Need help testing a Tube Tech MP-1A

[mrbowes]

I received a Tube Tech MP-1A from a member on this forum, and unfortunately it arrived with channel one not working. Channel two does work, and the seller insisted it was working 100% when he shipped it.

I've got a digital multimeter, but to be honest, I'm really not sure how to go about trying to find the point of failure.

Can someone describe how to use a multimeter to test a preamp like this? I opened up the case (will post pics later today) and everything is easily accessible.

I'm just not sure what settings to use on the multimeter, whether the MP-1A should be on/off while I'm testing...you know, the basic stuff.

Any advice would be greatly appreciated.

Edit: I will likely be taking this to a local tech when I identify what's wrong with it(if I can find one). I assume I can save a few bucks if I can go to him/her with exactly what needs to be replaced/fixed.

[zootdaze]

You don't think maybe it might just be a busted tube?

[mrbowes]

It could be, but all the tubes light up.

[juledude]

Originally Posted by mrbowes

I received a Tube Tech MP-1A from a member on this forum, and unfortunately it arrived with channel one not working. Channel two does work, and the seller insisted it was working 100% when he shipped it.

<stuff snipped>

Any advice would be greatly appreciated.

Dude.

First, you gotta know that diggin' in there can be very dangerous. I am not patronizing you here. It is impossible for me to know what your experience is, so forgive me. Nobody wants you hurt.

I have a friend who always wears latex gloves when inside any tube gear. Then when the sparks fly it ain't him that is frying.

Chances are though that it will not be a component failure. I would suspect something broken or loose. Visual inspection, very close, with the power unplugged is best. Tug at everything.

You may find out where the problem is using the meter by taking advantage of the fact that there are two discreet channels. One works. One doesn't. DC voltages should be very similar on both sides. Engineers I like are proud of being lazy, so I would suspect the physical layout of the two channels to also be discreet.

Please excuse my ignorance with that model, and laziness, but whatever tubes are in the beast, you need to find out what the plate and cathode voltages are to get an idea of what is happening. 12AX7 plates are pin1 and 6. Probably around 200V. Cathodes are 3 and 8. Just a few volts. Other tubes differ of course.

The voltages should be (and probably are) within a few volts DC of each other.

Good luck.

Jule

[Matt Syson]

Hi
Compare the working channel with the non working but look for the obvious loose bits first. Maybe a smashed switch if it was not packed well.
The cathodes are not 'only a few volts'. Part of U1 is at about 50 volts the other I can't remember. Similarly U2. The ECC82 cathodes should be about 32 Volts (easily measured across the 4K7 cathode resistors). Assuming the seller is correct in that it was working then expect violent physical damage or disconnection rather than subtle voltage differences. The HT is common to both channels. These units don't drop gracefully! If it has been dropped then there is the possibility of pins being broken on the output transformer if it is the PCB mounted type (T242/1 grey or /2 blue).
Matt S

[8thnote]

I got a used MP1a and had the same problem. Turned out that the DI 1/4" input needed cleaning. If it doesn't make a good connection it will not allow the pre to work properly. Find a good tech a let them do what they do.

BTW I love mine for the DIs on bass and keys.

[Matt Syson]

Hi
If the DI input works then spray a small amount of switchcleaner into the jack socket and plug something in a few times. Don't go mad with the switchcleaner you don't want it anywhere else!
Matt S