Fisher kx 100b hum...maybe..I guess

[MikeTest]

I realize volumes have been written and careers spent on susceptibility and emissions but ask your indulgence as I demonstrate my inexperience. My interest has recently turned to tube amplifiers with a seemingly fully functional kx 100b as my first victim. The only issue I know of is a slight 60 hz hum at the speakers (both channels) that gets louder and softer with the volume control. I am not calibrated to what's normal but don't think it excessive and figure I would chase it in the future with recap and the usual stuff (although I have scoped the power supply and it looks good to me....besides I thought power supply noise would show up at 120 hz...but I digress).

My intention was to study tube theory in parallel with taking measurements on the amp to gain some practical experience. I realize it is seldom a good idea to just poke around on stuff, especially stuff that can kill you so I am careful and methodical.

I did not get too far. I started at the phono preamp and put a scope probe one resistor away from the gate of 12AX7 (R9 for those that want to follow along). Looked normal to me i.e. looked like the input which I had at about 10mvac from signal generator. Tone volume responded accompained with the aforementioned underlying 60hz.

This is where things went off the rails. Not wanting to continually change the scope setting yet wanting to know dc as well as ac I connected a Fluke 77 to the same place as the scope probe. When I did so the 60hz got a lot worse (both thru the speakers and on scope).

So I go into disconnect and turn stuff off mode. In one of the iterations I noticed it was not the meter that caused the noise but the presence of a meter lead even without the meter. Result was the same with a single jumper cable like it acts like an antenna picking up 60 hz. But from what source ? There is nothing on any amp in or outs. The amp is off and not even plugged into ac. I shut off ac power to my garage/shop. There is nothing running on ac within 30 feet. Yet, when I clip a jumper on there is 60hz (with sufficient power to have an effect) showing up in the amp. You may ask how do you know if you have everything (including the scope) turned off ? Which is a good point and if this post weren't already ridiculously long I would go into the dozens of other observation I made in coming to this belief (not to be confused with a stone cold fact).

I would be happy to clarify or provide more information if anyone cares to weigh in.

 

[dcgillespie]

Hi Mike -- Welcome the The AK Fisher Forum! Nice to have you here.

Since you are (apparently) new to vacuum tube circuits, there is one thing you will need to learn about them, which is very important to your question: Compared to SS designs, vacuum tubes operate at very high impedance levels -- particularly at the tube's grid (the equivalent of a mosfet's gate). Therefore, without proper shielding, just touching your finger to the grid terminal, or adding a length of wire to the grid terminal will cause it to pick up all manner of stray 60 Hz noise that is all around us -- even if just in the wiring in the walls of our homes. Your observation then about placing a scope or meter probe on a grid terminal -- particularly that of the phono preamp tube where gain is extremely high (over 5000 from input to output with the volume control at max) -- is very normal.

Fisher amplifiers are normally very quiet with the volume control at minimum, and even advanced, typically only display a hissing sound -- as long as the bottom plate and any other shielding panels are in place. Power supply noise will in fact show up as 120 Hz noise, so if you are getting 60 Hz noise, look for lead placement issues (lead dressing), a missing power switch shield cap, or even a missing AC line bypass cap (if used). Also, try reversing the AC plug and see if that helps. 60 Hz hum is always the result of AC power wiring inducing noise into sensitive circuits where it should not be. Again, since vacuum tubes operate at high impedance levels, it is easy for sensitive leads to pick up noise if the build is sloppy or not properly executed.

I hope this helps!

Dave

 

[Tim D]

MikeTest, welcome to AK!

I'll step out of the way. The doctor is here. Carry on Dave.

 

[gadget73]

Also if you have any tube shields missing, re-install them. For testing it would be worth making up some shorting plugs for the input. Its just an RCA with the center lead tied to the shell. it helps make sure you're not picking up noise from the open input jack to screw with any testing.

 

[Tim D]

Most importantly BE CAREFUL! There are some very high voltages inside the unit. If you don't know how to be safe around high voltages, enlist someone that does know.

Hum can come from external source inputs too. Does it happen on all input selections? Have you unplugged all the input cables and does it still have the hum? Have you tried flipping the power plug at the socket? (The stock power cord came non-polarized). Do you know the status of the power supply caps? Have those caps recently been checked/replaced? I haven't used the shorting plugs myself and my KX-100 is silent without them and doesn't exhibit hum, but you could give the shorting plugs a try. Having said that if it is happening on all sources with all the sources cables unplugged then you probably have something going on internal. You might consider cleaning all the tube sockets and make sure they are all making good connection. Do an inspection of the underside wiring and look for damaged parts. You might clean all the slide switches and the selector switch. I used Deoxit to clean the contacts.

I ended up going through my KX-100 very thoroughly. I went so far as to replace all Power Supply (PS) caps, the rectifiers, coupling caps and power resistors. I also checked all of the resistors and found a few that were out of spec and replaced them. I also verified all the voltages throughout (against the available schematics) and fixed some problems along the way. Keep in mind these were built as kits originally and there may be some mistakes in the build. I found a couple of mistakes in mine and some later hacks that I had to undo. Mine isn't 100% stock anymore and I have a few of the more popular mods on mine. It's a nice unit and well worth restoring to good operation. Mine is in my main rotation now.

Good luck. The community is here to help, but Dave is the respected Doctor here. His expertise is one of the more entertaining things to watch and to learn from. (But he's not the only one to learn from. There are others that aren't life long technical, but they will share their experiences and help guide you almost as well as Dave can teach. While I'm not a total novice, I am in the category of just sharing what I've learned. I'm not the Doctor that Dave is.)

 

[MikeTest]

Hi Mike -- Welcome the The AK Fisher Forum! Nice to have you here.

Since you are (apparently) new to vacuum tube circuits, there is one thing you will need to learn about them, which is very important to your question: Compared to SS designs, vacuum tubes operate at very high impedance levels -- particularly at the tube's grid (the equivalent of a mosfet's gate). Therefore, without proper shielding, just touching your finger to the grid terminal, or adding a length of wire to the grid terminal will cause it to pick up all manner of stray 60 Hz noise that is all around us -- even if just in the wiring in the walls of our homes. Your observation then about placing a scope or meter probe on a grid terminal -- particularly that of the phono preamp tube where gain is extremely high (over 5000 from input to output with the volume control at max) -- is very normal.

Fisher amplifiers are normally very quiet with the volume control at minimum, and even advanced, typically only display a hissing sound -- as long as the bottom plate and any other shielding panels are in place. Power supply noise will in fact show up as 120 Hz noise, so if you are getting 60 Hz noise, look for lead placement issues (lead dressing), a missing power switch shield cap, or even a missing AC line bypass cap (if used). Also, try reversing the AC plug and see if that helps. 60 Hz hum is always the result of AC power wiring inducing noise into sensitive circuits where it should not be. Again, since vacuum tubes operate at high impedance levels, it is easy for sensitive leads to pick up noise if the build is sloppy or not properly executed.

I hope this helps!

Dave

View attachment 1242177

Hi Mike -- Welcome the The AK Fisher Forum! Nice to have you here.

Since you are (apparently) new to vacuum tube circuits, there is one thing you will need to learn about them, which is very important to your question: Compared to SS designs, vacuum tubes operate at very high impedance levels -- particularly at the tube's grid (the equivalent of a mosfet's gate). Therefore, without proper shielding, just touching your finger to the grid terminal, or adding a length of wire to the grid terminal will cause it to pick up all manner of stray 60 Hz noise that is all around us -- even if just in the wiring in the walls of our homes. Your observation then about placing a scope or meter probe on a grid terminal -- particularly that of the phono preamp tube where gain is extremely high (over 5000 from input to output with the volume control at max) -- is very normal.

Fisher amplifiers are normally very quiet with the volume control at minimum, and even advanced, typically only display a hissing sound -- as long as the bottom plate and any other shielding panels are in place. Power supply noise will in fact show up as 120 Hz noise, so if you are getting 60 Hz noise, look for lead placement issues (lead dressing), a missing power switch shield cap, or even a missing AC line bypass cap (if used). Also, try reversing the AC plug and see if that helps. 60 Hz hum is always the result of AC power wiring inducing noise into sensitive circuits where it should not be. Again, since vacuum tubes operate at high impedance levels, it is easy for sensitive leads to pick up noise if the build is sloppy or not properly executed.

I hope this helps!

Dave

My unit is the Stratakit kx100 for which I have an assembly manual but don't have (haven't found yet) a schematic. The schematic I am using is for a x-100-b, which I thought was the same as the kit. However, your comment about AC line bypass caps got me looking in this area. As far as I can tell there are no AC bypass caps at all on my unit nor is there any mention of them in the assembly instructions. On the other hand the x-100-b shows C33 (schematic below). I am not sure that is considered an AC bypass cap but my amp does not have it. In any event I wonder if it would be wise to add AC bypass to my unit just on the principal (in my past life I was trained to never use the S word).

 

[Tim D]

View attachment 1242177

My unit is the Stratakit kx100 for which I have an assembly manual but don't have (haven't found yet) a schematic. The schematic I am using is for a x-100-b, which I thought was the same as the kit. However, your comment about AC line bypass caps got me looking in this area. As far as I can tell there are no AC bypass caps at all on my unit nor is there any mention of them in the assembly instructions. On the other hand the x-100-b shows C33 (schematic below). I am not sure that is considered an AC bypass cap but my amp does not have it. In any event I wonder if it would be wise to add AC bypass to my unit just on the principal (in my past life I was trained to never use the S word).
View attachment 1242197

That is one of a couple of differences between the X100B and the KX-100. The C33 I think is for switching noise. I added one to my KX-100, but after the fact I was thinking that I should have done more research to get a cap that is designed to fail open rather than fail shorted. Maybe Dave might comment.

Here is a quote from one of my own posts on my KX-100 thread about the differences.
=====
Discovered differences between the KX-100 and X-100-B.

Just as a side note. I've been digging through the diagrams and parts lists for the KX-100 and the X-100-B. I have diagrams for two different versions of the X-100-B but I can't find any electrical differences in the two versions of the X-100-B. Here are the differences between the KX-100 and X-100-B that I've found so far.

1. The X-100-B has a single cap added to the power supply where the secondary of the power transformer joins the two diodes. This is listed as C33 which doesn't exist on the KX-100 and it runs from that junction to ground. It's listed as a "Molded 0.01 uF, 20%, 600V". I'm wondering if it is some kind of filter for high frequency (RF maybe?) that might come in on the line? Useful at all? [EDIT: Found a thread that talks about this extra cap from dcgillespie. It's in post #51 1st paragraph of this thread: http://audiokarma.org/forums/index.php?threads/fisher-kx-100-x-100-b-restoration.572641/page-3]

2. Values for C23 and 24 were changed from 100 pF (KX-100) to 180 pF (X-100-B). These are in between the two halves of each of V5 (and V6). I.e. Between the Plate of the first half and the grid of the second half. These caps go to ground. Same reasoning? Is this a filter to reduce RF interference? Is this useful at all?

3. The rest are relating to the difference in the bias circuits with the KX-100 (fixed) and X-100-B (adjustable - with a single adjustment for the entire output stage).
a. R69 (in the X-100-B) has been moved to where R70 was located (in KX-100).
b. R70 (in the X-100-B) has been added to a new location and is now a 5K pot.
c. R76 has been added in the X-100-B in the location where R69 was in the KX-100.

That's all I've found so far and it might be complete as I've gone through the entire parts lists of all 3 versions. (Assuming that there were no other part relocation's excepting for what I've already identified in the bias circuit for the output stage.)

 

[MikeTest]

That is one of a couple of differences between the X100B and the KX-100. The C33 I think is for switching noise. I added one to my KX-100, but after the fact I was thinking that I should have done more research to get a cap that is designed to fail open rather than fail shorted. Maybe Dave might comment.

Here is a quote from one of my own posts on my KX-100 thread about the differences.
=====
Discovered differences between the KX-100 and X-100-B.

Just as a side note. I've been digging through the diagrams and parts lists for the KX-100 and the X-100-B. I have diagrams for two different versions of the X-100-B but I can't find any electrical differences in the two versions of the X-100-B. Here are the differences between the KX-100 and X-100-B that I've found so far.

1. The X-100-B has a single cap added to the power supply where the secondary of the power transformer joins the two diodes. This is listed as C33 which doesn't exist on the KX-100 and it runs from that junction to ground. It's listed as a "Molded 0.01 uF, 20%, 600V". I'm wondering if it is some kind of filter for high frequency (RF maybe?) that might come in on the line? Useful at all? [EDIT: Found a thread that talks about this extra cap from dcgillespie. It's in post #51 1st paragraph of this thread: http://audiokarma.org/forums/index.php?threads/fisher-kx-100-x-100-b-restoration.572641/page-3]

2. Values for C23 and 24 were changed from 100 pF (KX-100) to 180 pF (X-100-B). These are in between the two halves of each of V5 (and V6). I.e. Between the Plate of the first half and the grid of the second half. These caps go to ground. Same reasoning? Is this a filter to reduce RF interference? Is this useful at all?

3. The rest are relating to the difference in the bias circuits with the KX-100 (fixed) and X-100-B (adjustable - with a single adjustment for the entire output stage).
a. R69 (in the X-100-B) has been moved to where R70 was located (in KX-100).
b. R70 (in the X-100-B) has been added to a new location and is now a 5K pot.
c. R76 has been added in the X-100-B in the location where R69 was in the KX-100.

That's all I've found so far and it might be complete as I've gone through the entire parts lists of all 3 versions. (Assuming that there were no other part relocation's excepting for what I've already identified in the bias circuit for the output stage.)

That is one of a couple of differences between the X100B and the KX-100. The C33 I think is for switching noise. I added one to my KX-100, but after the fact I was thinking that I should have done more research to get a cap that is designed to fail open rather than fail shorted. Maybe Dave might comment.

Here is a quote from one of my own posts on my KX-100 thread about the differences.
=====
Discovered differences between the KX-100 and X-100-B.

Just as a side note. I've been digging through the diagrams and parts lists for the KX-100 and the X-100-B. I have diagrams for two different versions of the X-100-B but I can't find any electrical differences in the two versions of the X-100-B. Here are the differences between the KX-100 and X-100-B that I've found so far.

1. The X-100-B has a single cap added to the power supply where the secondary of the power transformer joins the two diodes. This is listed as C33 which doesn't exist on the KX-100 and it runs from that junction to ground. It's listed as a "Molded 0.01 uF, 20%, 600V". I'm wondering if it is some kind of filter for high frequency (RF maybe?) that might come in on the line? Useful at all? [EDIT: Found a thread that talks about this extra cap from dcgillespie. It's in post #51 1st paragraph of this thread: http://audiokarma.org/forums/index.php?threads/fisher-kx-100-x-100-b-restoration.572641/page-3]

2. Values for C23 and 24 were changed from 100 pF (KX-100) to 180 pF (X-100-B). These are in between the two halves of each of V5 (and V6). I.e. Between the Plate of the first half and the grid of the second half. These caps go to ground. Same reasoning? Is this a filter to reduce RF interference? Is this useful at all?

3. The rest are relating to the difference in the bias circuits with the KX-100 (fixed) and X-100-B (adjustable - with a single adjustment for the entire output stage).
a. R69 (in the X-100-B) has been moved to where R70 was located (in KX-100).
b. R70 (in the X-100-B) has been added to a new location and is now a 5K pot.
c. R76 has been added in the X-100-B in the location where R69 was in the KX-100.

That's all I've found so far and it might be complete as I've gone through the entire parts lists of all 3 versions. (Assuming that there were no other part relocation's excepting for what I've already identified in the bias circuit for the output stage.)

That is one of a couple of differences between the X100B and the KX-100. The C33 I think is for switching noise. I added one to my KX-100, but after the fact I was thinking that I should have done more research to get a cap that is designed to fail open rather than fail shorted. Maybe Dave might comment.

Here is a quote from one of my own posts on my KX-100 thread about the differences.
=====
Discovered differences between the KX-100 and X-100-B.

Just as a side note. I've been digging through the diagrams and parts lists for the KX-100 and the X-100-B. I have diagrams for two different versions of the X-100-B but I can't find any electrical differences in the two versions of the X-100-B. Here are the differences between the KX-100 and X-100-B that I've found so far.

1. The X-100-B has a single cap added to the power supply where the secondary of the power transformer joins the two diodes. This is listed as C33 which doesn't exist on the KX-100 and it runs from that junction to ground. It's listed as a "Molded 0.01 uF, 20%, 600V". I'm wondering if it is some kind of filter for high frequency (RF maybe?) that might come in on the line? Useful at all? [EDIT: Found a thread that talks about this extra cap from dcgillespie. It's in post #51 1st paragraph of this thread: http://audiokarma.org/forums/index.php?threads/fisher-kx-100-x-100-b-restoration.572641/page-3]

2. Values for C23 and 24 were changed from 100 pF (KX-100) to 180 pF (X-100-B). These are in between the two halves of each of V5 (and V6). I.e. Between the Plate of the first half and the grid of the second half. These caps go to ground. Same reasoning? Is this a filter to reduce RF interference? Is this useful at all?

3. The rest are relating to the difference in the bias circuits with the KX-100 (fixed) and X-100-B (adjustable - with a single adjustment for the entire output stage).
a. R69 (in the X-100-B) has been moved to where R70 was located (in KX-100).
b. R70 (in the X-100-B) has been added to a new location and is now a 5K pot.
c. R76 has been added in the X-100-B in the location where R69 was in the KX-100.

That's all I've found so far and it might be complete as I've gone through the entire parts lists of all 3 versions. (Assuming that there were no other part relocation's excepting for what I've already identified in the bias circuit for the output stage.)

Thanks Tim D. Sorry for re-ploughing the same field. I realize now how much history their is in the forum I need to catch up on before I ask rookie questions.

 

[fred soop]

That is one of a couple of differences between the X100B and the KX-100. The C33 I think is for switching noise. I added one to my KX-100, but after the fact I was thinking that I should have done more research to get a cap that is designed to fail open rather than fail shorted. Maybe Dave might comment.
.....

There is much discussion about XY rated capacitors (the death cap) on the AC line, but actually, all AC applications (transformer secondaries) should use XY capacitors as well. This would be especially true for tube equipment with the high voltages, not so critical with 40 volt SS supplies.

 

[Tim D]

There is much discussion about XY rated capacitors (the death cap) on the AC line, but actually, all AC applications (transformer secondaries) should use XY capacitors as well. This would be especially true for tube equipment with the high voltages, not so critical with 40 volt SS supplies.

Yep Fred. That’s what I was talking about. XY. I’m going to look up my order and see what I bought for mine. Thanks.

 

[Tim D]

MikeTest,

BTW, there is no letter on the KX-100 model number.

 

[Tim D]

MikeTest,

Just saw a digital copy of the KX-100 assembly manual on the big auction site. I have no idea of the digital quality or whether it has the schematic. You can ask the seller questions though if you are interested.

 

[Tim D]

MikeTest,

Just saw an original copy of the KX-100 assembly manual on the big auction site. If you are interested you will want to ask if it includes the big schematic diagram. I don't see it in the pics. The prices looks good if it has that diagram. There are other diagrams that normally come with it that are helpful, but not absolutely necessary that were for the original assembly.

 

[dilburt]

I have a Fisher x-100b with the 60hz problem. It's hard to tell for sure if it's not 60hz + 120hz because it has a very sharp buzz overlaying the primary frequency. It's the same on all input selections. I tried the suggestions above but no luck. I'm not sure if this is a clue but the hum/buzz gets louder at 1/3 volume then quieter at 1/2 volume then louder again at 3/4 volume and up.

I've replaced all the original power filtering caps with new Nichicon caps. I've tested the remaining resistor and cap values and replaced everything that was out of spec so it's got to be a cable dressing problem but I'm not sure where to start looking or what I should be looking for since I grew up in the digital age. I'm assuming there is some cable carrying power that is running close enough to the preamp tubes that they are picking up the noise inductively. I assume all the shields should only be grounded on one side but what else should I be looking for? Does anyone have a trouble-shooting process they use when trying to find an inductive noise problem like this?

Thanks,

D

 

[Tim D]

What has been done to it during its' life?

How about some closeup screenshots of the underside? Many of the experts can tell what has been worked on and what might be out of place.

Barring that something is wired wrong if the electrolytic caps haven't been updated, then that is where I'd start.

 

[dilburt]

I picked it up for very cheap a couple of years ago as a project since the seller said it had hum problems. I'm a EE/CS and have designed many digital computer systems and even a few analog systems which were mostly controllers. However, I have successfully worked on several solid state amps and understand those designs enough to keep myself out of trouble but I am far from an audio expert of any type. I had the schematic for this amp and replaced all the caps filtering the power supplies. I used a cap tester to check some of the remaining caps connected to ground but they were all within spec. The new caps did make a difference but there is still a lot of hum that as I mentioned, peaks at around 1/3 volume then goes way down at 1/2 volume then back up at 3/4 volume and beyond. The only thing I could think of to cause this would be changes in circuit impedance at the different volume level that are reducing the inductively generated noise. I was hoping someone with far more analog troubleshooting experience than I have might have a few things they check for when running down problems like this. I do these types of projects to expand my knowledge in areas where I don't normally get a chance to work even more than I do them to hopefully restore a classic component which is why I'm looking for process tips more than the exact solution.

Thanks!

D

 

[Tim D]

I'd start by looking around the power supply again. If you have access to a scope you could check for 60/120Hz ripple in the DC. I'd check the rectifiers are good or simply replace them. There is a snubber cap connected just before the rectifiers you could remove or replace with an X/Y rated cap. If shorted or leaking it might cause problems.

Did you replace the bias cap (electrolytic)? Have you checked the tubes? Is the hum the same in both channels?

Failing all that it might be time to check that the power supply is wired correctly. The previous owner might have been in there making changes. Aside from your own changes, can you tell if someone has been inside before? You could go through it and check for bad solder joints and then re-solder them.

 

[larryderouin]

Also check, but verify with a meter to <1Ω, EVERY GROUND in the unit. I remember a problem similar to this a few years ago in the forum, and seems to me it was a grounding problem.

 

[dilburt]

Thank you for the feedback.

• No I didn't replace the rectifiers and they are real antiques so I will do that. Do you have a recommended part number?
• By snubber cap do you mean the c33 .01uf between the rectifiers? I did not replace that but will clip it as suggested. Do you recommend replacement if no difference?
• I don't have a scope but wasn't able to detect any measurable AC on the DC power output after the filter caps with a very high impedance DVM.
• By "Bias cap" I assume you mean the 100uf C4 across the filaments? If so yes. I did not replace the resistor in parallel with it but it tested fine.
• I replaced all the electrolytics in the power supply along with other key caps such as C4. I did not replace the small ceramic caps in the PS but tested most which were in spec
• I haven't tested all the ground points yet.
• There are two flexible metal conduits on each side of the chassis but the wires look properly twisted and routed.
• I couldn't find any evidence that any post manufacturing modifications had been done to this unit before I started on it looks like a professional assembler built it.

Thanks!

D

 

[dilburt]

Tim, I checked the diodes and snubber cap which were all fine. I also tested all the power supply voltages and they were well within spec - the ac ripple was less than 0.2%, too small to account for the hum.