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  1. drewbolce

    Dexoit'd my balance control, but it still acts up

    Deoxit is not the preferred lube for carbon tracks. Faderlube contains a small percentage of lubricant, Deoxit does not. Deoxit for switches, Faderlube for pots. If you cannot find Faderlube, depending on where you are located you could try adding a squirt of CRC "Electrical grade" lubricant...
  2. drewbolce

    Service Bulletin -- 800C

    Ok, on the 500c the 680k resistor goes to ground. This means that the "43355H Standalone" schematic is the correct one. Dave can you verify that your 800c has the 680k grounded? Of course Fisher moved this part of the circuit down by the power tubes on the 500c, just to make tracing it more...
  3. drewbolce

    Service Bulletin -- 800C

    Dave, don't get stuck on minor voltage differences. What we have here is a schematic error. I'll look at my 500c and see what the values are for each of the resistors.
  4. drewbolce

    Service Bulletin -- 800C

    R77 and R83 are swapped on the two schematics. I suspect one of them is wrong. I'll let Dave look into this further. Yes, there are errors on lots of Fisher schematics.
  5. drewbolce

    Fisher 800C question

    There are an enormous amount of coupling caps in these old Fishers. Somewhere around 30. You never see a unit with all of them replaced and none of the kits get near replacing them all. That being said, all refurbishers pick the ones they replace and what they replace them with based on...
  6. drewbolce

    Service Bulletin -- 800C

    I always thought that the small electrolytic C65 was the source of trouble in this area. It should always be replaced anyway, along with C67. If you have (not you, Dave) touched C66 (the only polystyrene) with your soldering iron this whole circuit will give strange and wonderful results. You...
  7. drewbolce

    Fisher 800C question

    At the risk of actually answering the original post, I would suspect the power filter caps. I would run the unit for a good period of time to "reform" the power caps and then reassess the problem. After a week or so post back here and we can help you with the other potential problem areas...
  8. drewbolce

    Help diagnosing Fisher 500C issue

    I'm not sure where Dave got the idea that you have JJ 7591's, but if that is the case this is a pretty common failure story with them. I have run them for hours without issue only to come back into the room and find a dead channel with an arc'd out tube. My experience and advice is just not to...
  9. drewbolce

    My Fisher 400 Restoration

    This is one of the key items that shows the 400 (later serial numbers) was the last in the series. I have never seen a 500b/c or 800b/c with the silicon rectifier. I wonder what other Fisher's had it?
  10. drewbolce

    Fisher 800c question

    The components in the stereo bulb circuit are prone to failure in all of the units that use it. This includes the 500c and 800c. Failure can come from the MPX failing to produce stereo (no 38k) or the relay circuit. Tune a strong stereo signal like a PBS station. Do you hear stereo when...
  11. drewbolce

    Converting Fisher 400 tape input to AUX - point me to the thread?

    Here you go, http://audiokarma.org/forums/showpost.php?p=6940765&postcount=165 This is David Gillespie's idea. I have not had the time to try it on my Fisher. I will be interested to hear how it goes.
  12. drewbolce

    Slight hum from 500C.

    Before you start playing with the tubes eliminate other connections as the source. Isolate the 500c by unplugging all connections other than the power. Turn on the aux input and see if there is a change in the hum. I once had an antenna connection create a low level hum such as you describe...
  13. drewbolce

    Slight hum from 500C.

    Assuming it still has the original power cord, first flip the plug in the outlet and see if the hum changes in any way.
  14. drewbolce

    Wiring size for IBAM board?

    Your wire size is fine. There is very little current in the circuit. 35ma is several orders of magnitude less than 7000ma (7 amps). Here is a link to my proposed IBAM startup procedure. http://audiokarma.org/forums/showpost.php?p=6504026&postcount=19 The main danger in a fresh IBAM...
  15. drewbolce

    Fisher 400 Individual Bias

    Larry's schematic reads a little better but a ground is a ground. The 0.1uf film cap is not needed and would not be a good idea. It would be mixing different bias designs and the results would be unpredictable. The bias design here is chosen using observed results and represents Fishers final...
  16. drewbolce

    Fisher 400 Individual Bias

    Those are removed. They are the original voltage divider circuit. They are R34/35 on the schematic I referenced in the IBAM diagram.
  17. drewbolce

    Fisher 400 Individual Bias

    I strongly suggest you do not leave and/or add a trimmer pot. The existing design has a built in safety to prevent red plating tubes. Adding a trimmer pot will interfere with or negate this important feature.
  18. drewbolce

    Fisher 800b reduced vol on MPX

    BTW, the highest voltage spec on the various designs is 202v at that location. At 115 volts and 20% tolerance that would be a high of 242 volts. However with 125 volts (modern power) it would be 263 volts. 280 volts is a little high, but nothing I would worry about. Irrespective of this the...
  19. drewbolce

    Fisher 800b reduced vol on MPX

    The MPX design varies slightly from model to model. It is 0.022uf on some schematics. Likewise R22, 223 can be 22k or 27k. On some V100 is a 12at7. Likewise power supply voltages vary. Just verify that is what you have on yours. I think my point on C210 is perhaps being missed. I upgrade this...
  20. drewbolce

    Fisher 800b reduced vol on MPX

    A 1uf 250v film cap will work in this location, a 50v will, of course, probably be toast. The 350v rating was for an electrolytic cap. Smaller 1uf 250v film caps were just too expensive at the time this was in production. The electrolytic in this location was another of several Fisher cost...
  21. drewbolce

    Fisher 800b reduced vol on MPX

    Ditto, I have used Sprague Orange Drops. If I remember correctly they are a real pita to replace. I believe I had to unscrew the assembly and push it part way out of the chassis for access. Risky maneuver at best, not for the faint of heart.
  22. drewbolce

    Fisher 800b reduced vol on MPX

    Reading your signature reminds me to point out that I am a Mechanical Designer from Oak Ridge, TN. Civil Engineers should be very, very afraid.
  23. drewbolce

    Fisher 800b reduced vol on MPX

    The de-emphasis fix is for the 87us (from my faulty memory, it is somewhere around that) that the Fisher circuit is designed for. Believe or not in the day this was within spec. The Fisherdoc's removal of the 38khz residual filter was to make the thing exact, at the expense of becoming a...
  24. drewbolce

    Fisher 800b reduced vol on MPX

    I strongly suggest you do not do the filter mod. It is there to remove the residual 38khz. A failed inductor destroyed a tweeter on my khorns. While most speakers are not this sensitive or delicate it can be very expensive to find out that yours is. This 38khz hash can be easily seen on a...
  25. drewbolce

    Fisher 400 Miller effect, another idea.

    Amazing, the tube I have been looking for with this feature is the 12dw7. They seem to be identical but I haven't seen them listed as subs. Are they?
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