Fisher 400 Silicone Bridge Rectifier replacement

[streetwise]

Hello all I bought a rebuild kit for my late model 70000 something Fisher 400 receiver. I've completed every part of the rebuild other than replacing the old silicone rectifier with the new one. Can someone walk me through what leg goes where in this receiver. From what I gather from the schematic that gets confusing in this section of the amp, the positive terminal goes to the 15ohm 5watt resistor in my rebuild kits case a GI856 diode is to go between the resistor and the rectifier for bias? Any Fisher Guru's have thoughts?
Thanks

 

[larryderouin]

If both rectifiers are 4 legged, 2 are AC and 2 are DC+ and DC-. The AC legs will have a Tilde mark (~) above it. On the AC LEGS it doesn't matter which wire goes to which lead on the rectifier. But it does on the DC SIDE. The DC+ will have a positive sign (+) and Negative DC will have a (-). Remove one at a time from the old and attach it to the new one.

If it's mounted to the side of the chassis, there should be a hole there or nearby. If the new rectifier looks like a table with legs, use a #6 x 3/4" screw and nut to fasten to the side panel. If it's mounted on the RF shield box, extend or shorten the wires to mount the new one behind the dual cap on the side of the chassis.

As for the rest it's fairly straight forward. Remember that the polarity of the BIAS and HEATER FILAMENT DC Voltage is Reversed (NEGATIVE VOLTAGE). The DUAL CAP will have the POSITIVE leads tied together, with the negatives separate.

I've got one on the bench I'll take a couple of pics with description of wires (where from and to). Don't take anything apart yet in the area.

You need for parts:
Rectifier
Diode is Optional as it's used supposedly for noise control, although I've never heard of this before, nor used one.
2 bias caps. Stock is 1000uf 35V. I use 2200uf 50V. Bit more filtering andbetter input voltage headroom.
Some wire of various colors
Heatshrink tubing of various sizes.

 

[streetwise]

Thanks Larry as always your the best. I wasn't totally sure I had an issue with a solid state piece that i replaced a rectifier on i did things backwards after spending several days working on a tractor designed in England with reversed polarities, so im extra cautious about doing rectifiers. I figured it out looking at pictures from the leestereo's restoration forum and staring at the schematic I built up the courage. Anyways things are running well voltages check out pretty well. The previous owner did all can cap replacements with original values, I just put in Jim McShane's rebuild kit. I have static on the right channel im trying to sort out right now. Im cleaning the 500k phase inverter control for that channel as I believe that is the source of the static since I adjusted that channel quite a bit following Dave's simple guide. Another question you might be able to answer... I plan on making a big mouser order to do all of Dave's mods EFB, IBBA, TONE BUFFER, RIAA. My questions is the only parts I didn't install of Jim's kit was 1k 2w screen stopper resistors, Dave has everyone doing 100 ohm resistors whats the difference?? Also not sure if it's due to the less efficient speakers im running or the optional diode/power supply rebuild & output stage rebuild, but the channel without static at the moment sounds dead quiet with volume maxed?

Thanks again
Paul

 

[streetwise]

Phase inverter pot doesn't seem to have changed the static issue. I notice sometimes fooling around with pin 9 of V14 the static seems to stop. May require a better cleaning with deoxit or reflow of solder on the pins?

 

[dcgillespie]

Paul -- Are these 1K 2W resistors replacing the original 1K resistors connected to the output tube control grids? Or, are these new additional components added to the output tubes -- that is, they are not replacing any components of the original design? If it's the latter, how are they instructed to be installed? I want to make sure that the resistors of Jim's that you're referring to are in fact for the screen grids.

Dave

 

[streetwise]

Paul -- Are these 1K 2W resistors replacing the original 1K resistors connected to the output tube control grids? Or, are these new additional components added to the output tubes -- that is, they are not replacing any components of the original design? If it's the latter, how are they instructed to be installed? I want to make sure that the resistors of Jim's that you're referring to are in fact for the screen grids.

Hey Dave, these are the ones in the pdf. i showed you on your improving the Fisher forum. New screen stoppers - go on screen pins of all 7868 the placement is N/A so these would be new product going in not replacements for the 1k. The parts list is the extent of his directions.

 

[larryderouin]

paul
1.) With regard to V14. If the 400 had EH7868's in it before the current tubes , the socket's will be stretched and stock tube pins will be too loose to fit snugly. And they can't be tightened enough to keep the old stock pins tight to the sockets. If it has Old stock and never had EH's it's either loose socket or dirt. Clean with DeOxit and a Dental Braces brush. Then follow up with a touch of Faderlube. Let dry and then tighten up the sockets using a dental pick or similar shaped tool. An Awl will work, as will a jewelers screwdriver.

If the tubes are EH's, the same applies for dirt and loose sockets. Because the pins onthe EH7868's are slightly larger in diam they stretch the sockets which will get looser over time. Tighten them up and clean them.

2.) I'm not familiar with Jim's kit, but I found your list of parts here. There was no mention of 1K 2W resistors for screen stability mods on it. OR is this not the correct list? If this is an add-on, me personally I would question Jim on the reasons for such a high value and wattage, and then throw the reply out for discussion. Seems like too much of a voltage drop for the screens to me, among other things I don't understand yet.

 

[streetwise]

They are in the pdf first thing under plate load/screen resistors. I haven't asked Jim yet, it seemed the output tubes were giving false positives on the hiss, turned out being something near the rca jacks was funky, dealt with the hiss by moving things around. I also have another issue after that one of channel imbalance, something to do with the loudness switch? when I disconnect the purple wire going to the volume control channels both are equal.

 

[dcgillespie]

Paul -- Thanks for the clarification. Using Jim's value for Screen Stability resistors would be a good move for stock late model 400s that use the higher impedance OPTs. In those receivers, it will raise distortion slightly due to the reduced regulation such a value will create, but it will also limit the screen grid dissipation which can be so dangerously high for the tubes in the later versions because of the higher load impedance used. Ultimately then, its a good trade off.

However, if you are going to install the EFB(tm) modification, note that screen stability resistors are not even required when that modification is installed, because besides providing EFB regulating action, the other thing that modification does is to correct the inherently wrong screen grid voltage found in the later versions, and in doing so, completely eliminates the need for any resistors at all. It really is the ultimate answer as with EFB, distortion is significantly reduced, power output is (in this case) maintained, tube life is significantly increased, and the output tubes are never subjected to overload conditions.

I hope this helps!

Dave

 

[streetwise]

Alright thanks Dave I will hold of on putting them in since the EFB project will be started next week hopefully, I doubt I will run the receiver enough to make it worth putting in the resistors for such a short while

 

[larryderouin]

ok I found them. turns out I didn't see the 2nd page. CRIPES! Those things gotta be HUGE! I think I'll take the risk and use the 100ohm 1/2w.