Hello Community!
I was recently gifted a pair of Klipsch KSP-400 speakers that both hummed and buzzed terribly. The previous owner described one speaker as sounding like there was an angry squirrel inside. The angry squirrel was actually the usual collection of failed capacitors in the Power Amplifier Module DC supplies.
The Service Manual does not include an amp VR101 Bias voltage adjustment procedure or spec. Can anyone in this community advise what it should be or is this an 'if it ain't broke, don't mess with it' item? Or, is there a way to determine a value from circuit topology / output transistor type? By the way, both amp heatsinks are warm to the touch when idle.
I was recently gifted a pair of Klipsch KSP-400 speakers that both hummed and buzzed terribly. The previous owner described one speaker as sounding like there was an angry squirrel inside. The angry squirrel was actually the usual collection of failed capacitors in the Power Amplifier Module DC supplies.
- Visual inspection showed C4 & C15 bulging slightly on both amps. No other capacitors were bulging or leaking.
- Voltage & Scope measurements at amp J16 Pins 1 & 3 revealed +/-12V supplies with huge ripple.
- Replacement of amp C3, C5, C9, C10, C4, C15, and for good measure, Control Panel PCB C14 and C15 cured the hum / squirrel noise from both speakers.
- One thing the stood out was how C3 from one amp tested open! See attached photo showing one internal terminal eaten away. This seemed like an unusually severe failure... Review of the circuit showed +/-25.2VDC on C3 and C5 that were spec'ed as 470uF @ 25V parts! TIP -> Replace amp C3 and C5 with 470uF 35V type.
- Finger temperature testing of amp U1 7812 & U2 7912 TO-220 regulators showed the 7812 was quite warm. As a precaution Digikey HS107-ND / 577202B00000G heatsinks were added to U1 & U2. A dab of RTV was added to keep the heatsinks from ever making contact with each other.
- As found Bias voltage across amp R30, R21, R33 and R36 on both amps was in the range of 4.8 - 5.6 mV.
The Service Manual does not include an amp VR101 Bias voltage adjustment procedure or spec. Can anyone in this community advise what it should be or is this an 'if it ain't broke, don't mess with it' item? Or, is there a way to determine a value from circuit topology / output transistor type? By the way, both amp heatsinks are warm to the touch when idle.
Attachments
Firstly well done of repairing the unit 👍 Caps run right up to their spec is not unheard of. Is this a linear PSU or a switching one? If linear then make sure any mains voltage selector is correctly set and also that the mains isn't to high. A 230v spec unit running on say 250 volts will produce rails that are noticeably high... on a linear supply.
It can be possible to determine bias current based on circuit design/topology/values etc but that in no way means the value is anything like the recommended setting from the manufacturer. Commercial constraints often mean the bias is lower than might be expected due to thermal constraints (small heatsinks) and to limit overall power consumption.
From a practical point bias currents of even a milliamp or two pretty much totally remove any audible distortion.
There is no reason at all to suspect the bias setting would be incorrect or have altered provided that no semiconductors within the power amp have been replaced.
Warm regs are fine, hot regs are fine. If they are close to sizzling water then they are to hot. If you can hold a finger on them then they are fine.
Mooly,
This is this a linear PSU design, LM7812/LM7912, with fixed 120VAC mains configuration. Mains voltage on my bench is 120VAC.
Based on your comments I'll leave the Bias pots as is unless someone chimes in with a factory value actually specified by Klipsch.
This is this a linear PSU design, LM7812/LM7912, with fixed 120VAC mains configuration. Mains voltage on my bench is 120VAC.
Based on your comments I'll leave the Bias pots as is unless someone chimes in with a factory value actually specified by Klipsch.