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Messages - ilyaa

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1
Amplifier Discussion / Re: Fender M-80 Chorus Bias/Avalanche
« on: March 05, 2021, 01:32:01 AM »
ebay.....am i bad?

2
Amplifier Discussion / Re: Fender M-80 Chorus Bias/Avalanche
« on: March 04, 2021, 04:29:51 PM »
wow great thinking everyone -

the heatsink compound WAS old and dried out -

i gooped some more on there and we are back in business! both amps work BUT they are not quite matched. the good amp that never had any issues idles at a lower current - about 24 mA - and is able to deliver more power. the fixed amp idles at a higher current - about 30 mA - but seems to have a bit less gain/does not get up to full power - there is still some headroom but compared to the other amp there is a significant difference, for sure.

i wonder: is this likely a transistor problem? i got what looked to be legit transistors to fix the bad amp but i dont think they were a matched pair....everything else about the amps is identical. any thoughts?

3
Amplifier Discussion / Re: Fender M-80 Chorus Bias/Avalanche
« on: March 02, 2021, 11:40:00 AM »
thanks, phatt!

ill give that a shot and see what happens!

4
Amplifier Discussion / Re: Fender M-80 Chorus Bias/Avalanche
« on: March 02, 2021, 12:22:19 AM »
this bias mod is in a fender service bulletin -

its in the schematic i attached!

and i have reason to believe the original issue (a blown output) was due to the overheating/runaway problems associated with the unmodded bias circuit

5
Amplifier Discussion / Re: Fender M-80 Chorus Bias/Avalanche
« on: March 01, 2021, 09:25:55 PM »
anyone have any thoughts??

6
Amplifier Discussion / Re: Fender M-80 Chorus Bias/Avalanche
« on: February 23, 2021, 12:35:48 PM »
yeah the power resistors are all good -

all voltages read pretty much exactly the same both sides until the bias starts to crawl up

7
Amplifier Discussion / Fender M-80 Chorus Bias/Avalanche
« on: February 23, 2021, 02:10:40 AM »
hey!

got one of these fuzzy carpet amps on the bench -

R155 and Q12 were blown - had -40 VDC on the left output (yowza!)

i replaced R155, Q12 and Q13, and then did the bias mod for good measure (swapped out R127, 129,154 and 156 with 2K resistors) -

the left side had some funky looking electrolytics so i replaced those, too, as well as Q10 and Q11 (just being thorough!)

now: amp turns on without blowing up, BUT if i measure bias across R165 or R166 i see it starts out okay (around 25 mV) and then starts to creep up, with no sign of stopping. i turn the amp off when it gets above 60 mV because i can tell itll keep going until kaboom.

any thoughts what might be the issue? the so-called avalanche diodes (the BYV 26Ds) measure OK but i have a feeling they are not, in fact, OK. however, i am not that familiar with this bias circuit so i am not sure best way to proceed -

thoughts/advice? thank you!

ilya

9
Amplifier Discussion / Re: Randall RM50 Channel Switching
« on: December 18, 2020, 12:14:35 AM »
dang that my only choice?

i dont want to wait till march!!

10
Amplifier Discussion / Re: Randall RM50 Channel Switching
« on: December 16, 2020, 06:47:43 PM »
hmmm

dang just not getting through -

im suspecting the relays - i can hear/feel one of them clicking but not the other (on the breakout/switching board)

but im having a damn hard time finding any of these:

SYS-S-112D -

any suggestions for suitable replace?? thank you!


11
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?


12
Amplifier Discussion / Re: Randall RM50 Channel Switching
« on: November 20, 2020, 12:31:13 PM »
Quote
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 work.....so what the heck??

13
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?

14
Amplifier Discussion / Re: Randall RM50 Channel Switching
« on: November 18, 2020, 02:00:29 AM »
hmm

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....


15
Amplifier Discussion / Randall RM50 Channel Switching
« on: November 17, 2020, 07:40:44 PM »
hey!

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!


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