Hi,
I recapped the capacitors in my Musical Fidelity A1. When I plugged the amp in after recap, and powered it on. The R2 resistor in the power amp section instantly burnt out.
its a 0.47 ohm 2w rated resistor, can anyone help to explain why this happens? Shall I just replace the resistor to a higher rated one?
I recapped the capacitors in my Musical Fidelity A1. When I plugged the amp in after recap, and powered it on. The R2 resistor in the power amp section instantly burnt out.
its a 0.47 ohm 2w rated resistor, can anyone help to explain why this happens? Shall I just replace the resistor to a higher rated one?
i have double checked, looks fine to me.
One strange thing is I desolder both R1 and R2, then power on, measured the voltage on both c3 and c1, it gives me 27V, while the schematic shows they should be 24v. But I didnt change anything except the capacitors
One strange thing is I desolder both R1 and R2, then power on, measured the voltage on both c3 and c1, it gives me 27V, while the schematic shows they should be 24v. But I didnt change anything except the capacitors
Sadly we see this a lot on the forum, a previously working unit rendered faulty.
Why did it happen... because you have done something you are not aware of. Perhaps a short you haven't spotted (solder blob or whisker of solder). Switch it off and measure resistance across C4. Is there a short?
Don't even think of replacing the resistor with a bigger one. What you should do is use a DBT (dim bulb tester) that will prevent such failure again even if there is a fault, Always use a DBT when working on something like this. The A1 is a Class A amp and so the current draw is high but the bulb should still allow you to determine if a rail is being pulled down, even if the rails don't come up to the normal values.
Its not unusual to hear of this happening after a recap that went beyond like-for like replacements. I saw it with smallish power amps that didn't need vast amounts of capacitance but newbies often assume that because capacitance is good for power supplies, then more capacitance must be better.
'Not suggesting this is the actual problem here but what size and grade of capacitors did you fit for C1,2,3,4?
'Not suggesting this is the actual problem here but what size and grade of capacitors did you fit for C1,2,3,4?
Sad to say, corner cutting on parts specs was something of a habit at Musical Fidelity. 'Can't say what the current ownership does - they seem to have gone etherial somewhere in Europe.
The replaced capacitors might be bad and leaky or there is short circuit on tracks while resoldering. You should not change the wattage of resistors at all as the fault is still persisting and might damage the transformer or the rectifiers.
As a thumb rule the max voltage rating of capacitors should at least 50% highr then the operating voltage. Bulb tester as suggested by Mr mool;y is a good option of testing.
As a thumb rule the max voltage rating of capacitors should at least 50% highr then the operating voltage. Bulb tester as suggested by Mr mool;y is a good option of testing.
As mentioned a dim bulb tester which can be purchased or made diy. Another option is a variac with a volt and amp meter which you can monitor while slowly turning up the voltage. My opinion here...any work done on a piece of gear should be monitored closely when starting up and for a period after initial startup.
I did carefully checked every capacitor I replaced after soldering if they wired correctly. They looked fine to me.
Double checked the wiring of big smoothing capacitors, cant find anything wrong with it, unless they are faulty at the start, but they are bought new.
However I do find the collector pin on TR10 PNP output transistor I replaced might have accidentally touched the heat sink, which is wired to the ground. This short circuit is intermittent, so I didnt spotted initially.
Could this be the reason caused the resistor to burn out?
The original transistor was 2N2955, I replaced it with MJ21193g.
Double checked the wiring of big smoothing capacitors, cant find anything wrong with it, unless they are faulty at the start, but they are bought new.
However I do find the collector pin on TR10 PNP output transistor I replaced might have accidentally touched the heat sink, which is wired to the ground. This short circuit is intermittent, so I didnt spotted initially.
Could this be the reason caused the resistor to burn out?
The original transistor was 2N2955, I replaced it with MJ21193g.
Can you show a top picture (component) including C3, C4 and R2 and the solder side picture of the same?
Replacing ONE faulty output transistor of a pair is foolish and not an option.
Us old and experienced techs always followed "by the book" and replaced with a matching pair to eliminate other problems.
It's the way it was done since transistors were invented.
Additionally, due to the careless nature of the shorted lead, the whole amp is possibly now suffering some damage.
It may not show up initially, but in my experience it can.
Us old and experienced techs always followed "by the book" and replaced with a matching pair to eliminate other problems.
It's the way it was done since transistors were invented.
Additionally, due to the careless nature of the shorted lead, the whole amp is possibly now suffering some damage.
It may not show up initially, but in my experience it can.
Sorry, I didnt make it clear, I did replaced all four of them thats 2 pairs of mj21193g/mj21194g