Fisher X-100-3 help

[phickman]

I'm working on a Fisher X-100-3 this unit has a strange circuit topology..
here is what happened an associate brought it in to me and complained that it was playing fine and it suddenly stopped in mid playing and he said he smelled someting burning so powered it off right away. On my bench I tested all the tubes, this one has the 7189 outputs..next bringing it up on a variac slowly nothing strange, all tubes light fine except the 12Ax7's V-1, 2, 3 these are the ones that have the strange fillament connections they go to the cathodes of the 7189's.. from what I can find in my research.. their should be about 30-40 vdc on the cathodes this is part of the topology that gets the v1,2,3, filliaments lit with DC.. I can only get about 1.4 volts..they are stone cold and no music is passing, I can directly injet sound at the 12AX7 phase splitters and get plenty of volume..as they have direct a/c from the transformer as the 7189's do. I have replaced the electrolytic in the Cathode circuit and tested the resistors and also cleaned all the tube pins and sockets.. still nothing. oh and the power supply B+ seems good at all critical areas

ok you Fisher pro's help me here.. I do not have a schematic as it is so rare
what can you tell me to check for

 

[dannyr]

Hey Phickman. The yahoo fisher group has the service manual for it in their file section. It may be named X-100-A which I guess is the same as the X-100-3.

http://launch.groups.yahoo.com/group/fishergroup/

I have a X-100-2 that I'm about to attempt to get going which seems pretty much the same as the x-100-3. Do you happen to know how hard the amp is running the output tubes? I'm wondering if normal el84's will work or be a bit stressed in it.

Also if anyone knows how to go about the phase inverter adjustment without anything more then a multimeter please let me know. I'm guessing to just center the pot. The KX-90 seems to be the same minus the phase adjustment sense it was a kit but I haven't found a schematic of it to just do the same on my amp.

 

[phickman]

Thanks for the help, I might be some help with the el84 vs 7189's the 7189's are a rugged version of the 6BQ5 they have a much higher plate voltage capacity upwards of 400 vdc on the plates and if your amplifier runs the higher b+ as some do that are built for 7189's the BQ5's will be stressed and could fail early or possibly arc internally.. fee:scratch2:l free others who know much mores about this to chime in here

 

[phickman]

OK, everyone here is the latest on the X-100-3.. Its now singing again!!
this was the issue, the cathode voltage was low on the 7189's 3.0 vdc
and would not light the fillaments on V1,2,3 so no gain stage was working.
I had checked the B+ on the power supply caps and it was good with the variac set for 100 vac.. the outputs and phase splitters were working and by sending signal in at the phase splitters I had music.. , so while I got sidetracked for a while, I kept wondering why I had good B+ and no catahode voltage. I noted that the 10 watt 2.2 k resistor was getting really hot (way hot) strange I thought.. so I checked the diodes and low and behold one was dead feeding the main input power supply cap. Replaced the diodes and like magic all is good 30+ vdc on the cathodes and the tubes light all is playing fine.
So I took to installing some under chassis lytic's and rebuilt the PS along with the Cathode B+ changed all the couping caps and this thing now sounds just super.. I will now delve into IM distortion adjustments and a few tweaks here and there

all is well , love them tube amps!!

 

[dannyr]

That's fantastic. I'm wondering how that I should go about the IM adjustment not having the equipment to pull it off. I'm looking for a KX90 schematic to see what value resistor is in the two adjustments pot places.

How hard is the amp running the output tubes?

Please keep us up to date on what you end up doing to the X-100-3.