audmod01
Super Member
I recently bought a Fisher 90-T which was originally sold in CA. It is surprisingly clean for one of its age. It was missing two 12AX7s and shields. I had both the tubes and the shields. It also was missing its EZ80 rectifier tube. I checked all my tubes and did not have any. I did have left over 1N5408 rectifiers that I had removed from the 800-C I have been working on (they were originally in the negative bias/preamp filament supply there). I decided to substitute two of the rectifiers for the rectifier tube and added a 100 ohm 10W resistor between the rectifier cathodes and the input to the electrolytic filter capacitor. I added one terminal strip to hold the rectifiers and one end of the resistor. I am leaving the socket for the EZ80 rectifier tube in place in case I ever want to install one and remove the solid state rectifiers.
I decided to try the 90-T connected to the AUX input of the 800-C. I connected with a Y cable and used a simple clip lead antenna to the FM antenna inputs of the 90-T. It came on and worked. The eye tube is nice and bright for its age and the FM band works well and is quite sensitive. There is some noise from the volume and tone controls that would be corrected by cleaning. There are numerous axial lead electrolytics in the tuner circuitry that need replacing and eventually the main ones in the power supply. I did not leave it on long, but it gave a good accounting of itself considering its age.
These 90-T models have 4 FM IF stages like the 500-B/C or 800B/C and some other units. A PC88 is the RF amplifier stage which if I remember correctly is a 7DJ8 (like a 6DJ8 but with a 7VAC filament structure). This was the early version of the "Golden Cascode" front end which boasted the highest FM sensitivity and lowest noise figure at the time compared to other RF amp tubes.
Unfortunately this one came without any knobs!
I think the above schematic which came originally from the Sam's folder for the unit was optimistic about the original tube rectifier supplying 400mA B+ since the tube manual says it only provides up to 90mA. More likely it provides about 40mA. I need to measure the voltage drop across the 100 ohm resistor I added and see how much current is actually flowing in it.
Joe
I decided to try the 90-T connected to the AUX input of the 800-C. I connected with a Y cable and used a simple clip lead antenna to the FM antenna inputs of the 90-T. It came on and worked. The eye tube is nice and bright for its age and the FM band works well and is quite sensitive. There is some noise from the volume and tone controls that would be corrected by cleaning. There are numerous axial lead electrolytics in the tuner circuitry that need replacing and eventually the main ones in the power supply. I did not leave it on long, but it gave a good accounting of itself considering its age.
These 90-T models have 4 FM IF stages like the 500-B/C or 800B/C and some other units. A PC88 is the RF amplifier stage which if I remember correctly is a 7DJ8 (like a 6DJ8 but with a 7VAC filament structure). This was the early version of the "Golden Cascode" front end which boasted the highest FM sensitivity and lowest noise figure at the time compared to other RF amp tubes.
Unfortunately this one came without any knobs!
I think the above schematic which came originally from the Sam's folder for the unit was optimistic about the original tube rectifier supplying 400mA B+ since the tube manual says it only provides up to 90mA. More likely it provides about 40mA. I need to measure the voltage drop across the 100 ohm resistor I added and see how much current is actually flowing in it.
Joe
