Meter polarity- early 4 fin 400

porkchop61

porkchop61

New Around These Parts
#1
Joined
Jul 12, 2022
Messages
48
Location
Mansfield, CT USA
#1
Hi all. Happy New Year.
First post here, but long time lurker.

I have an early 4 fin 400 that I’m replacing the meter lamp board with a new White Oak LED board.
The existing factory board meter lead solder pads are etched with a “+” on the PCB, but the lead goes to the meter on the non-positive
terminal. Both meters are wired this way, and the meters function correctly. (I say non-positive terminal because the meters only have a “+” mark)

I’m asking because when I install the new LED light board, do I wire the positive lead to the non-positive terminal, or wire it as marked?
I don’t want to ruin a new PCB, or smoke the unobtainium meters.

Has anyone else seen this on an early 4 fin 400?

Thanks
Glenn
 
Gepetto

Gepetto

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Staff member
#2
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May 15, 2011
Messages
14,018
Location
Sterling, MA
Tagline
Old 'Arn Enthusiast
#2
Glenn, it is damn hard to smoke a meter running through the display board, so much series resistance involved.

If your meters don't move in the positive direction, flip the meter leads. If you have an early 4 fin unit, PL was probably figuring it out at the time :)
 
porkchop61

porkchop61

New Around These Parts
#4
Joined
Jul 12, 2022
Messages
48
Location
Mansfield, CT USA
#4
Thanks for the reply.
I’ll probably just wire it the same way since it works correctly now.
The meters in this amp are different from most of the other 400’s I’ve seen anyway, that’s probably the difference.

I’ve got the amp working nicely. It originally had DC on the output left channel higher than I like (still within spec. per the manual

Replacing both differential transistor pairs fixed this. I actually replaced both channel transistor sets per the service manual recommendation. The existing pairs were ones that they said they had a lot of failures on.

Thanks again.
This place has been very helpful with all the information available.

Glenn
 
G

George S.

Veteran and General Yakker
#5
Joined
Feb 18, 2020
Messages
5,033
#5
I remember one of our members "Kyle" said he also had a issue getting his meters working on a early 400 4-fin. Wonder if he ever got them working?
 
porkchop61

porkchop61

New Around These Parts
#8
Joined
Jul 12, 2022
Messages
48
Location
Mansfield, CT USA
#8
I received and installed the new light board tonight.
The meters were working a little funny when I hooked them up the same way they were connected
to the old board. I swapped them to match the polarity on the meters and now they work correctly.

I’m not sure why it worked okay with the factory board and meters, strange.

I have to say that the new LED board is very well made. I really like the trim pot for lamp intensity,
as well as the jumper for meter sensitivity!
 
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