Fisher 800C consistent hum. Help needed!

[LPCOLLECT]

Greetings AK community.

my 800c has developed a consistent hum and it doesn't get louder as I increase the volume or play with any other controls.

the hum seems to stay at the same volume all the time.
When playing music, it's not as noticeable but still present.

any tips that might help narrow down the cause would be appreciated.
this is my all time favorite equipment that I own and I wish I can get rid of the hum somehow.

would it be better to have em go through by a tech?

I didn't notice any obvious solder joint cracks or leaking filter caps.
since it only allows me to upload only 10 photos here, I have uploaded more photos here.

https://imgur.com/a/HzKqoPI

but happy to zoom up on certain areas if needed.

all original fisher tubes and I was told that it was serviced when I purchased them two years ago from a fellow on FB.
have a great weekend!

 

[zackthedog]

It sounds like you have a bad power supply capacitor, one of the can caps. Are they original?

 

[LPCOLLECT]

It sounds like you have a bad power supply capacitor, one of the can caps. Are they original?

thanks for the response Zack. I'm not the first owner so I can't be sure but I was told that this unit was serviced by a tech.

are you referring to these blue caps?

 

[Mr Boochie]

Greetings AK community.

my 800c has developed a consistent hum and it doesn't get louder as I increase the volume or play with any other controls.

the hum seems to stay at the same volume all the time.
When playing music, it's not as noticeable but still present.

Could you upload a video to YouTube or some other host site? You cannot upload an AV file directly to this site. I upload to YT, then paste the address to the MEDIA (option at the top of the screen.

Post a file with baseline hum and how the hum stays the same even after changing the volume control. Hearing the hum frequency may help locate the part of the circuit that needs attention.

I uploaded this video with my camera and mixer setup to record my system. I used a Samsung S23.

 

[LPCOLLECT]

Could you upload a video to YouTube or some other host site? You cannot upload an AV file directly to this site. I upload to YT, then paste the address to the MEDIA (option at the top of the screen.

Post a file with baseline hum and how the hum stays the same even after changing the volume control. Hearing the hum frequency may help locate the part of the circuit that needs attention.

I uploaded this video with my camera and mixer setup to record my system. I used a Samsung S23.

absolutely. thank you for chiming in.

here is a video with sound.

https://imgur.com/MjiYU3E

the hum starts maybe 10 seconds after the unit is powered on.

 

[Mr Boochie]

absolutely. thank you for chiming in.

here is a video with sound.

https://imgur.com/MjiYU3E

the hum starts maybe 10 seconds after the unit is powered on.

Sounds like a 60Hz hum. That's from either the power transformer or the tube's heater supply line of the power transformer. For reference, I posted both a 60Hz and a 120Hz video. 120Hz hum is from other (post-rectified) sources. That could be a bad filter cap(s), ground loop, or bad layout of wiring leading to hum induction.

You could try swapping 12AX7 tubes around and see if the hum changes. It could be a noisy heater within a tube. That would be an easy fix — a new tube. Otherwise, the amp will probably need to go onto a tech's bench to narrow down the culprit(s).

 

[Mr Boochie]

Also, sometimes cleaning the pins and sockets of each tube fixes these kinds of problems. I like Deoxit D100. Put a little Deoxit on each pin and socket tang. Pull and replace each tube several times to work the solution into the surface.

I hope it works.

 

[LPCOLLECT]

Also, sometimes cleaning the pins and sockets of each tube fixes these kinds of problems. I like Deoxit D100. Put a little Deoxit on each pin and socket tang. Pull and replace each tube several times to work the solution into the surface.

I hope it works.

thanks for the suggestion. I've only got a D5 and fader lube. would Deoxit D5 also do the job?

 

[Mr Boochie]

thanks for the suggestion. I've only got a D5 and fader lube. would Deoxit D5 also do the job?

It'll work. Work the tubes in and out of each socket a few times.

Good luck!

 

[zackthedog]

thanks for the response Zack. I'm not the first owner so I can't be sure but I was told that this unit was serviced by a tech.

are you referring to these blue caps? View attachment 3435438

No, I'm referring to the tall silver can capacitors on top of the unit, at the back. If they are original then they're likely to be prone to failure. That would cause the kind of hum you are hearing.

Is it in both channels or just one channel? If it's in one channel it might be a bad tube. If it's in both channels it's probably a bad can capacitor.

Unfortunately, "serviced by a tech" doesn't mean a whole lot. A lot of techs will do the minimum to keep a unit running, but don't address problems like aging can capacitors that are likely to fail. It certainly can be fixed, but it would be wise to take it to someone who will do more than a quick fix.

 

[Mr Boochie]

No, I'm referring to the tall silver can capacitors on top of the unit, at the back. If they are original then they're likely to be prone to failure. That would cause the kind of hum you are hearing.

Is it in both channels or just one channel? If it's in one channel it might be a bad tube. If it's in both channels it's probably a bad can capacitor.

Unfortunately, "serviced by a tech" doesn't mean a whole lot. A lot of techs will do the minimum to keep a unit running, but don't address problems like aging can capacitors that are likely to fail. It certainly can be fixed, but it would be wise to take it to someone who will do more than a quick fix.

Cap can failure usually = 120Hz hum. His video sounds like 60Hz.

 

[zackthedog]

Cap can failure usually = 120Hz hum. His video sounds like 60Hz.

It's hard to tell from the video, I hear components of both. But if it's in both channels and remains fixed in volume that says power supply filter cap failure to me.

 

[gadget73]

yeah, cap failure is the most likely candidate. I see two newer caps in the power supply area, but the rest of that zone looks pretty old / original. Unless the can caps have been replaced or restuffed its a pretty good bet. The caps that have been replaced are low voltage too, so I'm guessing those are bias supply parts.

 

[LPCOLLECT]

No, I'm referring to the tall silver can capacitors on top of the unit, at the back. If they are original then they're likely to be prone to failure. That would cause the kind of hum you are hearing.

Is it in both channels or just one channel? If it's in one channel it might be a bad tube. If it's in both channels it's probably a bad can capacitor.

Unfortunately, "serviced by a tech" doesn't mean a whole lot. A lot of techs will do the minimum to keep a unit running, but don't address problems like aging can capacitors that are likely to fail. It certainly can be fixed, but it would be wise to take it to someone who will do more than a quick fix.

Hi Zack, sorry about the late response.
I believe you are talking about these two big silver capacitors.
It's coming from both channels for my case. the volume of the hum is exactly the same.
changing the volume with the pot also does not change the volume of the hum.

where exactly are the power supply filter caps? and what do they look like? perhaps I can carefully try desoldering one end and use my meter to check if it's measuring right.

 

[gadget73]

all 3 are involved in filtering the power supply at various points. If they are original from the 60s, I can save you some time on measurement and promise they are not good at this point. Measure if you like but expect to at least see high ESR and the caps being off value. High voltage leakage would be more telling in most cases though.



The black cap is the one highlighted here. Negative of that cap is at +214v to ground so its insulated. C91 A-D is the cap on the right. C87A-D should be the one on the left. C95A and C95B live underneath the chassis if I'm not mistaken.


Hayseed Hamfest can sell you a set of drop-in caps that match the original layout and values.

That black capacitor has been out at some point, the wafer was not screwed in originally.

 

[Mr Boochie]

Hayseed Hamfest can sell you a set of drop-in caps that match the original layout and values.

That black capacitor has been out at some point, the wafer was not screwed in originally.

Well, if he puts in new cap cans and it still hums, it's prolly a heater line issue.

 

[gadget73]

yes, but at least at that point its unlikely to emit the magic smoke because a 60 year old cap failed. Offhand I don't know what could go flukey in the heater circuit to create the same volume-independent hum in both channels though. Single channel H-K I can see but both seems somewhat less likely.

 

[Mr Boochie]

Offhand I don't know what could go flukey in the heater circuit to create the same volume-independent hum in both channels though.

The bias supply filter cap has dried out, and/or the diode has gone bad.

Heater line hum pots need cleaning, or it is bad.

A weak or corroded solder point of any part of the heater line can induce a hum in the unit.

Pilot lights with poor contact or are going bad — if they run off the heater.

The AC line into the chassis is compromised, and a 60Hz hum is injected into the unit.

I've seen amps in which one of the twisted pairs of heater line solder points to the socket of the output tube and gets hinky. That compromises the cancellation of the 60Hz hum from the push-pull pair.

 

[gadget73]

Pretty sure the bias cap has been replaced though, I see two modern blue caps under the deck where the old dual can would live. 35v, 2200uf seems like a reasonable sub for the original 35v, 1000uf parts. also frankly they don't make sense for much of anything else.

these don't have hum pots, it uses a pair of fixed resistors to ground

the others maybe, but equal in both channels just doesn't seem so likely to me.

The other big reason I question the caps is it sure looks like one of them is beginning to ooze.

https://audiokarma.org/forums/index.php?attachments/img_3922-heic_compressed-jpeg.3435359/

the bottom of the can basically center of the pic. That should be the black one.

 

[zackthedog]

Hi Zack, sorry about the late response.
I believe you are talking about these two big silver capacitors.
It's coming from both channels for my case. the volume of the hum is exactly the same.
changing the volume with the pot also does not change the volume of the hum.

where exactly are the power supply filter caps? and what do they look like? perhaps I can carefully try desoldering one end and use my meter to check if it's measuring right.


View attachment 3440000

Those can caps, or two of them at least, filter the rectified DC power to eliminate hum from the power supply. If they fail, you will hear a constant hum like what you have. I would not attempt to replace these yourself unless you are experienced with restoring tube amplifiers. There's a LOT that can go wrong and you don't want to destroy what seems to be an otherwise good working unit. I would take it to a professional repair shop.