Fisher 500c Left channel buzz

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I have a Fisher 500c that has been recapped and serviced. I have had it for the last few months and love it. I just recently replaced a couple of the old tubes for the high and low filters, tubes V14 and V15 with new Sovtek 12AX7's.

The sound of the receiver improved and it really opened up. However, there is now a noticeable buzz in the left channel. The buzz is present at zero volume regardless of source, and it is not altered at all by volume, tone, or balance controls. I am also using very efficient Klipsch speakers, so I know they are going to really pick up on any buzz from the receiver whereas my other speakers would not. The buzz doesn't affect sound quality when music is playing, but it bothers me when changing records, etc. The right channel is very quiet with no buzz at all.

I put the old tubes back in just to make sure it wasn't the new tubes themselves. The buzz was still present with the old tubes, I just didn't really notice it. The new tubes just make it stand out more, as the sound is much more clear.

Where should I be looking first for the buzzing sound? I am pretty new to troubleshooting equipment myself, but since it is independent of source, volume, and tone, it seems like it may just be some leaky voltage coming from somewhere? Thanks.
 
The tubes you replaced (V14 and V15) unfortunately would have nothing to do with the buzz, since they appear before the volume control in the circuit. Therefore, if the buzz is present with the volume control full down, they cannot have an impact on this issue. The tubes that would have an impact on your issues would be V8, V9, and V12 -- with the problem most likely associated with V12. Honestly, since you just discovered the buzz, it was in all likelihood present all along, and you have just now noticed it.

Try this: Remove V12, and then turn the receiver on. The Left channel of course won't work, but if the buzz is gone, that will prove it is associated with that tube. On the other hand, if the buzz is still present, then it must be associated with the output tubes V8 and V9. If that is the case, try swapping V8 and V9 with V10 and V11. If the buzz now moves to the other channel, then you have a well unbalanced pair of output tubes (originally in the V8, V9 position) that can also cause low level hum and noise.

I hope this helps!

Dave
 
The tubes you replaced (V14 and V15) unfortunately would have nothing to do with the buzz, since they appear before the volume control in the circuit. Therefore, if the buzz is present with the volume control full down, they cannot have an impact on this issue. The tubes that would have an impact on your issues would be V8, V9, and V12 -- with the problem most likely associated with V12. Honestly, since you just discovered the buzz, it was in all likelihood present all along, and you have just now noticed it.

Try this: Remove V12, and then turn the receiver on. The Left channel of course won't work, but if the buzz is gone, that will prove it is associated with that tube. On the other hand, if the buzz is still present, then it must be associated with the output tubes V8 and V9. If that is the case, try swapping V8 and V9 with V10 and V11. If the buzz now moves to the other channel, then you have a well unbalanced pair of output tubes (originally in the V8, V9 position) that can also cause low level hum and noise.

I hope this helps!

Dave

Dave, thanks so much for the guidance. I removed v12 and the buzz remained. I then swapped v8 and v9 with the other output tubes. It seemed to reduce the hum in the left channel but put a much slighter hum in the left channel so that both have a quieter hum. This seems to still point to the output tubes. The amp came with EH tubes but I have no idea how well matched they are.
 
So you know if your tech installed an Individual Bias Balance Modification to your receiver when it was recapped? If so, it will let you balance the output tube current to null out any hum from mismatched tubes.

Dave
 
So you know if your tech installed an Individual Bias Balance Modification to your receiver when it was recapped? If so, it will let you balance the output tube current to null out any hum from mismatched tubes.

Dave

Dave,

I do have a bias adjustment added in the receiver, but there is only one pot. Does that mean the channels cannot be individually biased? Does the one pot control both channels? See photos below of whole interior and one just focusing on the bias pot. I have adjusted bias on a couple of vintage Sansui SS receivers, but I have never done it with a tube amp. Is the process very different?

I will eventually invest in some new output tubes. Seems like the Tung-sol tubes require some modification to the circuits, however.

Interior Bias Pot.JPG Interior.JPG
 
So you know if your tech installed an Individual Bias Balance Modification to your receiver when it was recapped? If so, it will let you balance the output tube current to null out any hum from mismatched tubes.

Dave

Dave, I have been trouble-shooting this a little more, and I can't get anything I do to get them humm to go away. It stays mostly in the left channel still. It seemed to move slightly to the right channel when I swapped the power tubes, but the main humm is still in the left. I also tried swapping the phase inverter tubes and changing around all of the 12ax7's, but it made no real difference. I also contacted the person who installed the bias pot, and it adjusts the bias of all four tubes at once, so it's designed to be used with matched tubes.

Where do you think I should look next? Still seems like a problem in the left channel output section.
 
Correct. The original receiver was designed to use a matched quad of tubes of a pre-selected characteristic, with the bias voltage in the receiver set to a permanent value by fixed resistors. By installing the bias control as your tech did, it allows for tubes of somewhat different characteristics to be used (which occur due to manufacturing tolerances), but with only one adjustment, they still must be a matched quad for best performance, and to keep hum to a minimum. Next two questions then:

1. Do you know if it is a matched quad of output tubes installed?
2. Do you know if the power supply can caps have been replaced/

It would also be helpful if you can identify if it is 60Hz hum (the kind produced from an open ground), or 120 Hz hum (one octave higher). The latter is almost always generated by power supply issues, and so is a helpful troubleshooting tool.

Your 500C should be very quiet when turned to minimum volume, even with sensitive Klipsch speakers connected. If you would, please include a close up pic of the complete output tube area, and left channel phase inverter tube area.

Dave
 
Correct. The original receiver was designed to use a matched quad of tubes of a pre-selected characteristic, with the bias voltage in the receiver set to a permanent value by fixed resistors. By installing the bias control as your tech did, it allows for tubes of somewhat different characteristics to be used (which occur due to manufacturing tolerances), but with only one adjustment, they still must be a matched quad for best performance, and to keep hum to a minimum. Next two questions then:

1. Do you know if it is a matched quad of output tubes installed?
2. Do you know if the power supply can caps have been replaced/

It would also be helpful if you can identify if it is 60Hz hum (the kind produced from an open ground), or 120 Hz hum (one octave higher). The latter is almost always generated by power supply issues, and so is a helpful troubleshooting tool.

Your 500C should be very quiet when turned to minimum volume, even with sensitive Klipsch speakers connected. If you would, please include a close up pic of the complete output tube area, and left channel phase inverter tube area.

Dave

Dave,

Thanks again for being so helpful. I do not know how well matched the output tubes are, but after contacting the seller, I don't think they are very well matched. However swapping channels didn't really affect the humm in a significant way.

I did connect the left speaker to the speaker-2 terminals to try to isolate each channel more easily. The humm remains strongest in the left speaker, but the same humm is slightly present in the right channel as well. It helped to split them up so I could better hear the right channel humm with the left channel completely off. Neither are affected by volume or tone controls.

The humm sounds like a 120hz signal, not a ground humm, so that points to power supply. I know that some of the can caps have been replaced, but I don't know that all have been. I didn't have time to check this morning, and if they have been replaced then they were restuffed in the original cans, as the cans are still on top of the chassis. I plan to pull the receiver and open it up when I get home.

The only picture I have right now is the one I posted before, but I did go ahead and zoom in on the power tube and phase inverter section. I can get better photos later. Thanks again.

Screen Shot 2016-11-21 at 7.17.28 AM.png
 
Thanks for the pic -- they're always worth a thousand words. Your tech installed 10 Ohm cathode sampling resistors for each output tube so that you can measure the current draw of each tube individually. They also installed 100 Ohm Screen Stability resistors -- and this is where the problem lies: The resistors were installed well enough, but the wiring associated with their installation was not done correctly:

1. In the pic you provided, on the right most output tube socket as shown, there is a orange wire connected to pin 8. This wire needs to be disconnected from that terminal, and connected to either one of the terminals on the new 2 terminal T-Strip that has been installed, that has its terminals jumered together.

2. Similarly, on the output tube socket that is third from the right, there is a 1.2K 1W resistor that is connected to pin 8 of that socket, that also needs to be disconnected from that terminal, and connected to either terminal of the new 2 terminal T-strip as well.

There is potentially another issue that I cannot tell yet as the picture is not detailed enough, but concerns the black resistor and purplish looking resistors also associated with the new 2 terminal T-Strip. If you can provide a close up of their connections I can check that for you also.

Dave
 
Thanks for the pic -- they're always worth a thousand words. Your tech installed 10 Ohm cathode sampling resistors for each output tube so that you can measure the current draw of each tube individually. They also installed 100 Ohm Screen Stability resistors -- and this is where the problem lies: The resistors were installed well enough, but the wiring associated with their installation was not done correctly:

1. In the pic you provided, on the right most output tube socket as shown, there is a orange wire connected to pin 8. This wire needs to be disconnected from that terminal, and connected to either one of the terminals on the new 2 terminal T-Strip that has been installed, that has its terminals jumered together.

2. Similarly, on the output tube socket that is third from the right, there is a 1.2K 1W resistor that is connected to pin 8 of that socket, that also needs to be disconnected from that terminal, and connected to either terminal of the new 2 terminal T-strip as well.

There is potentially another issue that I cannot tell yet as the picture is not detailed enough, but concerns the black resistor and purplish looking resistors also associated with the new 2 terminal T-Strip. If you can provide a close up of their connections I can check that for you also.

Dave

Dave, you are the man! I will take some better photos of this section, as it seems like this may be where my issue is originating. I See the orange wire on tube socket V8 and I think the resistor on V10 that you mention. These would both need to go to either terminal on the T-strip that has been jumpered (to me this is the beige strip just below the large black and light-purple resistors you also mention). I will be sure to get a good image of where those resistors are going when I get home.

Do the terminals on the T-strip need to remain jumpered? I guess it wont matter which terminal then on the T-strip that I move the orange wire and the 1.2K 1W resistor to? Should I go ahead and take care of these connections or wait until you can have a look at the other resistors? Thanks again.
 
Yes, the terminals need to remain jumpered, and you are correct that it really doesn't matter which terminal you connect the orange wire and 1.2K resistor to -- both of the two terminals represent the same point in the circuit electrically. Those changes need to be made regardless, so you can go ahead and make those changes. You did correctly identify the 2 terminal T-strip I have discussed. When you send the closeup pics, then I'll have a look to make sure that the rest of the connections associated with this strip are correct.

Dave
 
Dave! I moved the orange wire and the resistor to the T-strip, and the hum is gone in both channels. The receiver sounds great. There is still a little volume-and-tone-dependent humm, but it is not noticeable at listening levels (it has to be turned wayyyy up to hear it) and I guess may have to do with poorly matched tubes.

Just curious, but would moving either of those parts have affected the temperature at which the amp runs? I am not sure if it is just me or if it is something different with the power tubes now, but they do seem to be pretty hot. Not that I have anything concrete to reference them by besides a general observation. Haven't felt their temp in a while, so could just always be that warm.

Here are some better photos of the other black and purple-gray resistor. They both connect to the T-strip on one end and another strip on the other end. Let me know if these connections are concerning to you.
IMG_5404.JPG IMG_5409.JPG
 
Glad that fixed it! You will always hear some hiss and possibly low level hum with the volume well elevated. It is simply the sound of amplification, but should be negligible at normal listening levels -- even with sensitive speakers. Since it only occurs by elevating the volume control, this has nothing to do with the matching of the output tubes, but everything to do with the phono preamp tubes (when selected), and the line/tone stage tubes. You might try different tubes in these positions to see if any other are quieter than the ones you currently have installed.

Changing the connections did -- in theory -- cause the two output tubes you removed the connections from to run hotter -- but only minutely so. The change caused those two tubes to increase their current draw by less than 2 ma each -- a very minor amount.

The connections with the black and light purple resistors are fine. There is supposed to be a single 1.2K 7w resistor in the location of the two resistors in question. Apparently, the tech did not have one, and therefore used a 2.2K and 2.7K resistor in parallel with each other -- a combination that porduces 1.212K -- pretty darn close.

Good luck with your 500C!

Dave
 
Glad that fixed it! You will always hear some hiss and possibly low level hum with the volume well elevated. It is simply the sound of amplification, but should be negligible at normal listening levels -- even with sensitive speakers. Since it only occurs by elevating the volume control, this has nothing to do with the matching of the output tubes, but everything to do with the phono preamp tubes (when selected), and the line/tone stage tubes. You might try different tubes in these positions to see if any other are quieter than the ones you currently have installed.

Changing the connections did -- in theory -- cause the two output tubes you removed the connections from to run hotter -- but only minutely so. The change caused those two tubes to increase their current draw by less than 2 ma each -- a very minor amount.

The connections with the black and light purple resistors are fine. There is supposed to be a single 1.2K 7w resistor in the location of the two resistors in question. Apparently, the tech did not have one, and therefore used a 2.2K and 2.7K resistor in parallel with each other -- a combination that porduces 1.212K -- pretty darn close.

Good luck with your 500C!

Dave

Dave, thanks again for the help. I have never checked or adjusted the bias on this receiver or any tube equipment. I think this is something I want to at least check on this unit, especially after the changes. I have a good multimeter that I have used with success on solid state gear. Do I need anything else to measure bias on a tube? Do I just need to identify which pins to measure in each socket?
 
Set your meter to DCV, and then put a lead on each end of the 10ohm resistor on pin 5 of each tube in turn. This will give you the reading in mv. take the reading and move the decimal 1 place to the left. FOR EXAMPLE your meter reads 0.350 (350mv). Move the decimal to the left one place and you get 0.035. This is the reading in ma. INSULATE THE PROBE LEADS with tape or heatshrink all the way to the tip. You just want the tip of the probe showing so you lessen the chance of a short circuit if you for example sneeze, or the probe slips.

Take those readings and write them down. Now put the black probe on a clean shiny part of the chassis and clamp it there(If your meter came with alligator clips even better!). Take the red probe and get a reading on the plate of each tube (pin 3).

To get the Wattage for each tube you multiply the pin 3 reading by the ma. NOT the mv. So for example you get 430v on pin 3, and your ma are .035. 430x.035=15.05 This is about as high as you want to take it with 80% being 15.2w.

Then there is the screen draw of between 2 and 3 ma to consider. Subtract the screen draw ( I use 3ma for safety purposes) and you get 12.05w.

SUBSTITUTE your Numbers in place of the example's numbers and you'll get your tube output's.
 
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Set your meter to DCV, and then put a lead on each end of the 10ohm resistor on pin 5 of each tube in turn. This will give you the reading in mv. take the reading and move the decimal 1 place to the left. FOR EXAMPLE your meter reads 0.350 (350mv). Move the decimal to the left one place and you get 0.035. This is the reading in ma. INSULATE THE PROBE LEADS with tape or heatshrink all the way to the tip. You just want the tip of the probe showing so you lessen the chance of a short circuit if you for example sneeze, or the probe slips.

Take those readings and write them down. Now put the black probe on a clean shiny part of the chassis and clamp it there(If your meter came with alligator clips even better!). Take the red probe and get a reading on the plate of each tube (pin 3).

To get the Wattage for each tube you multiply the pin 3 reading by the ma. NOT the mv. So for example you get 430v on pin 3, and your ma are .035. 430x.035=15.05 This is about as high as you want to take it with 80% being 15.2w.

Then there is the screen draw of between 2 and 3 ma to consider. Subtract the screen draw ( I use 3ma for safety purposes) and you get 12.05w.

SUBSTITUTE your Numbers in place of the example's numbers and you'll get your tube output's.

I think I should be able to handle this. Just a couple of questions:

1. Should I be shooting for around 15 watts of output on each tube, or should it be around 12w after my readings and calculation? Uncertain of whether I need to turn down the bias until I actually get a reading of 12w or if the subtraction down to 12w is taken care of by the screen and it's just useful to know the actual output.

2. Since the screen draw is 2-3 ma, should I subtract that from the final wattage calculation or subtract it from the initial ma reading? Seems like units are getting mixed up to me. Or is that just a guideline where we aren't actually subtracting units but just backing off the output by a few watts?

I do have alligator clips, so that is probably what I will use to take these readings. Thanks a lot for your help.
 
15.2w is 80% of max dissipation of the tube. This is the current recommended maximum wattage for Safe operation of the new (and old) tubes. 13.3w is 70% and recommended minimum. So anywhere in btwn is good for the final bias readings. In other words Once you set the bias for a predetermined value (btwn 70 and 80%) you're done. The subtraction is taken care of automatically by the screen. You don't need to do any corrections based on the screen draw.
 
15.2w is 80% of max dissipation of the tube. This is the current recommended maximum wattage for Safe operation of the new (and old) tubes. 13.3w is 70% and recommended minimum. So anywhere in btwn is good for the final bias readings. In other words Once you set the bias for a predetermined value (btwn 70 and 80%) you're done. The subtraction is taken care of automatically by the screen. You don't need to do any corrections based on the screen draw.

Got it. That's what I figured but thanks for the clarification. I'm still pretty new to tube gear, and so far I love it. But I have a lot of learning to do!
 
Glad that fixed it! You will always hear some hiss and possibly low level hum with the volume well elevated. It is simply the sound of amplification, but should be negligible at normal listening levels -- even with sensitive speakers. Since it only occurs by elevating the volume control, this has nothing to do with the matching of the output tubes, but everything to do with the phono preamp tubes (when selected), and the line/tone stage tubes. You might try different tubes in these positions to see if any other are quieter than the ones you currently have installed.

Changing the connections did -- in theory -- cause the two output tubes you removed the connections from to run hotter -- but only minutely so. The change caused those two tubes to increase their current draw by less than 2 ma each -- a very minor amount.

The connections with the black and light purple resistors are fine. There is supposed to be a single 1.2K 7w resistor in the location of the two resistors in question. Apparently, the tech did not have one, and therefore used a 2.2K and 2.7K resistor in parallel with each other -- a combination that porduces 1.212K -- pretty darn close.

Good luck with your 500C!

Dave

Dave,

I started noticing a little bit louder hum over the last couple of days. It seems to come and go between uses of the receiver and it is volume dependent and pretty equal in both channels. I tried different turntables and the aux input with a jumper cable and it remained. It is a bit louder with the phono input than aux.

I also noticed that there is a bit of a rhythmic rapid sputtering sound coming out of the right channel with the phono input selected, but when I touch tube v16, the phono left tube, it stops. I thought it may just be the tube but I swapped a few around and it remains. It seems like there is something going on with the phono section perhaps?

The transformer itself was also physically buzzing. I made sure the screws holding it together were tight and then reseated it on the chassis. I looked inside for anything suspicious, but I wasn't sure what to look for that would cause the transformer itself to buzz. It seemed to stop buzzing as much for a little while but it's starting to do it again. Is it electrically buzzing or mechanically buzzing? Would that have to do with the humm I'm getting?

Thanks again for any advice.
 
Transformer hum is a different issue from hum in the speakers. Transformer hum can be anything from loose mounting bolts to loose through bolts, to a loose coil form inside the transformer.

As for the hum from the speakers, the information regarding the Aux input is confusing. So:

With nothing connected to the Aux input, the Aux input selected, and the volume advanced, there should be little if any hum at all. Is there? Or is there not? Let's get this established first before moving on to the phono section.

Dave
 
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