Rare m-350 B Head serial #2043 [message #28498] |
Thu, 07 April 2022 16:20 ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
Guitarfinder
Messages: 8 Registered: April 2022
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Junior Member |
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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 #28523 is a reply to message #28498] |
Tue, 12 April 2022 17:46 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
stevem
Messages: 4764 Registered: June 2004 Location: NY
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Senior Member |
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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] Report message to a moderator
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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 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
stevem
Messages: 4764 Registered: June 2004 Location: NY
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Senior Member |
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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] Report message to a moderator
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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 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
rodak
Messages: 515 Registered: October 2001 Location: Georgia
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Senior Member |
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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 #28536 is a reply to message #28498] |
Sat, 16 April 2022 08:23 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) |
stevem
Messages: 4764 Registered: June 2004 Location: NY
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Senior Member |
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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] Report message to a moderator
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