Hi,
I am working on rebuilding the musical fidelity MA50, same circuit as A1 with higher rail voltage.
After my rebuild, one channel is good, but the other channel suffered from distortion problem about 10 seconds after power on. I measured the voltages, found the problem the problem could be abnormal voltage drop on R5.
As the schematics shows, r5 connects to emitter leg of TR1 and TR2. It measures about only 250mv and quickly dropped to 200mv or less. Compare to the other working channel, the voltage should be around 500-580mv. Voltage on R12 in the same channel voltage looks goods.
So I think the problem is bottom half of the circuit.
I think R5 voltage problem affected the wrong voltages on entire bottom half of transistors ( TR5, 6, 9)
I believe this is the source of the distortion.
My question is what determines the R5-TR1,2 voltage?
I measured almost the entire circuit check for wrong elements and connections, but can not find anything wrong. So I think its better to do some theoretical analysis based on what I found.
Thanks
I am working on rebuilding the musical fidelity MA50, same circuit as A1 with higher rail voltage.
After my rebuild, one channel is good, but the other channel suffered from distortion problem about 10 seconds after power on. I measured the voltages, found the problem the problem could be abnormal voltage drop on R5.
As the schematics shows, r5 connects to emitter leg of TR1 and TR2. It measures about only 250mv and quickly dropped to 200mv or less. Compare to the other working channel, the voltage should be around 500-580mv. Voltage on R12 in the same channel voltage looks goods.
So I think the problem is bottom half of the circuit.
I think R5 voltage problem affected the wrong voltages on entire bottom half of transistors ( TR5, 6, 9)
I believe this is the source of the distortion.
My question is what determines the R5-TR1,2 voltage?
I measured almost the entire circuit check for wrong elements and connections, but can not find anything wrong. So I think its better to do some theoretical analysis based on what I found.
Thanks
Voltage across r8 is 800mv and quickly dropping after power on. much higher than R9 which is around only 50mv.
Also checked the working channel, there R8, and R9 only have a few mv voltage, big difference
Also checked the working channel, there R8, and R9 only have a few mv voltage, big difference
TR1 placed correct / broken / bad soldering?
R8 solderings / ground path?
Current through R5 is set by +12V rail (ZD2), R7-R8-R6-R30 and feedback loop.
Voltages on R16 and R15 hints the feedback loops are working fine to get the amp in some balance.
All checked, a few times, voltage on R15 only measured -0.9 to -1 V, which is less then a half of normal voltage at about -2.5v
Voltage drop on R18 is the same as voltage on R27 as designed (TR5,6 as follower transistor providing no voltage gain).
but on the fault channel the voltage is 200mv, while the working channel is around 580mv. Also less than half.
so the whole current on the bottom circuit of the fault channel is less than a half current compare to the working one
but on the fault channel the voltage is 200mv, while the working channel is around 580mv. Also less than half.
so the whole current on the bottom circuit of the fault channel is less than a half current compare to the working one
Is voltage across R8 +800mV re ground? That would mean TR1 base is back-biased re emitter at +200mV. If R8 is at -800mV, that makes about 1V base-emitter, also unreasonable.
I suggest measuring voltages at R8, TR1 base, TR2 emitter, TR2 base, and R24 near TR3 collector. The TR1 base voltage should be slightly elevated re R8, confirming current really is flowing into T1 base. TR2 base and R24 voltages will tell us the bias current flowing through R24.
The voltage dropped on d10/d9 (ie the base voltage difference of the long tailed pair of transistor TR1 and TR2) is 380mv, while on the working channel it’s almost 0mv
yes, R8 voltage is -800mv, although when I measured it again this time, it sits at -500mv, each time when I power on, the voltage is different sometime measured -450mv
connect TR1 base to positive lead , r8 to negative lead, measured -10mv.
a new measure in one go:
r8 -330mv. ( vs working channel 25mv)
tr1 base -340mv. ( vs working channel 26.3mv)
tr2 emitter -110mv ( vs working channel 591mv)
tr2 base -740mv. ( vs working channel 16.9mv )
r24 left side, Sam as tr2 base, as the are connected.
r24 right hand side is +170mv. ( vs working channel -63mv)
please Note the voltage floats with time, so there might some inconsistency here, as I can’t measure quick enough in one go.
while in the working channel, the voltages are actually quite stable
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TR1 collector -31.1v ( vs working channel -30.5v)
TR2 collector -12.9v ( vs working channel -12.8v )
TR9 base -31.1 v ( vs working channel -30.4v)
TR9 collector 166mv ( vs working channel -66mv)
TR2 collector -12.9v ( vs working channel -12.8v )
TR9 base -31.1 v ( vs working channel -30.4v)
TR9 collector 166mv ( vs working channel -66mv)
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I also noticed that in the distorted channel: the voltage between TR10 collector and TR9 collector is 0
while is the working channel TR10 collector is 120mv higher than TR9 collector
so that means voltage drop on R31 and R30 both are 0, and it’s confirmed by measurement. Should have around 60mv voltage drop on each of them, could this be the problem? Not sure why this happened, measured resistance of r31 and r30, both are 0.47ohm as expected (slightly different than the 0r22 shown on the schematic, but that’s just due to differences different production batch varia ) same as the working channel resistance.
while is the working channel TR10 collector is 120mv higher than TR9 collector
so that means voltage drop on R31 and R30 both are 0, and it’s confirmed by measurement. Should have around 60mv voltage drop on each of them, could this be the problem? Not sure why this happened, measured resistance of r31 and r30, both are 0.47ohm as expected (slightly different than the 0r22 shown on the schematic, but that’s just due to differences different production batch varia ) same as the working channel resistance.
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"r8 -330mV & tr1 base -340mV", so -330mV -/- -340mV over R7 is 10µA flowing into TR1 or D9//D10 combo.
"tr1 base -340mV & tr2 base -740mV", so 0.4V over D9//D10 starts D10 conducting to...?
TR2 working ch is +591 -/- +17 = +574mV, TR2 wrong ch is -110 -/- -740 = +630mV -> in the differential TR2 is conducting heavily to find the 'balance in the loop'. TR1 is shut off, so base of TR5 is at the minus rail -24V... but you measured -22V. That's almost 1mA through R16 (2.2kΩ). Where does this 1mA comes from? Not TR1 as it is shut off by TR2.
Disconnect D9//D10 as these are for to large signal protection only.
Replace TR1-TR2 tb sure. Check R24//C15 in the feedback path. Bad solderings.
TR2 base is negative, C13 don't like that. C10 neither. Replace both. C13, C14, C10 bipolar caps preferred (C13, C14 both 63V).
They're both switched off, no bias current running. That's a result of the problem, not the cause.
please note, the printed numbers on the schematic comes with the Schematic (provided by https://www.markhennessy.co.uk/mf_a1/technical.htm), not what I measured.
it’s musical fidelity a1 schematic. I am working on ma50, which is the same thing, but with higher rail voltage, mine is at 32v rail (although I measured only around 31v), and output resistor being 0r47, rather than 0r22 on the schematic.
I got almost everything replaced, including the transformer.
Only the pcb, the metal case and a few resistors are original.
Lots of labor work but its fun. Almost finished, apart from the distortion problem.
Only the pcb, the metal case and a few resistors are original.
Lots of labor work but its fun. Almost finished, apart from the distortion problem.
Check C13 and C10 as they're exposed to a wrong dc voltage (did I type 'capaciter' in my lucky number #17 ???).
Build looks fine to me. R30-R31 are 0.33Ω 10% wirewounds... better use the flat non-ww types (but don't mount them on the hs!).
Styro's are superb, but keep them well away from the heat sources.
What are those darkened area's near ZD1/ZD2 on both channels? Looks critical.
Post the actual circuit better to avoid misinterpretations.
Build looks fine to me. R30-R31 are 0.33Ω 10% wirewounds... better use the flat non-ww types (but don't mount them on the hs!).
Styro's are superb, but keep them well away from the heat sources.
What are those darkened area's near ZD1/ZD2 on both channels? Looks critical.
Post the actual circuit better to avoid misinterpretations.