Hey folks.
I'm repairing a Fisher 400. Customer wanted new 7868 bases - done. I've replaced the filter caps with the Hayseed Hamfest excellent replacement set - fine and dandy so far. Voltages all fine.
However, I'm getting an oscillation when the PI and power valves are inserted (can even by one PI valve and one 7868):
Here is the relevant section of the schematic, have attached the whole thing hopefully:
Oscillation goes is the PI valves removed or the power valves are. It builds up as the valves come into emission. Theres a 1200 ohm 10W resistor which drops the HT down to the screen node which then goes off the other rest of the HT, the voltage drop without the oscillation is about 25Vdc, when both the PI adn power valves inserted that voltage drop goes to >100Vdc. I thought a short but cannot find one, even isolating other parts of the circuit.
I've rebuilt one side with new components - no help
There was a thread:
Which mentioned droping the grid leak resistors from 330k to 220k and increase coupling cap value from 0.47 to 0.1-0.2uF to account for newer valves
Feel like I'm missing something obvious but going slowly mad at same time. Anyone think those mods will help? Also wondered about adding in dedicated screen resistors for each output valve?
thanks for any help
I'm repairing a Fisher 400. Customer wanted new 7868 bases - done. I've replaced the filter caps with the Hayseed Hamfest excellent replacement set - fine and dandy so far. Voltages all fine.
However, I'm getting an oscillation when the PI and power valves are inserted (can even by one PI valve and one 7868):
Here is the relevant section of the schematic, have attached the whole thing hopefully:
Oscillation goes is the PI valves removed or the power valves are. It builds up as the valves come into emission. Theres a 1200 ohm 10W resistor which drops the HT down to the screen node which then goes off the other rest of the HT, the voltage drop without the oscillation is about 25Vdc, when both the PI adn power valves inserted that voltage drop goes to >100Vdc. I thought a short but cannot find one, even isolating other parts of the circuit.
I've rebuilt one side with new components - no help
There was a thread:
Which mentioned droping the grid leak resistors from 330k to 220k and increase coupling cap value from 0.47 to 0.1-0.2uF to account for newer valves
Feel like I'm missing something obvious but going slowly mad at same time. Anyone think those mods will help? Also wondered about adding in dedicated screen resistors for each output valve?
thanks for any help
You might be having PI problems as discussed in these threads. Apparently the 12AX7 used for the PI can greatly effect how it behaves and as designed requires a fairly strong old stock tube. DaveG suggests an improvement to allow it to operate properly with just about any AX7. He move the voltage feed to a higher tap point and modifies the circuit slightly.
https://audiokarma.org/forums/index...ose-revisited-for-500c-800c-receivers.857297/
https://audiokarma.org/forums/index...art-ii-for-400-and-500-800b-receivers.860275/
I believe this is the final schematic for all of his upgrades/changes. You can see the PI final result. Be sure to notice the voltage feed points for both the PI and VA stage.
https://audiokarma.org/forums/index...g-the-fisher-400.511867/page-18#post-12911459
https://audiokarma.org/forums/index...ose-revisited-for-500c-800c-receivers.857297/
https://audiokarma.org/forums/index...art-ii-for-400-and-500-800b-receivers.860275/
I believe this is the final schematic for all of his upgrades/changes. You can see the PI final result. Be sure to notice the voltage feed points for both the PI and VA stage.
https://audiokarma.org/forums/index...g-the-fisher-400.511867/page-18#post-12911459
Oscillating at 44Hz?
Start with simple stuffs if you have not. e.g. Check the B+ filter caps. Check the grid bias voltages for the output tube.
Is that possible you mixed up the transformer wires so that the negative feedback becomes to positive feedback?
Start with simple stuffs if you have not. e.g. Check the B+ filter caps. Check the grid bias voltages for the output tube.
Is that possible you mixed up the transformer wires so that the negative feedback becomes to positive feedback?
I would say you could have swapped the grids or the plates of the output tubes when redoing the bases.
thanks folks. its always the simple stuff you miss.......it was indeed a NFB being a positve feedback loop! On to the next issue of a very irritating diffecerence between left and right PI output! the battle continues
thanks all
thanks all
just bumping this up as need some more help (!). Once stable I then found that the bias voltage was 5V too low and teh 7868's were. biased close to 170%. with bias adjusted to -15Vdc as per schematic got it down to 85% dissipation.
However the output is incredibly low and unbalanced. took out the NFB loop but getting <10W per channel max clean output.
Measuring the DCR on the output primaries as aprt of getting plate current found that the left channel measured 265 ohm and 198 ohm either side of the CT and the right channel 245 and 111ohms. I know they don't have to match but seems big difference.
Worried that becasue of the oscillation and the high bias current that the OPTs are bust
Don't know anythign about the amp so maybe these DCR readings are fine
However the output is incredibly low and unbalanced. took out the NFB loop but getting <10W per channel max clean output.
Measuring the DCR on the output primaries as aprt of getting plate current found that the left channel measured 265 ohm and 198 ohm either side of the CT and the right channel 245 and 111ohms. I know they don't have to match but seems big difference.
Worried that becasue of the oscillation and the high bias current that the OPTs are bust
Don't know anythign about the amp so maybe these DCR readings are fine
It looks like your transformer with 265 and 198 might have an open winding on the 198 half. The tranny probably has 2 parallel windings per side and the high resistance compared to the other tranny with 111 means it has one coil open. It could be as simple as a broken solder joint where the coil ends come out to the CT or plate leads.
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