Improving the Fisher 400

Hi Robert -- If by modifying the "amp" section you mean strictly the power amplifier section, and further wish to use that section with other preamp units, then your first order of business (for any version) is to convert the old reverb jacks into Preamp Output/Power Amp Input jacks. Doing just this modification will then allow you to externally break the connection between the internal preamp and power amp sections of the 400, and then use a different external preamp to drive the 400's power amp section. Because the 400's internal power amplifiers are sensitive to drive impedance, your external preamp will need to display a low output impedance -- but this would be a characteristic of any well designed self contained preamplifier unit.

This modification would not even require you to install the small additional buffer stage, as that stage is technically a part of the internal preamp section, designed to make its output impedance uniformly low regardless of the volume control setting. By not installing the buffer stage, the 400's internal preamp is very much disadvantaged over that of a good external unit, and should not be used to drive any other external power amplifier, as the high, variable output impedance of the stock internal preamp design would produce very unfavorable results from the capacitance that the interconnect cables would represent, let alone the effects that a high drive impedance might have on an external power amp you might use. When the buffer circuit is added in addition to converting the reverb jacks to Pre Out/Power In jacks, not only does it improve the performance of self contained operation of the 400 (by providing a low impedance drive to the 400's power section), but also then lets the 400 properly interact with any external power amplifier or well designed preamp combinations you might wish to use by way of the converted jacks.

At this point, the EFB modification, as well as the NFB/stability circuit modifications is OPT specific, although I hope to provide alternate EFB values for use with the early version transformers in the near future.

If you are contemplating lowering the output stage grid return resistor values, then I would strongly suggest installing the modification I provided for the phase inverter stage (and specifically, just the phase inverter stage itself), and this stage is already challenged to provide adequate drive with the existing grid return resistor values used. The revised phase inverter stage will have no problems providing plenty of low distortion drive to the output stage even with lower value grid return resistors installed.

Good luck with your project!

Dave

Hi Dave, the power amplifier section was exactly what I wanted to do the modifications on and I already figured out it was OPT specific from reading
your previous post, is there any measurement I can do myself on my early transformers and post the results, I have a digital multimeter and I do not
have any problems with taking one of the OPT out for measurements or testing.

I also have the early service manual in PDF format and it has the early schematic for the 400.

When I looked at the modified schematic I also figured out the additional buffer stage would not need to be installed for what I am trying to do but
thanks for clarifying that.

I will absolutely convert the reverb jacks to Pre Out/Power In jacks, that's a simple mod and a good one.

I will study more on the phase inverter stage modification.

Robert

The application of EFB and installation of the NFB/stability circuit changes is very OPT specific. AudioDon was nice enough to send me an early version 400 OPT that I measured for impedance and power characteristics on, which will allow me to develop the changes required to the EFB circuit for use with the early transformer. Actually, that is sort of a low level project running along with but behind the scenes of the Fisher X-1000 integrated amplifier that I am currently rebuilding.

Unfortunately however, to develop any potentially improved NFB changes to the early 400 units would require having an actual early version 400 receiver to not only make tests on in stock form, but then develop any improved implementation on as well. I'm not opposed to doing that in the future at some point, but right now, the work bench is sagging as it is with all the work on it. Gotta get some of it cleared off!

Dave

I definitely see how involved this modifying is and your right you would need a whole early 400 to work with, if you come across one and find the time
to do that in the future that would be great.

I do have another Fisher 400 parts unit in storage and hopefully I can get out to where it is stored at in the next few weeks and maybe I will get lucky
and it's a later model 400 that I can use to install those modifications on.

Robert

Hello guys

Due to I read regarding the power switch is almost finished the stock where the thermisters are no enough to keep it usable, I decided in my restoration job to add a small SPDT relay to manage the current to the PT, and in that way minimize the current going thru the power switch.

So I add this relay in the drawings (Rev6).

This revision contains also the PS upgrade by Sony and the IBAM where I decide to include in my amplifier restoration as well.

Regards

Luis

The power/volume control has two problems. The arcing and eventual wearing out of the on/off switch is only one of them.

Every time you turn on the unit you are swiping the carbon traces. Over time they will wear unevenly and create differences in the channel volume. This wear is almost never linear and will be different at the various volume levels. It is almost always in the lower third of the control rotation.

After 40+ years of this wear almost all of them will have the problem. With sensitive speakers and low listening volumes it can be very annoying.

Here is a post I did for accurately testing the volume control using the headphone jack, a computer sound card, some free software and a few cables/adapters.

http://audiokarma.org/forums/showpost.php?p=5117229&postcount=49

My solutions was to add a Alps pot and move the on/off switch to the headphone jack location (on my 500c). I have found the pictures and will publish them in a separate thread (it is off topic and this one has gotten too long anyway),

Looks like I will have a project for the winter months, I went to my storage over the weekend and dug out my other Fisher 400 that I had gotten for parts
and was sold as is (nonworking), luckily I find it's a later model serial NO. 57534 V, after looking it over it apperas to be in better shape then I thought,
some pictures below.

I had already taken out the volume pot to use on my other early fisher 400 years ago, the pot is not the greatest and the tracking is off at low volumes but
it works and I will likely leave it in the other 400 for now, after reading what drewbolce has done with his volume control in his thread on Alps volume control
mod I will think about what I am going to do with that later.

I also took out the German ERL FOL II caps out years ago and had tested them and still have them stashed away, they did check out good although I may use
something else to replace them with.

All the tubes had already been removed before I got it and I think I have enough tubes from other Fisher 400s that I can use to get it going, I do know that
the FM tuner will need to be realigned but that can be done later down the road, the power supply will get rebuilt first.

The face plate has a few scratches and a couple of nicks and I'm sure it will clean up just fine not mint but good enough, the top of the chassis will need
a lot of cleaning and hopefully will look a lot better then it does now, surprisingly under the chassis looked really good and cleaned up very nicely with
just a blast of air.

Everything seems to be intact and in decent shape, I do have the knobs but they are missing the brass pieces, I will borrow the knobs off my other 400 for now.
The only other thing I noticed is that the loudness switch is broken although I do have the plastic part that broke off and just maybe it can be glued back on
and then again I may not even use the loudness circuit at all if I alter it more like what drewbolce has done to his 400.

Overall I think it will make a good candidate to modify from all this great info in this thread and a few other recent 400 mod threads.

I'm following this thread with great interest! So, for those of us not so engineering inclined (meaning me mostly ) would you mind documenting your restore as much as conveniently possible? I've been itching to do a 400 for a while and it'll help those who are like minded, I'm sure :thmbsp:

I got the EFB circuit completed so now I have to wire it up to the rest, figure out exactly how to swap over the tape head to a line level. I can't quite visualize how to move the 82k resistor. I'll need to reread the instructions a few times while both looking at the schematic and the unit itself. That should do it unless I made a mistake somewhere which is possible. There was a good amount of stuff to change. I'm worried about a ground loop with the extra cabling and buffer circuit though.

Btw, does anyone know what size/type of screw to buy that attaches the feet and will be long enough to use with the wood cabinet? I'm missing all four feet etc.

Thanks,
Dan

I figured out the screws. 8-32 x 1". If I use the wood case I may benefit from em being a bit longer. Speaking of wood case do you guys have any opinions on to use or not use it?

Dan

Danny -- Regarding possible ground loops, I used the ground from each area that the Preamp Output/Power Amp Inputs serve, and had no problems what so ever with ground loops. Therefore, it is a ground over near the input to the power amps that provides the ground for the Power Amp Input jack, and a ground from the volume control/buffer circuit that provides the ground for the Preamp Output Jack. When finished, my 400 produces no discernible noise in my Cornwalls at all with the volume at minimum.

It sounds like you're making great progress with it. Keep us posted!

Dave

Dave, okay. So you used the ground from power amp jacks to ground the buffer circuit only? I had the cable grounded like that but at the other end it was grounded to the lug where the two large sand cast resistors originally went by the volume pot. I have the moved tape loop cable grounded at both ends also as maybe some others. are there any cables that you only used a ground at only one end?

Thanks for the encouragement/help.

Dan

Danny -- To be clear, it will help to understand that ideally:

1. When you are applying an external signal to drive the power amplifiers, it should be applied between the input grid circuit, and the point where the cathode resistor of the input stage of the power amp is grounded. Therefore, on the power amplifier end of the shielded cable, the hot lead would attach to (if you are using my power amplifier modification) the point where the 10K input resistor and 1 meg grid return resistor meet. The ground lead of the cable would attach to where the 220 ohm resistor is grounded to the chassis. Also, the new 1 meg grid return resistor would ground there as well.

At the jack end of the cable, the hot lead goes to the center pin of the Power Amplifier Input Jack, and the ground lead connects to the ground terminal, which is otherwise isolated from the chassis by its mounting.

2. For the Preamp Output Jack, on the jack end, the shielded cable provides the ground connection for the jack -- just as it does with the Power Amp In jack as described above, while on the buffer end of the cable, the ground for the buffer, volume control, and cable to the Preamp Output jack is now made at the ground lug where the 25 ohm load resistors used to connect to, near the back of the volume control.

IIRC, the actual input jacks are grounded at their installation point, as are the ground leads of all the shielded cables connected to them there. On the other end, the ground are not used -- which I believe is as stock, but I would want to check this to be sure. I want to pull my 400 out next week to do some work on the cabinet, so I will check then.

I hope this helps!

Dave

notdigital said:

I'm following this thread with great interest! So, for those of us not so engineering inclined (meaning me mostly ) would you mind documenting your restore as much as conveniently possible? I've been itching to do a 400 for a while and it'll help those who are like minded, I'm sure :thmbsp:

Click to expand...

It will be awhile before I even get involved in restoring that 400, most likely during the winter months.

They're are some other very good threads here on AK on restoring the 400 with pictures too, if you search from google you can find links for them.

Right now I too will be following several threads in the Fisher forum to help me decide exactly what I will be doing to my unit, when I do begin the
work and post info and pics of my progress it will definitely be in a new thread so I don't disrupt Dave's thread.

Dave, thanks for the extra clarification. I have it wrong. I assumed that the jacks shared grounding between the in and outs. I'm going to have to desolder some grounds to figure out what is connected to what rca there.

Dan

Dave,

I"m lookin' at your 'revised' Fisher 400 power amp schematic, and I'm seeing the 10K resistor to the input grid on the first section of the 12AX7.....connecting directly to what I gather to be the RCA jack. But, no 1 Meg resistor on that diagram. Did you add it subsequently......or is it there on the stock model and you deleted it in your 'revised' version? Like I said, I don't have a 'stock' Fisher 400 schematic to work with. TSD

Tom -- check in a later post, where I added the grid return resistor. I inadvertently left it out on the first schematic I posted -- but not in the actual unit. I caught the error, and corrected it some posts later, providing a revised schematic.

Dave

I'm glad the 1meg resistor was brought up. I also didn't have it installed. It may be missing from Luis's schematics.

Okay. My unit is up and going. I only have to adjust the ac balance.

Dave, I noticed that I have to set the volume control higher than stock to get to the same level of output. It seems like it's a good amount less actually. Just wondering if I have to track down a issue or not.

I guess the next step should tell if I can't get the right wattage at the outputs to adjust the ac.

Update it may not actually be "a lot" but it seems less. I'll have to adjust the ac and hook it up to my normal set of speakers to be absolutely sure.

It's always something right? Sounds great so far btw.

Thanks
Dan

Okay, I have the dummy loads connected. While playing a 500mv 1khz tone and adjusting the volume so that I'm getting 12vrms at the speaker outputs I can't get the AC ballance zeroed between output pairs. It seems to give me 6-7 mv. At least I hope it was mv and not volts. I've looked for a error but I can't find one in the power amp selection. I've looked over the phase inverters like 10 plus times to be sure.

I'm hoping it's a flaw in the way that I'm trying to adjust things moreso than the amp itself.

Dan