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Rare m-350 B Head serial #2043 [message #28498]

Thu, 07 April 2022 16:20

Guitarfinder
Messages: 8
Registered: April 2022

Junior Member

Hi all, I am an amp tech but having trouble with this rare M-350 B head.
I have looked through all the technical stuff on this site with no luck.
This head has two channels, left side is Intensity, Speed, bass, volume and treble, with 2 inputs
right side is volume , bass, treble with 2 inputs.
It dies not have reverb or boost.
I can find no # on the board, it does have 2- Mallory 25v 4000mfd can caps.
Any help or info would be greatly appreciated.
I am new to old solid state amps but have all the gear to test...
HELP!!!!

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Re: Rare m-350 B Head serial #2043 [message #28501 is a reply to message #28498]

Thu, 07 April 2022 18:53

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

Hi to ya!
Does this amp have a small transformer mounted on the upper left rear wall of the amp as you view the chassis from the front?

Note that I will have to approve 4 more of your post before this site will allow you to just pop a reply straight up.

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Re: Rare m-350 B Head serial #2043 [message #28502 is a reply to message #28501]

Thu, 07 April 2022 18:57

Guitarfinder
Messages: 8
Registered: April 2022

Junior Member

Thanks for responding
There is a small transformer mounted on the base in the left corner.
Some of the wires go to the board, some go to the speaker Jack
I can send pics

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Re: Rare m-350 B Head serial #2043 [message #28503 is a reply to message #28498]

Thu, 07 April 2022 19:08

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

Pictures are not accepted by this site.
Send a bunch to my email if you would.
smag25ra5@yahoo.com.

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Re: Rare m-350 B Head serial #2043 [message #28505 is a reply to message #28498]

Fri, 08 April 2022 06:38

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

Thanks for the pictures.

Does the channel without the effects atleast work?

Also please send me one more picture.
Send a shot (1 shot ) of the whole inside of the chassis looking down from above.

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Re: Rare m-350 B Head serial #2043 [message #28507 is a reply to message #28498]

Fri, 08 April 2022 11:53

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

This is a very early made amp, maybe even a late 1964 amp.

I have worked on one of these in 2019 and I could not turn up a schematic for it, but fortunately it was all stock.

These amps are only about 18 watts in output power and they use output Transistors that are Germainum types and where used in car radios of the day, and they are not cheap when you do find them NOS these days!

Right off the bat I see two problems with the amp.
1) it looks like big Transistor in the non effects channel next to the blue wire is a silicon type and in this amp all the small signal preamp Transistors are Germainium.

2) on the effects channel there are not enough Transistors I see on the board for it to do what it needs to do, infact I think I see where atleast one is missing.

It looks like a ton of replacement caps have been stuffed into the amp and with the electrolytic types it's important that they where installed the right way.

Once again let me ask you this , does either channel work right now?

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Re: Rare m-350 B Head serial #2043 [message #28515 is a reply to message #28507]

Mon, 11 April 2022 13:36

Guitarfinder
Messages: 8
Registered: April 2022

Junior Member

Thanks very much for the input and help, when it came in the map had very, very low output on the right (simple) channel.
But now just a low hum...

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Re: Rare m-350 B Head serial #2043 [message #28517 is a reply to message #28498]

Mon, 11 April 2022 16:20

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

When you power it up are you reading a DC voltage on the speaker output jack, or no?

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Re: Rare m-350 B Head serial #2043 [message #28522 is a reply to message #28517]

Tue, 12 April 2022 11:27

Guitarfinder
Messages: 8
Registered: April 2022

Junior Member

Hi, thanks again for all the help.
Reads -27, If I switch the leads it reads +27 volts
B

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Re: Rare m-350 B Head serial #2043 [message #28523 is a reply to message #28498]

Tue, 12 April 2022 17:46

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

That amp only has a positive power supply rail just like a tube amp that's cathode biased, so I don't understand how your reading a negative.
If the amp has a shorted output transistor you should have a positive voltage on the output jack.

What amperage fuse is in this amp ?

You should test both output transistors and the driver transistor next to that small transformer.

I would also test out all 4 diodes on that heat sink plate

Note that this type of early output solid state output stage has no circuit to protect the output transistors from failing if there is a short across the speaker output.

[Updated on: Tue, 12 April 2022 17:53]

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Re: Rare m-350 B Head serial #2043 [message #28525 is a reply to message #28523]

Tue, 12 April 2022 18:57

Guitarfinder
Messages: 8
Registered: April 2022

Junior Member

3 amp for power and 4 amp for speaker.
I will test those, thank you
Could something be installed backwards?

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Re: Rare m-350 B Head serial #2043 [message #28526 is a reply to message #28498]

Tue, 12 April 2022 19:14

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

What do you have for voltage on the blue wire that feeds the primary of that small driver / phase splitter transformer?

It should be positive.

While your working on this amp I would derate that 3 amp fuse to 1.5 and that 4 amp speaker fuse to a 1 amp, both should be fast blow types.

[Updated on: Tue, 12 April 2022 19:16]

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Re: Rare m-350 B Head serial #2043 [message #28527 is a reply to message #28498]

Wed, 13 April 2022 09:47

rodak
Messages: 516
Registered: October 2001
Location: Georgia

Senior Member

I've been watching this discussion, and I'm very curious - what does this M-350B look like? I haven't seen that one anywhere. Can you point me to a photo of one? I didn't see it in the Literature section.

www.combo-organ.com

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Re: Rare m-350 B Head serial #2043 [message #28528 is a reply to message #28498]

Wed, 13 April 2022 10:24

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

From the pictures he sent me it looks just like a early Frank model with stove hat knobs.

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Re: Rare m-350 B Head serial #2043 [message #28529 is a reply to message #28498]

Wed, 13 April 2022 11:38

rodak
Messages: 516
Registered: October 2001
Location: Georgia

Senior Member

Wow, I'd love to see that.
Guitarfinder: could you send those pics to me too?

vintagekustom2@combo-organ.com

Robert

www.combo-organ.com

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Re: Rare m-350 B Head serial #2043 [message #28531 is a reply to message #28529]

Thu, 14 April 2022 16:37

Guitarfinder
Messages: 8
Registered: April 2022

Junior Member

Sent!

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Re: Rare m-350 B Head serial #2043 [message #28532 is a reply to message #28526]

Thu, 14 April 2022 17:09

Guitarfinder
Messages: 8
Registered: April 2022

Junior Member

Hey, If I understand you correctly the blue wire from the small transformer to the board reads .0015 Vdc

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Re: Rare m-350 B Head serial #2043 [message #28533 is a reply to message #28498]

Fri, 15 April 2022 06:26

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

Ok, what do you read on that red wire that goes to what looks like a 2.2K resistor?

That's the Collector terminal of the driver Transistor and it should have full power supply voltage on it which comes into that driver transformer thru that what looks like a blue wire also going to the primary side of that driver transformer..

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Re: Rare m-350 B Head serial #2043 [message #28534 is a reply to message #28498]

Fri, 15 April 2022 07:52

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

In terms of a schematic for this amp I have found none yet, but what I have found is that a schematic for a Vox T60 model from the 60s is very close at least in terms of the General power supply and output stage, even the preamp is the typical 3 Transistor set up used back then, but for the T60 being a one channel amp, and the T60 uses Germainium Transistors also.

The main difference is that the Kustom uses a cap coupled output stage the uses one of those big filter cans that the amp has.

The T60 schematic can be found at the tube store schematic listing .

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Re: Rare m-350 B Head serial #2043 [message #28535 is a reply to message #28533]

Fri, 15 April 2022 16:27

Guitarfinder
Messages: 8
Registered: April 2022

Junior Member

Howdy, I cleaned up some soldier joints and went over the board.
The red wire reads -25.72 Vdv
I don't understand the negative voltage.
I now get negative 25.72 at new points that were lower before.
I have discovered that I am getting voltage to new areas of the board that were dead before, although it is negative also...
Thanks to all helping!!

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Re: Rare m-350 B Head serial #2043 [message #28536 is a reply to message #28498]

Sat, 16 April 2022 08:23

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

Well unlike the mid year 1965 model that I have worked on it would seem that the model you have is built like a early table top solid state radio of the type that does not use a output transformer, or that Vox T60 model that I posted about in that they run on a negative power supply.

Proof of that would be the positive end of one of those filter cans going to ground.

If you indeed have a large negative voltage showing up on the hot of the speaker jack then you have a bad output transistor.

Have you swapped in lower amperage fuses yet?

[Updated on: Sat, 16 April 2022 08:46]

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