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Hello this is my first time posting a request for repair. I am working on a Randall RG-75D and it has burned up Q7,Q9, R15 & R16. I have found replacement transistors but I do not know the values of R15 & R16. I have tried to e-mail Randall but did not get any response. I saw a post similar to this but also there were no comments. The burned resistors read 12 ohm and 16 ohm. I have traced the board in Red and have drawn in the parts (may not be completely accurate) to give an idea of the layout. I will try to attach the tract I have done. Does anyone have a schematic of this board? I would greatly appreciate a copy ---- Thank you in advance! I have pulled Q6 through Q15 and all have read good (except Q7 & Q9). It looks like there was a shorted path from B+ to B- right through these two transistors!
I do not know if this attached properly. Thank you,---------------- OMguy
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I know, a necro thread resurrection... but for my defense this thread does pop up to searches and in case it happens to help or just interest anyone here's a schematic sketch. Some values (which were not included to original poster's sketch) are based on other Randall power amp circuits that follow an almost identical architecture. This may not be 100% accurate but it should get you to ballpark.Attached Files -
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Hi, I'm looking for the preamp schematics for Randall RH 100 G2 series. The power amp PCB is marked RG75-B, so the schematics is the one you posted above, while the preamp PCB is marked RG75-A. Do you have the RG75-A schematics? Thank you so much!!!!
Regards,
GiorgioComment
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RX120RH Power Amp Schematic
Hey, I have a Randall RX120RH with a RG75-B power amp, I need that schematic, this board has more than 28 resistors, and i recieved a file from randall (RG2-75-B) with only 28 resistor, if someone could send me a pic of the board where I can see R29 and R30, I will apreciate, I have the RX120D-A, RG2-75-B and RG75 power amp schematic from randall if anybody use them, I'm really desperate because in randall dont answer me no more my emails, nd I' from mexico dont have other way to contact them, I hope somen can help me, ThanksComment
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Please post the schematics that you have.
If the board that you have has a higher component count, I would not get exited about it.
It very well may be a minor change.
Then again it may not.
If GK cannot help you, you are on your own.
You have the board.
I guess you will have to figure it out.
(sometimes it sucks to be a tech)Comment
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here is the schematics
Hi , this is the schematics of the power amp section (I got two sligthly dfferent schematics from the Internet...)
Cheers
GiorgioComment
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Hi dmcastillor.
I'm sure the schematic must be the same or very close, but in different versions the same parts may be labelled differently.
I suggest you print a nice comfortable copy of the schematics posted here, and, pencil in hand (and a Sharpie pen too) you go over your actual board and tick part by part; putting a small sharpie dot besides the one you saw at the board and ticking with the pencil the same one identified on the schematic.
What´s important is "who is connected to who".
*Maybe* , say, C2 on the schematic is C4 (or whatever) on the board, but it will still be the 47uF/50V in series wuth Pin3 at Con1 (input) and connected to a 1K resistor, which might be labelled R1 ... or not.
When you are through, scan and post your corrected schematic for reference, although I´m sure that by then you will have identified the "missing" parts.
If not, they will be "orphans" that *still* will be connected to "known" parts, now you can add them by hand to the corrected schematic.
Mind you, your problem won´t be "just 2 burnt resistors" but other damaged parts, which you´ll have to troubleshoot.
Good luck.Juan Manuel FaheyComment
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Please upload the RX120D-A schematic, I believe R29 & R30 are on it. Not sure what you are looking for, are there burnt parts on the power amp board?Originally posted by dmcastillor View Post
Can you upload a photo of the damage?Originally posted by Enzo
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The RX120 power amp schematic is absolutely identical to the RH2-100 power amp schematic.Originally posted by g-one View PostComment
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I saw a schematic somewhere else labelled R120D-A. It was a preamp schematic that showed resistors called R29 & R30. As it is unclear what the poster is looking at I thought maybe that would help.
Edit: Doctor, I had jumped to the conclusion that "B" drawings were power amp, and "A" would be preamp. So when I saw that Randall had sent him R120-A, I thought this would be the preamp for the RG-75D. Perhaps I should have said R120-A preamp. I realize dmcastillor has specified power amp drawings, but the preamp is the only place I have seen R29 & R30.Last edited by g1; 01-09-2012, 06:39 PM.Originally posted by Enzo
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You are right, g-one. In fact Randall designates preamp schematics by suffix "A" and power amp ones by suffix "B".Originally posted by g-one View PostComment
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Hi, I attach you a pic the board, the resistors that I can see on the schematics and are burned are R29 and R20 beside R24 and R25 (5W0R15), I saw R29, R30, R31, 32 33, 34 and Q16 and Q17, I hope you can help me, I'm stuck on this. if someone have a pic of these board could help meOriginally posted by doctor View Post
ThanksComment
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Your picture does not show the board number. Maybe it is something other than RG75B as it has 17 transistors and the filter caps are 3300uf. Is there a part number on the board somewhere?Originally posted by Enzo
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In your picture R29 is burnt and missing, but R20 is not (4K3 ??).
I repeat my suggestion you compare your PCB with one of the published schematics, part by part, and relabel the ones on the schematic to match.
Only with that corrected schematic we can work.
In fact, you will find the answer yourself
"R29", "R20" means nothing on its own.
Hoping for somebody to have exactly the same amp, same version, so his picture matches *exactly* yours , is not impossible, but so unlikely that you should forget it.
Please do your homework and match PCB to schematic.
Good luck
Juan Manuel FaheyComment
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