Discussion in 'Fisher' started by dcgillespie, Jun 15, 2014.
Second style knobs, too.
Thanks for the info Sam. Min
The green caps are the .022 uF output coupling caps, while the dual .005 ceramic caps at each tube socket make up the .01 uF interstage coupling caps.
With the dual crossover design, the Output Mode Sw electrically appears at the input to the output amplifiers, making for a unit that is not nearly as prone to hum. Ditto with the earliest units, where this switch was primarily located electrically at the output of the output amplifier. In the units with the dual concentric tone controls and no crossover settings however, this switch is electrically located at the input of the line stage amplifiers, which makes for a unit with significant hum and ground loop problems. Glad the tip help to reduce the hum level in yours!
BTW, in your pic of the phono preamp section, the 390pF, .0018uF, 270K and 2.2M resistors make up the RIAA networks in each channel.
Dave, thanks for helping me elevate my Fisher 400C to levels I could only dream of! Min
Working on phono upgrade and I am trying to locate the 100K resistor (trying to move it from pin 2 and move to the other side of 10k resistor) in the preamp phono section. Is it connected to one of the 12AX7 phono tube socket? Do i just move one of the ends from pin 2 and solder it on the same 10k lead on the tube socket? I am also trying to upgrade the RIAA and was only able to purchase the 680PF 500V silver mica caps. Will it be a detriment to only change these caps and not change the 390PF to 350Pf? Thanks Dave.
There is a 100K resistor connected between pin #2 and ground on each phono preamp tube socket. The end of these resistors that are connected to pin #2 need to be removed from the tube socket terminal, and then connected to the other side of the 10K resistor also connected to pin #2 of each tube socket. Therefore, the 100K resistor would then connect between the end of each 10K resistor connected to the selector switch, and ground.
I hope this helps!
So Santa, err...The FedEx Man, delivered a 400C today. It is in really good condition, with a nearly impeccable faceplate and a chassis that has little oxidation. It's the same version 'C' as Dave's with the individual tone controls. It even came with the walnut wooden cabinet!!!
Supposedly it's had some work done with replaced caps, silicon rectifier, resistors and some lamp work.
Tonight is for enjoying it; tomorrow is for opening it up and seeing what's what.
I'm hoping to be able to get some guidance when performing the "Dave Transformation" as there are some of the upgrades I'm not certain exactly how to do. I'll also be updating my KX-200 thread and will be asking for some advice on how to turn 2 of the inputs into Preamp Out/Power in so I can use the 400C with it.
Pics and much more to come in the near future!
EDIT: Oops, wrong thread.
C22/C44 - you recommend changing those to 5000pF/.005uF. Would 4700pF/.0047uF be acceptable there? .005uF is a rather non-standard value.
That would be fine!
Any suggestions for preferred capacitors in the 400C/CA/CX/CX-2 series? Film or PIO?
I've always preferred film myself, but that has nothing to do with sound per say, but keeping physical size down to minimize interaction with other components and circuits.
Sorry to resurrect an old thread. I'm going to start these modifications on my fisher 400c (first version without the dual tone knobs).
The only thing that I'm confused about at this point, (and it's because I'm very green), is I don't understand the "lifted ground" for the power supply. If I get discrete caps for the power supply caps, where do I connect the ground leads? This is likely a loaded question, and hopefully it makes sense, but because Dave's photos are a little pixelated due to the upload quality, I can't quite put my head around the grounding scheme...
Thanks in advance for indulging me!
Hi Tysen -- The best way to imagine the grounding system of the modified DC Heater Power Supply is this way: The negative side of the DC heater supply rectifier, the negative side of the heater supply filter caps, and the connections from the negative side of this supply that connect to the heaters (and now indicator bulbs as well) are all still electrically connected together as they were in the original design, but no-where are any of these connections grounded to the chassis. The DC Heater Power Supply and the items it powers have no relationship to chassis ground at all at this point in the discussion.
Now, take a look at the third and fourth pics in my first post, showing the additional components added to the high voltage rectifier tube. In particular, I'm referring to the 270K 1W resistor, 68K .25W resistor, 10K .25W resistor, and 11 uF capacitor. These components receive power from the high voltage B+ power supply, and form an isolated voltage divider, whose positive output (of about 60+ volts or so) is then connected to the negative side of the DC Heater Power Supply circuit. With this connection then, if you use your volt meter to measure the voltage from the negative side of the DC Heater Supply circuit to the chassis, it will indicate 60+ volts. If you measure between the positive side of the DC Heater Supply (where it connects to the heaters) and the chassis, your meter will indicate about 85+ volts or so.
The purpose of doing this is that the section of the 12AT7 tubes that operates as a cathode follower have the cathode terminal operating at some 150 volts or so above ground. If the DC Heater Supply is connected directly to ground as it was in the original design, that can place as much as a 144 vdc potential between the heater and cathode elements within this section of the tube -- yet the 12AT7 tube is only rated for a maximum of 90 vdc potential to exist between these elements. As a result, a number of folks have reported that the original design can actually damage the tube in this location. By lifting the heater supply above ground by about 60+ volts, it reduces the potential between these elements to be within the amount allowable for the 12AT7 tube, and prevents the circuit from damaging them.
As for the DC heater circuit itself, it remains referenced to ground from an AC standpoint through the 11 uF cap (10 uF is fine, it's just what I had on hand) that decouples the voltage divider circuit to ground, which prevents any hum from developing were there no reference to ground for the DC Heater Supply at all. So with the heaters elevated by some 60+ volts, but still fully referenced to ground, you get your cake and eat it too: the heater/cathode elements in the 12AT7 tubes are no longer stressed, and the DC heater system still references ground for minimum noise.
I hope this helps!
That makes total sense. Thanks for responding. I'm going to start a new thread so as not to hi-jack this one.
Currently, mine has been restored...but after using it three times since getting it back from the tech, the right channel has cut out. Tried swapping tubes, to no avail. Hope it's not the volume control!
On thing that gets regularly missed on the 400C preamps is the Tape Monitor input jacks mounted in the chassis channel secured to the top of the rear of the chassis. These are very special jacks that require a longer than ordinary center pin to operate correctly. With nothing inserted into these jacks, the jacks allow a signal to flow through them. But when a long pin RCA jack is inserted into the jack, it breaks the path of the signal flowing through the jack, and allows whatever signal the long pin RCA jack represents to be heard. In effect then, the Tape Monitor jack becomes the Tape Monitor input jack, and the Tape Monitor switch all in one.
Over the years however, the jack contacts that allow a signal to pass through the jack when nothing is plugged into it can become compromised. When that happens, the channel cuts out. You might check the integrity of the contacts with these jacks, to make sure that the signal is not being stopped because their contacts are dirty. Just a thought.
I hope this helps!
I'll check those soon! Thanks Dave!
Dave - Can your post 22 mods be applied to a 500-C too? Thanks, Thorne
Here would be a better thread for your perusal.........
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