Recently restored a friends late 1980 RD-100 (6L6 power tubes) that was all kinds of beat up with intermittent problems. Now its solid and sounds great. I liked it so much (I'm primarily an RD50 man) I found a nice clean one cheap and bought it.
Mine is the second version (GD-2), uses EL-34 tubes. Was stock and everything worked. Did a full recap (21 electrolytic caps in total!), cleaned all jacks, pots, op-amp sockets, etc. Biased right up, sounds great, but one oddball thing - the Deep switch, which worked perfectly before, now acts as a volume boost.
I checked that all components in the Deep circuit seem in spec and connected properly, wondering if the switch itself suddenly went bad. Should the switch be closed in Deep or Normal mode?
I was going to pull the main board to double check solder joints, but the board seems stuck to the chassis. Even after removing all hold-down screws including the two small ones on the driver transistors, something is holding it mid-board to the chassis.
Suggestions welcome.
Deep Mode Switch
The SPST Deep Switch is closed for flat linear operation. It shorts out the .015uf cap. in op amp feed back circuit. Conversely, the Deep switch is open for Low Frequency Boost non linear operation. From looking at the feedback circuit, I'd expect a little bit of overall gain boost in Deep Mode. All of the thorough refresh work you did is letting you hear more of the true operation. Hope that helps. Keep up the good work. -mgriffin
Appreciate your insight.
Appreciate your insight.
Don't recall this much of a boost in volume from the Deep circuit on other MM amps I've worked on, not at all subtle here. Certainly much more pronounced/louder than turning on the Bright switch.
Think this is only the second 100 series amp I've worked on, have been through the 50s, 65s, 75s, 130s and150s. I see the 100 has a different circuit, not just switching a cap in and out.
FWIW, I did swap around some op-amps to see if I have a bad IC-1, but no difference.
Had my friend check the Deep
Had my friend check the Deep switch operation of his late 1980 RD-100 (6L6) and sure enough, he gets the volume boost as well.
Learn something new everyday. Now I can close mine up and enjoy it!
Deep is different yes
Indeed, the deep switch operation is different btw single channel and dual channel MM amps. The dual channel amps have a capacitor before the phase splitter stage - shorted out in 'deep' mode. The single channel models (that have this feature) have a switch shorting a capacitor in the feedback loop of the main gain stage - shorted in 'normal' mode. The filters (bright and deep) in the single channel amps are very noticeable when used, more subtle in the dual channel models.
Cheers,
Lars