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Fisher 400 Power Supply Refresh

audiodon said:
Sony6060,
I see that the top 680uf is on the far side of the choke in your correction. Hopefully I have that correct.

Are that 680uf cap and the 10uf ASC cap tapping off the same spot and terminating at the same spot? Is the 10uf acting as some kind of bypass cap? I don't understand that part. Can you help please?

The top 680uf appears to be replacing C56 in your schematic? The bottom 680uf cap appears to be replacing C57c.

Then you're using a 30/30/30/30 can for C52A and C52B with each being a 60uf combo of two 30uf sections paralleled?
I'm just trying to triangulate your posts on the other page with this schematic to make sure I understand this. I'd like to try it.

Also, if I understand it correctly, you're substituting a 30/30/30/30 for C52A-C52D because the upstream ripple is now far less.

That cap that you changed to a ceramic cap before diode CR3 - what is its circuit function? I thought that was the death cap. If not, what is is?
Click to expand...

I do not know how to express it different than as shown in schematic. Later version is correct. First version had wire going to 7868 audio transformers on wrong side of choke.

30/30/30/30uF was used because 40/40/40/20 was not available. I had no AC hum concerns because less AC input was being applied to the new filter with less capacitance thanks to the 680uF first caps, choke, then 10uF filter sections. The 10uF can also go to the ground vs top of lower cap. Either is ok.

CR3 is simply a transcient capacitor that shunts short term noise and high voltage spikes to ground vs thru the diodes. This adds some diode protection. The old cap may be subject to failure due to age so I changed it to new ceramic cap. I also believe the stock 600 volt rating a little on the low side, thus the 1000 volt rating of the new capacitor. Death caps are on the AC line, not B+. I removed the death cap and high value resistor from the AC line to chassis, but ran out of time to install 3-wire cord. If a two wire AC line design is present, the death cap should be installed from the neutral side of the AC power line to chassis. Problem is non-polarized plugs can not guarantee which side is neutral. Thus, the three wire cord is necessary with death cap removed. The death cap can reduce noise in high gain preamp sections on two wire AC systems. I'll perform the grounding type power cord later as I have wood floor presenting very little of a shock hazard. A concrete floor with bare feet is a hazard.
 
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Sony

Great job done onto your fisher 400.!!!!

Could you please inform the choke where is installed?, I just got from mouser the parts for do similar Job like you but without replace the cans. I'll put the new caps in the bottom of the chassis

Regards
 
eduarsan1 said:
Sony

Great job done onto your fisher 400.!!!!

Could you please inform the choke where is installed?, I just got from mouser the parts for do similar Job like you but without replace the cans. I'll put the new caps in the bottom of the chassis

Regards
Click to expand...

Choke located where the original bias dual capacitor was located. I removed the capacitor clamp and replaced the bias capacitor with pair of 4700uF snap-in type caps. See pictures I posted in original post.
 
Hi guys I just started my restoration project on my 400 amp where I decide to do also PS upgrade using similar components like Sony did, I included the Brigde rectifier as well

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This is the original rectifier section
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Original Second stage of the power supply where I suspect is becoming the heating noticed by sony in this thread (the resistors by convection are dissipating their heat for the can cap leads).
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Original 3rd stage of the power supply
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Instalation of Solen Fast Metallized Polypropylene Capacitor in the rectifier and AC area
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Second step is install the 680uF cap in the original paper can cap
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This is the result of the 680uF on to the can

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The next picture shows the 680uF cap, the thermisters, the 10uF Solen Fast Metallized Polypropylene Capacitor and the UF3007 diodes in the rectifier stage.
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Next one shows the another 680uF cap,the second power supply stage (30/30/30/30 cap), the choke installed, the 4700`s uF cap, the 100uF cap, the small SPDT relay and the new bridge rectifier.
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And finally the 3rd power supply stage (the another 30/30/30/30 cap), where the 330 ohm resistors and the 27 ohms resistor were moved to get more space for the unity gain buffer designed by dave.

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Results:

What can I say:

No hum detected and sonics is very good even in high volume where no noise was heard.

I still investigating the heating on to the can cap of the second stage mentioned upper in this thread.


Regards to AK community!!!!:music:
 
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Luis -- If I may, I would suggest that since you are using series connected diodes to (apparently) obtain the necessary PIV rating, you consider placing a small resistor and cap across each diode to help equalize the DC and transient voltage elements across each string that occur during the cut-off cycle. These components would typically have values of .01 uF for the caps, and about 220K ohms for the resistors. With these components added, cut-off value voltages will then be spread equally across the diode string, so that no one diode can have its PIV rating exceeded when the diodes are not conducting.

Also, if any of the increased capacitance installed appears at the output tube screen B+ supply point, please make sure that you have added the screen stability resistors at each output tube, to prevent any tendency towards output arcing when the original capacitance value is increased.

Finally, the 1.2K 1 watt resistor, and even the 2.2K 1 watt resistor at the second stage filter can are dissipating virtually at or near at their rated dissipation value. You cannot reduce the absolute dissipation element of course, but you can install higher rated components here to bring down the temps these components operate at.

The greater supply capacitance plan that Sony developed will add immeasurably to the transient performance of the power supply in the 400. Nice job on your install!

Dave
 
Two schools of thought regarding series diodes. Back years ago it was standard practice to add a .01uF & series resistor across each series diode to slow down the fastest diodes. This kept the reverse PIV more equal across each diode in the series string.

New school of thought is the new diodes such as UF4007 have a better controlled breakdown characteristic and do not require this shunt network. Some say over time the shunt network may drift further from one another causing other issues.

The second UF4007 in the Fisher power supply upgrade is so conservative design wise that it was placed with one diode possibly failing with an unusually rare line spike and continue performing with the other diode that survived.

Your thoughts Dave?
 
Dave

Your advice always be received well, not everybody has wrote articles in a magazine!!!

-The resistor and cap across each diode I suppose is in series right??, but is necessary to do this in a ultra fast diode rectifier where the PIV response is more effective??

-This week will be done the job on to the 100ohm screen resistors due to I was asking to larry and others (The IBAM experts) how to do it and install on to the amp, using a common bus wire.

-And finally regarding the heating in the second stage cap, also I detect is coming from the power transformer, so I´ll try to investigate "how to defeat this issue" and also follow your sugestion to increase the power rating on the resistors.

Once again thanks for your advice!!!:thmbsp:
 
If I may insert this. Why have any heat? I use this quiet fan that I can not hear to cool the entire chassis. The case only get slightly warm vs hot without the fan. The switched 120 volt AC plug is already on the Fisher chassis. The fan is installed on vintage Fisher feet with a sorbothane sticky dot on bottom of feet to increase isolation from the wood case.
 

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Orion Fans model number OA80AP-11-3WB. The sorbothane feet are needed to completely remove transmitted noise to wood cabinet. These fans are very quiet anyways.
 
Sony -- The network approach did certainly act to slow all the diodes within a string to the lowest common denominator with regards to switching speed, and it is also true that conformity of performance in nearly all SS devices manufactured today is far superior to that of yesterday. This would certainly act to reduce or eliminate the need of such equalizing networks for diodes of any speed class.

If the second diode is for overkill only, then I agree that no network would really be required -- unless you want to overkill the overkill design. My comments were based on the possibility that both diodes were necessary to achieve a PIV rating of normal safety factor -- where I would personally use the networks across diodes of any speed rating if that were the case. Since that is not the case in this instance, then the networks are really not required in this application. Thanks for filling me in on your design objectives!

Dave
 
Ok Dave. All of us like to refer to your knowledge base that you are so willing to share. It is extensive.
 
I was following along this conversation but the schematic referred to above is no longer showing up. Does anyone have a copy they can repost to this thread?
 
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