Another X-202-B In The House!!
- Thread starter notdigital
- Start date
Not -- As a suggestion, you might check out post #11 in this thread:
http://www.audiokarma.org/forums/showthread.php?t=551876
wherein I add a barrier strip to the rear of my own X-202B behind the power transformer, to effect test points when setting the bias. By adding them externally, it means never having to take the bottom cover off to check or set the DC bias and balance controls. The entire installation was done in a non-invasive fashion, with no holes drilled in the chassis to effect its installation.
Your can re-stuff job looks excellent! Isn't it amazing what a little love will do for these beautiful pieces of musical history?
Dave
http://www.audiokarma.org/forums/showthread.php?t=551876
wherein I add a barrier strip to the rear of my own X-202B behind the power transformer, to effect test points when setting the bias. By adding them externally, it means never having to take the bottom cover off to check or set the DC bias and balance controls. The entire installation was done in a non-invasive fashion, with no holes drilled in the chassis to effect its installation.
Your can re-stuff job looks excellent! Isn't it amazing what a little love will do for these beautiful pieces of musical history?
Dave
Bruce & Dave: thanks for the comments. These Fisher things are worthy of all the attention one can give. They certainly deserve it. And Don, again, many thanks for the switch.
As far as the test points are concerned, I don't know how I'm going to handle that as of yet. I have external test points on the 3 Fishers that I have now (for example: http://www.audiokarma.org/forums/attachment.php?attachmentid=401433&d=13577891160,) but I have to understand how the pots on the x-202-b work first. Do they each control one pair of 7591's? And doesn't that require that they then be "matched"? I haven't had a chance to really study the schematic or the operating instructions on the topic. However it seems to me that they contol one pair of tubes as channels A and B. And yes Dave, thanks for the pointer. I forgot all about that thread. I'll re-read it agin tonight as I ponder what to do next. Right now I know the tubes are running in the low 50's ma's. I don't care too much b/c ALL the tubes that came with this unit, including the outputs, are JJ's. So, I don't mind abusing them as testers. Once I finish up with the bias supply and other odds and ends, I'll turn my attention to fine tuning, etc.
As far as the test points are concerned, I don't know how I'm going to handle that as of yet. I have external test points on the 3 Fishers that I have now (for example: http://www.audiokarma.org/forums/attachment.php?attachmentid=401433&d=13577891160,) but I have to understand how the pots on the x-202-b work first. Do they each control one pair of 7591's? And doesn't that require that they then be "matched"? I haven't had a chance to really study the schematic or the operating instructions on the topic. However it seems to me that they contol one pair of tubes as channels A and B. And yes Dave, thanks for the pointer. I forgot all about that thread. I'll re-read it agin tonight as I ponder what to do next. Right now I know the tubes are running in the low 50's ma's. I don't care too much b/c ALL the tubes that came with this unit, including the outputs, are JJ's. So, I don't mind abusing them as testers. Once I finish up with the bias supply and other odds and ends, I'll turn my attention to fine tuning, etc.
Not -- There is one Bias and one DC Balance control for each channel. When you have individual 10 ohm cathode resistors installed, you simply adjust the DC Balance for 0 volts between the two cathodes of one channel, and then use the bias control to bring BOTH tubes (at once) up or down to the proper reading. It's really quite simple.
Since Fisher didn't include individual 10 ohm resistors, but one common one in each channel, their procedure is to adjust the DC balance control for minimum hum. But it is much better to install individual resistors, and use a meter to set the control.
Dave
Since Fisher didn't include individual 10 ohm resistors, but one common one in each channel, their procedure is to adjust the DC balance control for minimum hum. But it is much better to install individual resistors, and use a meter to set the control.
Dave
Finally finished off the bias can. Now onto fine tuning....
Before messing with the bias/ac bal pots, I took measurements off pin 5 and WOW: All 4 in low to mid 50's.
brought them all down to between 32 & 35. Good: the bias pots work. However, I can't seem to get any response from the AC Balance pots. I found this thread re making adjustments
http://mail.audiokarma.net/forums/showpost.php?p=7248271&postcount=13.
I (almost) zipped out the bias. However, when I went back to the AC Balance pots I get no response. It fluctuates wildly when I turn the pot but it always settles where it started. :scratch2: :scratch2:
Anyway, I have had it playing now for about an hour: pretty impressive. Tomorrow I'll check the voltages around the unit to make sure everything is in order. If so, I'll finish off the cosmetic stuff and seal it up :yes:
Before messing with the bias/ac bal pots, I took measurements off pin 5 and WOW: All 4 in low to mid 50's.
brought them all down to between 32 & 35. Good: the bias pots work. However, I can't seem to get any response from the AC Balance pots. I found this thread re making adjustmentshttp://mail.audiokarma.net/forums/showpost.php?p=7248271&postcount=13.
I (almost) zipped out the bias. However, when I went back to the AC Balance pots I get no response. It fluctuates wildly when I turn the pot but it always settles where it started. :scratch2: :scratch2:
Anyway, I have had it playing now for about an hour: pretty impressive. Tomorrow I'll check the voltages around the unit to make sure everything is in order. If so, I'll finish off the cosmetic stuff and seal it up :yes:
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Not -- you cannot adjust the AC balance control at the cathode terminals without using a more complicated procedure that involves driving the amplifier to a high level of power output from a test source.
As a very good alternate approach however, I offered a procedure to Larry some time ago that he has now posted in a sticky at the top of the page regarding the Fisher phase inverter adjustment, which is what the AC balance control is. You can use that procedure to get a reasonably close setting using nothing but an ohm meter, while the unit is turned OFF.
Great work!
Dave
As a very good alternate approach however, I offered a procedure to Larry some time ago that he has now posted in a sticky at the top of the page regarding the Fisher phase inverter adjustment, which is what the AC balance control is. You can use that procedure to get a reasonably close setting using nothing but an ohm meter, while the unit is turned OFF.
Great work!
Dave
Hmmmm. I got ahead of myself. The pots between the low and high voltage doublers: What are they for?
Dohhhhhhh!!!!!!!!!! This is what I get for 1) getting to bed too late and b) gettting up too early. Coffee anyone 
On it. Thanks Dave.
On it. Thanks Dave.
Those are the left and right channel DC balance pots.
Dave
Dave
Yeah, I know, now that I woke up. Gonna get more cafe.
Aw, screw the Coffee. :tongue: Stick some tobacco pieces in the corner of your eye's. Guaranteed to keep you awake! Vision will be a bit fuzzy, but you're awake!!:nutz:
I laugh at myself sometimes: Get the boy out the house to his course, get the girl to soccer, read a schematic, remember not to get electrocuted, check the broken garden hose so I don't flood the neighbor, pay my AK subscription. It's hilarious sometimes.
Anyway, just balanced the inverters; now onto the DC pots that I just found!!! Hahahahaha. Hilarious
Anyway, just balanced the inverters; now onto the DC pots that I just found!!! Hahahahaha. Hilarious
Very nicely done!! Did you make the little box the colored test jacks are mounted into, or was that something you conveniently had on hand for the job?
Dave
Dave
COOL! LEGO Test points!
Seriously, Nice job. I like the 4ply braided leads underneath.
Seriously, Nice job. I like the 4ply braided leads underneath.
I have small sheets of acrylic layin' around from aquarium projects from many years ago. Kept them cuz they come in handy once in a while
Thanks Larry!!
Not so fast Beaver!!!
Looked everything over, decided I was done for now, put feet on the botttom cover, flipped it over and connected a tuner, turned my head and.......
SMOKE!!!!!!!!!!!
Arghhhh. One of the bolts I used to attach the rubber foot was just a tad too long shorting out the resistor. Good thing a had a 1.5k 7watter in the parts tin.
Here it is doing standup duty in the background. No smoke, all good :thmbsp:
Oh yeah, wondered why it didn't blow a fuse. Checked it and: 10 amps!!!! Oh Clippy, Clippy.....
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This looks incredible! Very nice job. Can't wait to see the final shots.