Looks great!
Dave
Dave
Fisher X-202 Rebuild
- Thread starter 2dbit
- Start date
At this point, I am going to begin the initial powering and adjustment to the amplifier.
Prior to first power:
-All tubes are removed
-DC balance pots adjusted to approximately center of travel
-Bias Adj. pots are set to full CW position
Phase Inv. Pots have been adjusted via DC Gillespie's advice on a Fisher 400 adjustment procedure: (adjusted to fit this amp)
-Alligator clip lead between C55A and ground (the B+ supply source to both phase inverter tube circuits)
-DVM common lead connected to ground
-Measured resistance at pin #1 of left and right phase inverter tube sockets(V7 and V8) (18.09K for A and 18.63K for B)
-Measured resistance at pin #3 of left and right phase inverter tube sockets, then adjusted phase inverter pots so that V7 pin 3 read 18.09K and that V8 pin 3 read 18.63K.
I ran out of time to go back and perform any additional verification checks, but it would seem that following these initial adjustments, I can place a dim bulb tester at the AC input and apply power.
With all tubes still removed, I'll check for:
-Output voltage from silicone rectifier
-Voltage at pins 4 and 6 of the tube rectifier socket(V13)
-Voltage at pins 2 and 8 of the tube rectifier socket(V13)
-Voltage at pins 4 and 5 of V7, V8, V9, V10, V11, and V12
If all voltages check good, then I planned on installing the rectifier tube and checking the following voltages:
-~290V at C46B,
-~295V at C55A,
-~273V at C55B,
-~240V at C55C,
-~195V at C55D
I realize they will all be above those values due to increased line voltage and lack of tube load, but making sure the voltages are present would likely be a good starting point.
If all those check good, I'll probably take the leap and install all tubes.
I could at this point install a variac on the input and bring it up using that, or just keep the dim bulb tester installed and see what I get with that.
I suppose if the bulb behaves properly, then I'll finally remove the bulb, plug directly into the wall, and cross my fingers.
I have not yet installed the CL-70 inrush limiter. I wanted to make sure that everything was behaving properly before I performed any modifications. If the amp comes to life and has all its initial adjustments complete, I'll likely add it in after that.
Prior to first power:
-All tubes are removed
-DC balance pots adjusted to approximately center of travel
-Bias Adj. pots are set to full CW position
Phase Inv. Pots have been adjusted via DC Gillespie's advice on a Fisher 400 adjustment procedure: (adjusted to fit this amp)
-Alligator clip lead between C55A and ground (the B+ supply source to both phase inverter tube circuits)
-DVM common lead connected to ground
-Measured resistance at pin #1 of left and right phase inverter tube sockets(V7 and V8) (18.09K for A and 18.63K for B)
-Measured resistance at pin #3 of left and right phase inverter tube sockets, then adjusted phase inverter pots so that V7 pin 3 read 18.09K and that V8 pin 3 read 18.63K.
I ran out of time to go back and perform any additional verification checks, but it would seem that following these initial adjustments, I can place a dim bulb tester at the AC input and apply power.
With all tubes still removed, I'll check for:
-Output voltage from silicone rectifier
-Voltage at pins 4 and 6 of the tube rectifier socket(V13)
-Voltage at pins 2 and 8 of the tube rectifier socket(V13)
-Voltage at pins 4 and 5 of V7, V8, V9, V10, V11, and V12
If all voltages check good, then I planned on installing the rectifier tube and checking the following voltages:
-~290V at C46B,
-~295V at C55A,
-~273V at C55B,
-~240V at C55C,
-~195V at C55D
I realize they will all be above those values due to increased line voltage and lack of tube load, but making sure the voltages are present would likely be a good starting point.
If all those check good, I'll probably take the leap and install all tubes.
I could at this point install a variac on the input and bring it up using that, or just keep the dim bulb tester installed and see what I get with that.
I suppose if the bulb behaves properly, then I'll finally remove the bulb, plug directly into the wall, and cross my fingers.
I have not yet installed the CL-70 inrush limiter. I wanted to make sure that everything was behaving properly before I performed any modifications. If the amp comes to life and has all its initial adjustments complete, I'll likely add it in after that.
Well, aside from the 3.2A slowblo fuse being blown (not terribly surprised given the mode of failure on this unit) I began powering the unit up.
Voltages were: (all tubes removed)
-Heater circuit downstream of the silicon bridge rectifier: -32.51v (schematic: -21v)
-Pin 4 of tube rectifier socket: 390vac (schematic: 370vac)
-Pin 6 of tube rectifier socket: 390vac (schematic: 370vac)
-Pin 2 to 8 of tube rectifier socket: 5.40vac (schematic: 5vac)
-Heater circuit of V7 through V12: 7.00vac (schematic 6.3vac)
With these voltages checking good, I went ahead and installed the tube rectifier.
Voltages downstream of the tube rectifier were:
-C46c: 265.6vdc (schematic: -290vdc) HOPE schematic is wrong on this one!
-C55a: 333.1vdc (schematic: 295vdc)
-C55b: 332.5vdc (schematic: 273vdc)
-C55c: 330.5vdc (schematic: 240vdc)
-C55d: 318.7vdc (schematic: 195vdc) HOPE the discrepancy is due to lack of loading...
So, for today, I'm going to leave this as is. I am going to hold off on installing all the tubes and taking additional readings until I get some community feedback on these initial findings.
The 75watt bulb I installed worked like a charm, without the tube rectifier installed, the bulb didn't light when checking the voltages, and with the tube rectifier installed, it pulsed on dimly at first, went out, then once the tube was warm, emitted a soft glow.
Voltages were: (all tubes removed)
-Heater circuit downstream of the silicon bridge rectifier: -32.51v (schematic: -21v)
-Pin 4 of tube rectifier socket: 390vac (schematic: 370vac)
-Pin 6 of tube rectifier socket: 390vac (schematic: 370vac)
-Pin 2 to 8 of tube rectifier socket: 5.40vac (schematic: 5vac)
-Heater circuit of V7 through V12: 7.00vac (schematic 6.3vac)
With these voltages checking good, I went ahead and installed the tube rectifier.
Voltages downstream of the tube rectifier were:
-C46c: 265.6vdc (schematic: -290vdc) HOPE schematic is wrong on this one!
-C55a: 333.1vdc (schematic: 295vdc)
-C55b: 332.5vdc (schematic: 273vdc)
-C55c: 330.5vdc (schematic: 240vdc)
-C55d: 318.7vdc (schematic: 195vdc) HOPE the discrepancy is due to lack of loading...
So, for today, I'm going to leave this as is. I am going to hold off on installing all the tubes and taking additional readings until I get some community feedback on these initial findings.
The 75watt bulb I installed worked like a charm, without the tube rectifier installed, the bulb didn't light when checking the voltages, and with the tube rectifier installed, it pulsed on dimly at first, went out, then once the tube was warm, emitted a soft glow.
The only other (important) voltage to confirm is that at pin #2 of each output tube. If you're seeing at least -10 volts or more at each socket on this terminal, then it would appear you are good to go!
Dave
Dave
I'll be sure to check those voltages prior to tube installation.
Next will be putting together a test apparatus. I've got a couple of 8ohm non-inductive power resistors, I will get those hooked up, and see how all the voltages are.
I'll probably then install the CL-70 and without the bulb tester, perform all adjustments for bias and balance
Next will be putting together a test apparatus. I've got a couple of 8ohm non-inductive power resistors, I will get those hooked up, and see how all the voltages are.
I'll probably then install the CL-70 and without the bulb tester, perform all adjustments for bias and balance
The voltage at each terminal is determined (and therefore adjusted) by the setting of the DC bias and DC Balance controls on the chassis for each channel respectively. Turn the appropriate DC bias control so that the terminals read something greater (i.e. more negative) than -10 vdc. It appears that all is good at this point.
Dave
Dave
It's a bit hard to see, but I've installed wires from the cathode of each output tube socket to a now unused terminal on the block. The wires used are red...
I did my best to route them to be inconspicuous (not that it really matters at this point given how much obvious work has been done, but looks a bit cleaner)
I did my best to route them to be inconspicuous (not that it really matters at this point given how much obvious work has been done, but looks a bit cleaner)
Also, I went ahead and installed the CL-70. I was not a fan of creating an air connection for this installation, but I did my best to minimize any lead length. At least then it could be a more solid installation. Additionally, incase of catastrophic failure of the CL-70, I wanted to minimize lead length to reduce the risk of shorting.
I would have preferred to use the other AC input wire off of the fuse, but given its position, it would have been difficult to get the CL-70 out where it can have the least exposure to other components, and maximum cooling space.
I would have preferred to use the other AC input wire off of the fuse, but given its position, it would have been difficult to get the CL-70 out where it can have the least exposure to other components, and maximum cooling space.
So, at this point... I believe it may be time to install some tubes and see what happens. I still have the potentiometers set to have -18v on pin 2 of each output tube. I hope that's a good starting point and I can then adjust bias to be... .27v? The manual says that across the original 10 ohm resistor which was setup with two tubes, to have .6v. So, .3v each...
These tubes aren't quite as plentiful and inexpensive as they once were, perhaps dialing back the bias would be a wise decision.
Since I have each output tube wired for individual bias measuring, would balance be set by putting a DVM across both output tube cathodes and adjusting for as close to 0v as possible?
These tubes aren't quite as plentiful and inexpensive as they once were, perhaps dialing back the bias would be a wise decision.
Since I have each output tube wired for individual bias measuring, would balance be set by putting a DVM across both output tube cathodes and adjusting for as close to 0v as possible?
Absolutely! Set the DC balance for 0 volts ACROSS the test points for each channel, and then adjust the DC bias for the current draw you want, measuring between either test point, and ground. The controls are somewhat interactive between each other, and each channel as well. But you can get it dialed in in no time.
Looks great!
Dave
Looks great!
Dave
Was just reading the adjustment procedure published by Fisher, and found their instructions to be somewhat peculiar.
Currently I have no jumper wires of any kind on the terminal strips. Per their instruction, they want a jumper wire between S and 16 on channel B, and the load resistors (I have 2 100watt 8ohm resistors) between G and 16 on channel A and between L and H on channel B.
Originally I thought I'd just be putting the load resistors between G and 8 on each channel.
How close do I follow this?
Currently I have no jumper wires of any kind on the terminal strips. Per their instruction, they want a jumper wire between S and 16 on channel B, and the load resistors (I have 2 100watt 8ohm resistors) between G and 16 on channel A and between L and H on channel B.
Originally I thought I'd just be putting the load resistors between G and 8 on each channel.
How close do I follow this?
If your load resistors are only 8 ohms (the standard today, but not back in the day), then you can still use the Fisher procedures provided you connect the jumper for channel B between the S and 8 terminals, then use the L and H terminals for your 8 ohm load resistor as before. Connect your channel A 8 ohm load resistor between the G and 8 terminals for that channel.
These connections are nothing more than how you would normally hook up 8 ohm speakers to the unit, using speakers in place of the 8 ohm load resistors. Such connections then allow the front panel Phase Reverse switch to operate.
Alternately, you can in fact simply hook your load resistors between the G and 8 for each channel -- as you can your speakers as well in normal operation. However, such connections won't allow the Phase Reverse switch to operate. That may be a moot point in this day and age with the careful attention to speaker connections that is common today. However, back in the day, when single (i.e. mono) speaker systems were common place, the need for an ability to easily reverse the phase of the speaker connections was paramount, as people were still just getting used to the new sound of stereo, let alone any hole in the middle effects that might come along for the ride due to improper phasing between the channels.
Dave
These connections are nothing more than how you would normally hook up 8 ohm speakers to the unit, using speakers in place of the 8 ohm load resistors. Such connections then allow the front panel Phase Reverse switch to operate.
Alternately, you can in fact simply hook your load resistors between the G and 8 for each channel -- as you can your speakers as well in normal operation. However, such connections won't allow the Phase Reverse switch to operate. That may be a moot point in this day and age with the careful attention to speaker connections that is common today. However, back in the day, when single (i.e. mono) speaker systems were common place, the need for an ability to easily reverse the phase of the speaker connections was paramount, as people were still just getting used to the new sound of stereo, let alone any hole in the middle effects that might come along for the ride due to improper phasing between the channels.
Dave
Quite stressful putting all the tubes in and powering it for the first time!!!
All went well, I am a little curious why V7 flashes on bright when power is first applied, it lights up bright and then dims down to be equal to the rest as the whole unit warms up...
Having terminals C and X on each channel freed up for adjustments is quite possibly the best thing there ever was. Talk about making life easy! I was able to get both channel A and channel B adjusted to 270mV(+-0.2mV). I was also able to get the DC balance adjusted to less than 0.2mV. Fantastic!
I'll post back later on when I have some actual tonal tests completed
All went well, I am a little curious why V7 flashes on bright when power is first applied, it lights up bright and then dims down to be equal to the rest as the whole unit warms up...
Having terminals C and X on each channel freed up for adjustments is quite possibly the best thing there ever was. Talk about making life easy! I was able to get both channel A and channel B adjusted to 270mV(+-0.2mV). I was also able to get the DC balance adjusted to less than 0.2mV. Fantastic!
I'll post back later on when I have some actual tonal tests completed
