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Fisher Four-hundred Fotos?

Right, just like BKville's shown in post 15. The brn and org are the AC from the power transformer bias winding. They go to the AC IN lugs of the bridge (usually marked with a squiggle/tilde/sine wave on the bridge). The "-" terminal of the bridge (negative DC side) goes one of the 1000 uF cap sections -- remember the bias voltage is negative, so everything's sort of 'backwards' in the bias circuit. The "+"terminal of the bridge goes to that 15-ohm power resistor.
 
Are you taking voltage readings while powering up??? From the sounds of it...I would not be surprised if the bias filter cap is shorted. Voltage readings will (most likely anyways) tell some of the story.

I would not bother troubleshooting anything. Rebuild the unit, and then power it up. Most likely...the problem will be gone.

If not...you have ruled out a lot of potential culprits and this will make troubleshooting much easier. Although the later 400 (Serial # above 30,000) comes equipped with a silicon rectifier- does not mean that it is operating properly. I have seen enough bad silicon diodes to prove this.

These units are old enough that even if they do work in stock form...something usually goes wrong after using it for awhile.
 
Just a quick question while we're on the subjectof SS rectification. Would I be correct in assuming that on these older Fisher units the tubes get subjected to full B+ voltage before they warm up? Just curious, Jay :scratch2:
 
Fisher 400

Yes. Please post pics. I have one of these I bought at auction for $5.00 and that included a Garrard Lab A-70 turntable and AR speakers. I don't remember the model # of the speakers but they are a 2-way with an 8" and a tweeter. I got this in the .80s and I love it but all of the components need a complete going over. I am still using it in my family room to listen to my Dean Martin and Al Goodman records. Any recomendations and advise would be greatly appreciated. Thanks
 
The most straightforward replacement will be one that is the same lead style as the original which is still made. Mouser has one listed part #583-RS207L...which is a 1000V 2A part.

I use a BR-36 (1000V 3A) bridge which mounts via a hole and a bolt/nut in the chassis. In all other Fisher equipment this is very easy to do...since the selenium rectifier is mounted via two bolts (or one bolt in some cases). In the late model 400 I actually drill a hole and mount up the BR-36 bridge.

If you type in BR-36 in the mouser search page...you will be directed to a pdf of the catalog page which will show you line drawings of the different style rectifiers that are available.
 
jaymanaa said:
Just a quick question while we're on the subjectof SS rectification. Would I be correct in assuming that on these older Fisher units the tubes get subjected to full B+ voltage before they warm up? Just curious, Jay :scratch2:
Click to expand...

Yes you are correct. Some months ago found a nice schematic of a HV delay unit which I build and installed into my X202B. Works nicely, and should be good for any other Fisher, etc. Original schematic is here:
http://www.nervenet.info/HT_delay/htr214xp.html

BTW, I have no connection with the original author (also named Alex).

-Alex
 
The cap that the B+ diodes is connected to is a single section 200uf 250V (or 300V) can cap. This cap is NOT grounded...because it is wired in series with another 200uf section on another can cap. This was done because a 100uf 500V (or 600V) can cap was either to large in 1963 or to much money for the bean counters at Fisher Radio Corp. The above caps are the voltage doubler in the 400.

Both B+ diodes should be replaced. There are a lot of people who feel that silicon diodes do not go bad...and I also belonged to this school of thought until one actually went bad on me. Plus the whopping $.60 cents for two uprated diodes is worth the peace of mind to me.

It is a good idea to replace the voltage doubler caps in your 400. They are frequently fine...but after the bias filter caps...the voltage doubler caps are the most likely to fail in a Fisher receiver.

It is very easy to underwire two 220uf 250V caps in series (or larger voltage caps) in a 400.

As long as you are replacing parts in the power/bias supply...make sure to replace the two .01 600V bypass caps, and the other small parts in the power supply/bias like the 100uf electrolytic and the 15ohm 5W resistor.
 
Many months ago, when I first checked my X202B, I found that one of the voltage doubler caps (the one that is grounded) had gone short-circuit and took out (shorted) one of the silicon diodes, and toasted the transformer HV winding. Of course the 30Amp fuse that I found was in the fuse holder instead of the standard 3Amp was most probably the reason such extensive damage occurred. Looks like the person that was testing the amp at the time thought that if the 3amp fuse was going, better try with a 30-Amp one to try and cure the problem.

But even if you're running your amp with a "legal" 3-Amp fuse, it definitively pays to always replace the HV doubler caps as standard procedure, along with the dual 1000uF, and the 50uF bias supply filter caps that most certainly will be dried out because of the heat of the 15-ohm 5W resistor that sits just beneath them. To reduce the emitted heat, I replaced the original 15-ohm unit with two series wired 7.5-ohm 10W units, and mounted them away somewhat to the side of the caps as much as possible.

If you can, better replace all HV caps in the resistor divider string. If your amp still has the originals, these caps will be at least 40 years+ old. Specially check the last 20uF one, a brown paper envelope cap that sits by it's own bellow the chassis. In my amp this capacitor showed signs of venting, as there was a noticeable bulge with a small hole on the positive side cover cap.

-Alex

el-37 said:
The cap that the B+ diodes is connected to is a single section 200uf 250V (or 300V) can cap. This cap is NOT grounded...because it is wired in series with another 200uf section on another can cap. This was done because a 100uf 500V (or 600V) can cap was either to large in 1963 or to much money for the bean counters at Fisher Radio Corp. The above caps are the voltage doubler in the 400.

Both B+ diodes should be replaced. There are a lot of people who feel that silicon diodes do not go bad...and I also belonged to this school of thought until one actually went bad on me. Plus the whopping $.60 cents for two uprated diodes is worth the peace of mind to me.

It is a good idea to replace the voltage doubler caps in your 400. They are frequently fine...but after the bias filter caps...the voltage doubler caps are the most likely to fail in a Fisher receiver.

It is very easy to underwire two 220uf 250V caps in series (or larger voltage caps) in a 400.

As long as you are replacing parts in the power/bias supply...make sure to replace the two .01 600V bypass caps, and the other small parts in the power supply/bias like the 100uf electrolytic and the 15ohm 5W resistor.
Click to expand...
 
It is not uncommon to see streaked chassis on Fisher equipment...some of them have plating defects.

I do not use polish when cleaning the chassis...due to the fact that it will remove the lettering if your not careful. But if it works for you...all the better!!! Since there is no danger of losing any lettering in between the IF cans and the tube sockets...you can use pretty much any cleanser of your choice on a Q-Tip or a similar object. Just make sure to follow up with a water moisted Q-Tip to remove any cleaner residue so it will not discolor the plating after a while.
 
You shouldn't worry too much about the streaks. Right now you are looking at the amp with all the tubes and tube shields pulled. You can see broad expanses of shiny metal on the top and each imperfection catches your eye. Once you replace the tubes with the metal shields on them, the view will be broken up and it won't be as noticeable.

I am not being so agressive with the polishing of my X-1000, but I did get some parts to shine. Because I wasn't agressive, there are some stains around some of the lettering that I did not want to mess with but I think it will look fine when I get the tubes on. If I were to polish anything to an obnoxious shine, maybe it would be the tube shields. It can't hurt anything, and it would really draw your eye away from other imperfections.

--gh
 
The 1000V caps are fine.

Yes, you want the 220-ohm resistors to ground directly, else you'll get hummmm.

Measure the voltage drop across the resistor(s). The voltage drop in mV x 10 (ohms) will be the bias current in mA. Thanks to a little thing I like to call "Ohm's Law".
 
umm, seems like that's a little high; Craig (NOSValves) suggested 30 mA and you're at 32-38 mA. You should indeed probably change the resistor. The lower resistance will give you a more negative bias voltage, which is what you probably want. The absolute value of the voltage is higher with the lower resistor. The bias current will be determined by the tubes; but will depend on the applied bias voltage.

Mine were all a little different with four matched EH 7868's, but slightly closer together than your spread (ca. 10% variation between the 4).
 
Don't worry about the voltage you're measuring; it's a consequence of the bias current flowing in the tube, governed by Ohm's law. When the cathodes were directly grounded, "R" was too low for you to measure "I". With 10 ohms of R, you can measure E and calculate I. You want E in this case to be about 300 mV (i.e., a bias current of 30 mA). you are looking at the 'output', if you will. The 30 mA bias current you want will require a bias voltage applied to the tube of on the order of -19 or -20 V (the exact voltage would vary from tube to tube, and for the exact value of other components in the circuit). The Fisher design bias voltage was -17V, which gives higher bias current. Whether it sounds better is a matter of debate; but there's no doubt that the higher-biased tubes run hotter and probably have a shorter lifespan than they need to.

I am really not qualified to explain any more or better than the above. :-(
 
Dunno... you can check the 'true' capacitance of your new ones if you have an LCR meter. Are they hooked up correctly? Nothing touching anything it shouldn't be (sorry for the triple negative!)? Neat "lead dress" on the new caps? Long wires and protruding leads from solder joints can be antennas!
 
The only thing that springs to mind is that you might want to measure the bias voltage. You can pull the 7868's out of their sockets and put the receiver on a Variac to 'dial down' the B+ in the reciever's unloaded state. If you had "zero" bias voltage (i.e., a dead bias supply), you might have the kind of trouble you describe. I had this problem, transiently, on the Marantz 8B I re-habbed (due to a bad bias supply rectifier). Fortunately 1) I was using a variac and 2) the mains fuse popped, so I didnt' cook output tubes or transformers.

EDIT: oooh, I forgot the most important part! Check, double check and triple check all the work you did against: the schematic, your before photos, and check for inadvertant shorts, poor solder joints, stuff like that.

I do otherwise defer to experts.
 
When you changed the 100uf cap in the voltage doubler did you mistakenly ground the capacitor???

It is wired in series with the 200uf section on the can cap in mounted near the front of the unit.

Plate voltage is 405V the 365V is the grid #2 voltage.

You can measure the negative bias voltage at either pin 2 of the 7868's or you can measure it right after it comes out of the bias supply.

Double and Triple check your work...observe that you have polarity correct on the B+ diodes, and also observe if the electrolytics in the bias supply are installed correctly. I once installed the 100uf 25V cap backwards by mistake and had zero bias voltage....luckily I caught the problem immediatly before it cooked the output tubes.
 
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