Carbsandcaps
Super Member
I thought that I would post this as a separate thread from my initial post in case someone else is having this problem, and thank you to the people that helped me out with it.
I picked up a 700t relatively cheap “for parts and repair”. To my delight it actually turned on and produced sound by it was evident there was something wrong. The transistors almost immediately got hot to the touch. I measured the bias and it was at .2v and wouldn’t budge with the bias adjustment pot.
I checked the “stabistor” diodes and they seemed to be open. After checking the service manual it showed 2 different stabistors in 2 different schematics. Checking pictures online of other 700t’s it looked like mine was missing the stabistor modules completely So I came on here to ask if anyone had any experience with them. After getting 2 responses it turns out there are at least 3 different versions of stabistors: a separate module with diodes in it that mounts to the back plate, the same module with an added external diode, and then my version which was just what looked like a single diode. So I figured it was time to get into it.
I removed the stabistor diodes from the circuit on mine and found them both to be at 1.5v so now I’m assuming there’s 3 diodes in there. The part number was MAL8056 and that came up with nothing. I was also reading online that the stabistor is more of short circuit and overheating protection and doesn’t set the bias so probably wasn’t the problem. I then looked at the schematic and saw there’s a 2 amp diode and a 6.8v zener diode in the circuit mounted to the driver board. In that spot on mine and others there’s just a black box that looked like a film capacitor and no info online when running part numbers. I removed all 4 of them (2 per channel) and tried to test them with inconclusive results. I figured since it’s in the spot where the 2 diodes should be I ordered a 1 watt zener diode assortment from Amazon and assembled the diodes. I only had 1n5408’s laying around so they are a little bigger than necessary but they fit the bill. Installed the diodes, recapped the board while it was off and replaced the other diode on the board. Measured the bias and BOOM! 40mv! I may still replace the stabistor with 3 1n4001’s but it’s working for now so I’ll probably just leave it.
I decided to try to open up the modules to see if there was in fact 2 diodes in there and sure enough there was. Hopefully this helps anyone else struggling with biasing on these or similar fisher models.
I picked up a 700t relatively cheap “for parts and repair”. To my delight it actually turned on and produced sound by it was evident there was something wrong. The transistors almost immediately got hot to the touch. I measured the bias and it was at .2v and wouldn’t budge with the bias adjustment pot.
I checked the “stabistor” diodes and they seemed to be open. After checking the service manual it showed 2 different stabistors in 2 different schematics. Checking pictures online of other 700t’s it looked like mine was missing the stabistor modules completely So I came on here to ask if anyone had any experience with them. After getting 2 responses it turns out there are at least 3 different versions of stabistors: a separate module with diodes in it that mounts to the back plate, the same module with an added external diode, and then my version which was just what looked like a single diode. So I figured it was time to get into it.
I removed the stabistor diodes from the circuit on mine and found them both to be at 1.5v so now I’m assuming there’s 3 diodes in there. The part number was MAL8056 and that came up with nothing. I was also reading online that the stabistor is more of short circuit and overheating protection and doesn’t set the bias so probably wasn’t the problem. I then looked at the schematic and saw there’s a 2 amp diode and a 6.8v zener diode in the circuit mounted to the driver board. In that spot on mine and others there’s just a black box that looked like a film capacitor and no info online when running part numbers. I removed all 4 of them (2 per channel) and tried to test them with inconclusive results. I figured since it’s in the spot where the 2 diodes should be I ordered a 1 watt zener diode assortment from Amazon and assembled the diodes. I only had 1n5408’s laying around so they are a little bigger than necessary but they fit the bill. Installed the diodes, recapped the board while it was off and replaced the other diode on the board. Measured the bias and BOOM! 40mv! I may still replace the stabistor with 3 1n4001’s but it’s working for now so I’ll probably just leave it.
I decided to try to open up the modules to see if there was in fact 2 diodes in there and sure enough there was. Hopefully this helps anyone else struggling with biasing on these or similar fisher models.
