fisher doc changes - good ideas?

[madwing]

i've been doing a variety of things to get ready for my coming fisher 400, including reading all i can about refurbing it.

i found the attached document on this site: http://www.hhhyde.com/music/fisher.html called "fisher doc restore instructions". i made a .pdf out of it for convenience.

i read through it, and laid it against the information actually found on that site, and most of it matches (hhhyde seems to have gone higher on voltage values, while keeping capacitance values on point). however, there are some things the fisher doc recommends removing altogether, or changing the value of from original, that i am curious to hear peoples' opinions on:

• removing resistor r94 (bias supply shunting resistor) to bring up the negative bias voltage to -20 to -24;
• removing l102 and l103 from the multiplex chassis (these are the 19kc filters);
• decreasing the resistance of r63 from 47K to 39K or 41.2K for accurate mono de-emphasis
• decreasing the resistance of r222 and r223 on the multiplex chassis from 27K (or 22K, depending on the original) to 16K for accurate fm stereo de-emphasis.

i understand there are valid reasons for not sending him any equipment for repair, or perhaps even avoiding him for the purchase of resto-paks, but that's not at issue here. i am concerned about his actual recommendations. are they valid? are they accurate? are they advisable (i.e., will they do what he suggests they will?). i guess it comes down to knowing whether he's actually a doc, or a quack.

thanks.

 

[BuzzK]

I've made these changes and I think they are valid. I install a pot in the shunt circuit to better fine-tune the bias. With the higher line voltages, you'll run the tubes hard if you don't compensate.

None of the changes will hurt, IMHO.

Buzz

 

[sgmlaw]

Al's recommendations are generally sound. His philosophy appears to only change values for the sake of longevity or safety (and not to spike performance), which I tend to agree with. Even though it may sound more "ballsy" running a stock hot bias, popping up the bias voltage is a good idea. It's easier on the outputs and easier on the main supply components. Getting just around -20 gets close to optimal in my experience and translates to a good mA read in most instances.

I think as to the de-emphasis mods, it's ok to change the mono resistor value. No big deal there.

But I think the MPX adjustment a waste of time. Just changing the resistor is a half-ass approach, as the choke values need to be adjusted as well to get it right. Just tearing out the chokes, as some advocate, introduces carrier harmonics into the audio output. Frankly, in the face of today's generally poor commercial FM quality, the sonic effects of the stock de-emphasis error is hardly noticeable. If it were mine, I'd leave it be.

Some of the other performance orientated mods, such as doubling cap values and other "tweaks" sometimes introduce circuit consequences that the modder didn't contemplate. But to each their own.

 

[Dadbar]

Removing r94 is sort of bad advice. You should install the 10 ohm resistors on your output cathodes and measure idle current in circuit....then adjust r94 accordingly. I found in my Fisher 400 that with the limited samples of EH 7868's that I've tried, grid bias voltage needs to be ~17V to get 29ma of current to flow at idle. If you adjust for -20V with these tubes you might be running them too lean.

YMMV, of course, with whatever matched quad of 7868's you use.

 

[BuzzK]

Installing 10 ohm resistors is recommended by Fisher Doc.

 

[madwing]

thanks for the commentary, folks. i've posted a list in a different thread of the parts i need to do what i've decided. i'll change the mono deemphasis, leave the stereo and the multiplex chassis resistors/chokes alone, and leave the r94 for now.

i will be adding the 10Ω resistors, a 2A, 120Ω thermistor as a current inrush limiter, and replacing the silicon diodes with a pair of IXYS ultrafast soft recovery FREDs (12A/1000v). this all in addition to the chassis caps and the selenium rectifier bridge replacement.

thanks bundles!

 

[larryderouin]

madwing; did you note the CAUTION on the 1st paragraph concerning the instructions and the fact that these are for units with a serial # of 29999 and UNDER?

Depending on the Service manual you use, a part would have a different part number. So make sure you have the correct manual for your unit, and the correct modification sheet if you do Al's mods.

I went thru this a couple of months ago and it drove me nuts as I couldn't find a part # he listed on my 400. With mine, it's a different number and if you don't know the difference you could do some serious damage. Luckily I got hold of Drew Boyce and he,along with a few others, helped me thru the problem, .

Larry

 

[LinuxGuru]

I have restored my Fisher 400 quite long time ago. You can find detailed walk-through at:

http://www.vacuum-tube.eu/audio_fisher_400_restoration_en.html

 

[madwing]

madwing; did you note the CAUTION on the 1st paragraph concerning the instructions and the fact that these are for units with a serial # of 29999 and UNDER?

yes, indeed. and the one i'll pick up from roy55 is in the 27K range.

Depending on the Service manual you use, a part would have a different part number. So make sure you have the correct manual for your unit, and the correct modification sheet if you do Al's mods.

yep, did that, thanks for the warning, though!

bill

 

[madwing]

I have restored my Fisher 400 quite long time ago. You can find detailed walk-through at:

http://www.vacuum-tube.eu/audio_fisher_400_restoration_en.html

thanks, yours was one of the sites i looked at first! actually got the service manual pdf from your site, and the idea for stacking the cans. where'd you get those clamps?

and i'll be looking at it again and again, too.