Knight mono block with volume static question, help needed

[SwingModern]

Hello,
I have this Knight monoblock. When I turn up the volume I get static and distortion. At low volume it sounds fine.

I replaced most of the capacitors already and used deoxit.

Any help in identifying the problem is greatly appreciated. Thanks!

 

[AlexHempel]

I'd try bypassing the volume pot entirely with a jumper (to give max volume) and seeing how that sounds. The pot could have issues on part of its adjustment range. Keep in mind the volume pot doubles as the grid leak resistor in some designs so you might need to install a temporary one.

 

[SwingModern]

I'd try bypassing the volume pot entirely with a jumper (to give max volume) and seeing how that sounds. The pot could have issues on part of its adjustment range. Keep in mind the volume pot doubles as the grid leak resistor in some designs so you might need to install a temporary one.

I'm still a newb at this. How would I go about bypassing the volume pot? Thanks!

 

[cademan]

Which capacitors have you not replaced yet?

 

[gadget73]

exactly which model is this? and/or do you have a schematic to post? Would be easier to do this with a common point of reference.

 

[SwingModern]

Which capacitors have you not replaced yet?

There are two marked RK559 10-25 DC that I haven't replaced yet. Thanks.

 

[SwingModern]

exactly which model is this? and/or do you have a schematic to post? Would be easier to do this with a common point of reference.

I don't know what model it is and don't have a schematic unfortunately. I could upload a photo of the underside with all the wiring if that would help. Thanks

 

[AlexHempel]

A picture would help. The pot should have three tabs coming from it, one of which is grounded and the other two of which are not. You can determine which is which by tracing the wires or using a DMM in resistance mode. In order to bypass the pot, you'd connect the two non-grounded terminals together temporarily. Now that I think about it, bypassing the pot in this manner doesn't remove the resistance to ground, so there isn't any potential issue with grid leak resistance.

 

[zackthedog]

It's an Allied-Knight 20 watt kit amplifier. There's an assembly manual available on eBay. I don't think I can post the link here, but here's an image of the manual. Maybe one of our experts can source a copy:



ETA: Ah, it's a model S-750:

https://www.radiomuseum.org/r/allied_rad_20_watt_amplifier_s_750.html

Schematic attached.

 

[zackthedog]

I have a feeling it's not the volume pot, but I could be wrong. Sounds like the amp is distorting at higher power output. Could be the 20uF cathode bypass cap has gone bad, which is probably the one the OP hasn't replaced. But I'm just guessing. ;-)

 

[SwingModern]

It's an Allied-Knight 20 watt kit amplifier. There's an assembly manual available on eBay. I don't think I can post the link here, but here's an image of the manual. Maybe one of our experts can source a copy:

View attachment 2568970

ETA: Ah, it's a model S-750:

https://www.radiomuseum.org/r/allied_rad_20_watt_amplifier_s_750.html

Schematic attached.

Thanks for this! I couldn't find the model before.

 

[SwingModern]

I have a feeling it's not the volume pot, but I could be wrong. Sounds like the amp is distorting at higher power output. Could be the 20uF cathode bypass cap has gone bad, which is probably the one the OP hasn't replaced. But I'm just guessing. ;-)

I probably haven't! I'll look for that cap on the underside. Thanks!

 

[gadget73]

Looks like its part of the can cap. Honestly if you haven't done the cans and don't have a way to test them you probably ought to replace them.

 

[SwingModern]

Here are some pics of the bottom. I've ordered more capacitors to finish up the job, including the DC caps.

 

[Selmerdave]

Yeah it looks like the can caps are still in the circuit, definitely good idea to replace them.

 

[SwingModern]

Yeah it looks like the can caps are still in the circuit, definitely good idea to replace them.

So, I replaced the two caps marked Sprague and now instead of static and sound, I only get a tapping sound.

Ideas on what I did wrong? Thanks!

 

[Selmerdave]

So, I replaced the two caps marked Sprague and now instead of static and sound, I only get a tapping sound.

Ideas on what I did wrong? Thanks!

I'm not sure what your level of experience is with this stuff but do be careful with lethal voltages etc. If you replaced the Sprague electrolytics with proper value caps with the same polarity those should be fine. The power supply caps that would normally be high on the priority list for replacement are the can caps on the top of the chassis, circled below.

 

[SwingModern]

I'm not sure what your level of experience is with this stuff but do be careful with lethal voltages etc. If you replaced the Sprague electrolytics with proper value caps with the same polarity those should be fine. The power supply caps that would normally be high on the priority list for replacement are the can caps on the top of the chassis, circled below.View attachment 2572451

Thanks for that information.

What I'm trying understand it going from static with sound to replacing the sprague caps and now no sound. It seems like it's something in that change that's causing the problem. Thanks

 

[zackthedog]

Did you wire the new caps in the correct polarity?

 

[tuberadios69]

Definitely a wiring issue. Check for cold solder joint(s), too.