New OT into '77 2203 problem

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Jab
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New OT into '77 2203 problem

Post by Jab » Fri Oct 27, 2017 7:00 pm

Hello,

I have '77 jmp 2203 that had a non-original OT. I ordered a replacement from Classictone (~200€ to europe, yikes) and just installed it.

I'm getting ugly ratty overdrive and shrieking sounds. Used the attached diagram to the best of my ability. Wonder what wire I've got in the wrong place?

Biased to 60% but bias shoots up to 115% when I hit a chord.

Here are a few pics. Blue wire should be ground, red and white to the tubes? Wires to the ohm selector should be correct.


https://www.dropbox.com/sh/e2tblsp8vh5y ... tiuya?dl=0

Thanks,
Janne

harleytech
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Re: New OT into '77 2203 problem

Post by harleytech » Fri Oct 27, 2017 9:30 pm

It should be the blue and the red to the tubes and white to the board...
I have wired up a few of these... Swap the white wire and the blue around...
Leave the red where it's at...

Jab
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Re: New OT into '77 2203 problem

Post by Jab » Sat Oct 28, 2017 3:19 am

harleytech wrote:
Fri Oct 27, 2017 9:30 pm
It should be the blue and the red to the tubes and white to the board...
I have wired up a few of these... Swap the white wire and the blue around...
Leave the red where it's at...
Thanks so much Harleytech!
I was initially going to run white to ground as on the previous OT but I thought the schematic indicated otherwise. Should learn to read electronics better. I hope the blue lead is long enough. Does it matter which way round the blue and red go?

harleytech
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Re: New OT into '77 2203 problem

Post by harleytech » Sat Oct 28, 2017 5:47 am

Yes, Blue should go where the white wire is now at the tubes.. Leave the red wire where its at...

Jab
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Re: New OT into '77 2203 problem

Post by Jab » Sat Oct 28, 2017 9:44 am

harleytech wrote:
Sat Oct 28, 2017 5:47 am
Yes, Blue should go where the white wire is now at the tubes.. Leave the red wire where its at...
Ok, I'll do that. I just meant if there's a difference to which side the blue and red go, generally.
Thanks!

harleytech
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Re: New OT into '77 2203 problem

Post by harleytech » Sat Oct 28, 2017 5:11 pm

yes... there is..

Jab
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Re: New OT into '77 2203 problem

Post by Jab » Mon Oct 30, 2017 8:33 pm

Working properly now and sounding pretty good. Thanks!
I put in a resonance mod and a .68 over v2a while I was in there. Just hope I didn't damage the tranny when wiring it up wrong at first..

harleytech
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Re: New OT into '77 2203 problem

Post by harleytech » Tue Oct 31, 2017 2:09 am

Very Good.. ! You should be ok on the OT.. They can take a beating...
Time will tell...

Jab
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Re: New OT into '77 2203 problem

Post by Jab » Tue Oct 31, 2017 7:44 am

Did notice something strange. Playing leads higher on the neck doesn't sound right. Note decay is fizzy and won't sustain well. Wonder if it's the tranny or something else. Am using an Spl Reducer, but it works fine on the super bass.

harleytech
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Re: New OT into '77 2203 problem

Post by harleytech » Tue Oct 31, 2017 3:59 pm

check your solder joints and was it doing it before the mod... ?
Might want to make it all stock and get it playing great, Then mod it...
So you can isolate any problems like this...

Jab
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Re: New OT into '77 2203 problem

Post by Jab » Thu Nov 02, 2017 5:47 pm

harleytech wrote:
Tue Oct 31, 2017 3:59 pm
check your solder joints and was it doing it before the mod... ?
Might want to make it all stock and get it playing great, Then mod it...
So you can isolate any problems like this...
Yep, that was it. The fixed resonance mod. Thanks!
This amp has 100k nfb to 8ohm tap. Is that the usual setup for an early 2203? Sounds good in combination with the .68 cap over v2. I think I'll leave it in for now.

harleytech
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Re: New OT into '77 2203 problem

Post by harleytech » Fri Nov 03, 2017 7:09 am

Great ! That's correct... 100K to 8ohm tap..
:thumbsup:

Jab
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Re: New OT into '77 2203 problem

Post by Jab » Mon Nov 13, 2017 2:24 pm

Oops, just noticed I have no output on the 8ohm tap, just buzzing OT. 16 and 4 work. Could this be because of my earlier wiring?!
Hoping it's just no contact at the selector.

Jab
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Re: New OT into '77 2203 problem

Post by Jab » Mon Nov 27, 2017 9:13 pm

Ok, just a broken impedance selector.
Need a new (nos) oasis cap type complete selector. None on ebay or reverb.
Anyone have one to sell?
Anyone reading this...

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