The 400-C Returns!!
- Thread starter notdigital
- Start date
I did see that detail mentioned in your thread Dave so thanks for the reminder. Without such reminders, I sometimes jump ahead of myself only having to backtrack.
Well now it's an easy decision
And thanks for the schemo Sam
I've now completed the wiring of the indicator bulb array and the modification of the grounding points
Re-mounting the Power Indicator Bulb was a fairly straight forward job. As an extra precaution, I shaved down the sides of the bracket to help avoid accidental contact with the sides of the chassis. A brief power up established that the new 28v power indicator bulb did work. The other indicator bulbs failed to light however. Happily, as I was reviewing other threads, I came across a note by cbox83 wherein he mentioned that the polarity of the wires might be switched at the bulb. Indeed that was the case and a quick switch solved the issue. Finally, as Dave mentioned in his "Transformation" thread, the newer 8384 might not ride as deep as the original lamps. To correct this issue, I chose to extend the existing contact by carefully adding more solder. Done!
The ground point modification, too, was straightforward with the drilling being the most time consuming. For future reference, I've pictured and numbered the drilled out points as well as the corresponding ground points of the other channel.
With these two stages completed, I started taking voltage readings but I couldn't make sense of what I was getting. Perhaps I was tired, or perhaps I simply don't understand what I'm reading, or perhaps a combination of the two. In any case, I'll give myself a break, re-measure, and post the results when time allows. From what I HEAR so far, this is on special piece of equipment!!!!!
Re-mounting the Power Indicator Bulb was a fairly straight forward job. As an extra precaution, I shaved down the sides of the bracket to help avoid accidental contact with the sides of the chassis. A brief power up established that the new 28v power indicator bulb did work. The other indicator bulbs failed to light however. Happily, as I was reviewing other threads, I came across a note by cbox83 wherein he mentioned that the polarity of the wires might be switched at the bulb. Indeed that was the case and a quick switch solved the issue. Finally, as Dave mentioned in his "Transformation" thread, the newer 8384 might not ride as deep as the original lamps. To correct this issue, I chose to extend the existing contact by carefully adding more solder. Done!
The ground point modification, too, was straightforward with the drilling being the most time consuming. For future reference, I've pictured and numbered the drilled out points as well as the corresponding ground points of the other channel.
With these two stages completed, I started taking voltage readings but I couldn't make sense of what I was getting. Perhaps I was tired, or perhaps I simply don't understand what I'm reading, or perhaps a combination of the two. In any case, I'll give myself a break, re-measure, and post the results when time allows. From what I HEAR so far, this is on special piece of equipment!!!!!
Attachments
Thanks MarZ. This piece is most likely the last major Fisher in my collection. I know, famous last words......
And Dave, when you get the chance, can you please comment on my voltages:
All voltages taken volume fully down @117vac, chassis ground:
C-9 182.6
C-18c 253
C-32c 185.2
C-42c 255
C-49-A 345
C-49-B 324
C-49-C 302
C-50-A 113.6
C-50-B 108.8
V-3
Pin1 253
Pin2 153.7
Pin3 156.5
Pin4 109
Pin5 97.9
Pin6 123.7
Pin7 2.5mv
Pin8 1.1
Pin9 103.6
V-6
Pin1 128.5
Pin2 1.9mv
Pin3 1.12
Pin4 109
Pin5 98.1
Pin6 255
Pin7 142.5
Pin8 145
Pin9 103.4
V-2
Pin1 172.8
Pin2 2.5mv
Pin3 1.15
Pin4 97
Pin5 86.2
Pin6 153.4
Pin7 5.0
Pin8 12.3
Pin9 91.6
V-5
Pin1 170.8
Pin2 -.03
Pin3 1.26
Pin4 108.4
Pin5 97.5
Pin6 142.4
Pin7 6.0
Pin8 14.5
Pin9 103.0
V-7
Pin2 86.6
Pin3/4 345
Pin6 84.6
Pin9 345
As pictured, I have a grounded 56k resistor connected to pin 6 of V-7 instead of a 68k resistor.
As I've read through the 400c restoration threads, I've come to understand that the bias supply modification addresses the imbalance created in the 12AT7 tubes in the original design. I've hit a brain-block on a) whether I have the appropriate voltages on these tubes and b) whether the voltages on V-7 are likewise appropriate. As far as anything else, there is a very slight hum in both channels. However, for now, I attribute that to the bottom being off, the tubes being unsheilded, and the unit being plugged into the variac that shares the circuit with my bank of 4 overhead flourescent lamps. For now, I'm not too concerned.
Thanks!!!
And Dave, when you get the chance, can you please comment on my voltages:
All voltages taken volume fully down @117vac, chassis ground:
C-9 182.6
C-18c 253
C-32c 185.2
C-42c 255
C-49-A 345
C-49-B 324
C-49-C 302
C-50-A 113.6
C-50-B 108.8
V-3
Pin1 253
Pin2 153.7
Pin3 156.5
Pin4 109
Pin5 97.9
Pin6 123.7
Pin7 2.5mv
Pin8 1.1
Pin9 103.6
V-6
Pin1 128.5
Pin2 1.9mv
Pin3 1.12
Pin4 109
Pin5 98.1
Pin6 255
Pin7 142.5
Pin8 145
Pin9 103.4
V-2
Pin1 172.8
Pin2 2.5mv
Pin3 1.15
Pin4 97
Pin5 86.2
Pin6 153.4
Pin7 5.0
Pin8 12.3
Pin9 91.6
V-5
Pin1 170.8
Pin2 -.03
Pin3 1.26
Pin4 108.4
Pin5 97.5
Pin6 142.4
Pin7 6.0
Pin8 14.5
Pin9 103.0
V-7
Pin2 86.6
Pin3/4 345
Pin6 84.6
Pin9 345
As pictured, I have a grounded 56k resistor connected to pin 6 of V-7 instead of a 68k resistor.
As I've read through the 400c restoration threads, I've come to understand that the bias supply modification addresses the imbalance created in the 12AT7 tubes in the original design. I've hit a brain-block on a) whether I have the appropriate voltages on these tubes and b) whether the voltages on V-7 are likewise appropriate. As far as anything else, there is a very slight hum in both channels. However, for now, I attribute that to the bottom being off, the tubes being unsheilded, and the unit being plugged into the variac that shares the circuit with my bank of 4 overhead flourescent lamps. For now, I'm not too concerned.
Thanks!!!
Not -- The only thing that's a little puzzling is the voltage at pin #6 of V7. This point is the source of DC heater supply bias. In your unit, that bias is derived from a point that you indicate is 302 vdc. Now, with the divider values you are using (270K/56K), it means that the voltage at pin #6 of V7 should be 51.9 vdc if the components you used for the divider string are accurate -- except that you are indicating 84.6 vdc at that point, and even higher at pin #2, where it should normally read slightly lower than the value at pin #6. The higher reading at pin #2 could be explained if you accidentally swapped those two readings in posting them, but the nearly 35 vdc differential between what should be at pin #6, and what (likely) IS at pin #6 should be investigated. The purpose of the heater bias is to address the rather low heater/cathode voltage differential rating of the 12AT7 tube. This is specified to be a maximum of 90 vdc, yet the stock design exceeds this value by typically some 158% in the CF section of these tubes, which can lead to early tube failure.
I hope this helps!
Dave
I hope this helps!
Dave
THAT reading on pin 6 of V-7 is what I couldn't understand and is what I was referring to in post #43. I took 2 sets of readings and V-7 came up the same so I concluded that I goofed somewhere. Both the divider resistors are new 1% jobs so I put up the numbers in post #45 with the hope that you could zero me in. And you did.
It turns out that the Bugle Boy EZ-80 that came with the set resulted in wonky readings. I popped in a brand new RFT, and took a quick reading: 50.1 vdc which is right in line with what it should be. I suspect that with this correction, the heater/cathode will tighten up to < 90vdc for both 12AT7's and all I'll have to do is fiddle a bit to get to the target 55-56 vdc you specified in another thread. I imagine that running at wall voltage will raise C-49-c sufficiently to get to that target but that experiment, together with a re-measure of all the tubes will have to wait until I have a block of time. For now, with your guidance, I'm happy that I don't have to wake up in the middle of the night thinking about it!!
As always, many thanks!
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Humm -- Nothing I can find suggests that pins #2 & #6 of the EZ80 cannot be used as tie points as they are in this case, so it must be something goofy with that tube -- unless anybody can offer any information to the contrary......
Glad you found it!
Dave
Glad you found it!
Dave
I have the pre set up on my bench in the basement connected to a pair of Optimus speakers via a NAD receiver's main out. I haven't really lent an ear to critical hearing because I'm still working on aspects of restoration. Once I'm confident everything is running as it should, I plan to pair it up with my SA-100 and give it a go with my KLH Sixes. From what I'm hearing on the test bench, it'll be wonderful !
I saw no need to replace a few of the caps that had been replaced previously. However, I have no clue as to the manufacturer. The big maroon .1 and its little brother .01 Anyone have a guess?
Never mind: El Menco "Brown Drops."
GAWD ND! That sounds like leftover's from a case of Monteczuma's Revenge.
Fairly good caps tho.
LOLOLOL! That image is perfect right after lunch!! 
You gotta stop eating those Bean and Cheese Burrito's for Lunch!
Had a chance to retake voltages on the 12AT7 tubes and the cathode/heater difference:
V-3: 84vdc
V-6: 72vdc
All other voltages line up pretty darn close to spec!
Today/tomorrow I'll just be running juice through on CD & FM. So far so good with, happily, no balance or volume issues. If nothing jumps out as irregular, what's left is faceplate and cabinet work.
V-3: 84vdc
V-6: 72vdc
All other voltages line up pretty darn close to spec!
Today/tomorrow I'll just be running juice through on CD & FM. So far so good with, happily, no balance or volume issues. If nothing jumps out as irregular, what's left is faceplate and cabinet work.
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400-CA Operating Instructions now in Digital Docs
thanks for posting, I have been looking for this for quite awhile
Gary
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Well, I think I'm finally done. At least for now. The only real fiddling that's left is to the phono stage of this wonderful instrument. I've got it upstairs in the living room now and, together with the SA-100, I can't begin to tell you how happy I am with its performance.
Special thanks to Matt (rufleruf) for providing the missing shield, to Larry for satisfying my obsessive need for documentation, and of course Dave, whose masterful guidance is priceless!
Special thanks to Matt (rufleruf) for providing the missing shield, to Larry for satisfying my obsessive need for documentation, and of course Dave, whose masterful guidance is priceless!
