No, the manual tells you to use an oscilloscope.
That needs some practice and can be very wrong if the amplifier is still sick. There is a voltmeter clue on the schematic.
Attachments
I connected dummy loads to both channels and let the amp warm up for about 20 minutes, volume at minimum, and as per the service manual, both bias pots fully counter-clockwise.
Left channel...only had to turn the pot less than 1/4 turn to reach .05 v dc
Right channel...had to turn pot about 3/4 turn to reach the same
Checking other voltages in the amp section I found the following,..... Note: spec'd voltages from schematic are in (parenthesis)
_____________________Q210 (left channel)__________________Q310 (right channel)
Base___________________42.0 (29.5 v)__________________________24.0 (29.5 v)
_____________________Q211 (left channel) __________________Q311 (right channel)
Emitter________________.05 v (.05 v)___________________________ .05 v (.05 v)
Base____________________.6 (.55 v)______________________________ .6 (.55 v)
Collector_______________40.9 (29 v)_____________________________23.5 (29 v)
I then connected a signal generator to the auxiliary input, 1K sine wave with output of .25 VAC and connected an oscilloscope to the speaker terminals.
Left channel output showed a sine wave of slightly more magnitude than the right.
......Left channel started to clip at the top around the 1 o'clock position of the volume pot
......Right channel started to clip at the bottom around the 2 o'clock position
To achieve symmetrical clipping,
the LEFT channel BALANCE pot is almost fully counter-clockwise, just like the left channel BIAS pot
the RIGHT channel BALANCE pot is almost fully clockwise like the right channel BIAS pot
........................................................................................................................................................................................
The +- 40 v (left) and +- 24 v (right) seems to be a problem. Any suggestions of what to do next?
Last edited:
I removed the dummy loads, reset the bias and checked the voltages again
..... Note: spec'd voltages from schematic are in (parenthesis)
_____________________Q210 (left channel)__________________Q310 (right channel)
Base___________________36.0 (29.5 v)__________________________26.0 (29.5 v)
_____________________Q211 (left channel) __________________Q311 (right channel)
Emitter________________.05 v (.05 v)___________________________ .05 v (.05 v)
Base____________________.6 (.55 v)______________________________ .6 (.55 v)
Collector_______________36.0 (29 v)_____________________________26.0 (29 v)
Left channel still has higher voltages than specified and Right channel has lower than spec'd.
..... Note: spec'd voltages from schematic are in (parenthesis)
_____________________Q210 (left channel)__________________Q310 (right channel)
Base___________________36.0 (29.5 v)__________________________26.0 (29.5 v)
_____________________Q211 (left channel) __________________Q311 (right channel)
Emitter________________.05 v (.05 v)___________________________ .05 v (.05 v)
Base____________________.6 (.55 v)______________________________ .6 (.55 v)
Collector_______________36.0 (29 v)_____________________________26.0 (29 v)
Left channel still has higher voltages than specified and Right channel has lower than spec'd.
Check the power supply thoroughly and how the rails feed the transistors.
Could be a leaky current path.
And you changed the BC109 transistor on the left channel?
Swap it to right channel or put a new one there too.
So much mismatch in voltages is not normal.
Could be a leaky current path.
And you changed the BC109 transistor on the left channel?
Swap it to right channel or put a new one there too.
So much mismatch in voltages is not normal.
Thank you for the response. Yes, I checked the ps voltages and they are within a few volts of spec, I also replaced the the right side pair of BC109's.
Then you will have to do it the hard way, disconnect each and every pot and switch, check those.
Then check every resistor, actual to schematic.
Capacitors, you have a meter to check?
So much voltage difference is not at all normal.
Then we need to check the transistors at the last step.
Then check every resistor, actual to schematic.
Capacitors, you have a meter to check?
So much voltage difference is not at all normal.
Then we need to check the transistors at the last step.
BIAS voltages match to a T : 0.05V across emitter resistors
ONE Rail voltage is grossly wrong (36V vs 29V).
Only possible way to have that is amplifier is a dual monobloc with separate supplies, which I doubt, so remaining possibility is measuring error.
As nareshbd suggested, trace voltages backwards and forward from collectors to supply big capacitors, step by step, use always the same point as ground reference for both channels, you are making some measuring error.
Rail voltage difference is NOT a Bias problem.
NareshBrd, all but four of the electrolytics have been changed. I just checked all of the transistors on the Pre-amp and Tone-boards.
The main difference I see is Q202 (left channel) has a gain of 352 while Q302 (right channel) has a gain of 218 .
JMFahey, thank you for the advice. Will do but may have a few more questions very soon
The main difference I see is Q202 (left channel) has a gain of 352 while Q302 (right channel) has a gain of 218 .
JMFahey, thank you for the advice. Will do but may have a few more questions very soon
Another electronics newbie question..... is "rail voltage" another term for power supply voltage ?
Last edited:
I have not gone through the manual.
Q202 and Q302, where are they located?
How did you measure gain?
If in doubt, remove and check, first resistance with DMM, then Hfe.
Do they affect the power supply voltages?
Again, can you describe the power supply?
In my Kenwood amp, there is a single supply for both channels, one voltage, about 14V, and small current, is used for the pre-amplifier section, and also supplies the tape deck and receiver.
The higher current 27 V supply is for the STK 4141 chip amp.
Both are Zener or transistor regulated....I forgot, but that is a common arrangement.
The main point is that the same supply is used for both channels, that is normal, unless it is a dual mono block unit, with separate power supplies for both channels.
For me the starting point would be to track down the voltage fault, then do whatever adjustments are needed, because with faulty supply voltages, most efforts are bound to fail.
Look closely, either channel may have a faulty voltage regulation. one is not being pulled down to 29.5 from 36, and the other is being pulled down too much from 29.5 to 26.
26 is still okay, your output may be a little less, bearable.
But the 36 is the important fault, fix that first.
Q202 and Q302, where are they located?
How did you measure gain?
If in doubt, remove and check, first resistance with DMM, then Hfe.
Do they affect the power supply voltages?
Again, can you describe the power supply?
In my Kenwood amp, there is a single supply for both channels, one voltage, about 14V, and small current, is used for the pre-amplifier section, and also supplies the tape deck and receiver.
The higher current 27 V supply is for the STK 4141 chip amp.
Both are Zener or transistor regulated....I forgot, but that is a common arrangement.
The main point is that the same supply is used for both channels, that is normal, unless it is a dual mono block unit, with separate power supplies for both channels.
For me the starting point would be to track down the voltage fault, then do whatever adjustments are needed, because with faulty supply voltages, most efforts are bound to fail.
Look closely, either channel may have a faulty voltage regulation. one is not being pulled down to 29.5 from 36, and the other is being pulled down too much from 29.5 to 26.
26 is still okay, your output may be a little less, bearable.
But the 36 is the important fault, fix that first.
Last edited:
Oh, and a tip, if you have a damaged probe, cut off the tip, and solder an alligator clip.
Useful for poking around in circuits.
Like here, same point can be kept connected while you poke around.
And keep just the tip open, too easy to blow transistors with too long probes.. Motherboard, it was old....845, cheaper to buy old than replace all the Mosfets.
Most probes fail at the ends, and I use a paper pin to check.
Ended up with a pair of probes with alligator clips, so I can watch the voltages, and use my hands elsewhere.
I NEVER do this on mains circuits, too risky!
Useful for poking around in circuits.
Like here, same point can be kept connected while you poke around.
And keep just the tip open, too easy to blow transistors with too long probes.. Motherboard, it was old....845, cheaper to buy old than replace all the Mosfets.
Most probes fail at the ends, and I use a paper pin to check.
Ended up with a pair of probes with alligator clips, so I can watch the voltages, and use my hands elsewhere.
I NEVER do this on mains circuits, too risky!
In case you have to work a long time, solder a piece of wire to the ground track, and use that to connect to the meter, in my case, the alligator clip.
Less chance of measurement error.
And clean up the track side with a fiber glass brush to remove build up that may cause reduced current flow to the probes.
Less chance of measurement error.
And clean up the track side with a fiber glass brush to remove build up that may cause reduced current flow to the probes.
Q202 and Q302 are located in the pre-amp board. I tested them with a dedicated semi-conductor tester Peak Atlas brand.
Regarding your power supply question, I will have to study the schematic a little later today and hopefully be able to explain what is going on.