AMPEG vh140c Guitar Head Power Issue

[TENTACLES]

Hi,

I've Been playing this head for about 6 years now with no problems whatsoever, I love this head. Today unfortunately I ran into an issue with the head loosing power.
When the amp has a guitar cable attached, the unit will brown out or power will cut out for a few seconds then comeback. This happens every few seconds and makes it impossible to play.

The one thing i noticed is when this power loss occurs, only the LED light for the reverb and the selected channel go out, but the red power on LED stays on.

Maybe it's not the power supply and might just be the preamp that's loosing power?

I took a look at the wiring schematic online to see where maybe I could be loosing power, but I'm an amateur at reading such things and need to open the unit to really understand what I'm looking at.

Any input on this would be greatly appreciated, I would like to fix the unit myself and have a basic knowledge of the mechanics involved.

Thank you

-J


UPDATE: I powered the unit up just now to mess with it and the issue seems to be a loose connection somewhere internal. When I have the unit turned on with no other cords attached, if i bump the unit or hit it firmly the LEDs on the face (but not the power LED) go out, tap it again or jiggle the head power comes back.

Plugging the instrument cable in seems to make it go out too so maybe the loose connection is around that terminal? Also not sure what voltage powers the preamp, will have to look at the prints some more. Going to open her up tomorrow and see what i can find. Thanks in advance for any thoughts you might have...

[g1]

Are these the schematics you are using? http://music-electronics-forum.com/t12554/
The first one shows a couple 10 watt power resistors in the low voltage supply, R75 and R76, near the lower right of the drawing. Check their solder connections, they probably run fairly warm.

[TENTACLES]

Thanks for the reply, I am using that diagram and will give a look at those resistors you mentioned. Will post what I find out.

[Ronsonic]

UPDATE: I powered the unit up just now to mess with it and the issue seems to be a loose connection somewhere internal. When I have the unit turned on with no other cords attached, if i bump the unit or hit it firmly the LEDs on the face (but not the power LED) go out, tap it again or jiggle the head power comes back.

This is called "percussive diagnostics" and yes, a very valid technique. This is an SLM product and has bad solder joints. That amp is extraordinarily robust and is usually given to young metal heads after their first missed haircut. It's almost iconic as an affordable brute of an amp. They will last forever once you've done the clean up and reflow that these all seem to need.

Forget the schematic, address the amp in front of you. There's no reason to try to track down THE fault. Disassemble and reflow any connection that is either important or looks dodgy. Basically anything larger than a 1/2W resistor. All the jacks, all the controls, all the pins on the internal conx, the caps, power zisters, everything. If you want to study and learn look at the joints from the solder side closely, use magnification if needed. Now press or wiggle the component with a finger tip from the other side, if you see movement it's a bad joint. If you see a ring around the lead, it's a bad joint.

Once you've satisfied yourself that you know what bad joints look like, realize that you can reflow a dozen joints in the time it takes to hunt for one. Just start reflowing. Put a hot, clean well wetted tip on a joint, heat for a second and feed in just a touch of fresh solder, hold for a second and go to the next. Do all boards. Wave soldering is weak on large components and components on the edges of the board. Notice that that means jacks and pots will have the worst problems.

Get a can of Caig DeOxit D5. Spray every jack and exercise it. A burnishing tool does great work on the contact leafs on the jacks, especially the FX loop. Hit every internal cable connector with a short jet of DeOx and exercise the connector.

Reassemble, securing all hardware as you go. This is about $150 worth of work, so take your time and be thorough.

You'll be good for years.

[TENTACLES]

This is called "percussive diagnostics" and yes, a very valid technique. This is an SLM product and has bad solder joints. That amp is extraordinarily robust and is usually given to young metal heads after their first missed haircut. It's almost iconic as an affordable brute of an amp. They will last forever once you've done the clean up and reflow that these all seem to need.

Forget the schematic....................You'll be good for years.

Thanks alot for the info! I have a friend who is an electrical engineer helping me with a complete reflow on friday, so we'll get this thing back to tip top shape.

haha yeah and haven't cut my hair in about 9 years