Mindprint mic pre "blown" ! UPDATE FIXED

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MartyMart

Well-known member
Joined
Feb 28, 2006
Messages
2,340
Location
Berlin for a while
Hi all, this is my first post and a request for some assistance I'm afraid !.

I've found you, after spending some time at arons "DIY stompbox" forum
and have many good builds/designs under my belt, including a couple of
valve projects.
I have not touched much over 18 volts though !

I have a "Mindprint Envoice MK I" valve mic-pre, which at the moment is half dead.
Pre amp/48v phantom and all controls are "working" ( LED's etc ) but
the eq/compression/saturation block is not.
( I dont have a schematic for it )
Inside the unit there's evidence of a few burnt up components :
1 - 15 ohm 2 watt power resistor ( measures 15 ohms removed )
2 - 2x 22ohm half watt reistors and 1N4007 coming from the transformer
toward the valve section.
These also measure correctly, once removed.
I'm wondering what to look at next.
Would you advice replacing these components anyway ?
There is a pair of 2,200uf 16v caps between the 22 ohm/1N4007 and the
valve section, perhaps these have also been damaged ...?

Could a "section" of the transformer have blown ( powering the eq/comp)
seems odd that all else including the 48v line is working !
The 12AX7 also lights up.

Many thanks for any pointers/advice, this seems like a great place :D

Cheers,

Marty.
 
Maybe it is the tube? For all I know the tube is an integral part of the signal path, and the tube is part of the compression/saturation circuit.

Mindprint have their own support forum at www.mindprint.com. If you ask nicely, they might send you a schematic.
 
The hot resistors measure right, so they're probably still fine. They just got a bit hot.

Sounde like too much current being pulled, -Try swapping the tube, that's why they're usually socketed after all!

After that, perhaps look elsewhere.

Keith
 
[quote author="MartyMart"]...Inside the unit there's evidence of a few burnt up components...15 ohm 2 watt power resistor...2 - 2x 22ohm half watt reistors and 1N4007...[/quote]
Yep - been there, done that. I think it's a design flaw - these resistors are not rated at high enough wattage for some tube's heater current. Replace the tube for sure, but I'd also up the 15ohm to 5 watts and the 22s to at least a watt. Install them so that they are not touching the board. That way the heat won't damage the PCB.

Oh, and when it start's cutting out intermitantly, check all the "insert" jacks. :wink:

Other than that, it seems to be a great unit - built well and sounds very good.
 
Thanks for the replies chaps :D
Well, I need to do some more testing and get some voltage readings from the tube too.
I have a new 12AX7 in and have "made good" what I can .. still dead !
This tube and the one removed both still work, so at least it's not £8 for
every "power up" !!

Seems like the whole "eq/comp/sat" is based around the tube, so if I
can get that up and running.... BINGO.
I'll try the mentioned uprating of the resistors too.
Odd that the compession LED ladder still operates as normal, there's just
no output when the eq/comp is out of bypass ...

Cheers,
Marty.
 
Tim helped me out with this quite a bit, so many thanks :D
New 240v/15v 2VA transformer and some uprated resistors, now it's good to go !

Thanks to all who replied,

Marty.

BTW: That's my first "240v" fix !
 
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