Hi Mark,
Thanks for the PSU schematic.
Thanks for the PSU schematic.
I checked the soldering and tracks. They all performed 100% conductivity. I also checked the bridge rectifier KBPC2504. At least the conductivity test shows all (+) current goes to C3 and (-) goes to C1. When it conducts, the resistance towards C1is slightly higher showing (6.58/7.69MOhm) while towards C3 is measured (5.82/6.54MOhm). But I don't believe it could be the reason for 42V. Perhaps, grounding is not ok?
Fixed!!!
Today, easter holiday was over and shops opened. I bought 3.15A fast fuse for R2, where 0.47ohm resistor was supposed to be located. Ex-owner might have modified it and a fuse clip is there instead.
Then the mystery of 42V thing was suddenly resolved. All reservior capacitors showed 34~35V across them. Very good sign.
I was also suspicious of the transistors. As I described in the earlier post, the conductivity test showed a little strange behaviour when they were wired to the PCB. B<->C, C<->E, B<->E, you know that stuffs... When only either way should conduct, somehow both directions conductivity existed. I decided it is normal, as the circuit seems to connect eventually.
So I put the lid on, and gave it a go. Amazingly, my speakers started to sing again! Great!
It was a long journey, and in conclusion I think the replacement of the transistors (strange combination of A2P/A2N and MJ15003/MJ15004) were effective. I think it was me who blew the fuse during the repair by slipping the probe when measuring the voltage across C3. Perhaps I somehow blew it again by the same mistake, but for some reason it escaped from my further inspection...
Anyways, thank you very much for all your help and suggestions. Finally I have A100 with all 105degrees rated electro caps. Certainly the biggest benefit is that I gained vast knowledge of my own amp!
My A100 resurrected on the day when Jesus did. I believe it must be baptised...
One last thing.
Before it went down, I had a problem of the volume control. When volume is set 0, there still was sound from one channel while the other is silent. The first suspect, as pointed by Mark, is the potentiometer. However, the potentiometer showed the same resistance from the both channel when it is set 0, when I measured. Strange, uh?
Today, easter holiday was over and shops opened. I bought 3.15A fast fuse for R2, where 0.47ohm resistor was supposed to be located. Ex-owner might have modified it and a fuse clip is there instead.
Then the mystery of 42V thing was suddenly resolved. All reservior capacitors showed 34~35V across them. Very good sign.
I was also suspicious of the transistors. As I described in the earlier post, the conductivity test showed a little strange behaviour when they were wired to the PCB. B<->C, C<->E, B<->E, you know that stuffs... When only either way should conduct, somehow both directions conductivity existed. I decided it is normal, as the circuit seems to connect eventually.
So I put the lid on, and gave it a go. Amazingly, my speakers started to sing again! Great!
It was a long journey, and in conclusion I think the replacement of the transistors (strange combination of A2P/A2N and MJ15003/MJ15004) were effective. I think it was me who blew the fuse during the repair by slipping the probe when measuring the voltage across C3. Perhaps I somehow blew it again by the same mistake, but for some reason it escaped from my further inspection...
Anyways, thank you very much for all your help and suggestions. Finally I have A100 with all 105degrees rated electro caps. Certainly the biggest benefit is that I gained vast knowledge of my own amp!
My A100 resurrected on the day when Jesus did. I believe it must be baptised...
One last thing.
Before it went down, I had a problem of the volume control. When volume is set 0, there still was sound from one channel while the other is silent. The first suspect, as pointed by Mark, is the potentiometer. However, the potentiometer showed the same resistance from the both channel when it is set 0, when I measured. Strange, uh?
I've been reading this thread and I wonder if the A100 circuit is
the same as the A220 which I just fixed last week. The A220 is
also hot as wok but it uses the Buz900/905 mosfets instead of
15003/4 BJTs. The PSU is also +-42 volts. As I posted somewhere
here the A220 is a clone of the 1977 Hitachi 100w amp and had
been one of the popular diy Maplin's amps. The only difference is
that the Vbias resistor is turn way high turning it into class-A.
Someone, however, posted that it's actually class-AB.
the same as the A220 which I just fixed last week. The A220 is
also hot as wok but it uses the Buz900/905 mosfets instead of
15003/4 BJTs. The PSU is also +-42 volts. As I posted somewhere
here the A220 is a clone of the 1977 Hitachi 100w amp and had
been one of the popular diy Maplin's amps. The only difference is
that the Vbias resistor is turn way high turning it into class-A.
Someone, however, posted that it's actually class-AB.
I've been reading this thread and I wonder if the A100 circuit is
the same as the A220 which I just fixed last week. The A220 is
also hot as wok but it uses the Buz900/905 mosfets instead of
15003/4 BJTs. The PSU is also +-42 volts. As I posted somewhere
here the A220 is a clone of the 1977 Hitachi 100w amp and had
been one of the popular diy Maplin's amps. The only difference is
that the Vbias resistor is turn way high turning it into class-A.
Someone, however, posted that it's actually class-AB.
the same as the A220 which I just fixed last week. The A220 is
also hot as wok but it uses the Buz900/905 mosfets instead of
15003/4 BJTs. The PSU is also +-42 volts. As I posted somewhere
here the A220 is a clone of the 1977 Hitachi 100w amp and had
been one of the popular diy Maplin's amps. The only difference is
that the Vbias resistor is turn way high turning it into class-A.
Someone, however, posted that it's actually class-AB.
same circuit?
Hello Efren,
I do not know the circuit of the A220, but I do know the circuit of the generic A1/A100 etc. The P170/270/370 is completely different from the A100.
The bigger musical fidelity amps used mosfets' driven by an opamp. I guess this circuits is similar to the Maplin amp? Can I conclude then that the A220 also uses mosfets driven by an opamp (for voltage amplification in the proper power amp)?
You can easily attach a file, it must no be that difficult to scan your diagram and have it set to .jpg format and show it in the thread.
Rudy
Hello Efren,
I do not know the circuit of the A220, but I do know the circuit of the generic A1/A100 etc. The P170/270/370 is completely different from the A100.
The bigger musical fidelity amps used mosfets' driven by an opamp. I guess this circuits is similar to the Maplin amp? Can I conclude then that the A220 also uses mosfets driven by an opamp (for voltage amplification in the proper power amp)?
You can easily attach a file, it must no be that difficult to scan your diagram and have it set to .jpg format and show it in the thread.
Rudy
Hi Rudy,
Yes, the A220 has mosfets, 2 pairs of Buz900/905 instead of the
discontinued 2sk135/j50. It's a 1977 Hitachi clone and I believe
one of Lindsey Hood's books has this circuit as a Maplin diy.
No, there's no op-amp input IC, if that's what you mean. This
Hitachi design uses long tailed pair in the input and in the VAS,
the voltage amp stage, before the mosfets.
I believe this is still a good alternative to the full complimentary
mirror-image design as it provide 2 routes of DC feedback for
stabilty.
The A220 has the bias resistor turn high making the fets 'class A.'
I saw the A100 circuit and it's different from the A220. This
company seems to like class-A! Is this because people think
class-A is better than class-B?
I was impressed though with the mosfets. They could really
take a lot of heat! When I opened the case the PCB was
discolored from years of heat. I fixed a lot of amps for PA use
& they always have fans, but the A220 doesn't have a fan &
it really gets hot. Even the PCB area where the fets are soldered
were blackened but the fets are all fine.
Yes, the A220 has mosfets, 2 pairs of Buz900/905 instead of the
discontinued 2sk135/j50. It's a 1977 Hitachi clone and I believe
one of Lindsey Hood's books has this circuit as a Maplin diy.
No, there's no op-amp input IC, if that's what you mean. This
Hitachi design uses long tailed pair in the input and in the VAS,
the voltage amp stage, before the mosfets.
I believe this is still a good alternative to the full complimentary
mirror-image design as it provide 2 routes of DC feedback for
stabilty.
The A220 has the bias resistor turn high making the fets 'class A.'
I saw the A100 circuit and it's different from the A220. This
company seems to like class-A! Is this because people think
class-A is better than class-B?
I was impressed though with the mosfets. They could really
take a lot of heat! When I opened the case the PCB was
discolored from years of heat. I fixed a lot of amps for PA use
& they always have fans, but the A220 doesn't have a fan &
it really gets hot. Even the PCB area where the fets are soldered
were blackened but the fets are all fine.
I tried what rmgvs suggested a while ago. 0.47ohm output resistors played nice music with much less heat. It certainly made my A100 much more reliable and hasssle-free.
Perhaps mosfet(lateral mosfets) version of class-a will be somewhat more reliable...but they usually require higher quiescent bias currents, and the original A100 already seems to have way too high quiescent current that frequently causes breakdown...Reducing this current to about 650mA seems to be the best solution for the best sound quality and reliability. thanks rmgvs.
Perhaps mosfet(lateral mosfets) version of class-a will be somewhat more reliable...but they usually require higher quiescent bias currents, and the original A100 already seems to have way too high quiescent current that frequently causes breakdown...Reducing this current to about 650mA seems to be the best solution for the best sound quality and reliability. thanks rmgvs.
Another A100 rebuild
Hi All,
Another A100 will be resurected..........eventually
So far in the amp section I have replaced all the electrolytics with 105degrees, as well as all the high wattage resistors (all with higher rated 2W & 3W types) & most of the transistors.
I will be replacing the resistors in the power supply as one measures closer to 0.6 than 0.22 & one of the main caps is showing as near enough a dead short.
My question for the experienced is this. I can get either 40v 85degree or 35v 105degree capacitors to fit in the space provided. Which one would be a safer bet? I believe that the caps originally used are too close for comfort on both the voltage & heat rating.
That said, while I agree with the opinion of 105degree caps in the amp itself, I am more inclined to go for the 40v 85degree caps because of the fact that the fans blow cool air directly over the power supply caps.
Any input is greatly appreciated
Cheers
Paul
Hi All,
Another A100 will be resurected..........eventually
So far in the amp section I have replaced all the electrolytics with 105degrees, as well as all the high wattage resistors (all with higher rated 2W & 3W types) & most of the transistors.
I will be replacing the resistors in the power supply as one measures closer to 0.6 than 0.22 & one of the main caps is showing as near enough a dead short.
My question for the experienced is this. I can get either 40v 85degree or 35v 105degree capacitors to fit in the space provided. Which one would be a safer bet? I believe that the caps originally used are too close for comfort on both the voltage & heat rating.
That said, while I agree with the opinion of 105degree caps in the amp itself, I am more inclined to go for the 40v 85degree caps because of the fact that the fans blow cool air directly over the power supply caps.
Any input is greatly appreciated
Cheers
Paul
just one tip for troubleshooting A1 and relatives 
but also any other amp-during fixing it's smart thing to pull out (desolder) output transistors ; in that case all you have is miniscule of original amp,but faults are there-DC on output etc....in any case idle current is small and you can measure and thinker all night
but also any other amp-during fixing it's smart thing to pull out (desolder) output transistors ; in that case all you have is miniscule of original amp,but faults are there-DC on output etc....in any case idle current is small and you can measure and thinker all night
Dead Musical Fidelity A100.
Anyone knows which fuse I should get to replace this amp? I had mine 48 hours ago, it was playing fine. I played for 10 minutes, gave it a rest about half hour until it cooled down. Switched it back on, light went on, a strange noise, and it went dead.
Opened the fuse capsule, it was blown, it had 3.15A on it, so i went to the shop and replaced it but the fuses blew again 3 times and i gave up investigating.
What's wrong with it? Please help. Brisbane Australia does not have specilist in this.
Thanks.
Zul
Anyone knows which fuse I should get to replace this amp? I had mine 48 hours ago, it was playing fine. I played for 10 minutes, gave it a rest about half hour until it cooled down. Switched it back on, light went on, a strange noise, and it went dead.
Opened the fuse capsule, it was blown, it had 3.15A on it, so i went to the shop and replaced it but the fuses blew again 3 times and i gave up investigating.
What's wrong with it? Please help. Brisbane Australia does not have specilist in this.
Thanks.
Zul
