I bought mcintosh c20 preamp as is
Both channel sounds okay just little crackling noise when I turning the volume potentiometer
I cleaned the entire unit with contact cleaner and
I sprayed deoxit on each volumes and switches.
I dried three days.
Now, one channel is working perfect but
another channel has buzzing noise.
When I turn the main volume potentiometer to louder, the buzzing noise become louder.
Also crackling noise time to time.
When I turn the main volume potentiometer to very silent, I cannot hear the noise.
I switched inputs and nothing changed.
What could be the issue? is it potentiometer? or something else?
If it's not volume potentiometer, shouldn't be buzzing noise same level regardless of turning the volume?
Both channel sounds okay just little crackling noise when I turning the volume potentiometer
I cleaned the entire unit with contact cleaner and
I sprayed deoxit on each volumes and switches.
I dried three days.
Now, one channel is working perfect but
another channel has buzzing noise.
When I turn the main volume potentiometer to louder, the buzzing noise become louder.
Also crackling noise time to time.
When I turn the main volume potentiometer to very silent, I cannot hear the noise.
I switched inputs and nothing changed.
What could be the issue? is it potentiometer? or something else?
If it's not volume potentiometer, shouldn't be buzzing noise same level regardless of turning the volume?
Please forgive me if this is too obvious, but in case it's not: When cleaning with D5 it's important to mechanically operate the switches and controls over their ranges while applying. You can even do this while listening to the device - safety first, of course. Rotate the controls and rotary switches lots while wet, and work the slide switches.
With it turned off and unplugged, also clean the valve sockets.
It's very unlikely that anything could have gotten damaged in the first cleaning, and it may just need another cleaning. Worth a shot.
All good fortune,
Chris
With it turned off and unplugged, also clean the valve sockets.
It's very unlikely that anything could have gotten damaged in the first cleaning, and it may just need another cleaning. Worth a shot.
All good fortune,
Chris
Thank you for the advice.
I cleaned with
I just cleaned with isopropyl alcohol 99%, still same.
I am just curious.
if I turn the main volume louder, the buzzing noise become louder.
Is it most likely potentiometer or could be something else.
I am also curious
only one side buzzing noise is possible when the volume potentiometer is failed.
The volume Potentiometer has alot of rust, so it might get damaged during cleaning.
I ordered new volume potentiometer already but
I just wondering i need to find something else too if it’s doesn’t fit the symptom above.
I cleaned with
Thank you for the advice
I just cleaned with isopropyl alcohol 99%, still same.
I am just curious.
if I turn the main volume louder, the buzzing noise become louder.
Is it most likely potentiometer or could be something else.
I am also curious
only one side buzzing noise is possible when the volume potentiometer is failed.
The volume Potentiometer has alot of rust, so it might get damaged during cleaning.
I ordered new volume potentiometer already but
I just wondering i need to find something else too if it’s doesn’t fit the symptom above.
Alcohol is a very strong de-greaser, so can potentially remove the lubricant for the rotating shaft's bearing surface. There was an America-wide incident in the mid-1970's when the McIntosh Clinic traveling show, in the days of the great Davy O'Brien ("DOB" on the clinic sticker) when Blue Shower was first introduced. A few weeks after the clinic came to towns, lots of controls seized up, all across the country. So, you may want to re-lube the shafts. D5 is safe and recommended.
It sounds like the volume pot is working as it should. If the buzz decreases all the way to zero when fully CCW, then I'd not worry about it. The rust issue is more troublesome, and unpredictable at this distance.
I have seen some issues with McIntosh's crimped jumpers between the coax cable shields and their destination connecting points. These are normally shrink-wrapped, so invisible, but a poor connection here will give your symptoms.They carry signal ground too, so troubleshooting is tricky. The solution is to cut open the shrink wrap and to solder the crimped connection. These materials don't like solder, so you have your work cut out for you. If this machine has spent decades near the ocean or other metal-inhospitable place, it may be the better part of valor to just shotgun all of the crimps. Not fun, but.
All good fortune,
Chris
It sounds like the volume pot is working as it should. If the buzz decreases all the way to zero when fully CCW, then I'd not worry about it. The rust issue is more troublesome, and unpredictable at this distance.
I have seen some issues with McIntosh's crimped jumpers between the coax cable shields and their destination connecting points. These are normally shrink-wrapped, so invisible, but a poor connection here will give your symptoms.They carry signal ground too, so troubleshooting is tricky. The solution is to cut open the shrink wrap and to solder the crimped connection. These materials don't like solder, so you have your work cut out for you. If this machine has spent decades near the ocean or other metal-inhospitable place, it may be the better part of valor to just shotgun all of the crimps. Not fun, but.
All good fortune,
Chris
Sometimes a buzz can occur with heater to cathode coupling. Suggest you swap the input ECC83's over to see if the buzz follows the valve.
Deoxit should be used sparingly, do not overdo it. It may leave residue itself. Perhaps some cleaning, wiping the remaining deoxit is in place.
Taking tubes in and out while turned off should clean the sockets.
Taking tubes in and out while turned off should clean the sockets.
I just received notification that two users left comments, so I came here
but I can't see any new comments. weird.
Anyway, Since I am here, I would like to share something.
I found two electrolytic capacitors (CAN CAP) have lower capacitance than spec. Almost half of the value.
Not sure improper "the cleaning process" causing this or not. It was the issue of the noise.
Since I decided to replace the caps, I replaced whole caps include ceramic caps and bumble bees.
Also, I rebuilt all the potentiometers.
Everything working perfect now except one issue
Only issue is one channel is much louder than another.
I tried to turn the balance knob and it adjusted the balance of channels but
one channel cut the sound suddenly before perfect balance.
The issue was I had used audio pots for balance control
I though I bought linear pots but I made mistake and didn't know until I played a music after total rebuilt.
For who doesn't know difference between audio pots and linear pots.
Linear pot resistance value increase and decrease linearly during turning the knob
Audio pot resistance value increase and decrease exponentially during turning the knob
C20, series of two pots control channel balance.
The right and left channel wires are connected in reverse to each pot.
So, the middle value of each pot should be same to make even balance at the middle position of balance knob.
Middle position of Linear pot has half resistance value of spec but audio pot don't have half value in middle position.
That's the issue.
I am waiting two 500k linear pots to deliver now.
but I can't see any new comments. weird.
Anyway, Since I am here, I would like to share something.
I found two electrolytic capacitors (CAN CAP) have lower capacitance than spec. Almost half of the value.
Not sure improper "the cleaning process" causing this or not. It was the issue of the noise.
Since I decided to replace the caps, I replaced whole caps include ceramic caps and bumble bees.
Also, I rebuilt all the potentiometers.
Everything working perfect now except one issue
Only issue is one channel is much louder than another.
I tried to turn the balance knob and it adjusted the balance of channels but
one channel cut the sound suddenly before perfect balance.
The issue was I had used audio pots for balance control
I though I bought linear pots but I made mistake and didn't know until I played a music after total rebuilt.
For who doesn't know difference between audio pots and linear pots.
Linear pot resistance value increase and decrease linearly during turning the knob
Audio pot resistance value increase and decrease exponentially during turning the knob
C20, series of two pots control channel balance.
The right and left channel wires are connected in reverse to each pot.
So, the middle value of each pot should be same to make even balance at the middle position of balance knob.
Middle position of Linear pot has half resistance value of spec but audio pot don't have half value in middle position.
That's the issue.
I am waiting two 500k linear pots to deliver now.
It's fairly rare to actually need to replace pots, even in very old equipment, partly because proper cleaning & lube fixes almost all problems and partly because adapting replacement pot's shafts to existing knobs is a major pain. Are you sure you need to do all this? Seems like you started with a noisy pot, most likely only needing cleaning, and have replaced parts shotgun fashion, adding problems along the way. Breathe deep, relax, study the situation?
FWIW, the original CTS pots weren't great (and McIntosh loyally stayed with them until the model C32 era, mid 1970's, before switching to Alps), but were custom made for the job. Balance controls were made with half tracks lossless and half tapered - no loss at center position.
All good fortune,
Chris
FWIW, the original CTS pots weren't great (and McIntosh loyally stayed with them until the model C32 era, mid 1970's, before switching to Alps), but were custom made for the job. Balance controls were made with half tracks lossless and half tapered - no loss at center position.
All good fortune,
Chris
My C20 project started with terrible condition unit.
Rust all over the outside pots and shafts, chassis, chassis covers.
A lot of dust, grimes, rat poops and hays everywhere inside and outside.
When I remove the rust and clean the unit, the chemical cleaning stuff may effect on inside of pot.
I saw partial carbon track has been fallen off from every pot.
I rebuilt them all already with Alpha pots. I only use carbon track pcb from the Alpha pots.
Only problem was to find 300k pots. Current available 300k pot normally has 250kish values.
I used two 250k pots which measured 249and 251k.
Only need to re-do 500k as state above.
Could you please explain more about
"Balance controls were made with half tracks lossless and half tapered - no loss at center position."
means?
I could not quite understand this because I don't have enough knowledge.
I don't wanna make mistake again and re-do.
Pots made specifically for use as balance controls had half of each track carbon (lossy) and half metal (not lossy). Of course these were mirror-imaged between the two channels' tracks. Since you have plenty of gain, the 6dB gain loss due to using full-track linear pots won't be a problem. The difference between using 300K and 250K pots is also unimportant. You'll be fine.
All good fortune,
Chris
All good fortune,
Chris