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Messages - ilyaa

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Amplifier Discussion / Re: Randall RM50 Channel Switching
« on: November 20, 2020, 02:52:56 PM »
i guess maybe what would be helpful:

anyone here think they can explain how the channel switching is supposed to work in this thing?

Amplifier Discussion / Re: Randall RM50 Channel Switching
« on: November 20, 2020, 12:31:13 PM »
One would therefore assume that power for U2 comes from the 12v rail present on QC7 or QC8 on page 3.

i know and i have been assuming that! the only problem is: it doesnt!

with any amp like this, im assuming it worked properly at one point, so i dont think a connection has somehow magically disappeared! but since i am not getting +12V at the input of U2, and i cant even find where it WOULD come from, it makes the troubleshooting quite difficult.....

when i got the amp, the previous tech - who had got the amp to work but not fixed the channel switching, had rigged a little wire from QC3 on page 5 (the heaters of V2) to go over and deliver +12VDC to the input of U2. but thats clearly not how things are SUPPOSED to what the heck??

Amplifier Discussion / Re: Randall RM50 Channel Switching
« on: November 19, 2020, 11:23:39 PM »
hmm unless there is an error in the schematic, i do not see how that would go to the input of U2 on pg 6, g1.....

sorry to be dense, but how would it get there?

Amplifier Discussion / Re: Randall RM50 Channel Switching
« on: November 18, 2020, 02:00:29 AM »

page 3 Qc7 goes to:

page 1 Qc4
page 5 Qc3

the relay contact on page six goes to page 1 Qc7, which just goes to R2 - not +12VDC

unless im missing something i do not see how that gets 12V to the relay....

Amplifier Discussion / Randall RM50 Channel Switching
« on: November 17, 2020, 07:40:44 PM »

got an RM50 head on the bench -

things are working but it wont switch channels. there are some signs of 'boom' on the rear/footswitch/output tube PCB and someone has been in there before, but looks like they patched things up OK.

i am struggling with a specific issue:

see U2 on page 6 of the schematic - its a 7805 - i am not getting any voltage on the input, so, obviously, none on the output either. the problem is, im having a doggone time figuring out where the voltage on the input would be coming from in the first place. am i just missing something really obviousy?? im guessing itd be around 12VDC or so, but where the heck would it come from?

thank you!

Amplifier Discussion / Re: fender ultra chorus distortion
« on: August 20, 2020, 06:06:32 PM »
it's all moot now!

i had a FEELING i was barking up the wrong tree - joecool's early hunch was right!

after a ton of pointless tinkering, turns out C47 had come just evverrrr so slightly desoldered. argh! anyway, secured it and we are good to go - i knew it would have been something like that - whenever i find myself too deep down the 'doesnt make sense' rabbit hole, it turns out to be some mechanical nonsense -

thanks, guys!!

Amplifier Discussion / Re: fender ultra chorus distortion
« on: August 20, 2020, 02:11:00 PM »
Schematic I posted is correct!

Amplifier Discussion / Re: fender ultra chorus distortion
« on: August 20, 2020, 01:22:37 PM »
No dice -

U6 replaced, Q6 and Q7 replaced, C41 and C42 replaced, C89 and C88 replaces -

No change - man what is behind this?

The reason I suspect the effect itself is that if I turn the chorus trim pot I can hear the distortion getting worse as i turn it away from the center position in either direction -

Amplifier Discussion / Re: fender ultra chorus distortion
« on: August 19, 2020, 02:32:17 PM »
Haven’t found any similar complaints -

I’m not too familiar with chorus chips - here is what seems odd maybe:

The P-P square at pins 2 and 4 of the 3101 and 2 and 6 of the 3007 should be like 15V peak to peak but it’s more like 30V peak to peak -

I’ve got about 1VDC at pin 8 of the 3101 at pin 4 of them 3007. That ok?

It’s confusing because data sheet says pin 8 should be 14/15 of pin 3, but pins 1 and 3 are swapped in this design - with 1 being the supply instead of ground and vice versa with 3....

And at pin 5 and 6 - the input of the 3101 - there is some weird looking oscillation - but thats supposed to be there according to data sheet

Oh, and no noticeable ripple of low voltage rails

This whole thing feels like I’m barking up the wrong tree - but this noise is def chord related and it’s def bad!

Amplifier Discussion / Re: fender ultra chorus distortion
« on: August 18, 2020, 04:40:25 PM »
what a strange problem -

i replaced all the chorus-related ICs - checked for loose/cold stuff but didnt see anything

thought problem went away but it didnt.

so just to reiterate: amp works great but once you turn on the chorus effect any loud-ish transients break up in a very non-musical way. more like bad connection/busted speaker cone kind of way. but speakers are good and connections seem fine, too.

the chorus controls do not affect the issue - even with depth all the way down the distortion is still there. as soon as chorus is off it goes away.

the square waves at TP34 and TP35 have a little squiggly oscillation right at the top of each square.

when i scope the input to the power amps, it looks fine on both channels. clean sine wave on clean channel and chorus-y, wobbly sine wave on FX channel, but NO distortion. BUT the distortion is triggered by transients - so maybe i should put a square wave through the amp and see what that looks like...

the +/- DC bias to the op amps is a little high - almost +/- 18VDC when should be 16, but everything else in the amp works fine so im not really suspecting that....

any thoughts???

Amplifier Discussion / Re: fender ultra chorus distortion
« on: August 13, 2020, 02:09:38 PM »
its Clear the distortion is coming from the chorus effect -

If I mess with the chorus bias, I can hear it getting better or worse, but I can’t get it to go away. It kind of sounds like the chorus effects internal gain or makeup gain is too hot - but what causes this and how to repair?

Amplifier Discussion / fender ultra chorus distortion
« on: August 13, 2020, 03:00:10 AM »

got one of these:

works great UNTIL i turn the chorus then -

the chorus itself works and sounds great, EXCEPT if i hit a note hard enough it gets this nasty sounding distortion. like the chorus effect is clipping - not good.

speakers are good - when amp is cranked w/o chorus on, sounds crisp and strong and clear.

any thoughts? not troubleshot an issue quite like this before.....

thank you!

Tubes and Hybrids / Re: Mesa Triple Rec Woes
« on: March 24, 2020, 04:23:45 PM »
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?

Tubes and Hybrids / Re: Mesa Triple Rec Woes
« on: March 18, 2020, 01:49:56 PM »
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??

Tubes and Hybrids / Mesa Triple Rec Woes
« on: March 16, 2020, 07:13:54 PM »
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 change!

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