Bypass Headphone Jack on Micro Terror

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2n5088
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Bypass Headphone Jack on Micro Terror

Post by 2n5088 » Thu Oct 26, 2017 9:20 am

I have recently acquired a micro terror that has had the PCB connections around the headphone jack damaged while trying to replace the jack. I figure it is probably easier to bypass the headphone jack entirely then trying to repair the PCB and replace the jack.

Does anybody have experience with this? I doubt the original warranty covers this, but I suppose I could take the unit in to a dealer to have them look at it.

Any advice is appreciated, thank you.

bclaire
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Re: Bypass Headphone Jack on Micro Terror

Post by bclaire » Thu Oct 26, 2017 10:23 am

Do you need the headphone jack? If not, I would leave it if the amp is working ok otherwise.

But if you really want it, I'd be careful. If you bring it to someone for repair, the cost of repair will probably be very close to the cost of a new MT and not worth it.

Rlw59
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Re: Bypass Headphone Jack on Micro Terror

Post by Rlw59 » Thu Oct 26, 2017 8:31 pm

The preamp signal runs to the headphone jack, then through a shorting contact in the jack ("normally closed" contact) then out to the power amp. When you insert a headphone plug, the shorting contact opens up, sending the preamp signal to the headphones instead of the power amp, silencing the speaker.

Educated guess since there's no schematic available. It could be wired/switched differently. But inserting a plug into the jack does silence the speaker, and I'm assuming the OP feels the need to jumper the jack because he's not getting output from the speaker jack.

But no schematic available. Not sure I could draw one up if I disassembled my MT and there's some risk of breaking things trying to get the PCB out of the chassis.

A tech's bench time just to disassemble it will be more than the cost of a working used MT.

So nothing to lose trying to fix it yourself. Stored voltages in the caps are much lower than in a tube amp (the 12AX7 is run in low voltage starved-plate mode) and I gather you've already taken it apart. But be careful anyway.

With a multimeter, determine which contacts are normally shorted together in the jack. Find the corresponding terminals on the board and install a jumper.

Worst case, you fry something in a dead "beyond economical repair" amp. Then use it as a chassis for a DIY pedal effect.

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Re: Bypass Headphone Jack on Micro Terror

Post by Rlw59 » Thu Oct 26, 2017 9:53 pm

Alrighty, opened mine up to see what I could see without removing the PCB.

All three contacts are normally shorted closed. So use 3 jumpers, just straight across from one side to the other.

The tip and ring contacts are parallel connections so you really only need one of those connected. But if the traces are messed up using two will double your chances of having a good connection.

Rlw59
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Re: Bypass Headphone Jack on Micro Terror

Post by Rlw59 » Thu Oct 26, 2017 10:03 pm

OK, with it powered on I put a plug partway into the jack. First click (the ring/ground contact) shut off the speaker, so you definitely need a jumper there. If those traces are bad, follow each to the next components and jumper across those.

Probably still need at least one of the other jumpers. I can't test that without more disassembly, but regardless they can't hurt.

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Re: Bypass Headphone Jack on Micro Terror

Post by Rlw59 » Thu Oct 26, 2017 10:36 pm

Been having the urge to roll tubes again anyway, so opening it up wasn't totally altruistic.

Do hope you'll keep us updated. Curious if it works out for you, and I don't want Billy to worry I got you electrocuted. :wink:

2n5088
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Re: Bypass Headphone Jack on Micro Terror

Post by 2n5088 » Fri Oct 27, 2017 6:33 am

Thank you for the input, y'all!

Alright, going to try to run some jumpers, the second and third contacts run to a couple of resistors, I am just not sure where the initial input is coming from, I don't believe there is an actual headphone preamp, so I think it should be possible to short the jumper before the terminals completely.

Will let you know how it goes. Think the chance of electrocution is pretty minimal... I think...

2n5088
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Re: Bypass Headphone Jack on Micro Terror

Post by 2n5088 » Fri Oct 27, 2017 7:44 am

Well no luck running jumping wires between the two sides and connecting the resistors as the trace would, I imagine it has to do with the damage to the pads themselves. Tried to trace back to see if I can just short over the headphone section entirely, but it gets hairy with the two sided pcb, as I said I am not the most experienced with working on PCB. Will give it a shot tomorrow.

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