I received this TA-800 for restoration. The front panel is beautiful and is more detailed vs the later Fisher receivers.
The Fisher TA-800 is similar to the TA-600. The main differences are the TA-800 changed from the TA-600 GZ34 rectifier tube to a solid-state voltage doubler, changed from power tube cathode bias to fixed bias and replaced the EL84s with 7591 power tubes.
Using the dim bulb tester, the 300 watt bulb immediately went to full brightness. The suspect HV short was found in one 200uF capacitor. The original doubler capacitors were replaced with two 680uF capacitors for an effective 340uF filter. All other HV power supply capacitors were replaced with original values. I have changed from CE can caps to a JJ can caps due to the CE capacitor now costs $47 per can. I only indulge due to owners request of the CE can original metal appearance or the CE four section cap due to under chassis space restrains.
The top hot style HV rectifiers were replaced with UF4007. Using two in series provides 2KV per leg. Interesting that a snubber capacitor was not used on the HV AC winding as installed in the newer Fisher 400, 500 & 800 receivers. I installed a .01uF @ 2KV ceramic capacitor from the HV winding to ground.
Next, the power supply switch was unloaded with a relay interface and use of a thermistor for soft start. The 120 volt AC line input cap to ground was replaced with a .01 @ 2KV ceramic. I use the neutral to ground vs hot to ground across the .01uF capacitor. The same value capacitor was placed across the relay contacts to increase contact life. A three wire power cord was installed.
The bias supply dual 1000uF can cap was replaced with two 4700uF capacitors. The original Siemens bridge rectifier was replaced with four UF4007 rectifiers. The output voltage increased to 27 volts under load. Two diodes in series with the 12AX7 tube filament string brought the voltage down to 25.2 volts. The 12AX7 tubes receive 12.6 volts each. The 7591 power tubes fixed bias was adjusted to -19 volts using a 4.7K resistor in parallel across the original resistor that is connected to the 330K power tube grid resistors.
Later 400, 500 and 800 receivers use an additional 100uF bias filter after the fore-mentioned resistor. I added a 4.7uf capacitor at this position for additional filtering. The upgrade from the dual 1000uF can capacitor to two 4700uF filter by itself is superior to the Fisher original design.
The 330K grid resistors were changed to 220K. This provides superior protection for guard the 7591 tubes against thermo run away.
Upon power up, one channel was out and the other was distorted on bass notes. One open resistor was found at the phase splitter. With both channels up and running, one channel still had flabby bass and the other originally dead channel produced an annoying hum. Other resistors were within specifications.
Placing a scope on the phase splitters, the channel with the flabby bass was far out of balance. After adjustment, the flabby bass was corrected.
Next, was replacing the cathode caps on the 12AX7 input preamp and 12AX7 tone control amplifier tubes. I use Elna Silmic II audio grade capacitors. The hum in the one channel was resolved with the new capacitors.
Finally, all other coupling capacitors less the tone PEC network were replaced with K40Y-9. As some other AKers mentioned, a few ceramic capacitors left in the tone section seems to maintain some of that Fisher magic. Other coupling capacitors that were replaced with the K40Y-9 adds clarity & depth to the soundstage. The original power tube .047uF grid coupling capacitors were replaced with .068uF due to the reduction of the 330K to 220K power tube grid resistors.
The AM & FM tuners were aligned. I did not find the tuners far out of alignment, but the sensitivity was brought back to factory specifications. The TA-800 does not have an on board FM multiplexer. I recommend Fisher stand alone multiplexers for best sound.
The original Fisher label Sylvania power tubes were healthy enough not to warrant replacement. I measured 24 watts output per channel. The four 12AX7s were replaced with two Westinghouse 12AX7 narrow short plate in the phono section and two RCA 12AX7 long plate in the preamp and tone control amp.
Further improvement came forth replacing the RCA 7199 phase splitter tubes with Amperex 6BL8. This change requires 6GH8 to 7199 socket adapters commonly found on ebay.
The stock Fisher TA-800 sounds good. I prefer it to the stock Fisher 400, 500 and 800 receivers. I suspect the TA-800 phase inverter is a superior design with higher gain and better drive with the lower plate resistance triode section vs the 12AX7. The treble is brighter vs the Fisher 400, 500 and 800 receivers. Changing the 7199 to 6BL8 provides superior sound.
Edit- one of my best tools is using lowly Boston A70s speakers with crossover upgrades for evaluating amplifiers. I never heard a speaker in the bass section go to hell as quickly producing flabby bass with amplifier issues. Normally the bass is well defined and clear. The phase splitter issue above produced awful bass whereas some speakers would still sound ok. The tweeter section is terribly unforgiving as well. A little on the bright side if anything is wrong in the amplifier treble response these tweeters will cry out like a stuck pig. Of course serious listening on my highly modified Klipsch Forte brings forth the best. At times a trip to my friends music room (my second lab) using JBL L300s & ultra high-end sources will ruin an audiophile. JBL L300s costs about $5K.
