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Fisher 400 both channels distortion

So what are we saying here? is there something else to try or test before switching the tubes? My EFB trimmer is maxed out at -13.5v I only can go between -13.5v to roughly -15.4v with that trimmer
 
streetwise said:
So what are we saying here? is there something else to try or test before switching the tubes? My EFB trimmer is maxed out at -13.5v I only can go between -13.5v to roughly -15.4v with that trimmer
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What is the currently installed value of the resistor connected between the EFB trimmer and “first” transistor base? You will need to reduce that value to reduce negative voltage output from the EFB. That will allow you to increase the current flow through the output tubes.
 
I believe that is the 75k resistor that was originally a 39k before the IBBA boad was installed. I also had that as 150k before
 
streetwise said:
I believe that is the 75k resistor that was originally a 39k before the IBBA boad was installed. I also had that as 150k before
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Try a value in between 39k and 75k and see if it will allow you to reach 21mA across each cathode resistor. When you get to that point, you’ll need a scope and signal generator to drive amplifier to the max output voltage before output start to clip. That will tell you the true life level of the output tubes.
 
I changed the value to 57k, I have -20.5v on the and 211mv across the cathode resistors. I was looking at the "fisher 400 with EFB,IBAM,PS,PH_INV and control section update_REV7.pdf" I noticed they have a 47k resistor in that position and they show -17v supplying the IBAM? should I lower the EFB to this -17 and reset the tubes to 210mv?
 
It doesn’t matter at this point as long as you are able to dial on the specified current through each output tube. Changing that resistor on EFB will change the physical position of wipers on your IBBA trimmers. Ideally, they need to be in the middle of their travel.
Do you have a scope?
Dummy loads?
Signal generator?
 
grindfix said:
It doesn’t matter at this point as long as you are able to dial on the specified current through each output tube. Changing that resistor on EFB will change the physical position of wipers on your IBBA trimmers. Ideally, they need to be in the middle of their travel.
Do you have a scope?
Dummy loads?
Signal generator?
Click to expand...
I don't have any of those. I rarely do anything that I would use a signal generator or scope for, the only other time I had considered getting them was for tuner alignment but than i would still need even more gear. Im up about as far north as you can get in new york and I don't know anyone around here or vermont that does electronic work anymore either.
 
Theoretically, someone with experience could probably close estimate power output across dummy loads with voltmeter without scope. At some point the output voltage will stop increasing because it will clip. Actual “clean” power output will be below that of course but if AC output stops increasing above, say 10v across 8ohm load on 8ohm tap, you know your output tubes are done. 10 x 10 : 8 = 12.5 watts. 1KHz tone can be found online.
If I didn’t have equipment to do it properly, I’d probably give it a careful try if I was desperate to know the basic condition of my outputs.
 
grindfix said:
Theoretically, someone with experience could probably close estimate power output across dummy loads with voltmeter without scope. At some point the output voltage will stop increasing because it will clip. Actual “clean” power output will be below that of course but if AC output stops increasing above, say 10v across 8ohm load on 8ohm tap, you know your output tubes are done. 10 x 10 : 8 = 12.5 watts. 1KHz tone can be found online.
If I didn’t have equipment to do it properly, I’d probably give it a careful try if I was desperate to know the basic condition of my outputs.
Click to expand...

interesting what else would could potentialy happen during testing the output power other than the output tubes being damaged? If i were to use the receiver as is for the time being are there any concerns with possibly worn tubes? Would I get noticeable distortion listening to music. I never have had the volume over the 12o'clock position my speakers are roughly 103db efficient I typically listen with the knob at 9 o'clock
 
With solid state amp clipping you can easily damage tweeters. Tube amps clip softer but also less obvious. Still possible to cause damage to speakers, I think.
 
grindfix said:
With solid state amp clipping you can easily damage tweeters. Tube amps clip softer but also less obvious. Still possible to cause damage to speakers, I think.
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hmm interesting, maybe I should get a scope, I was looking at threads and noticed a thread from a few years ago about updating the power supply again on late model fishers ...as well as possibly other riaa mods. Looks like going further down the rabbit hole
 
scopes are a very useful tool to have, and if you aren't picky about it being modern, an older analog scope can often be had for small money.

the down side is they eat bench space. Digitals are smaller, and PC-based ones are real small, assuming you have a computer in the shop already. You also get nice capture abilities. My "digital capture" is me trying to take pictures with my phone's camera.
 
I have plenty of space to put things in storage when not in use so that wouldn't be a problem, I would love to get more into learning to fix electronics just haven't had the time and or money. I see some newer scopes for as low as 40 dollars online not sure how good they are. I love old equipment the only concern is how reliable it is, I have read abit about people doing work on there signal generators and fm calibration gear but needing more gear to align there gear.

I certainly have a bunch of broken stuff that could be fixed up
NAD Tuner
2x Parasound JC1 mono amps
Harman Kardon Citation 17 preamp
Carver amplifier.

I have been playing the Fisher 400 and haven't had any problems that I've noticed I will test the output tubes in the amp when I get a scope and dummy loads but they seem to be in good enough condition as far as listening tests. This receiver is sounds so much more dynamic than the Denon AV receiver I was using while fixing the Fisher.
 
It would be a good idea to re-measure all vital voltages, including the AC input at which DC voltage readings are taken. With output tubes drawing correct current, assumingly 21mA, B+, and screen voltages will likely be lower than you measured them last time. I think you mentioned plate voltage being below 400v without output tubes. Without digging, from memory, I think Dave's specification was 420v when outputs are drawing 21mA each. I wonder why your unloaded (by outputs) voltage is so low. Makes me wonder what your heater voltages are as well.
 
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