Akai M7 monoblocks meter problems

[Omegaman]

I would go through and double check the replacement capacitors for correct polarity

Okay good idea! I double checked the caps and they are all oriented correctly according to the original pictures as to polarity. I have a couple of hours in playing these amps and they performed as expected

I did notice that someone previously had worked on the good meter with one bad lamp. The calibration screw paste had been scratch out to access the screws in back

But now I am concerned about the amps and don't want to play them till I know more. I certainly don't want to install new meters and then burn them up myself.

It could be that without the lamp circuit hot (SS test amp) the meter works but once the lamps are powered up the meter shorts. I could check continuity between the lamp and meter circuits and see if they short

I need to get it on a scope (dont have one) and see if the output signal is as crazy as the meter says

One thing. I used a polarized power cord and the wide plug end wire goes thru the power switch to 0 on the PT. The narrow end of the polarize plug is wired thru a fuse block and then to 117v on the PT. I did guess on this mod

 

[RWood]

You did right on the power cord, although on most modern gear it is the hot lead (narrow plug) that is both switched and fused. Yours will work though.

How bout hooking up the questionable meter but leave the lamp disconnected? That would tell you if the lamp circuit is messing with the meter.

 

[Omegaman]

I tried cutting the lamp circuit but I only cut one side which might not be correct for a short elimination but having the meter work with the SS amp when the meter was pulled or lamps powered off seemed to work

I have to go screed and lay paving stones in the 108 degree Sun and cannot play with this amp today

 

[jaymanaa]

I tried cutting the lamp circuit but I only cut one side which might not be correct for a short elimination but having the meter work with the SS amp when the meter was pulled or lamps powered off seemed to work

I have to go screed and lay paving stones in the 108 degree Sun and cannot play with this amp today

Looks like you're good at whatever you put your hands to.:thmbsp: I lived in Tempe for a year while my fiance was attending ASU. It would get so dang hot I would have to wear gloves to use wrenches up on the roof top ACs. I try to think of that whenever we have snow and sub zero temps here.

 

[Omegaman]

I like to be positive and say "At least it's a DRY blistering inferno"

Yeah, I was up in my attic at 5:30 this morning. It was only 80. I pulled the cover on the AC air handler and used a can of FOAMING COIL CLEANER on the condensation coils. I call it SPACEWALKING to space station AC. My wife was complaining of foul odor ( I don't smell anything) and thats the cure. works great

My wife bought me a roll-around tool chest for our first Xmas and tools for every birthday and quanza since

Thats why I buy her Dyson, Sub Zero, T-Fal and Pyrex

 

[Omegaman]

Okay, back to the meters

I got some replacement meters from an AK brother and both new meters pass the SS amp test.

I was looking at the schematic and saw that the VU lamp 6.3 volts AC came from the transformer winding and passed thru a VR4 potentiometer

Does this pot adjust the voltage or current?

I was wondering if this pot (which is maxed out to the right) could have caused the lamps to over heat and cause the failure?

I hooked up my AC meter to the twin blue wires of the lamps and it measured 6.71. Adjusting the one pot that the blue wires passed thru did NOT change the voltage


Is the 6.7 reading high?

What does the pot do?

 

[Omegaman]

One meter done

I took it apart and cleaned the glass and reattached the glass to the frame. I was going to use the original meter face but the parts were slightly different

 

[RWood]

What does the pot do?

That is a hum balance pot for the one filament winding. It creates a reference to ground for the winding, which is sometimes done with a resistor from each leg to ground. The nice thing about using a pot is that it can be dialed in to create identical resistance - and lowest AC heater noise - from each leg to ground.

Another clever technique they used here is to elevate the reference to ground by allowing it to go through the power tube's cathode resistor. So it's about 10 vdc above ground, which helps isolate the heater ground noise potential from signal ground.

The pot has everything to do with the audio circuit, not so much for the lamps.

The other filament winding has one, too.

RW

 

[Omegaman]

Wood cases made on the cheap

My first attempt at case building.

If you buy a 2' x 4' piece of 1/2" birch plywood, the guys at House Depot or Loew's will cut it for you. As many cuts as you like

I drew up plans (Thanks to my 7th grade shop teacher- Mr Hecht) and had the guy use their awesome vertical table saw to make 10 cuts

Then I glued it all together using tightbond

Using the finest available tools for precision work

square is square

 

[Omegaman]

I don't know how to veneer

Long story short

I used TOLEX

pretty cool

 

[RWood]

Love the tolex finish work - they look fantastic. Thanks, too, for the cabinet plans; those dimensions would work for all M6, M7, M8 amps (and many Roberts 770 etc). Now I am itching to tear into mine!

Do you detect any difference in sound between this pair and the last two you did?

RWood

 

[Omegaman]

Thanks, I thought I was blacklisted as no responses were forthcoming

I have not done any critical listening, (busy, busy, busy) of this unit yet

This unit retains the original NEC tubes including the 12ad7 where the other 2013 model has 12ax7's and mullards

Cabinet plans by MoVal Engineering

I love that I can get all the cuts done for free. I was going for low cost.

Next up a "tooled leather" set of cases

 

[derekva]

Looks very nice! I'm working on a set now, although I'm going to do a single stacked chassis (with about 1.5" between the top of one and the bottom of the other). I'm thinking teak veneer for that vintage Danish modern look.

-D

 

[Omegaman]

Derek - Looking forward to your work on the tone controls

 

[pete1729]

I like to be positive and say "At least it's a DRY blistering inferno"

Yeah, I was up in my attic at 5:30 this morning. It was only 80. I pulled the cover on the AC air handler and used a can of FOAMING COIL CLEANER on the condensation coils. I call it SPACEWALKING to space station AC. My wife was complaining of foul odor ( I don't smell anything) and thats the cure. works great

My wife bought me a roll-around tool chest for our first Xmas and tools for every birthday and quanza since

Thats why I buy her Dyson, Sub Zero, T-Fal and Pyrex

Harambé.

(forming a foundation all day in New Orleans.)

 

[derekva]

Derek - Looking forward to your work on the tone controls

Depending on how things go, tone controls may be out-of-scope in V1. It looks as if in order to do tone controls and not lose a ton of gain, I'd have to have a gain stage that's out-of-circuit when the tone controls are off and in circuit when tone controls are on. Not sure if I should try that by splitting the 12AD7 in half (since both stages are theoretically too much for the circuit) or ????.

Sorry to thread jack here, just spitballing. Maybe Dave Gillespie or Kegger have suggestions.

-D

 

[Omegaman]

The Guitar amp look

Here are the back plates where the I/O goes. The front jacks are still active and one mod is required to bypass the 1/4 plug-in jack discriminator

The amps are "handed" left and right

 

[derekva]

That looks fantastic! I'm going for a single chassis (for better or worse), but I'm reconsidering now...

-D

 

[mrwood200]

Nice work! I like the vintage pro look!
Thanks for sharing your woodworking plans.

 

[zandru]

Don,

What a Beautiful Job and a Great Look, I hope they sound fantastic & serve you well...