Mindprint Envoice problem

A friend of mine gave me his mindprint envoice to look at. It's got a weird problem where the inputs sometimes stop working untill you put a loud noise through it. Then it works until the input goes quiet for some time again. After that you need to feed it some loud noise again to get it working.

I tried the mindprint website but to download service manuals there you have to have a username and password. Does anyone have schematics or service manuals for this beast?

Yeah. And it's kinda hard to find the problem. Everytime i touch something with a probe I cause a pop loud enough to 'trigger' the input en everything works again. Until I leave it alone for a while then i hear a sputtering noise and the input goes quiet again...
But right now It has been working for about half an hour.

What could this be. Do caps go like this? Is it an active component? Is one of the opamps slowly dying?

I just started playing with electronics. But now all my friends start bringing me stuff they break. :?

I bet on bad solder joints or oxidated pcb connectors!
Have you checked the tube?

I have ISA 430 on the way in with the same kind problem, sometimes dead, sometimes it sounds like frying eggs!

I had the same problem with a vipre. It was caused by overheating and the vipre would shut down. It got so bad the only way to use it was turn it off and back on again. I ended up getting a replacement heatsink much larger from groove tubes for free. install was painless. cound be what is happening with your mindprint.

The power rais look fine. Allso the regulators run really cool so I don't think it's them overheating.

There's a diff-pair of transistors before the first opamp. It could be there's something wrong there. I noticed the first opamp got about +8v dc on both it's inputs. It's a MC33078N could that cause any problems?

I really have to buy myself a decent scope. I have a 2nd hand one now without good probes... That doesn't really help.

just googled the mindprint. I never worked on one. Is the 12ax7 a china one? you might have a bad tube.

Does it use a DC to DC converter for the tube? if so check around the tube for good voltages.

I hope it does not use 48V for the tube?

When you have to run a large signal to make a circit work one of the things that comes to mind is bad bias.

You could have a bad connection does it use connectors inside? if so reseat them. Also check all conections related to ground

Or you could have a leaky cap causing a voltage shift with signal level

Can you post some picture of the inside that could give some of us ideas for what to look at.

I'm quite sure it's not the tube. You can bypass the tube-drive circuit. It will sound the same with the tube bypassed.

I found out yesterday that the instrument-input on the front-panel seems to work ok. I still don't have schematics. But it seems the mic-input and line-input go through two sides of the same 2 opamp-ic befor being routed to the input select switch on the front-panel. After that the signal goes throug the instrument-input so that contact wil be broken when you put a plug in there. So there's something wrong with that opamp ic. Could be one of the capacitors on the power rails or the ic itself.

Other than the unit being broken it looks like a real first class piece of kit to me. The tube works with 250V (there's an extra 15V to 250V transformer) (about 200V more than my noisy presonus bluetube all the magazines raved about)
They use mc33078 opamps throughout the unit. Some THAT chips near the compressor so they're probably VCA's. And a nice big toroid for power. I'll post some pics soon.

mendelt wrote

"I found out yesterday that the instrument-input on the front-panel seems to work ok. I still don't have schematics. But it seems the mic-input and line-input go through two sides of the same 2 opamp-ic befor being routed to the input select switch on the front-panel."

I think this is a big hint. Check the switch and look at the instrument input jack.

It could be the input jack, sometimes the input jack passes signal via one of the contacts tip or ring. When the plug is in it lifts a leaf away from the contact (disconnecting the preamp?)and the tip is connected to the circuit. The metal leaf in some of the jack gets weak and does not make good contact.

Is it a plastic jack like used in marshall amps?

It's indeed a plastic jack. And the signal is connected/disconnected as you say.

The leaf looks like it's doing it's job. but there seems to be some dirt on the contact. That could be it!

I don't really have time to test it right now but i'll let you know if it works. Thanks for the tip.

Kinda funny. I'm looking for an electronic problem all the time so i don't see a possible mechanical solution.

if it has a send-return loop, try inserting a cable. I'm sure the problem will go away. A lot of early 90's Marshall Valvestate had identical problems, what were cured after inserting a cable in the send-return. Replacing the REAN jack inputs solves the problem longtime. Sometimes it is enough to bend the contacts of the REAN's a little to let them make better contact in open position. In case the Mindprint has no loopconnectors, I would look at the other "plastic" inputs involved.

Good luck,

Tony

Sounds like a dry contact/contamination problem as suggested early on---oxides, sulphates, adsorption of nonconductive organics, etc. Cleaning, a bit of rolfing of springy things anywhere in the signal path ought to do it.

For years the home hifi in the Wonder Years had an old JBL SA600 integrated amp, brainchild of Bart Locanthi. Periodically it would begin to emit a loud hum, and my father would curse and go and switch the power on and off to clear it. He would then vow someday to get into it and fix it, then go back to his scotch and eventual snoring on the couch.

I took him at his word at the time that it was a failing electrolytic in the PS, that would somehow "reform" when the power was cycled. Years later after I inherited the piece I discovered that the 'lytics were fine---it was the cadmium-plated RCAs that had gotten decrepit and the shells intermittent due to an abundance of cadmium oxides and Gus knows what other toxic compounds.

Switching the power off and on would produce a transient sufficient to cut through the muck, and provided a convenient reinforcement to an incorrect diagnosis.

I'm quite sure it was the instrument input jack now. Yesterday I cleaned it, plugged in a microphone and put it next to my keyboard. Put on my headphones and just listened to myself typing for an hour.. the problem seems to be cured.

Thanks for all the help!

Glad you located the weak spot :wink:

Cheers,

Tony

[quote author="tony dB"]Glad you located the weak spot :wink:
[/quote]

Gus located it .. I was still trying to scope the opamp power rails with a faulty scope and no real probes :grin:

I owe you some :sam: or :guinness: if you ever get to the Netherlands :thumb: