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Improving the Fisher 400

Dave

Apologies with the mistake. I was confusing about that, but now is OK.

Please find as follows the revision #5 with the jacks corrected.

Regards

Luis
 
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Hi Luis -- Other than placing a small "TM" after EFB in the acknowledgement section, it looks perfect!

Thanks for all your hard work to put this altogether. Your work is really superb!

Dave
 
Dave: Can you tell me what version of the manual this came from. The serial # series on Front is sufficient.

Thanks

Larry
 
eduarsan1 said:
Ok Dave

You got it

Till you say another thing this drawing will be the final version.

Regards
Click to expand...

Reviewing the schematic, I notice a small error. I measured 420 volts after the 1HY choke at zero signal output (zero volume). The voltage at the tube plates would drop to a value less than 420 volts due to the audio transformer primary winding internal resistance. I did not measure voltage at the plates, but would estimate 410 volts. The 420 volts would vary depending upon AC line voltage input, but not necessary to make note of that IMO.
 
Larry -- If you're talking about what version the schematic originally started life as, you'd have to ask Luis to make sure -- but it appears that it's the schematic for serial numbers 48001 and up.

Dave
 
Hi, first I want to say thanks for sharing your adventure/expertise with us. I have a Fisher 400 and you've got me inspired to check into if I could mimic your work. I have a few questions though that maybe someone can help me with.

I couldn't find any ksa10130BU at mouser, digikey or ebay. Can you point me to a place to aquire them in small numbers or a drop in replacement? Also where do I find the L-brackets that you used? I didn't see any phono mods, did i miss them? Would a TL072 work good in the buffer circuit? I have a few around.

Thanks.
Dan
 
Hi Dan -- Thanks for the kind words. The components available at the large parts houses seem to change daily. The KSA10130BU is the Korean version of the basic A1013 device. Crossing this (for example) to an NTE device (the old ECG) comes up with an NTE 32. This is a 160 volt PNP silicon transistor rated for 1 amp, with a min hFE of 100, and comes in a TO-92M package. Any device rated along these lines will work just fine.

Since this was a one-off project, originally started to prove out certain elements of a "universal" EFB circuit I am working on, the board and mounting bracket all came from stuff I had on hand. The board was a portion of a larger R/S perf board, and the bracket was a small L-Bracket I had in my hardware box. I would think that the big box retailers carry small brackets that would work, or they could be fabricated easily enough from some thin aluminum stock.

The TL072 should work just fine in the buffer circuit.

I did not do any phono mods for a number of reasons:

1. Those circuits don't impose the significant compromises made in the power amplifier/power supply sections that I was dealing with regarding the screen dissipation, phase inverter drive, feedback/stability circuits, and input stage Miller considerations.

2. The modifications to the line/tone stages were primarily based on bringing the 400 into the 21st century, turning obsolete features into useful ones.

3. I had to stop somewhere!

In the phono preamp section, one modification that would be useful is to convert the Tape Head input into another Aux level input which can be done with some fancy rewiring of the selector switch, and input jacks.

If you do implement any of these modifications, I'll be happy to answer any questions you have along the way.

Thanks for the interest!

Dave
 
Larry, thank you so much for clearing that up for me. Also that's a great idea on making a bracket. Do you happen to know if I should change the grid resistors/coupling caps to 200k/.1uf with Dave's mods or should I leave them alone?

Dan
 
Hi Danny.

Is not necessary to change the grid resistors and the coupling caps due to the EFB controls the screen voltage and the tubes will run very cool.

In the other hand. Kindly request you guys take more patience, i promise to finish my job regarding the mods in the drawings (make only one with the old one and the modified one in a only file). After finish I'll send all the files to Dave.

Regards!!
 
Dave, thanks for the offer to answer any questions that I have. I'm sure I'll be taking you up on it when some parts arrive and I get off my butt. I'm going to have into how to repurpose the tape head input. I have a few vintage pieces that would also benefit from it.

eduarsan1, thanks for clearing up my grid resistor/coupling cap questions. I wasn't sure if that I popped in some new production 7868's and turned up the amp if the larger grid resistor would come into play even with Dave's EFB installed.

Anyway, this is an exciting project and a bit of a challenge and I'm guessing a large learning opportunity for me. I'm going to be first recapping the amp's power supply, then adding the EFB and if that works out doing the buffer/line stage changes.

Dan
 
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Danny I´m in the same purpose to do those mods on my fisher 400. That´s why I´m supporting Dave making the drawings. I got the parts from mouser and they´ll arrive soon to my home, hope to start this project in around 1 month, so if you have questions regarding this just ask.

Regards

Luis.
 
Luis,
We're looking forward to your project and your opinion of the results.

I've got two 400s to do, but the family expects me to paint the first floor inside the house this summer, so all bets about getting tube work done are off until late October.
 
Hi Don

I know what you mean. I had waited for do this for more than 5 months, because my wife has been warning me to expend my time in this due to similar duties like you have.:tears:

So hope to do this next month


Luis
 
Just wanted to get my name on the notification list for this thread. My 400 works with the normal basic restorations, but this project looks like the hot potato. I'll start gathering parts so I'm ready when fall arrives.

Thank you Dave and all the others for the project.

John
 
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