BobW RD50 mod problem

stratube

Newbie here with a first version (I think) RD50 from December 1980. It has the 27 ohm cathode resistors. No schematic found to match what I have. My problem is that I did the BobW liliter channel mods and love the drive when it's full on but when backing down the gain, it sounds somewhat like blocking distortion in the limiter channel. I'm wondering if I may have some circuit differences from the later versions that he modded. Any suggestions on what to look for? Thanks in advance for any help.

I followed this list of changes to the T:
1. disconnected the cathode grounds and ground leads from C10,R12 at V1-8.
2. Added a 5uf and a 1.8k at V1-8 and connected the grounds to the C10,R12,ground node.
3. replaced plate resistors R9 (470k) with a 180k, and R11 (4.7M) with a 150k.
4. replaced R10 with a wire jumper.
5. removed grin wire at V1-7 and added a 150k series grid resistor.

Initially also added a 470pF across the Clean Vol pot, and a 1000pF across the Limiter Vol Pot.

Full Name
Ian Azrikan

Member for

10 years 9 months

inertian Thu, 02/08/2018 - 19:39

I've restored/modded quite a few RD50's, my personal October 1980 110 pretty much matches the Rev A schematic on this site. Is it possible someone made some changes before you? Clean side working properly?

Has the amp been fully recapped? If not, it is overdue. Check out this thread to see what is recommended:

http://pacair.com/mmamps3/node/2141

Do you have a schematic with the Bob W mod? If you have checked your work against it, one additional thing I do is disconnect C6 (.047/250v) from ground, effectively taking it out of the circuit. Try that and see if it helps.

Good luck!

Full Name
Ian Azrikan

Member for

10 years 9 months

inertian Sat, 02/10/2018 - 07:14

...have a late '81 RD 50 here I haven't recapped or modded, it's Distortion sounds ok at higher settings, increasingly less so as you lower the Gain, gets congested and less musical. Again, I would do the recap first and then check the mod changes.

Full Name
James Meeks
Music Man Equipment
First version 112 RD50, 1978 RP100

Member for

6 years 11 months

stratube Sat, 02/10/2018 - 12:47

I figured out that the idiot who modded the limiter channel( Me!) wired it wrong. I did compare my amp to the Rev A schematic. Differences are minimal. I now have the mod properly installed and the limiter channel is great! Thanks for the suggestions/help!

Full Name
Ian Azrikan

Member for

10 years 9 months

inertian Sat, 02/10/2018 - 12:52

Glad it was an easy fix.