DSL 401

[Valvehead]

On page 2 of the schematic in area C-3. There are lines called ODMUTE, V2A-IN, OD1G1.
Check that they are changing in voltage depending whether clean or dirt.
Have you tried with the footswitch or are you using the panel switches?

i will check that--

no fsw--the panel switches seem to switch the relay fine--i can hear it click --just no OD signal

[Valvehead]

ok....

if volts change - here we go,


ODMUTE ---YES

V2A-IN = YES

OD1G1..... which leg is the gate ? looking at the transistor, flat facing me ...the left leg - yes , the middle leg = no , the right leg =yes

[Jazz P Bass]

J174 P Channel Jfet: D G S



A Jfet is normally 'closed' D to S.
It needs the gate voltage to 'open' it.

[Valvehead]

ok...what am i missing-------the gate connection goes nowhere on the scem..?? ?? im going to go trace it ..

[Enzo]

No, the schematic says the gate connects to OD1G1. Look on the other page just right of T10,11. There is OD1G1 right beneath CLNG1.

[Valvehead]

i just found it while you posted---sorry...

so we need voltage from T2 yes ? and im looking for 24v or bit lower ?

sorry , trying to get these transistors--so these are sending signal to ground when they are switched in?... now i see T10 called ODmute

[Valvehead]

Im lost. Heres what Im getting , testing the j174's with the amp fired up and switching channels -

T9 (j174) gate has 20v clean or OD
clean -- D+S = 47K
OD D+S = 47K


T2 (mpsa13) HAS 28V , CATH OF D18 HAS 20V CLEAN / 0V IN OD

T10 (j174) GATE HAS CLEAN = 0V / OD 19V
CLEAN D+S = 43 OHMS
OD D+S = 300K


T11 (j174) GATE HAS CLEAN 0V / OD = 18V
CLEAN D+S = SHORT
OD D+S = SHORT

shouldnt T11 open with that 18v ??

[DrGonz78]

Just a quick question... Have you cleaned the JS-1 foot switch jack? I figured you probably did but just had to ask.

[Valvehead]

Just a quick question... Have you cleaned the JS-1 foot switch jack? I figured you probably did but just had to ask.

yes--ill hit it again to be sure

so if T10 is getting its gate voltage...it should "open" , ? because i have 1 leg going to ground still --is that it maybe ?

[Enzo]

T10 seems to be working. With zero v on the gate the JFET reads 43 ohms - a low resistance. Put 19v on the gate for OD, and resistance goes up to 300k.

T11 is controlled by the same gate signal, and so I expect it to behave the same as T10. Looks bad.

T9 always had gate voltage? Then go back to OD!G! source and find out why. Remember to plug something into the input jack, because it has a mute contact involved with this. Empty input jack and R100 is grounded at the top, and that is the 47k you are reading, I'd wager. meanwhile I'd expect T9 to still vary once that is corrected. it looks like it isn't. SO I suspect those two transistors at this point.

The MPSA13 is bipolar and doesn;t test this way.

[Valvehead]

T9 still has 20v at gate with or without something in the input jack.

i dont know what switches the gate volts on/off at T9 . T2 bad?

T2 = E = switches 25v to 9mv
B = switches 25v to 0.6v
C = switches 26v to 27v

i have swapped out T9 and T11

[Jazz P Bass]

"T9 still has 20v at gate with or without something in the input jack.
I don't know what switches the gate volts on/off at T9"

Either the Ft Sw Ring normally closed contact (going open) or the Fr Pnl Sw (going open) will remove the +20V from the gate of T9.

[Valvehead]

I inserted a cable in the fsw jack- it does switch and the T9 gate volts go to 0.2v when in od mode......still no sound though--also tried pressing down on the contacts

[g1]

You had a fet that looked bad, T11. You swapped it with T9.
Do the resistance checks on both again. When the voltage is at the gate the D & S should not be shorted.
If one of them is shorted all the time, remove it and see what the amp does with it removed.

[Valvehead]

You had a fet that looked bad, T11. You swapped it with T9.
Do the resistance checks on both again. When the voltage is at the gate the D & S should not be shorted.
If one of them is shorted all the time, remove it and see what the amp does with it removed.

ive swapped out T 9 / 10 / 11 at this point with a J175 (that should work as a 174 yes? ) = no change

with 20v at gate of T9 or T11 -- D+S is open on T9 / T11

before i swapped T10 out i tested like above and D+S was 1.8k so i tried swapping it = nothing....

if i pull T11 out do i need to leave t9/10 in ...or can i pull more than 1 ?