Discussion in 'Fisher' started by tekuhn, Apr 10, 2018.
Kinda like the black caps. How did you clean the chassis?
I used Brasso there too. It works pretty well and didn’t touch the silk screening. The chassis had a few spots where something was spilled on it and corroded it. I still need to work on that a bit.
Kinda like darker streaks? That's the cadmium from when the chassis was dunked originally. Some chassis streaked and some didn't. Go figure. When you get done shining it up to USMC standards, you'll need to put some straight carnauba wax on it. Butcher's Bowling Alley Wax or Johnsons Paste Wax (yellow can) will do it. Give it a couple coats as the brasso has stripped off alot of the cadmium plating.
The chassis on mine was so bad Brasso wouldn't touch it. It was pitted through the cadmium layer down to the iron underneath all over. I ended up using a rust remover, but I did put wax on it when I was done to try to keep the rust from coming back. A lot of my stenciling is unreadable now. I think I'm going to find stenciling fonts and try to replace some of the missing text.
Thanks for the tip on the carnauba wax. Mine mostly had lots of oxidation - especially where finger prints were left, but there are about 3 sections where something got on the surface and attacked it and then dried. A razor blade worked pretty well on those, but can't get to all of it. Not a big deal - I care a lot more about how it sounds than looks, but still like to polish stuff up where I can.
I plan on converting the Spacexpander jacks to Pre-Out and Amp-In Jacks. There is a resistor network prior to each jack. My question is where to insert the shielded cable? I'm wondering if one way or the other would make the long cable runs less susceptible to picking up stray signals and audio degradation?
My guess is B for the Pre-out side and A for the Amp-in side to keep the signal level in the cables as high as possible
Tek -- In my unit, the shielded cable was installed at points A and C. The components of the buffer -- including IC1, the .22 uF cap, 100Ω, and 220K resistors -- are all installed on a small perf-board type circuit board, with shielded cable running from the junction of the 100Ω and 220K resistors on the board, and the preamp output jack. For the power amp input jack, the 1M resistor is mounted at the power amp input jack, with the cable running between the jack and R100, which is located at the tube socket.
As for which ends of the cables are grounded, as a rule, it is always best to ground only one end -- unless continuity is required in the ground lead to carry the ground to the other end of the cable. In my unit (as I recall), the ground for the new preamp output jacks was already established at the jack end from the original build, which I maintained in their conversion to Pre-out jacks. The shield for the cable to this jack was therefore grounded at the jack end, and left floating at the circuit board end. For the power amp input jack the cable is clearly grounded at the driver tube end, and (I believe) carries through to provide the ground for the Power Amp In jack as well.
I hope this helps!
Exactly what I needed - thank you!
Everything is really tight in the corner where the input jacks are, so I decided the drill the rivets and remove the Spacexpander panel and do a good job of cleaning things up to prepare for new cable. I'm glad I did and I already have in my head the path I'm going to use for the cable runs. The inventor of desoldering braid deserves to be rich.
I received most of my parts this week. Here's one channel of the power amp finished. I find with a high wattage gun, the chassis will take solder just fine. I am using that technique to add terminal strips as necessary to support new components. I used a piece of desoldering braid to attach the upper end of the metal shield to the chassis after desoldering it from the High Filter switch.
I have the rear jacks and Tape Mon switch wired and assembled the buffer board....
I decided I really needed a variac to power this and several future projects up safely, so I bought the cheapest made-in-USA one I could find on eBay. It wasn't pretty, but I see the potential. It's a very early Superior Electric Powerstat 116. It's almost finished once I find a suitable box to replace the damaged bakelite original.
I remember when Fair Radio had those things by the car load for $5 each. Same with good transformers.
More progress... Picture #1 - I have the components installed for the two negative supply lines for the buffer board and the reduced DC filaments - 1N4007 diode, 1000@35v (2), .1uf cap, and 10Ω resistor. Picture #2 - HV diodes are 8amp 1600piv hexfreds (soft/fast=quiet). I tied both returns from the power switch to the amp pt (as well as keeping rear outlets wired too) so that everything gets the current carrying benefit of both sides of the switch. Installed CL-80, Power supply cap values remain same except the quad 40/40/40/20 is now 50/50 and 50/50. I also split the single 200uf into two 100uf so I could spread them out to available vacant spaces. Power lead has been added to the lower tap for the reworked AF stage. Picture #3 - EFB (tm) board is almost complete. It just needs 2 more resistors added and the leads for the MOSFET which I plan to mount remotely.
I finished wiring the repurposed SpacExpander jacks and riveted the assembly back in. I ran the "Amp In" cables to the power section. I completed the EFB (tm) board and attached wiring. The board layout came out pretty well with no cross jumpered traces. This could be made into a single-sided PCB pretty easily.
Question for Dave....... I added the Gate, Drain, Source labeling to your schematic using Luis' schematic as reference. Can you please confirm that the labeling is correct? In my mind, the Source and Drain seem backwards, but I suspect it's just due to me not fully understanding how the circuit works. Also, do you see a problem with me remote mounting the MOSFET to the chassis? The leads will be about 3 inches long and twisted. Thank you!
The labels on the mosfet leads are in fact correct for that device. When I first worked with mosfets, I always thought the labeling was backwards as well, but it is best understood by remembering that electron current flow is always in a direction going from negative to positive, not the other way around -- even though for the purposes of reading a schematic, it used to be taught backwards! In this case, with actual flow moving from negative to positive, the Source lead then is the lead providing the "source" of electrons to the device, while the electrons exiting from the device then "drain" off via the Drain lead. The definition of the Gate lead is of course self explanatory. The remote mounting of the device should be fine with leads of only three inches.
Your work looks beautiful!
That explanation makes sense - thank you!
I'm curious if you have looked into the 6P41S ? They are being sold on eBay as a 7868 substitute. I see lower max voltages and max plate dissipation, but wonder if they would work, especially in a 400 w/ EFB. I found a thread where people are successfully pushing them well beyond ratings.
Type Output beam tetrode
Application application in TV scan modules,power amplifiers and generators
Cathode type oxide,indirect heating
Filament voltage,V 6,3
Filament current,A 1,0-1,2
Anode voltage,V 190
Anode current,A 0,056-0,076
Anode power,W 14
Grid2 voltage,V 190
Reverse grid current,uA 1
Microphnic noise,mV 500
Socket type rsh24-1
TEK. Keep an eye on those jumpers. They like to get pushed in and then short to the outer casing. I have a habit of heatshrinking the top and about 1/4 of the way down the legs. This isolates the outer casing and prevents shorts. Or dip them in liquid plastic for tools. Something to think about.
That's not a bad idea, thanks Larry!
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