can see clearly in Steve's picture that one of the caps is a 22uF but the other is smaller and could be a 10uF...care to clarify that?

[sluckey]

The smaller electrolytic is 1µF @ 450V. It's not a filter cap. It is a coupling cap that connects the plates of the reverb driver tube to the reverb tank. I would replace it with a 1µF. Look at the schematic for your M10 and you will see a component labeled "delay line". This is the reverb tank.

Be aware that the M10 and M10A are different. Your amp has a transistor reverb recovery circuit. Mine uses a tube for recovery. There are other differences also. You should be using the M10 schematic.

[TubeGeek]

No transistor's in this amp, the reverb recovery is a 12ax7.  I see that the M-10 also has a tweeter....no tweeter in this one.  I also do not have any 12DW7's in the amp.  They have been replaced by 12ax7's.  I have been referring to both schematics.


The power supply resistors match up with an m-10a in my amp too, not the m-10 values.


Safe to say I have an M-10A in front of me.


I'll have to order in a few of the 1uF 450V caps.  I assumed they would both be changed to 22uF but you know what they say when you assume . That's ok though, turns out I am going to need a 12DW7 too.

[sluckey]

I should have looked at the first pic in your original post. That's definitely a M10A.

There are so many significant differences between the M10 and the M10A that they are really two different amps. They are just two very different siblings of the same family.

Did you notice all the extra holes in the chassis, especially the front panel? They used the same chassis for all models of those custom suitcase amps.

[TubeGeek]

The cabinet says M10 on it.  I didn't realize there was an m10a and m10 until I began looking things over.  Looks like a previous tech just installed what he had around in this amp and didn't pay attention to the 12dw7, which is a detail I think needs to be original.  Two of the 12au7's were also 12ax7's.


I do see the holes you mention.  Makes sense they'd use one chassis for the line.

[terminalgs]

The cabinet says M10 on it.  I didn't realize there was an m10a and m10 until I began looking things over.  Looks like a previous tech just installed what he had around in this amp and didn't pay attention to the 12dw7, which is a detail I think needs to be original.  Two of the 12au7's were also 12ax7's.


I do see the holes you mention.  Makes sense they'd use one chassis for the line.

that is  a M10A:

• 12 knobs on a M10A, 10 on an M10.  the extra two knobs are from the single 'tone' knob change to bass & treble.
• tube recovery for the reverb tank on M10A, transistor on an M10.
• 12" speaker on a M10A, 8" on an M10.
• 10 tubes on an M10A, 8 tubes on an M10.
• treble-boost circuit on M10A's (preamp has three triodes), simple mellow/normal/bright switch on M10.
• and I'm 99% sure the M10A's were all silver motif, and the M10's were gold motif.

Another 1mfd electrolytic is for the "E" power supply node.   Lots of amp circuits the first two triodes share a single power supply node (Fender AB763, Ampeg Gemini's, etc).  Estey engineers opted to provide a little separation with the 1mfd and 10K resistor.  I'm not sure why, It cost more. and predecessor amps like the 480 and 460 didn't do it. maybe they found it to make a quieter amp? (as much as the wiring layout looks like it'd make a noise amp, these tend to be pretty quiet given their age).


One thing to know about these amps is the company was small, and the engineering team handled warranty repairs.  If a distributor or musician called with a complaint or suggestion, they probably talked directly with an engineer.  they also played every finished custom amp with a guitar plugged in.  All that is to say that there were running changes to the circuit that didn't wait on the printed in-amp schematics to be updated.  Some Estey schematics have the revisions marked, approved, and dated in the upper righthand corner,  while others don't have this at all.  If you amp is a late M10A,  but the schematic you are using doesn't have the updates, you'll likely find that not all resistor or cap values in your amp match the schematic.  study the change before swapping it out, it might have been to improve the amplifier.

[TubeGeek]

The cabinet says M10 on it.  I didn't realize there was an m10a and m10 until I began looking things over.  Looks like a previous tech just installed what he had around in this amp and didn't pay attention to the 12dw7, which is a detail I think needs to be original.  Two of the 12au7's were also 12ax7's.


I do see the holes you mention.  Makes sense they'd use one chassis for the line.

that is  a M10A:

• 12 knobs on a M10A, 10 on an M10.  the extra two knobs are from the single 'tone' knob change to bass & treble.
• tube recovery for the reverb tank on M10A, transistor on an M10.
• 12" speaker on a M10A, 8" on an M10.
• 10 tubes on an M10A, 8 tubes on an M10.
• treble-boost circuit on M10A's (preamp has three triodes), simple mellow/normal/bright switch on M10.
• and I'm 99% sure the M10A's were all silver motif, and the M10's were gold motif.

Another 1mfd electrolytic is for the "E" power supply node.   Lots of amp circuits the first two triodes share a single power supply node (Fender AB763, Ampeg Gemini's, etc).  Estey engineers opted to provide a little separation with the 1mfd and 10K resistor.  I'm not sure why, It cost more. and predecessor amps like the 480 and 460 didn't do it. maybe they found it to make a quieter amp? (as much as the wiring layout looks like it'd make a noise amp, these tend to be pretty quiet given their age).


One thing to know about these amps is the company was small, and the engineering team handled warranty repairs.  If a distributor or musician called with a complaint or suggestion, they probably talked directly with an engineer.  they also played every finished custom amp with a guitar plugged in.  All that is to say that there were running changes to the circuit that didn't wait on the printed in-amp schematics to be updated.  Some Estey schematics have the revisions marked, approved, and dated in the upper righthand corner,  while others don't have this at all.  If you amp is a late M10A,  but the schematic you are using doesn't have the updates, you'll likely find that not all resistor or cap values in your amp match the schematic.  study the change before swapping it out, it might have been to improve the amplifier.

Thanks for the helpful info.  The only change I am looking to make in the amp is to up the value of the E node filter cap to 22uF and the bias cap to 47uF.  Other than that, I am going to leave it alone.

[TubeGeek]

Update:

I got the amp working but it has issues with it flickering on and off.  Looked into the connector and the pins are loose and this is causing intermittent signal problems.

Decided to replace the 12 pin connector because I found the crimps were not correct.  The pins were not staying locked in the connector and I discovered that one side of the retaining clip was broken off too.  I waited a few weeks for the new molex connector to arrive, got held up in customs, finally showed up so I am back to work on the amp. 


This amp is getting a whole new connector assembly with a newer updated style of barrel pins, which means it shouldn't have anymore intermittent connection problems.  Funny thing is I keep finding problems the more I work on the amp. The end must be close though.

[TubeGeek]

It is back up and running but I need to troubleshoot the power switch wiring.  Steve, would you mind describing how you have yours wired up?


Looking into this further and while trying to remove the chassis from the top of the amp, I learned that there have been some pots replaced previously.  At least one of them is the incorrect value.
Next frustration is two of the knobs have been glued to the shaft of the pots. I cannot get the chassis to separate from the top housing.  Now it's definitely not going to be a simple job.


I am thinking the best thing to do here is to install a 3 way off/standby/on switch on the bottom power supply chassis and be done with the on/off switch on the top chassis.  I'll leave it there but disable it.


Time for a break.  I'll come up with a plan to fix this.


The good news is that it is sounding much better after a cleaning of all pots and switches and replacing 3 incorrect tubes.  I am getting close to having this amp back to normal.

[sluckey]

What exactly seems to be the problem with your switch? You 'may' be able to fix it without pulling the plastic shell off the chassis.

There are 3 sections to the switch. Section A provides the standby function by breaking the ground path of the output tubes in OFF and STBY positions.

Section B actually switches the main power on and off.

Section C enables one of two line filter caps (death caps) by providing a ground connection to one or the other cap in positions ANTI-HUM 1 or ANTI-HUM 2.

My switch had a bad wiper on section A so my amp was always in STBY. I disconnected the line filter caps from Section C and rewired that good section for the STBY function. (NOTE... One line cap is on the main chassis. The other line cap is on the power supply chassis.

All this stuff is noted on my pdf schematic file.

[TubeGeek]

Turns out the power switch is not working, it has continuity in both switch positions. 

Both line caps are gone.

A connects to the ext speaker jack ground

B connects to nothing. It has a short brown wire hanging in the open.

C has a broken off lug here, nothing connected here.

D: The brown wire on the switch connects to the neutral/pt primary

E: The black wire on the switch connects to the line/fuse.

F: The small black wire connects to the cathodes of the 7189's through a 100Ω resistor.

G is connected to chassis ground

As soon as the power cable is connected to the wall, the pilot light and filaments are on.

[TubeGeek]

Scratch that....since the switch was dead this is the path I am taking...


I used my heat gun to soften the glue on the power switch knob...eventually I got it to come off.


I installed one of Doug's 3 position switches.  I will wire it up tomorrow, getting late here.


Would there be any reason to not connect the cathode to ground switch as the standby function in this amp?  It's either the PT CT of the high voltage winding or the cathode to ground switch.  Does it matter?

[sluckey]

The wires you had labeled D and E will be the primary AC side of your new switch. The STBY side of the switch should have one terminal connected to the 100Ω resistor that goes to the cathodes. The other terminal will connect to ground.

You may be able to repair that rotary switch. I believe the AC portion of the switch simply screws onto the back of the wafer switch. I think they were originally intended to be a replaceable component of that switch. Might be worth a try if you want to keep it original. Heck, you may even be able to find that switch part.

[TubeGeek]

It's all finished up.  Going back to the owner tonight.  Thanks for all the help.