RD50 Zener Question

twangdang

RD50 head from 82.It has a slight fuzzy sound in the background similar to a blown speaker.This sound occurs on both channels.Reverb and footswitch are disconnected.Have tried different speakers and output tubes with the same result.I realize I am not a tech and do not own any test equipment except a DVM.That being said,All voltages on the schematic seem close
Pin 3-611v
Pin 4-305v
Pin 5-20v[should be 30v?]
R64-16.26/7.46v
R67- -16.42/-7.6v
R62-40v/20v[should only be dropping 10v?]
so,could the D9 zener be at fault?

thanks,
Doug

Full Name
Mike Griffin
Music Man Equipment
MM 112-RD-65

Member for

10 years 9 months

mgriffin155 Sat, 07/20/2024 - 09:18

Yes. Voltage at D9 should be 30V. Zeners get "soft" as they age so change it out. Zeners also get hot so replace with higher wattage value and stand it off the motherboard 2mm for better heat dissipation. Good luck. -mgriffin

Full Name
Doug Coffman
Music Man Equipment
5-RD50 112s,two in white pinwhale with EVM12Ls
1-RD50 Head
1-212HD150

Member for

1 year 1 month

twangdang Wed, 07/24/2024 - 18:42

I now have 32v at D9.Background "fizz" is gone.Amp has more fidelity and power now.Thanks, mgriffin,for your help!
Doug