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Author Topic: Randall RM50 Channel Switching  (Read 367 times)

ilyaa

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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!


Jazz P Bass

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Re: Randall RM50 Channel Switching
« Reply #1 on: November 18, 2020, 01:35:40 AM »
+12Vdc comes from Page 3 Qc7.
It must pass through Relay 1 (page 6) normally closed contact to get to U2, the 7805 input.

ilyaa

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Re: Randall RM50 Channel Switching
« Reply #2 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....


g1

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Re: Randall RM50 Channel Switching
« Reply #3 on: November 18, 2020, 01:33:40 PM »
I think the source is Qc6 at top right of pg.3 ?

ilyaa

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Re: Randall RM50 Channel Switching
« Reply #4 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?

phatt

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Re: Randall RM50 Channel Switching
« Reply #5 on: November 20, 2020, 06:39:07 AM »
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?
No you are not dense,, these type of schematic can be very confusing. xP
On Page3, The 12v output of U1 goes to CN1/Fan + QC7(BP_QC4) + QC8(RR_QC3)

One would therefore assume that power for U2 comes from the 12v rail present on QC7 or QC8 on page 3.
If power came from QC6 as that is 17V rail which is a large voltrage drop for the 5 volt reg. so more likely comes from the 12volt reg.
Phil.

ilyaa

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Re: Randall RM50 Channel Switching
« Reply #6 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??

ilyaa

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Re: Randall RM50 Channel Switching
« Reply #7 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?


g1

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Re: Randall RM50 Channel Switching
« Reply #8 on: November 20, 2020, 08:07:04 PM »
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....



It has to go through a couple relays, on page 1, and page 6.
pg3 QC7 > pg1 QC4 > pg1 RL1 (above D1 lower left corner) > pg1 QC6 > pg6 QC17 > pg6 RL1 > pg6 U2.