Laney AOR help

[Valvehead]

Ive been working on this amp...and im stumped.
its a Laney AOR series 2 50w head.

heres the scem -

file:///C:/Users/km/Desktop/laney-aor-50-series-ii-circuit-diagram.pdf

The amp has a low freq hum on both channels. the volume affects it, as in no volume - no hum.

Ive replaced the filter caps--there are 4 or 5 on the board, and one 50/50 can cap.

ive swapped out all tubes

if I pull out V1, still hums , but pull out V2 = no hum.

there is no AC on the grids of V1 or V2 . the only ac i have is on pin 2 of V3 , and its 3vac

I cant find the source ---can you guys throw some ideas at me ?

possibly one of the 2 H11f1 chips ? if so---i happen to have 2 of those in my bin !

[Enzo]

Your link didn;t work, is this it?

http://bmamps.com/Schematics/Laney/l...50_series2.pdf

The low input completely bypasses V1 anyway, so unlikely to be V1.

Which volume control turns the hum down, the preamp volume or the master?


Grid of V3 has 3v of hum? Pull V2, is that 3v still there?


The schematic has DC voltages on it, how close are yours in the V2 area?

[Valvehead]

yes---thats the correct scem Enzo.

illl try to test it tonight------cheers

[Valvehead]

the gain / volume controls =

AOR channel (dirty)
mv on
pre off =hum

mv off
pre on = no hum

Ch1 / clean
same as dirty ch.

hum present with either input jack


pulling out V2 , now i have around 9mv ac now instead of 3v

the voltages look good at V2 except at pin 8 i have 1V dc and the scem shows 0.2v

[Jazz P Bass]

I would sugest that you check V2A's blocking cap for leakage. (C9)

It may be applying a voltage to V2B's grid.

[g1]

the voltages look good at V2 except at pin 8 i have 1V dc and the scem shows 0.2v

That's a schematic error. They show 120V drop across R21, which would make tube current around 1mA. That should give around 1V across R20.

[Valvehead]

I would sugest that you check V2A's blocking cap for leakage. (C9)

It may be applying a voltage to V2B's grid.

i check that by measuring ac on each side--correct ?

[Jazz P Bass]

I was thinking Vdc.

I have not a clue where Vac could come from.

Unless you have a bad rail capacitor.

[Valvehead]

235 vdc one side and maybe a mv on the other

[Jazz P Bass]

What is the Vac (ripple) measurement on both sides of C9.

[Valvehead]

meter says =

across it 04.4 vac

on each end to ground =

003.5 mvac

07.36 vac

[Jazz P Bass]

Well surely the cap will pass the Vac from the plate side of V2A.

I would look at replacing the rail cap.(C17)
New or not, something is wrong there.

[Valvehead]

tried another cap at c17 -- no change

[g1]

Which side of C9 has the 7VAC, the side connected to R15?

[Valvehead]

yes.

but im not so sure thats 7v

the meter says 07.4vac , but if i measure one of the heaters (pin 9) meter says 3.3vac / not 03.3

im confused (as usual)

wouldnt that be just under 1 v ?