New to Tubes...800c score...help restore.
- Thread starter marloubow
- Start date
marloubow
Best Day Ever
Kory...not almost...am done!
At least with the component replacement.
Final phase. Multiplex Decoder.
Replaced 4 caps. C210, C214, C221 and C222. All in very tight quaters.
C210 is polarity oriented, so of course check and blah, blah, blah.
Both C210 and C214 I used "pigtails" to slip over old leads.
On C222 and C221 I desoldered one lead of any component in the way. Most notably the 2 big red orbs. I have no idea what they are…I’m sure someone here does. C221 had to be relocated as seen in the photo below.
Radio is playing strong and locking into stereo. Not CD quality, but is really quiet in the gaps.
Any volunteers to walk me thru biasing?
Martin
At least with the component replacement.
Final phase. Multiplex Decoder.
Replaced 4 caps. C210, C214, C221 and C222. All in very tight quaters.
C210 is polarity oriented, so of course check and blah, blah, blah.
Both C210 and C214 I used "pigtails" to slip over old leads.
On C222 and C221 I desoldered one lead of any component in the way. Most notably the 2 big red orbs. I have no idea what they are…I’m sure someone here does. C221 had to be relocated as seen in the photo below.
Radio is playing strong and locking into stereo. Not CD quality, but is really quiet in the gaps.
Any volunteers to walk me thru biasing?
Martin
audiodon
Addicted Member
Those sonicaps might be pretty nice. The older kits had orange drops. They're fine, but too many of one kind of coupling cap type can be too much of a good thing. I used to use all orange drops, but now I break the coupling caps up a bit so there's not one single sonic signature due to one coupling cap formulation. Using sonicaps on the outputs may help you quite a bit. I'm not too familiar with them myself, but they're probably substituted based on experience by the kit builder and I'd leave 'em. If you're happy with the sound, great. One nice thing about them is that they seem pretty small.
Martin,
I recommend adding a 10K pot in place of the 5.6k bias resistor coming off the negative leg of the 1000uf filter caps. This allows you to adjust the negative grid voltage for all 4 output tubes. If you do install the pot, make sure you connect the unused wiper to the center lug. Then use the center tap and remaining tap. Think of this pot as the "throttle" for the whole rig. Make sure you measure resistance of the pot and set it around 5k before installation so your output tubes aren't running wide open when you first fire up the Fisher.
To measure the bias set your DMM to the appropriate dc voltage range. (I use 2 volts on mine). Attach the negative probe to anywhere on the chassis, and touch your positive probe to the Pin 5 side of each 10ohm cathode resistor. You will get a voltage reading such as 0.31 which means that output tube is drawing 31ma approximately. Since I find it rare a quad of tubes is perfectly matched across the board, I shoot to have all 4 in the 29-35ma range. Sometimes on pair will bias 29,31 and the other 33,35. I just find the tube that is drawing the least current and set the bias pot so it draws the least amount of current acceptable to me (29-30ma) then the remaining 3 tubes should be right in the acceptable range.
Sometimes I'm not the best at putting all of this into words but feel free to ask questions and someone else might be able to explain it a bit better. Hell if you want I'd be glad to walk you through it over the phone if that would be easier.
Kory
PS: A note of safety: It would be wise if you don't have steady hands and good lighting to get a set up clips to connect to the probes of your DMM so you eliminate the chance of slipping off and shorting something or shocking yourself.
I recommend adding a 10K pot in place of the 5.6k bias resistor coming off the negative leg of the 1000uf filter caps. This allows you to adjust the negative grid voltage for all 4 output tubes. If you do install the pot, make sure you connect the unused wiper to the center lug. Then use the center tap and remaining tap. Think of this pot as the "throttle" for the whole rig. Make sure you measure resistance of the pot and set it around 5k before installation so your output tubes aren't running wide open when you first fire up the Fisher.
To measure the bias set your DMM to the appropriate dc voltage range. (I use 2 volts on mine). Attach the negative probe to anywhere on the chassis, and touch your positive probe to the Pin 5 side of each 10ohm cathode resistor. You will get a voltage reading such as 0.31 which means that output tube is drawing 31ma approximately. Since I find it rare a quad of tubes is perfectly matched across the board, I shoot to have all 4 in the 29-35ma range. Sometimes on pair will bias 29,31 and the other 33,35. I just find the tube that is drawing the least current and set the bias pot so it draws the least amount of current acceptable to me (29-30ma) then the remaining 3 tubes should be right in the acceptable range.
Sometimes I'm not the best at putting all of this into words but feel free to ask questions and someone else might be able to explain it a bit better. Hell if you want I'd be glad to walk you through it over the phone if that would be easier.
Kory
PS: A note of safety: It would be wise if you don't have steady hands and good lighting to get a set up clips to connect to the probes of your DMM so you eliminate the chance of slipping off and shorting something or shocking yourself.
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Martin,
Looks like you're heading in the right direction but try to ignore setting a specific negative voltage. It's a guideline only, no single number is the gospel because all tubes vary. I find old stock 7591's need less negative voltage to get the target cathode current and new stock need more. Pay more attention to how much current the OP tubes are drawing when you adjust that bias resistor.
Kory
Martin,
I'd swap V11 and V14 to have better matched pairs. After the swap it should look something like this.
V10 .13
V11 .15
V13 .16
V14 .17
This way you have better a better matched pairs. Also you need to decrease negative voltage a tad by increasing the resistor value a bit to get the outputs to draw in the neighborhood of 30-32ma. Right now you're biased ice cold. I know some folks here bias their tubes cooler but with the availability of perfectly capable new production 7591's I stick to a little hotter biasing. Try it cooler and hotter and see if you can notice any difference. If you don't then cooler can't hurt!
Kory
O.L. - is that "over limit" ?
Did you change the range of your DMM to take that measurement? Anything over 500V should suffice. It ought to tell you what a little about the state of the voltage doubler capacitors which I find are often tired and of need of replacement more than any other caps in the power supply. 2/4 of the previous Fisher receivers I've worked on were only producing a B+ of 300-360 volts due to bad voltage doubler caps. Once replaced it returned rock solid in spec.
Kory
I'm a little skeptical that -16.87V with the spec 430V of B+ is only causing the OP tubes to draw such a low current.