Mesa Triple Rec Woes

[ilyaa]

hey team!

trying to stay busy and keep working as the world burns....

and this stupid mesa triple rectifier is not helping my mood!

it has a weird hum (ground-esque) -

mostly on channels 2 and 3 but sliiight hum on channel 1, as well.

it stays, but quiet, if i pull V1 and goes away completely if i pull V2 -

its def in the preamp (half-split the amp, etc.) -

if i scope i see weird noise on pins 1, 7, 8, 6 of V3 that is totally not knob dependent - still there with knobs all the way down, and only the gain knob affects it. but if i cut out the previous part of the circuit, around V2, it goes away.

SO its clearly something around V2. i suspect some of the jfet/darlington muting stuff, but its gotten kind of hard to troubleshoot because i only have a dual rec schematic and it doesnt seem to match 100%.....

does anyone have a good triple rec schematic?

PS its not the preamp tubes i swapped em all and ive already replaced most of the Jfets and Darlingtons and smaller filter supply caps.....no change!

[Jazz P Bass]

Here is the schematic

[ilyaa]

thats the one ive been using...

whats mystifying me is that the circuit around V2a is just not matching up between schematic and amp

on the schematic there is a jfet there across that 475k going to the grid - on this amp, a relay goes across that resistor, and with the amp off that resistor measures short across...

that 15 uf from the cathode is also going to a jfet, not a relay....

in other words, that part of the schematic does not match up, and it seems to be where the problem is coming from but im sort of reaching in the dark here....

any thoughts??

[trobbins]

Using the 'Preamp Part 1" schematic for reference in the link you provided, can you temporarily short the grid to the local cathode of V2A, to confirm it is not a problem from then on?

Perhaps then temporarily short a further back point like the wipers of CHx Gain when they are selected.

Similarly you may be able to short across the tone pots (top to bottom of series string of pots on CH1), and across the 2M2 loading resistor for V1A.

The initial aim being to include/exclude certain sections of circuitry.

[ilyaa]

kay so V2:

if i ground pin 7, hum dies

if i short pin 7 to pin 8, hum dies

if i ground pin 2, hum gets quieter but doesnt die

if i short pin 2 to pin 3, hum dies

if i open the 475k going to pin 2, no noticeable change

if i open the 475k or the .02 uF going to pin 7, hum dies

so sounds like problem is in V2, but where???

ive tried opening all of the components to no avail.....wudya guys think?

[trobbins]

Was the test for grounding pin 2 done with a short link from the pin 2 terminal to the ground end of the 1k8 cathode resistor of V2a ?  Is the hum drop on CH.1 and on CH2/CH3 ?

Is the hum killed on CH1 if you lift one leg of the .07u cap coming from the relay to V1b?

Can you connect another 15 to 100uF cap across the 1k8 cathode resistor on V2a?

[duckduckgoose]

"whats mystifying me is that the circuit around V2a is just not matching up between schematic and amp"
wonder if it could be an intentional error lol