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djmeverett wrote:Well I'm hoping someone can help me with some issues and wire replacements, restoration questions I'm doing on this unit and some schematic and tube circuit interpretation.
Once I got this thing opened up and separated into it's main components (turntable, speaker, antenna, tube chassis), I started to document the tube chassis, pins, caps, resistors and controls etc.
While checking with the schematic as I went I found a couple things not referenced on there.
There is a "tone" control pot (25K) which is mounted under and part of the main power switch. I believe it's supposed to add or adjust some brightness during recording...not sure if it also works for everything else like the radio or phono playback?
The pot itself only uses two poles (center wiper and one end, other end is NC).
One end goes to a .05uf/400 cap to ground. The wiper of the pot appears to be a shielded green wire that goes to the 6V6GT tube pin 3. That pin 3 (plate) also goes to the audio output transformer input tap and a .01uf/1000V cap to ground.
I'm hoping a tube circuit guru or Meissner 9-1065 expert could tell me if that is the correct hookup of that tone pot and how that works in terms of brightness control. Most tone control I've seen are done at the preamp stage and this seems to be connecting at the output plate stage of the 6V6GT power tube???....so I'm looking for help with that!
The second item I can't seem to figure out on the schematic and appears to be an additional circuit (maybe not) is a .01uf/400V cap, 470K, 100k resistors connected together at pin 6 of the 6V6GT. Now per the tube datasheet pin 6 of the 6V6GT is a N.C. pin.
I suspect they used that pin as a common tie point?
So....the other end of the .01/400V cap goes to ground, the other end of the 470k resistor goes to the + side of the 15uf/450V 2nd filter cap (B+) and the 100k goes to pin 5 of the 6SC7 tube.
Well...on one schematic I have it shows pin 5 of the 6SC7 with a tie to pin 2 and on another schematic I have there is no connection tie at pin 5 to pin 2 at all?
So again some confusion there as to where and what that's doing in the circuit and if that is correct? I have attached both schematics (pin 5 NC) and (pin 5 connected to pin 2) of the 6SC7.
One final bit of confusion and knowledge I need is on the two 6J7 tubes in the schematic each have a grid pin shown as a square off the tube socket (it's G1 grid). Is that actually pin 6 on the socket itslef and any idea why the Meissner schematic would show it between pins 3 & 4. The RCA tube data sheet shows that G1 tab between pins 5 & 7. Just need some verification on that....
I would really appreciate any help especially from any tube circuit people who are familiar with this kind of stuff....
djmeverett wrote:Mark....you know on that 6J7 tube which is what is listed in the schematic, that has a completely different pin layout than what's in there which are 6SJ7GT's!!
The schematic diagram shows a 6J7 and wiring.
When I look at the data sheets for the 6J7 the Cathode (K) is shown on Pin 8 and that G1 pin is on the cap of the metal MT* case, so I double checked the markings and these are Sylvania 6SJ7GT and the Plate (P) is listed as Pin 8!
They are all completely backwards...
None of the pins actually match a 6J7
I'm really confused now and wonder if someone put the wrong tubes in.
I'm not sure what to do now!
Do I rewire?...or try and find some 6J7 or 6J7GT's?
I think this is just a case of your unit being a newer revision. I would just ignore the older Riders schematic other than to get a general idea of how the unit works. In reality, you really don't need the schematic to restore this unit. The general idea is to replace all paper caps with modern film versions. Replace all electrolytic caps as well. Power up and see if you can get the radio portion working. Replace tubes and other parts as needed (don't rely on a tube tester). Getting the radio working is a good thing because this uses all of the circuity (the record and playback functions just re-arrange some of the circuit blocks).
I've never owned one of these, but I have a good deal of experience restoring old radios. You can probably bank on the cutter head and pickup being dead. They used Rochelle salt crystal elements. These typically don't age well. Getting the head rebuilt seems to be an issue these days. Gib at West-Tech is looking to retire soon.
Do a search here and you'll see. Good luck with the restoration.
The replacement should be fine. You might have to adjust the weight to keep from killing the pickup since the supplied tone arm tracks at over 2X the replacement. I doubt you'll need to make any mods to the electronics. I'd try it and see.
I doubt those will be good either. The problem is the Rochelle Salt crystals in these pickup and cutter heads have a limited life. A PZT based ceramic is the way to go.
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