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New to me Fisher 400!

Whaleman

Whaleman

Hi-Fi Rescue Mission
Just acquired a Fisher 400 and I am quite pleased with the sound. I have a bit of a weaker left channel, but beyond the sound is crystal clear. The unit came with a hefty invoice for service, but I am not sure what was replaced other that a couple of tubes and a can capacitor.

In the underside it appears all original? I am considering a rebuild kit from metalbone?

Any help or comments appreciated.

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I don't know who metalbone is but why not just order your parts from digikey, mouser, and/or AES yourself? I'd figure that would be cheaper and you can make sure that you get the quality parts that you want. Are they offering recommendations/instructions for modifying the circuit or..?

That 400 looks nice and clean.
 
mhardy6647 said:
http://www.audiokarma.org/forums/showthread.php?t=78700
Click to expand...

Thanks Mhardy, spent a couple of hours going through the thread that took on a flavor of a forum. I think your posts were of most value, but your pictures tapered off later. I'd hope to see more. Nevertheless, it is a great start for me. Thanks a bunch for offering the information.

Does anyone have a parts list?

  1. I found the rectifier bridge from mouser: 583-BR36
  2. I can find the Orange Drops at AES
  3. The two diodes I may have from Radio Shack, are they 1N4007?
  4. The electrolytics are easy to find, especially the dual section underneath
  5. Anything else?

As far as adding the bias resistors, do you have better / closeup pictures of them? If all goes well, I would post a parts list and photos of the work. I have a good camera in my shop.
 
It looks like the bare minimum was done to keep it running by replacing what may have been the selenium rectifier (some were factory silicone). Read up on the restoration threads. That lytic in the MPX should also be replaced. That's a nice unit. Once up to specs and aligned these are incredible receivers and they are fairly easy to work on.
 
Here are my orders for rebuilding the Fisher 400. I decided to break out the big multisection cans except for the 4 multisection power one that has already been replaced. These orders are from www.mouser.com and www.tubesandmore.com and are close or exact matches. They include the rectifier bridge, all electrolytics, mylars, and moded capacitors.

2 - 1N4007 diodes from Radio Shack
4 - 10 ohm 1/2 watt carbon film resistors from Parts Express

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Per Gordons Recommendation:
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The underside appears all original, except for the bridge rectifier being replaced with discrete diodes. No sign of any cap replacement at all, especially comparing the "quality" of the solder work done on the diodes vs. the high-voltage section can cap.

I wonder what the hefty service invoice was for? Unless they replaced all the knobs that is, they are beautiful!
 
BrassTeacher said:
The underside appears all original, except for the bridge rectifier being replaced with discrete diodes. No sign of any cap replacement at all, especially comparing the "quality" of the solder work done on the diodes vs. the high-voltage section can cap.

I wonder what the hefty service invoice was for? Unless they replaced all the knobs that is, they are beautiful!
Click to expand...

As far as I can tell from the invoice, 2 old Fisher tubes, 1 NOS cap (the high voltage 4 section can towards the front), and some other illegible part. The diodes were done already, not very nicely. The knobs were recapped by the previous owner. Given that, I should be able to pay myself at least 8 bills for my work. :D
 
Whaleman said:
As far as I can tell from the invoice, 2 old Fisher tubes, 1 NOS cap (the high voltage 4 section can towards the front), and some other illegible part. The diodes were done already, not very nicely. The knobs were recapped by the previous owner. Given that, I should be able to pay myself at least 8 bills for my work. :D
Click to expand...

Are you talking about the tubular cardboard cap? That doesn't look very NOS to me, almost like it was pulled from another piece of equipment.
 
fsjonsey said:
Are you talking about the tubular cardboard cap? That doesn't look very NOS to me, almost like it was pulled from another piece of equipment.
Click to expand...

No, the can behind the tuning eye tube towards the front of the unit, but you got me thinking, it could have just come from a parts unit.
 
Whaleman said:
Here are my orders for rebuilding the Fisher 400. I decided to break out the big multisection cans except for the 4 multisection power one that has already been replaced. These orders are from www.mouser.com and www.tubesandmore.com and are close or exact matches. They include the rectifier bridge, all electrolytics, mylars, and moded capacitors.
Click to expand...

Two problems I see with your parts list. One of them is potentially SERIOUS! :nono:

The SERIOUS problem: those .047 caps (for the coupling between the 12AX7 drivers and the 7868 output tubes) NEED TO BE 630V. NOT 400V. At least the two that already ARE 630V in the original, in any case. I like to replace ALL of the with EXACTLY THE SAME 630v caps... can't hurt to have them exactly matched.

The second issue- which seems to crop up more now than ever- is the need to UPGRADE the output tube bias supply, to keep output tubes from "running away" and turning red ("red-plating", a type of severe overheating). If you're replacing the coupling caps, I'd upgrade them to .1uf (instead of .047uf) anyway. This will allow you to use the bias voltage upgrade that I've been recommending (and was recommended to me by some VERY ASTUTE people, such as Jim McShane and Tom Bavis!). In addition to the .1uf 630v coupling caps (4 of those), you will also need 4 220K ohm resistors (I prefer using 1 watt, for stability and safety), and an 820 ohm 1 watt resistor. The 4 220K ohm resistors will REPLACE the 330K ohm resistors that connect to the output legs of the coupling caps (were .047, now are .1uf) and to the bias supply wires- one for each output tube. The 820 ohm resistor will replace the 1K ohm resistor that is connected to the big cap in the bias supply (between it and the smaller 100uf cap, IIRC). This will increase bias voltage from about 15-17 volts, to between 18 and 20 volts negative. Enough safety margin, to use ANY decent 7868 tubes, INCLUDING modern replacements.

Also, I'd replace that big bias supply cap, and the 100uf secondary bias supply cap, ASAP. If those go bad, the outputs will melt down. I'm not kidding. Seen it happen. Don't want to blow up those output transformers!!
 
Thanks for the advise Gordon, will follow it. Thought I was done ordering parts :drool:

Yes those 630 V did baffle me since the schematic calls for 400 V and there were 630 V caps stuck in there. Might as well go with the .1 uF @ 600 Orange Drops for all four.

Also, I am replacing all eletrolytics, I think that is what you meant by that 100 uF bias cap? It is the Nichicon in second list.

Will order the resistors you mention too, AES has them and will replace that four section can while I am it. I suspect the service person just pulled it off a parts unit to put in here, but can't be sure. It has the pressed on lettering which I've not seen in new caps.

The 1K resistor you mention should be changed to 820 ohms, it is R35 in the attached schematic?
 

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Hi I think he is talking about R106,107,108,109 = 820 ohms R98, 99, 100, 101 =220k ohms and C62,63,64,65 = 0.1uf
I am restoring my 400 also.

Would like to know what exactly dose this modification do ???

I have read other thread where one of the more important safety upgrade is to add a 10 ohm 1/4 to 1/2 watt resistor on output tubes pin #3 to ground this serves as a fuse also it the tube shorts saving other parts and to use this to measure bais current wich should be 35mA max with 5% tolerance between all output tubes. Hence Bias Voltage will need to be adjusted to get this.

I also thinkt he Bais voltage can be adjusted somewhat with R102 the original is 5.6k ohm and changed mine to 2.7k ohm. Please check other threads on this as I am a newbe and have killed on tube already. :tears:
 
Whaleman said:
The 1K resistor you mention should be changed to 820 ohms, it is R35 in the attached schematic?
Click to expand...

Yep! That's it.

bowtieman427 said:
Hi I think he is talking about R106,107,108,109 = 820 ohms R98, 99, 100, 101 =220k ohms and C62,63,64,65 = 0.1uf
I am restoring my 400 also.

Would like to know what exactly dose this modification do ???
Click to expand...

Actually, those 1K resistors to the 7868 sockets (otherwise known as "grid-stop" resistors- they keep the output tubes from oscillating uncontrollably) need to STAY at 1K. Changing the 330K resistors to 220K is what you want to do.

What this does, is to make sure that there's enough current available for the grid bias on each tube. 330K doesn't control the bias voltage to each tube well enough... it's actually OUTSIDE the recommended range for a 7868 or 7591 (300K max!). The 220K is fine, to keep things in spec, no matter what tube vendor you use.

bowtieman427 said:
I have read other thread where one of the more important safety upgrade is to add a 10 ohm 1/4 to 1/2 watt resistor on output tubes pin #3 to ground this serves as a fuse also it the tube shorts saving other parts and to use this to measure bais current wich should be 35mA max with 5% tolerance between all output tubes. Hence Bias Voltage will need to be adjusted to get this.
Click to expand...

That's always a good idea, in theory. I haven't done that to the one here (was keeping it as close to original as possible), but if you're building one up as a KEEPER, then yes... that'd be a good idea!

Regards,
Gordon.
 
Thanks for the input gordon. I am looking at an early 400 schematic so the numbers do not lign up with the previously posted schematic. In the early 400 there is a 10k ohm drop down in parrall with the 100uf cap vs a 2.2k ohm and the 1k ohm you mention droping to a 820 ohm will not work on the early version as it has an origianl 5.6k. I changed the 5.6k to a 2.7k and have also replaced the 100uf cap also.

Won't the .1uf coupling caps filter out more low frequencies in the output amplifier stage ??

The 10 ohm resistor from pin 3 to ground is a very good move. :yes: Good insurance It can be used to measure the bias current using ohms law. I = V/R. This should be idle 35mA max.
As far as keeping it original just like cars once you start messing with stuff it is not longer original. At least with these you are only messing with what's under the hood.

Have you completed the coupling mods ??? WHAT output tubes are you using ? Currently I have originals but will be replacing them. I am looking at EH.
 
bowtieman427 said:
Won't the .1uf coupling caps filter out more low frequencies in the output amplifier stage ??
Click to expand...

My HH Scott amps use .1 uF 400 volt coupling caps, I can't tell any bass is missing but this is a interesting question for Gordon / the experts.

bowtieman427 said:
Have you completed the coupling mods ??? WHAT output tubes are you using ? Currently I have originals but will be replacing them. I am looking at EH.
Click to expand...

Haven't done anything yet except order parts. The amp is playing nice now without a single issue. I know I risk self destruction and thermonuclear meltdown but I figure when the guy serviced it, at least he caught the obvious. I am using 100% stock tubes, branded Fisher outputs. Say if you rid yourself of the remaining good original output tubes, PM me, I may be interested in a spare Fisher. I have 9 Electro Harmonix output tubes for the HH Scott. They are nice.

BTW, I ordered the parts Gordon recommended. I did not find a close match to the 20/40/40/40 uF multisection can, so I am going to assume that the person who serviced the unit actually put something worth what the owner paid. The unit is absolutely free of any hum
 
bowtieman427 said:
Won't the .1uf coupling caps filter out more low frequencies in the output amplifier stage ??
Click to expand...

Actually, the opposite. The bigger cap, since it's inline with the signal, will pass MORE low frequencies. This will be mitigated a bit by the lower-value bias resistors (which will "sap off" a bit more), but the end-result will be SLIGHTLY more low-bass extension "bandwidth".

bowtieman427 said:
Have you completed the coupling mods ??? WHAT output tubes are you using ? Currently I have originals but will be replacing them. I am looking at EH.
Click to expand...

The EH tubes worked great here, once the new resistors and caps were in place!

Regards,
Gordon.
 
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