Hi, please see the voltage drop values from bottom upwards:
bad good
R 22 1.39V 1.24 V
SFR 2 2mV 232mV
D6 2.17V 2.16V
R 20 2.02V 691mV
In a good channel SFR2 was left as is (not turned to zero)
bad good
R 22 1.39V 1.24 V
SFR 2 2mV 232mV
D6 2.17V 2.16V
R 20 2.02V 691mV
In a good channel SFR2 was left as is (not turned to zero)
Voltage drop is usually difference of voltages.
You could get better response if you would do that calculation yourself and not expect the forum readers to do it 😉
Voltage across R20 looks very suspect. Around 700mv is what I would expect and the good channel confirms that.
Without overthinking anything at this point I would suspect a possible issue with the NPN driver transistor Q6. It seems to be pulling far to much current through R20. Check very carefully for cracked print on the collector lead where it is soldered. You could also very carefully (one slip and it all goes pop) the base/emitter voltage of Q6 and compare to the other channel.
Without overthinking anything at this point I would suspect a possible issue with the NPN driver transistor Q6. It seems to be pulling far to much current through R20. Check very carefully for cracked print on the collector lead where it is soldered. You could also very carefully (one slip and it all goes pop) the base/emitter voltage of Q6 and compare to the other channel.
It is much safer to solder pins to the points you plan to measure in advance and hook the proves to those. as needed.
Takes some minutes but prevents slips 😉
These driver transistors have rather long legs so I can just hook probes before switching on the voltage.
"you probably meant calculation of current values based on respective resistances" - not quite.
I meant the directly measured voltage across pins of the selected component (resistor, diode etc). From that voltage you can roughly judge the failure of a component (diode, transistor) or calculate the current based on component resistance.
And the currents need to "add up" along the chain and make sense.
From your results:
"SFR 2 2mV 232mV"
there seems to be ca 230mV (0,23V) across SFR2 at its minimum position and that I would consider suspicious. 0 V would be expected...
I meant the directly measured voltage across pins of the selected component (resistor, diode etc). From that voltage you can roughly judge the failure of a component (diode, transistor) or calculate the current based on component resistance.
And the currents need to "add up" along the chain and make sense.
From your results:
"SFR 2 2mV 232mV"
there seems to be ca 230mV (0,23V) across SFR2 at its minimum position and that I would consider suspicious. 0 V would be expected...
Solder pins under the board and hook up the DMMs to these. Safest way I know today.
madis 64 those 232mV were from a good channel where I didn't turn SFR2 to zero (I left a comment below) sorry once again
"The voltage drop over a diode is fixed. It is always 0.7 V, no matter which current flows. (Some people say it is 0.6V)."
http://www.frankshospitalworkshop.com/electronics/training_course_diode.html
How do you get 2,16...2,17V across D6?
http://www.frankshospitalworkshop.com/electronics/training_course_diode.html
How do you get 2,16...2,17V across D6?
D6 it is STV-4H diode acting as temperature sensor screwed to heat sink. When it is broken people replace it with three or four regular diodes in series or with a transistor e.g.BC550 with 3.3k and 1k resistors.
Hi, I have measured BE voltage on npn transistor Q6 and came to strange results:
on a good channel the value is +605mV
white on bad one it is negative -650mV
on a good channel the value is +605mV
white on bad one it is negative -650mV
Transistor taken out has following parameters:
hfe 83
Ie 0.68m
Vbe 559mV
As hfe range is 60 to 200 so looks good.
Maybe connections to pcb were broken.
Should I look for a new one or put it back but firmly connect ?
hfe 83
Ie 0.68m
Vbe 559mV
As hfe range is 60 to 200 so looks good.
Maybe connections to pcb were broken.
Should I look for a new one or put it back but firmly connect ?
Refitting may not prove much because the act of heating and removing it can often temporarily 'fix' suspect parts. Those are old obsolete devices and many offered for sale could be fake. I would try something like an MJE340 replacement. MJE15032 is another.
You should verify that it actually is NPN transistor and that the pinout matches the board, nowadays you never know...
Success !
I have replaced both driver transistors 648/668 with new ones. The seller claimed that they are genuine Hitachi. Don't know since their printing is black while mine was white. Their hfe is higher av 170 and the old ones was av 70. Nevertheless the are working good and you can easily set the desired bias value.
Thank you Mooly and madis64 for your help in putting that beautiful amplifier back to life.
I have replaced both driver transistors 648/668 with new ones. The seller claimed that they are genuine Hitachi. Don't know since their printing is black while mine was white. Their hfe is higher av 170 and the old ones was av 70. Nevertheless the are working good and you can easily set the desired bias value.
Thank you Mooly and madis64 for your help in putting that beautiful amplifier back to life.
