I have a JVC 4vn-990 amplifier which has an annoying hiss from all channels plus both right channels have increasing hiss as the volume is turned up.
I am going to replace all electrolytic capacitors plus some transistors. I will order replacement transistors for all but only replace those that need replacing. One of these transistors is the 2SC458ALGC (20 of them) which is apparently known for being a bad unit. However I am confused at the replacement unit, KSC1845, that is suggested by many sources.
The 2SC458 has
Ic = 100 mA, hfe = 160 > 320 and ft = 230 MHz
The KSC1845 has
Ic = 50 mA, hfe = 200 > 400 and ft = 110 MHz.
Why would a smaller capacity (Ic) unit be used?
I assume hfe is not a problem unless it differs greatly.
What is the current gain bandwidth (ft) and what should I be looking for when replacing transistors?
FYI. The 2SC458's are used in both the pre-amp and main amp, if that would make any difference in deciding on replacements.
Any help is much appreciated.
I am going to replace all electrolytic capacitors plus some transistors. I will order replacement transistors for all but only replace those that need replacing. One of these transistors is the 2SC458ALGC (20 of them) which is apparently known for being a bad unit. However I am confused at the replacement unit, KSC1845, that is suggested by many sources.
The 2SC458 has
Ic = 100 mA, hfe = 160 > 320 and ft = 230 MHz
The KSC1845 has
Ic = 50 mA, hfe = 200 > 400 and ft = 110 MHz.
Why would a smaller capacity (Ic) unit be used?
I assume hfe is not a problem unless it differs greatly.
What is the current gain bandwidth (ft) and what should I be looking for when replacing transistors?
FYI. The 2SC458's are used in both the pre-amp and main amp, if that would make any difference in deciding on replacements.
Any help is much appreciated.
Hi SK33,
Normally I would use 2SC1775 to replace the 2SC458. The KSC1845 / 2SC1845 would also work fine. Since these are signal transistors, it would be rare to find them dealing with more than 10 mA of actual current. Most tend to run around 1 ~ 2 mA, and if you look at the noise graphs, you will see why.
So, how are your soldering skills? I only ask because "recapping" audio equipment is generally done by non-professionals. There are many more things to do than blindly replacing all capacitors in sight. That and there is no "one best capacitor" type.
If you have access to a real capacitor LCR meter, you will be looking at dielectric absorption (D/A), not ESR (which is useless in signal applications). Real LCR meters, or even better, and LC bridge, are made by General Radio, Keysight (HP / Agilent) and a few others. A built-in capacitance function on most hand held meters is next to useless in determining capacitor quality. A skilled technician is worth his or her weight in gold.
If you are buying your parts from Ebay, stop. Just take the unit outside, pour gasoline into it, and light it up. At least you'll get something in return for the money spent. Only buy parts from the proper distribution network (Newark, Mouser, Digikey and others like that).
Best, Chris
Normally I would use 2SC1775 to replace the 2SC458. The KSC1845 / 2SC1845 would also work fine. Since these are signal transistors, it would be rare to find them dealing with more than 10 mA of actual current. Most tend to run around 1 ~ 2 mA, and if you look at the noise graphs, you will see why.
So, how are your soldering skills? I only ask because "recapping" audio equipment is generally done by non-professionals. There are many more things to do than blindly replacing all capacitors in sight. That and there is no "one best capacitor" type.
If you have access to a real capacitor LCR meter, you will be looking at dielectric absorption (D/A), not ESR (which is useless in signal applications). Real LCR meters, or even better, and LC bridge, are made by General Radio, Keysight (HP / Agilent) and a few others. A built-in capacitance function on most hand held meters is next to useless in determining capacitor quality. A skilled technician is worth his or her weight in gold.
If you are buying your parts from Ebay, stop. Just take the unit outside, pour gasoline into it, and light it up. At least you'll get something in return for the money spent. Only buy parts from the proper distribution network (Newark, Mouser, Digikey and others like that).
Best, Chris
Thanks for your reply.
That makes sense why the replacement transistor has a lower Ic capacity. I did a calculation for one transistor and it was 2.6 mA. I should obtain some software that can calculate voltage and current around the whole circuit!
My understanding was that electrolytic capacitors that are 41 years old would need replacing. Having very little knowledge or experience with electronics, I was just going to get audio quality capacitors of the same voltage and capacity and solder them in. Is this not good enough? Taking it to a repair shop is not really an option due to costs. As for soldering skills, it can be very good and other times like a dog's breakfast.
I realise a lot of Ebay sellers have inferior parts. In fact I have never used Ebay.
That makes sense why the replacement transistor has a lower Ic capacity. I did a calculation for one transistor and it was 2.6 mA. I should obtain some software that can calculate voltage and current around the whole circuit!
My understanding was that electrolytic capacitors that are 41 years old would need replacing. Having very little knowledge or experience with electronics, I was just going to get audio quality capacitors of the same voltage and capacity and solder them in. Is this not good enough? Taking it to a repair shop is not really an option due to costs. As for soldering skills, it can be very good and other times like a dog's breakfast.
I realise a lot of Ebay sellers have inferior parts. In fact I have never used Ebay.
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