Ashdown EVO500 ABM500 Help needed.

[andywragg]

OK folks, before I go ripping this amp to pieces does anyone have any pointers for an amp, which according to the owner, looses volume during a gig? Amp is an Ashdown Evo 500 model ABM500 power section board is dated post 2011.

Best regards

Andy

[Leo_Gnardo]

I'll bet that amp has an effects loop. Start here: http://music-electronics-forum.com/t28549/

[andywragg]

OK a bit more information for you. When I crank the Clean/Grind pot the volume diminishes, looking at the manual this blends the solid state preamp pot=zero with the ecc83 valve stage pot on full. Changing the valve makes no change.

Edit

I do have the correct schematic I just needed to look at the other pages in the file I have. RTFM FFS.

[Mick Bailey]

You could have either a faulty pot, or more likely one of the solder joints on one of the pot legs may have cracked. A clue to this is usually a loose pot nut. In some cases the leg can break off level with the component side of the board.

This could be a red herring - I sometimes find such a fault has been long-standing and ignored, and is often not the main problem. My money is still on the FX loop, but check that all the pots are working OK.

Also, make sure those screw wiring blocks are good and tight.

[andywragg]

OK Now I have the right diagram I have no HT supply (+350v) to the valve - rectifier circuit is shagged. Going to replace the 4 diodes, resistor and 2 fiter caps as I have them all in stock.

[Enzo]

Before you throw a lot of parts at it, is there high voltage AC going INTO those rectifiers? Is ther DC on the filter cap that isn't reaching the tube circuits? Is the little high voltage fuse open?

You could be right, of course, but having all the rectifiers fail open would be exceedingly rare. They generally fail shorted anyway, which would likely blow a fuse. Likewise the caps, if they failed open, you'd still have voltage, just not smooth - in other words tons of hum. If they fail short, again we have blowing fuses. Open resistors would be a better bet.

[andywragg]

AC was going in and nothing coming out. I replaced the whole circuit (22uf 400v Caps x 2, 4x 1n4007 diodes and 1 x 10K resistor) as I had to lift them off the board anyway to test the individual components. Turned out to be the resistor that was open but it's between the 2 caps and hard to get to. Sounds fat now.

[Mick Bailey]

Was that the actual fault causing the volume to drop?

[andywragg]

OK to reset the initial diagnosis (based on conversation with one of the band) - the volume wasn't dropping, it dropped once during the gig and never recovered.

This is explained by the Clean/Grind pot and is actually how I came across the fault. They play with a blend of solid state (clean) and valve (grind) preamp gain. Once the valve stage went down (due to the power supply) the volume dropped. By winding the pot back to fully clean the volume could be recovered somewhat.

Once I repaired the valve stage it was back to full volume and a real beast at that.

Hope this helps.

[Mick Bailey]

Ah, yes. The reason I asked is that sometimes I come across the tube side not working and the customer only uses the SS side.