Hi
first let me thank @BSST , @Mooly , @pitbul again for all the effort on my previous experience with a pair of Kenwood L08M.
https://www.diyaudio.com/community/threads/kenwood-l08m-circuit-discrepency.400634/
I have another pair which I am working on and need some insight and guidance.
Current Status:
All caps have been replaced (except the main power supply caps)
The 10 ohm and 2.7k 3W resistors have been replaced.
New relay installed
Q25/Q26 installed
R51/R52 installed
all other components installed
D8-D9 Shorted
C17 shorted
Input shorted
Q29/Q30 (drivers installed)
all relays click and dim bulb off.
The not so good news
In this state the base of Q29 and Q30 are both -0.4v, where as I would expect that Q30 should be +ve rather than -ve
I should also add that there is no voltage across R51/R52, or R57/R58 which implies the front end is working as it should.
additionally when the 2SA output is installed- the relays click and the dim bulb goes off, suggesting that the negative rail side is OK,
however when the 2SC is installed, the Dim bulb goes on and no relay click
any help greatly appreciated
Peter
first let me thank @BSST , @Mooly , @pitbul again for all the effort on my previous experience with a pair of Kenwood L08M.
https://www.diyaudio.com/community/threads/kenwood-l08m-circuit-discrepency.400634/
I have another pair which I am working on and need some insight and guidance.
Current Status:
All caps have been replaced (except the main power supply caps)
The 10 ohm and 2.7k 3W resistors have been replaced.
New relay installed
Q25/Q26 installed
R51/R52 installed
all other components installed
D8-D9 Shorted
C17 shorted
Input shorted
Q29/Q30 (drivers installed)
all relays click and dim bulb off.
The not so good news
In this state the base of Q29 and Q30 are both -0.4v, where as I would expect that Q30 should be +ve rather than -ve
I should also add that there is no voltage across R51/R52, or R57/R58 which implies the front end is working as it should.
additionally when the 2SA output is installed- the relays click and the dim bulb goes off, suggesting that the negative rail side is OK,
however when the 2SC is installed, the Dim bulb goes on and no relay click
any help greatly appreciated
Peter
So having replaced D11-D15 and R53 and R57
with the 2SA installed - relays click dim bulb goes off
however, when I connect the 2SC output the dim bulb starts rising after the initial surge and settle down
The transistor tests good on the Peak Transistor tester, though of course this can be misleading.
I am running out of ideas.
Before the bulb gets too bright the base voltage on the 2SC is about 0.3v and on the emitter 100 or so mv
Any guidance or insights greatly appreciated
Peter
with the 2SA installed - relays click dim bulb goes off
however, when I connect the 2SC output the dim bulb starts rising after the initial surge and settle down
The transistor tests good on the Peak Transistor tester, though of course this can be misleading.
I am running out of ideas.
Before the bulb gets too bright the base voltage on the 2SC is about 0.3v and on the emitter 100 or so mv
Any guidance or insights greatly appreciated
Peter
First thing I would do is check to see if the problem is actually very high frequency oscillation that occurs when the outputs are in place. If the base is at around 0.3 volts and the emitter at 0.1 volts then you have only around 0.29 volts across the junction. That should be far to low to cause current to flow and that again is another suspicion of oscillation.
The final driver stage of Q31 and Q32 without the outputs fitted look as though they should be able to correctly set all DC conditions and also deliver (without a load) a normal output voltage swing. So that's something else to check. The voltage across the 33 ohm emitter resistors should respond to bias setting and you should be able to set around 0.7 volts across each.
The final driver stage of Q31 and Q32 without the outputs fitted look as though they should be able to correctly set all DC conditions and also deliver (without a load) a normal output voltage swing. So that's something else to check. The voltage across the 33 ohm emitter resistors should respond to bias setting and you should be able to set around 0.7 volts across each.
I would try it first with C17 shorted and the short on the diodes doesn't matter if the cap is shorted.
With no outputs fitted the DC offset should be zero and the voltage across the 33 ohms close to zero. If that's OK remove the short on the cap and the diodes and starting with the preset on minimum resistance see how it behaves. Offset should be zero and voltage across each 33 ohm should adjust from below lets 0.4 volts to around 1 volt or more, perhaps a lot more with no outputs to limit it. Once you get to around 0.6 volts that would be the point the outputs start to conduct in a normally working amp.
With no outputs fitted the DC offset should be zero and the voltage across the 33 ohms close to zero. If that's OK remove the short on the cap and the diodes and starting with the preset on minimum resistance see how it behaves. Offset should be zero and voltage across each 33 ohm should adjust from below lets 0.4 volts to around 1 volt or more, perhaps a lot more with no outputs to limit it. Once you get to around 0.6 volts that would be the point the outputs start to conduct in a normally working amp.
Hi
with C17 shorted and D8/D9 shorted R61 0mv R62 24mv
With C17 not shorted and D8/D9 shorted R61 195mv R62 -4mv
With C17 not shorted and D8/D9 not shorted and Bias set to minimum R61 0.7v R62 -0.7v
in all the above relays all click as they should
With the bias pot fully clockwise I get +/-0.48v
Fully anticlockwise (which I thought was minimum) I get the +/-0.7v
Perhaps the bias pot needs replacing?
I think is is an oscillation, though what is causing this has me flummoxed
Thanks
Peter
with C17 shorted and D8/D9 shorted R61 0mv R62 24mv
With C17 not shorted and D8/D9 shorted R61 195mv R62 -4mv
With C17 not shorted and D8/D9 not shorted and Bias set to minimum R61 0.7v R62 -0.7v
in all the above relays all click as they should
With the bias pot fully clockwise I get +/-0.48v
Fully anticlockwise (which I thought was minimum) I get the +/-0.7v
Perhaps the bias pot needs replacing?
I think is is an oscillation, though what is causing this has me flummoxed
Thanks
Peter
Its good no oscillation shows.
R62, 33 ohm. When you have 4 mv across it there is no way that can turn on the outputs.
The circuit shows R65 as the other 33 ohm (I can't see an R61) The voltage across R65 should also be low.
If the circuit is OK in that state and has an issue when the outputs are fitted then you need to check for oscillation once the outputs are back in place.
R62, 33 ohm. When you have 4 mv across it there is no way that can turn on the outputs.
The circuit shows R65 as the other 33 ohm (I can't see an R61) The voltage across R65 should also be low.
If the circuit is OK in that state and has an issue when the outputs are fitted then you need to check for oscillation once the outputs are back in place.
with 2SA installed only- relays click dim bulb off
with 2SA and 2SC installed relay does not click and dim bulb goes very bright
the input is shorted
the voltage across R65/R62 is 0.6v
C17 is not shorted
D8/D9 are not shorted
With the 2SA and 2SC fitted - on the scope I can see a sine wave and using the DMM as frequency counter I get 688Khz- so I guess it is oscillating
First the dumb question- why?
and the second question how to diagnose?
with 2SA and 2SC installed relay does not click and dim bulb goes very bright
the input is shorted
the voltage across R65/R62 is 0.6v
C17 is not shorted
D8/D9 are not shorted
With the 2SA and 2SC fitted - on the scope I can see a sine wave and using the DMM as frequency counter I get 688Khz- so I guess it is oscillating
First the dumb question- why?
and the second question how to diagnose?
some progress- I have replaced the 470 ohm 3W R94/R92- and with one 2SC fitted (so far)- no oscillation/dim bulb out/relays clicking
the pulled resistors measure fine, but I am wondering if it is a bad solder joint
will report back more fully
the pulled resistors measure fine, but I am wondering if it is a bad solder joint
will report back more fully
Thanks again @Mooly - That was the problem, the relays click, dim bulb goes off all outputs installed no DC on output
I will bias and test with a dummy load for signal next.
Then onto the second one of the pair, which has 27.5v on the output (with no outputs fitted) and a couple of blown output transistors, though with this one now working as a reference I am hopeful that it will be straight forward.
I then have a Kenwood L01M to work on- which appears to be a stripped down version of the L08C and L08M in a single integrated amp
Again many thanks for your insights, wisdom and assistance
I will bias and test with a dummy load for signal next.
Then onto the second one of the pair, which has 27.5v on the output (with no outputs fitted) and a couple of blown output transistors, though with this one now working as a reference I am hopeful that it will be straight forward.
I then have a Kenwood L01M to work on- which appears to be a stripped down version of the L08C and L08M in a single integrated amp
Again many thanks for your insights, wisdom and assistance
well great plans of mice and men
Input not shorted
amp not on dim bulb
offset set at 0v on VR1
the bias starts at 0.03mv and when I raise to anything over 0.3mv then amp starts drawing loads of current- so there is still a problem
I have scope set to 100mv with a load resistor attached- not connected to the amp- and when the current draw goes high the scope display shows a strong signal- which suggests the amp is broadcasting RF and the scope/load resistor is picking it up.
The Bias pot is original and measures OK.
Input not shorted
amp not on dim bulb
offset set at 0v on VR1
the bias starts at 0.03mv and when I raise to anything over 0.3mv then amp starts drawing loads of current- so there is still a problem
I have scope set to 100mv with a load resistor attached- not connected to the amp- and when the current draw goes high the scope display shows a strong signal- which suggests the amp is broadcasting RF and the scope/load resistor is picking it up.
The Bias pot is original and measures OK.
The voltages on Q20 and Q22 collector are 1.7v/-1.7v
Q25/Q26 emitter voltages are 1.6/-1.6
Q29/Q30 emitter voltages are 1.2/-1.2.
Q25/Q26 emitter voltages are 1.6/-1.6
Q29/Q30 emitter voltages are 1.2/-1.2.
If it seems to be doing that and radiating a lot of high frequency signal then it is unstable as you increase bias and once tht happens the DC voltage measurements become less reliable because the voltages are no longer static but are AC voltages superimposed on a DC value.
I have the PCB partially out of the case, so if I reconnect and make sure all the earths are connected (I have flying leads at present), that may address this?
These are a true PITA to work on
These are a true PITA to work on