Hi all,
I have a Goodmans One Ten Receiver.
It crackles when the volume is rotated in several places.
New pot hooked up still does it.
cleaned pot no fix
Seems there is DC offset of 250mv on the input of the fist stage IC amplifier.
Disconnected the coupling cap and the output ressitor so chip isolated, still dc offset present?
Replaced the IC with same part, still DC offset is there on input????
The mains filter caps have been replaced as have all the electrolytics couplers.
The resistors around the chip were all out of spec so replaced, the ceramic capacitors removed and all are tested as OK.
At a loss now, nothing odd on the scope regarding noise and the DC voltages are all correct.
suggestions please.
thanks, and Happy New Year to all.
I have a Goodmans One Ten Receiver.
It crackles when the volume is rotated in several places.
New pot hooked up still does it.
cleaned pot no fix
Seems there is DC offset of 250mv on the input of the fist stage IC amplifier.
Disconnected the coupling cap and the output ressitor so chip isolated, still dc offset present?
Replaced the IC with same part, still DC offset is there on input????
The mains filter caps have been replaced as have all the electrolytics couplers.
The resistors around the chip were all out of spec so replaced, the ceramic capacitors removed and all are tested as OK.
At a loss now, nothing odd on the scope regarding noise and the DC voltages are all correct.
suggestions please.
thanks, and Happy New Year to all.
Well, try a coupling cap series the volume pot to the IC input. About 10 uf 25 v is traditional. If the voltage is plus to the speaker ground, put the plus of the capacitor towards the IC. At this low voltage, hanging out in the air with one end soldered to the pot lead is probably okay.
I'm not precisely sure what indianajo is suggesting. But to clarify, placing a cap between the volume pot wiper and the IC3 pin 8 isn't workable--- there would be no bias path for the opamp input.
Hi, Thanks for the suggestion, but the volume pot is after the first IC and if I disconnect the resistor R177 the IC is disconnected from the next stage IC3, which was also replaced with a new one.
Disconnecting R177 and C107 Isolates the IC completely, still the hi DC is present on pins 5 & 6???
The crackle is present on whatever inoput is selected, but it does seem slighly louder on the right channel.
I have even checked the PCB around the chip for leakage, as I had another amp a while back that read 12.5meg ohm between an IC pin and the PCB.
Thourough cleaning fixed that little problem.
There is a large and clear Schematic on it's own at
https://www.hifiengine.com/manual_library/goodmans/model_one_ten.shtml
The schematic not the manual as that is faded.
Many thanks to both replies.
What now I ask )))))))))))))))))))))))))
regards
Disconnecting R177 and C107 Isolates the IC completely, still the hi DC is present on pins 5 & 6???
The crackle is present on whatever inoput is selected, but it does seem slighly louder on the right channel.
I have even checked the PCB around the chip for leakage, as I had another amp a while back that read 12.5meg ohm between an IC pin and the PCB.
Thourough cleaning fixed that little problem.
There is a large and clear Schematic on it's own at
https://www.hifiengine.com/manual_library/goodmans/model_one_ten.shtml
The schematic not the manual as that is faded.
Many thanks to both replies.
What now I ask )))))))))))))))))))))))))
regards
Op you will measure approx 250mV on the input to IC1 its the voltage drop over R99 (1M ohm) caused by the input bias current . You need to be looking around IC3 as that is where the volume pot is connected , thou I fear you will be on a fruitless mission as the volume pot is in the negative feedback loop and carries bias current for the negative input and so is destined to be noisy . It should more accurately be called a gain control as that is the way it is connected .
Happy New Year, Everyone!
epicyclic has nailed it--- you should be looking at IC3. I suspect if you monitor IC3 pin 13, you'll see an erratic trace as you rotate the volume pot through the noisy positions.
Would you report a few voltage measurements, both at full volume and at 0 volume:
DCV at IC pin 13
Junction of C113 and R110
Any variation in the C113,R110 readings between the two pot extreems is likely due to leakage through C113. Would you also measure the voltage at IC3 pin 9? It will be only a few negative mV.
I've referenced only the right channel. Would you also measure analogous readings in the left channel?
I'm optimistic that you'll be able to reduce the reduce the scratchy volume problem with a few mods. Any chance the opamp is already socketed?
Thanks!
epicyclic has nailed it--- you should be looking at IC3. I suspect if you monitor IC3 pin 13, you'll see an erratic trace as you rotate the volume pot through the noisy positions.
Would you report a few voltage measurements, both at full volume and at 0 volume:
DCV at IC pin 13
Junction of C113 and R110
Any variation in the C113,R110 readings between the two pot extreems is likely due to leakage through C113. Would you also measure the voltage at IC3 pin 9? It will be only a few negative mV.
I've referenced only the right channel. Would you also measure analogous readings in the left channel?
I'm optimistic that you'll be able to reduce the reduce the scratchy volume problem with a few mods. Any chance the opamp is already socketed?
Thanks!
Hi
Re measurements
Pin 1 and 13 = 0v until pot rotated, then jumps up and down up to about 89mv either channel.
Pin 9 & 6 is 0 volt all the time
C113/R10 both channels = 718mv fixed
Trace is erratic as suggested.
No one seems to question why when R177 is disconnected in both channels that the high voltage on the IC 2 is still present?
How can IC3 effect IC2 when it is not connected to it as the resistor is out of circuit both channels.
I am a tad confused here.
thank you all
regards
P
Re measurements
Pin 1 and 13 = 0v until pot rotated, then jumps up and down up to about 89mv either channel.
Pin 9 & 6 is 0 volt all the time
C113/R10 both channels = 718mv fixed
Trace is erratic as suggested.
No one seems to question why when R177 is disconnected in both channels that the high voltage on the IC 2 is still present?
How can IC3 effect IC2 when it is not connected to it as the resistor is out of circuit both channels.
I am a tad confused here.
thank you all
regards
P
I believe R177 and the bias present at IC2 are unrelated to the noisy volume pot.
The voltage at the junction of C113/R10 both channels is unexpectedly high: "C113/R10 both channels = 718mv fixed". I can't reconcile the 718mV with the 89mV you report at pin 13. But please conduct the following experiment:
Set volume pot to 0. There may be a few mV present at pins 13 and 1. By way of explanation, U3's bias will now flow through the pot wiper directly to the CCW end of the volume pot and connect directly to the opamp output (pin 13); ideally, there will be 0V developed across the pot's resistor element, but I suspect otherwise. With volume at 0, please measure with respect to ground voltages at all three pot terminals (CCW, wiper, CW) and junction C113/R10. Also, manipulate the Contour switch; it should have no effect upon the DC voltages. Similar measurements for both channels.
Thanks.
The voltage at the junction of C113/R10 both channels is unexpectedly high: "C113/R10 both channels = 718mv fixed". I can't reconcile the 718mV with the 89mV you report at pin 13. But please conduct the following experiment:
Set volume pot to 0. There may be a few mV present at pins 13 and 1. By way of explanation, U3's bias will now flow through the pot wiper directly to the CCW end of the volume pot and connect directly to the opamp output (pin 13); ideally, there will be 0V developed across the pot's resistor element, but I suspect otherwise. With volume at 0, please measure with respect to ground voltages at all three pot terminals (CCW, wiper, CW) and junction C113/R10. Also, manipulate the Contour switch; it should have no effect upon the DC voltages. Similar measurements for both channels.
Thanks.
That is interesting--- maybe C113 is shorted? I'm suspicious of leakage in that cap, but if the junction of C113,R110,R112 is 718mV, I can't explain ~89mV at IC3 pin 13 or pin 9.
Am I missing something?
Am I missing something?
Thanks, epicycle!
I guess we'll have to wait for Phil50 to give feedback. Just to clarify, it's anticipated that the emitter of VT20 should be about -718mv and should connect to the negative terminal go C113; positive terminal of C113 should connect to R110 and ideally should measure ~0V.
I guess we'll have to wait for Phil50 to give feedback. Just to clarify, it's anticipated that the emitter of VT20 should be about -718mv and should connect to the negative terminal go C113; positive terminal of C113 should connect to R110 and ideally should measure ~0V.
A tall order for the weekend chaps, very interesting.
I will be back on the bench on Moday/Tuesday and will post what I find.
Thank you all, and have a ncie weekend.
regards
P
I will be back on the bench on Moday/Tuesday and will post what I find.
Thank you all, and have a ncie weekend.
regards
P
Hi All,
I am afraid time and money is now over budget on this receiver.
It is being retired, too much time too much bother.
If it was a QUAD/Radford?Rogers etc it would have been over and done with, but this prblem is inherent.
I have had a colleague check everything over and he could not come up with a solution.
Somethings you just can't fix.
I would like to thank everyone who was helpful and by the way I do not put capacitors in the wrong way round.
thank you all.
Until next time
P
I am afraid time and money is now over budget on this receiver.
It is being retired, too much time too much bother.
If it was a QUAD/Radford?Rogers etc it would have been over and done with, but this prblem is inherent.
I have had a colleague check everything over and he could not come up with a solution.
Somethings you just can't fix.
I would like to thank everyone who was helpful and by the way I do not put capacitors in the wrong way round.
thank you all.
Until next time
P