Transistors checked in situ or off board?
In situ can be affected by nearby parts.
In situ can be affected by nearby parts.
I scanned my copy of the schematic and joined both scans into one 11 x 17 document for ease of viewing. I then tested the remaining transistors of the circuit and re-installed them. I found one transistor, Q206 (Left channel) to be faulty. The Current Gain Hfe was 19 with a Leakage Current of .064 ma, Q306 (Right channel) had an Hfe of 216 with zero leakage. Could this be the source of the higher than normal voltage in the Left channel?
I then tested all points noted on the schematic and noted the measurements at each location.
I then tested all points noted on the schematic and noted the measurements at each location.
The replacement transistor should arrive today, hopefully it will solve the problem.
I chose this amplifier to restore as a learning tool because it seems relatively easy to work on. If someone would be so kind as to look at the schematic in the previous post and let me know if there are any other points that should be measured with a DMM or Oscilloscope, or any other tips, I would greatly appreciate it.
I chose this amplifier to restore as a learning tool because it seems relatively easy to work on. If someone would be so kind as to look at the schematic in the previous post and let me know if there are any other points that should be measured with a DMM or Oscilloscope, or any other tips, I would greatly appreciate it.
Replacing the defective Q206 (left channel) transistor brought the voltage down to 28 VDC which seems to be withing normal limits. I replaced Q306 (right channel) while I was at it. When injecting a sine wave into the amp, there is still a channel imbalance between right and left, left being a little higher than the right.
Although I know it's not complicated, I haven't yet calculated output power using an oscilloscope. I'll take a stab at it tomorrow evening.
Left channel is 25 Watts RMS at symmetrical clipping
Right channel is 20 Watts RMS
but, if I turn the balance knob slightly past the 12 o'clock position, not quite 1 o'clock, I get the same amplitude in both channels on the scope
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You changed both the BC109s or only on one channel? The gain difference could be the cause of the balance issue.
Or:
Change the balance pot, or add a preset in series, to get them both right.
Do this after a thorough check / flow all joints / clean and spray all switches and pots in the signal path.
At least now you have decent sound...
Or:
Change the balance pot, or add a preset in series, to get them both right.
Do this after a thorough check / flow all joints / clean and spray all switches and pots in the signal path.
At least now you have decent sound...
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Yes, both right and left channel BC109's were changed and all pots and switches were sprayed and worked back and forth. Re-flowing all solder joints is a good idea, will do that next.
What exactly is a "preset in series"?
What exactly is a "preset in series"?
That means an extra preset in series with the main pot, to make both equal.
But your difference is, shall we say, within production tolerances, so an extreme solution.
I would bring the supplies up to specification of 29 before doing that.
Live with it, enjoy the sound with the satisfaction of having made it work.
But your difference is, shall we say, within production tolerances, so an extreme solution.
I would bring the supplies up to specification of 29 before doing that.
Live with it, enjoy the sound with the satisfaction of having made it work.
When I first powered up the amp in my system, I was disappointed in the sound which was rather grainy, so I removed it and hooked it up to a tuner and cheap pair of speakers in my office where it could just play while I worked. After over a week of running 8 or so hours a day, I put it back in my system last night and WOW, what a difference, the sound is really incredible. This little amp has a full, rich sound to it. I found myself enjoying music that I rarely listen to like Reggae which was playing on a local college radio station. I then switched to a classical station which was broadcasting Bach "Magnificat", the vocals were simply sublime.
Thank you again to all who helped me along with this project, i don't think I would be enjoying this amp as much if I had simply sent it to a repair shop.
Thank you again to all who helped me along with this project, i don't think I would be enjoying this amp as much if I had simply sent it to a repair shop.
Dear respected JmFahey sir I pull out some toshiba transistors from my old amp then I check it on digital multimeter the reading on meter is showing approximately same from base to emitter or from base to collector I just wanted to know that my transistors are ok or not????
Thanks..
Dear Zohaib, you are very kind and extremely polite, no need for over praising here 🙂
We need numbers here 🙂
Set meter to diode scale and measure EB BC CE , both ways on each, son 6 numbers per transistor, what is the number displayed?
Inn general display shows voltage drop with 1 or 2 mA current, so we expct some 650-700 mV forward and "open" the other way, and open both ways CE
But in principle avoid pulling parts just to be measured "outside", more damage than good, I prefer measuring voltages on a working or trying to work amplifier, and only remove parts when many signs point to it being bad, for confirmation.
Never at random.
What about the gross rail voltage difference?
EDIT: I do not find you in this thread. ¿¿¿¿????
Are you talking about another amplifier?
Suggest you open a separate thread for it.
Which is?
We need numbers here 🙂
Set meter to diode scale and measure EB BC CE , both ways on each, son 6 numbers per transistor, what is the number displayed?
Inn general display shows voltage drop with 1 or 2 mA current, so we expct some 650-700 mV forward and "open" the other way, and open both ways CE
But in principle avoid pulling parts just to be measured "outside", more damage than good, I prefer measuring voltages on a working or trying to work amplifier, and only remove parts when many signs point to it being bad, for confirmation.
Never at random.
What about the gross rail voltage difference?
EDIT: I do not find you in this thread. ¿¿¿¿????
Are you talking about another amplifier?
Suggest you open a separate thread for it.