Hi all:
I thought this would be a relatively simple project for the long weekend, but I am stumped on how to proceed and would be grateful for help/pointers.
This is a Kenwood KA-3700 amplifier with really low output on the Right channel; Left channel is good. The unit passes DBT check. DC offset is 41.5mV/43.1mV (will be replacing the input differential pairs Q1/Q3 and Q2/Q4 with newer devices). Outputs and drivers for both channels test OK. I've compared voltages between good and bad channels; all appear similar except across diode Dm1 (good channel) is -343mV and Dm2 (bad channel) is -373mV.
First big clue: Bias Left (good) channel is 20.3mV while Right (bad) channel is 36.6mV. There is no provision/potentiometer to adjust Bias.
In the bad channel I replaced the Q6/Q8 pair but saw no change. This is a relatively simple amplifier with few components. What am I missing ... and how do I adjust the bias?
Thanks in advance for your help.
Mayank
PS I've cleaned the volume, balance and other potentiometers with Deoxit already.
I thought this would be a relatively simple project for the long weekend, but I am stumped on how to proceed and would be grateful for help/pointers.
This is a Kenwood KA-3700 amplifier with really low output on the Right channel; Left channel is good. The unit passes DBT check. DC offset is 41.5mV/43.1mV (will be replacing the input differential pairs Q1/Q3 and Q2/Q4 with newer devices). Outputs and drivers for both channels test OK. I've compared voltages between good and bad channels; all appear similar except across diode Dm1 (good channel) is -343mV and Dm2 (bad channel) is -373mV.
First big clue: Bias Left (good) channel is 20.3mV while Right (bad) channel is 36.6mV. There is no provision/potentiometer to adjust Bias.
In the bad channel I replaced the Q6/Q8 pair but saw no change. This is a relatively simple amplifier with few components. What am I missing ... and how do I adjust the bias?
Thanks in advance for your help.
Mayank
PS I've cleaned the volume, balance and other potentiometers with Deoxit already.
Attachments
Last edited:
1) was the amp operating as described above prior to making any component changes?
2) have you replaced any caps?
3) cleaning switches is just as important as cleaning pots
4) long shot guess....Cm10
2) have you replaced any caps?
3) cleaning switches is just as important as cleaning pots
4) long shot guess....Cm10
Last edited:
I would not expect Q6 Q6 to have any real effect on offset bias. Their effect is swamped by the gain of Q2 Q4. Which are nominally where offset is compared.
However many-dozen mV of offset may be "normal". While the Q2468 quad does balance emitter currents, base current is subject to hFE variability, and the base resistors are *different* Q2 and Q4 (100K vs 57K).
Another note: the *entire* system (minus phono), line amp tone amp power amp, is done in one giant stage. IMHO, doing "too much at once". And generally a sign that the amp was designed-down to a price. I could be wrong, it may be elegantly simple and minimal. But small "flaws" like an insignificant (and variable) offset are to be expected. And probably not Problems.
Diodes Dm12 should be like 1800mV, not 400mV. This would cause "hoarseness at low volume", play OK when loud.
Bad Cm10 would be high output, not low. Cm14 seems more to the point.
However many-dozen mV of offset may be "normal". While the Q2468 quad does balance emitter currents, base current is subject to hFE variability, and the base resistors are *different* Q2 and Q4 (100K vs 57K).
Another note: the *entire* system (minus phono), line amp tone amp power amp, is done in one giant stage. IMHO, doing "too much at once". And generally a sign that the amp was designed-down to a price. I could be wrong, it may be elegantly simple and minimal. But small "flaws" like an insignificant (and variable) offset are to be expected. And probably not Problems.
Diodes Dm12 should be like 1800mV, not 400mV. This would cause "hoarseness at low volume", play OK when loud.
Bad Cm10 would be high output, not low. Cm14 seems more to the point.
I think the bias voltages are a red herring. They sound good to me.
The problem is elsewhere.
Given the amp does work at least you can apply a signal and scope it through until it the signal drops off.
I had a similar problem with an amp, it just wasn't very loud.
I kept going back to the same transistor where things didn't seem quite right.
I checked all the components using a diode test and they were fine.
I couldn't check two transistors for being correct as the numbers had worn off with age.
So I carefully diode tested again and it suddenly hit me the transistor where I thought the fault was is a pnp and not an npn !!!!
Someone had tried to fix the amp previously and put in a wrong transistor.
I had a struggle finding the transistor but one turned up on ebay and that fixed it.
The problem is elsewhere.
Given the amp does work at least you can apply a signal and scope it through until it the signal drops off.
I had a similar problem with an amp, it just wasn't very loud.
I kept going back to the same transistor where things didn't seem quite right.
I checked all the components using a diode test and they were fine.
I couldn't check two transistors for being correct as the numbers had worn off with age.
So I carefully diode tested again and it suddenly hit me the transistor where I thought the fault was is a pnp and not an npn !!!!
Someone had tried to fix the amp previously and put in a wrong transistor.
I had a struggle finding the transistor but one turned up on ebay and that fixed it.
