I am restoring a KSA100 amplifier, I have cleaned everything, replaced the capacitors and had everything running well. The amp then wasn't used for 8 months and when I turned it back on 1 channel gets very hot and then shuts down.
When measuring the voltage across the output resistors I am getting 2.3V! The bias trimpot does not make any difference. So before I start stripping the amplifier down does anyone have any idea what could cause this and what to check?
Any help would be much appreciated.
When measuring the voltage across the output resistors I am getting 2.3V! The bias trimpot does not make any difference. So before I start stripping the amplifier down does anyone have any idea what could cause this and what to check?
Any help would be much appreciated.
Do you have the original schematic
if no
download from the link
KRELL KST100 Service Manual download, schematics, eeprom, repair info for electronics experts
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there might be something wrong in the bias circuitry,
check it for open circuitry, resistors, transistor Q60 15030, bad joints.
when switch on and not sure the problem solved,
and you don't want the power transistors to run hot,
try use a wire to short the C E of Q60 15030,
this will stop bias current flow to the drivers ( 15024, 15025 ) thus no bias to the output transistors.
doing so is to find the fault of the DC offset without the output transistors to run hot.
also leakage of the drivers and output transistors could cause the heating and DC offset,
the input circuitry also matters.
check if there were dirty spots on the amp board, moisture, dead bugs...
after checking bad joints, resistors, use diode mode to check all transistors.
once you find the fault and fixed , the wire should be removed and adjust the bias.
if no
download from the link
KRELL KST100 Service Manual download, schematics, eeprom, repair info for electronics experts
****
there might be something wrong in the bias circuitry,
check it for open circuitry, resistors, transistor Q60 15030, bad joints.
when switch on and not sure the problem solved,
and you don't want the power transistors to run hot,
try use a wire to short the C E of Q60 15030,
this will stop bias current flow to the drivers ( 15024, 15025 ) thus no bias to the output transistors.
doing so is to find the fault of the DC offset without the output transistors to run hot.
also leakage of the drivers and output transistors could cause the heating and DC offset,
the input circuitry also matters.
check if there were dirty spots on the amp board, moisture, dead bugs...
after checking bad joints, resistors, use diode mode to check all transistors.
once you find the fault and fixed , the wire should be removed and adjust the bias.
after fixing the heat and DC offset still persist.
compare the voltages of the good and bad channels to narrow down the fault.
check all the voltages of the power supplies of the input circuitry.
compare the voltages of the good and bad channels to narrow down the fault.
check all the voltages of the power supplies of the input circuitry.
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Replace the bias transistors and trim pot on both channels. They have been run hard for 40 years. You must clean all the output and bias transistors of thermal compound (probably powder by now) and replace it with new compound.
If the emitter resistor is 1 ohm on the output devices you want to see 625 mv. Total bias current is 2.5 amps after 15 -20 minutes. It starts higher and comes down and stabilizes after a half hour or so.
Which capacitors did you replace ? I hope you replaced all the Frakos on the PC board for sure.
If the emitter resistor is 1 ohm on the output devices you want to see 625 mv. Total bias current is 2.5 amps after 15 -20 minutes. It starts higher and comes down and stabilizes after a half hour or so.
Which capacitors did you replace ? I hope you replaced all the Frakos on the PC board for sure.
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Thanks for the advice.
I'll try removing of the bias circuit to see what happens.
I replaced all of the electrolytic caps, I take it there are a few more to replace, should I just replace everything?
Thanks Trampo
I'll try removing of the bias circuit to see what happens.
I replaced all of the electrolytic caps, I take it there are a few more to replace, should I just replace everything?
Thanks Trampo
other than the bias circuitry problem,
oscillation, parasitic oscillation may also cause the heating and DC offset problems.
oscillation, parasitic oscillation may also cause the heating and DC offset problems.
OK so I shorted out c and e of q60 and the output resistors now measure 0v. I take it that implies q60 is faulty? I will order new ones together with the trim pots and see what happens.
I have replaced the bias transistor together with the trim pot and I can now trim down to 640mV bias at the end stop of the trim pot. DC offset now sits at 170mV. Looks like the problem also existed elsewhere.
Would the DC offset prevent the bias from being trimmed properly?
Are there any common faults that could cause the DC offset?
Would the DC offset prevent the bias from being trimmed properly?
Are there any common faults that could cause the DC offset?
if you could not use the dc offset trim pot to adjust the output offset to 0v.
normally we compare the two good and bad channels to find out the differences in voltages and resistance to locate the fault. and probe the transistors with diode mode on the DMM to check for faulty one.
1) trim the dc offset trim pot, see if it could adjust the output to 0v,
check and clean the trim pot, it might be faulty. look for bad solder joints.
2) measure and compare the voltages on the diodes, zeners in the input circuitry.
3) measure and compare the voltages on the caps, resistor joints in the input circuitry.
normally we compare the two good and bad channels to find out the differences in voltages and resistance to locate the fault. and probe the transistors with diode mode on the DMM to check for faulty one.
1) trim the dc offset trim pot, see if it could adjust the output to 0v,
check and clean the trim pot, it might be faulty. look for bad solder joints.
2) measure and compare the voltages on the diodes, zeners in the input circuitry.
3) measure and compare the voltages on the caps, resistor joints in the input circuitry.
Just checking voltages etc and have found some differences. I have the mkII version which is slightly different from the mkI. KRELL 100-MK2 SCH Service Manual download, schematics, eeprom, repair info for electronics experts
Measuring the voltage across the 47R5 resistors I am getting around 2.2V on the faulty side and about .8V on the working side.
If I were to replace any transistors are there any that would need matching? and if so what parameter would need to be matched?
Measuring the voltage across the 47R5 resistors I am getting around 2.2V on the faulty side and about .8V on the working side.
If I were to replace any transistors are there any that would need matching? and if so what parameter would need to be matched?
ksa 100 schematic
https://www.pinterest.se/pin/712061391064456297/?amp_client_id=CLIENT_ID(_)&mweb_unauth_id={{default.session}}&_url=https%3A%2F%2Fwww.pinterest.se%2Famp%2Fpin%2F712061391064456297%2F&open_share=t
does the above schematic match your amp?
what number is the resistor that you mean?
use an oscilloscope to see if there were oscillation .
https://www.pinterest.se/pin/712061391064456297/?amp_client_id=CLIENT_ID(_)&mweb_unauth_id={{default.session}}&_url=https%3A%2F%2Fwww.pinterest.se%2Famp%2Fpin%2F712061391064456297%2F&open_share=t
does the above schematic match your amp?
what number is the resistor that you mean?
use an oscilloscope to see if there were oscillation .
No, its the schematic I posted before. It has more transistors, but the resistors are the same and are R22 and R26.
OK so I removed the mosfets and found one to be faulty. After replacing both vn0206n5 s the amp is now stable with the correct bias and dc offset. I do now however have some mains hum. Could this have anything to do with the matching of the mosfets or if not does anyone have any ideas?