I'm trying to help a friend who removed the 250k alps pot from his Almarro a318b and attempted to replace it with a 100k tkd pot before realizing he was in over his head. Now I'm in over mine. Unfortunately there are no pictures of the pot before it was removed. I tried wiring the input to pin 1 L&R and the output to pin2 L&R, with the ground from the source selector switch on pin 3 R and the ground from the output side on pin 3 L. All I get is a loud distorted buzz from the speakers. The signal wires also have shields that look like they were soldered somewhere on the original pot, I tried coupling them with their respective ground wires and it made no difference, both shields are floating on the ends not connected to the pot. Any help is appreciated even just a picture clearly showing the stock wiring. I know there were originally some DC blocking caps on the input, but those were discarded before I got the amp. Here's what I have without the ground wires soldered in. The red wire on pin 2 broke while I was pulling it out for a photo, but there's enough slack to reconnect.
Looks like the TKD pots are configured the same as an alps. https://www.hificollective.co.uk/sites/default/files/tkd-2cp2511-datasheet.pdf
Post #3 in this thread shows the alps. https://www.diyaudio.com/community/threads/wiring-up-an-alps-rk27-50k-potentiometer.280137/
So in your pic, the top pins would be the input, the center pins would be the output, and the bottom pins would be ground. Looks like the inputs and outputs are wired wrong.
jeff
Post #3 in this thread shows the alps. https://www.diyaudio.com/community/threads/wiring-up-an-alps-rk27-50k-potentiometer.280137/
So in your pic, the top pins would be the input, the center pins would be the output, and the bottom pins would be ground. Looks like the inputs and outputs are wired wrong.
jeff
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That's how I have it wired, input on the top, output center, ground on bottom. Input coming from the source selector and output going to the first tube in the gain stage.
When I tested it the pot was firmly mounted to the chassis, I just pulled it back out for the picture. The buzz was much louder than just background noise, there was no signal coming through besides the loud distorted buzz. It wasn't strictly 60 or 120hz either, lots of harmonics in there.
Use a DVM to check for continuity between the bottom terminals (that should go to ground), and ground.
A full circuit schematic is not available for this amplifier, only a basic sketch. There is a 5 ohm resistor that 'floats' a star point of bunched earth leads above the true chassis ground point. The earthy end of the volume pot should be made to this chassis ground.
Just checked. With all the wires connected, there's continuity from the ground pins to chassis ground. I also checked the 2 ground wires disconnected from the pot, the ground from the output side is connected to the main ground bus and has continuity with the chassis, but the input side, which runs back from the source selector to the common ground for the RCA ins does not. I'm guessing that connection is first made at the volume pot because the RCA jacks are mounted to an insulated material on the back of the chassis.
Sometimes the two pot "ground" terminals are connected together at the pot. Try that.
I decided to test the tubes, which I should have done earlier. There's a short in the 6SL7, replacement is on the way, I'll see if the problem persists with a new tube.
Resurrecting this, the new pot is in and wired correctly and the amp plays fine with the new tube, but it now has a background hum. It's present and the same volume with or without an input connected, but it goes away at minimum and maximum volume. I read somewhere after some searching that a high value resistor from the wiper lug to ground/common lug could fix the issue, so I tried a 10M ohm and at first I thought it reduced the hum, but now I'm not sure, I think it's unchanged. I also tried some better shielded signal wire from the pot output to the first tube in the gain stage, but that didn't help either. Any ideas for troubleshooting the new issue?
That's usually pickup on the signal after the pot. When its at min or max the impedance is low. The amount of pickup depends on the value of the pot, screening etc. It can also be pickup from the heaters to the grid of the first tube. It can also be a tube. Hope this helps.
It's a tdk pot with plastic body, but I'll check the bushing. If the noise is in the heaters can I do a 100r from each heater pin to a 0.1uf cap to ground for a filter, and is there any disadvantage or better way to get rid of heater noise?
Make sure all the pot wiring is correct, and the cables placed away from hum sources.
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Good to see a schematic. The 250K log pot will be worse than 100K log pot as the impedance is higher. The grid pin 1 on the 6H9C is next to a heater pin 8. It takes <1pF to couple the signal in. If you don't need the impedance on the inputs to be so high (i.e. driven be modern equipment) you can replace with a much lower value say 22k log.
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I have now used a shorter run of shielded signal wire from the pot output and made sure to dress it away from the heater pins of the 6SN7(6H9C), I tried adding a 100 Kohm resistor from the input lug to ground lug on both channels to reduce pot impedance, and added a 100 ohm resistor from each of the 6SN7 heater pins running to a 0.1uF cap to ground to try to filter any extra noise there, none of it has removed the hum. Today I noticed a mechanical hum from the power transformer. I will fire it back up and check voltages tomorrow. Should I try filters on the power tube filaments instead of the 6SN7? I've done that to remove hum in the past, but in that case the heater voltage for the whole amp was daisy chained off of the 1st power tube, where instead the Almarro looks to have positive leads from the PT to each tube with a common negative, I'm not 100% on that, will check again tomorrow when I have the amp opened up again.
Try just the first stage heater off a bench supply or battery and see what happens.
The heater supply for the 6H9C is taken from a separate centre tapped 6.3 volt winding on the transformer. A good tube should not be the cause of hum. It is a normal procedure with this amplifier to replace the 100 mFd electrolytic capacitors fitted to each of the 6C33B O/P Triodes. Use 105 degree rated types and place them as far as possible from the cathode resistors and the O/P tubes. Adjust the bias to 0.18 volts after a 20 to 30 minute warm up from cold.