Fisher 700 T rebuild
- Thread starter jcon109
- Start date
Not that I'm aware of. I used what I had in stock when I did mine. The small value electrolytic caps (1uf 70V) all got replaced with ECQ 1uf 100v films. You can use films, provided there is room. Usually anything under 4.7uf I replace with a film. 1.) sounds better, and 2.) I won't have to worry about replacing it in 20-25 years. If you have an imbalance in volume where the balance pot is about 3:00 or so to even the channels out, check the PEC's on the TONE Control Board. PC50187-25 is the board (1 per channel.) the biggest suspect is the 10K resistor that bridges Pin 1 and pin 7 internally. Ohm your's out and if it's off you can parallel another resistor to get 10K, or just build a new PEC on a piece of perf board. The small transistors are socketed, and are in dire need of DeOxit in the sockets. Spray and insert and remove about 10 times. Pots' and switches as usual. As for the caps, it would be advisable to bump the voltage up one step on any / all caps as Fisher liked to run the voltages right close to the voltage rating on the cap. A step from 50V to 63 is fine.
Make sure you have the correct manual for it. Serial # Beginning 10001 and beginning 30001. HIFIENGINE has both.
Make sure you have the correct manual for it. Serial # Beginning 10001 and beginning 30001. HIFIENGINE has both.
fred soop
Super Member
If controls, switches, and socketed transistor leads are clean, then having volume equalized when in mono would indicate that the weak channel problem is ahead of the mono switch as the signal MUST be the same on both channels at that point.
Connect a jumper between pins 1A and 1C on the preamp board. Mode selector in STEREO.
1 - If the channels are balanced, the problem is ahead of that point.
2 - If they are now unbalanced, the problem is in the mode selector, reverb jacks (check and clean the jumpers) or REC or TAPE jack wiring. Check R19 - R22.
If the result from the above is (1) then, remove the 1A-1C jumper, mode selector in STEREO, and connect a jumper between pins 1D and 1M on the preamp board.
1 - If the channels are balanced, the problem is the source selector switch, input jack wiring, or program source itself.
2 - If the channels are unbalanced, then there is a problem on the preamp board.
While the tone control PEC could have the typical problem, it is after the MONO switch, so is not the direct cause of the unbalance.
Do a 6way test on each of the transistors. http://audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/ Replace any bad ones. If none are bad, (I doubt all are good at this point) replace the electrolytics, and check/replace any bad resistors.
fred soop
Super Member
You can also swap the transistors between channels, but it is possible that another failure damaged a transistor.
So, try the following: Remove the "good" transistors from the left channel and set them aside. Move the transistors from the right channel to the left channel. Test again. If the left channel is now dead, then one of those transistors is bad. If the left channel still works, then there is another problem in the right channel.
That preamp board also has a PEC. The resistance values can be checked using the schematic for reference. There are quite a few electrolytic capacitors, all of which are 50 years old. Any of them are suspect and if they are good today, they may not be tomorrow. The 0.5 µF can be replaced with 0.47 polypropylene if there is room.
So, try the following: Remove the "good" transistors from the left channel and set them aside. Move the transistors from the right channel to the left channel. Test again. If the left channel is now dead, then one of those transistors is bad. If the left channel still works, then there is another problem in the right channel.
That preamp board also has a PEC. The resistance values can be checked using the schematic for reference. There are quite a few electrolytic capacitors, all of which are 50 years old. Any of them are suspect and if they are good today, they may not be tomorrow. The 0.5 µF can be replaced with 0.47 polypropylene if there is room.
6v
Probably not a bad idea to go to 10 or 26v.
It buys some headroom which you may (probably don't need as it lasted 50 years). New ones tend to be smaller so it might be an easier fit (closer physical size match to original).
It buys some headroom which you may (probably don't need as it lasted 50 years). New ones tend to be smaller so it might be an easier fit (closer physical size match to original).
Solid state stuff very rarely goes over 100V except maybe the filter caps (if it's one of the bigger monster amps from the 70's) The 60's Fishers were fairly low voltage, with the highest voltage caps the 1uf 70V Electrolytics. Those plastic Gray cans are electrolytics. Mallory's and the infor is on the top of the cap. 1st number is value in uf / 2nd is voltage So a 160/6 would be 160uf/6V
fred soop
Super Member
The 160/6 is an emitter bypass and is subject to very low voltage. The board has 30 V input, dropping through R119 to 11 V at the collector of Q105. That works out to 3.4 mA collector current. 3.4 mA through R117 would produce 0.51 V. A 6 V capacitor is fine. A 10 V would be almost overkill. The voltage ratings of the originals should be in the parts list. (I assume you DO have a service manual?)
220uf is fine. Most brands jump from 150uf to 220uf. If you look hard enough you might find a 180uf. But it will be more expensive than the standard caps.
