Imanoldee
Super Member
Hi, I have been lurking for some time on the AK forums.
I have learned quite a lot about audio repair over the last couple of years.
My thanks to all the audio repair and restorers on the AK forums for their great posts.
Recently I found out I could join the discussions without paying the annual fees.
Silly me, I once tried to join and thought the fee was a requirement to join the discussions, so I backed out of the process.
As a bit of an intro I am an old EE, as you may figure out from my AK user name Imanoldee.
I have been restoring and re-capping old audio gear for sometime now.
Recently, I found an error in the SX-1050 protection circuit (AWM-090) that I have not seen discussed before.
While looking at the circuit it struck me that R25, a 2.4k ¼ Watt resistor, had a large voltage drop across it and the resistor value seemed low for that, since it was only a ¼ watt rated resistor.
So I ran a quick set of calculations.
The schematic shows a voltage drop of (60v-33.5v) = 26.5v across that 2.4k resistor.
The current through that resistor would be I=V/R or 26.5v/2400 = 11mA.
Using one of the equations for power, in this case, P= V*I then the power through that 2.4k is 26.5v*11mA = 292mW!
Or you can use this equation for power as well.
P = (V*V)/R which gives you the same 292mW.
I checked how hot this resistor was on one of the SX-1050’s had restored and re-capped.
First I turned on the SX-1050 and let it warm up a minute or two, then I simply put my finger on the R25 resistor in question.
It was very hot, as I would expect if it was dissipating 292mW, on a 250mW rated resistor.
I then pulled the resistor lead and measured it, the meter showed it was indeed 2.4k.
FYI, the band colors showed a 2.4k value as well.
I then checked the other SX-1050 unit I have yet to restore, and it too was uses a 2.4k resistor.
Needless to say I bought some 2.4k, ½ Watt resistors the next time I had an order to Mouser, and switched out the under wattage resistors from the protection circuit.
This seems to be a double error. When I look at the schematic the value shown is 2.4k, on the foil image it is also shown as 2.4k, however on the parts list for the AWM-090 it shows a value of 3.4k (RD ¼ PS342J) which would be a 1/4 watt resistor per their coding scheme.
So, either the original design called out a 3.4k that was later changed to 2.4k or the designers missed 2 errors, the more critical to me would be the power handling of the 2.4k.
The second error would be the parts list showing 3.4k.
A bit of speculation would be that if the original value had been 3.4k then a ¼ watt resistor would have been ok since the current through a 3.4k would be 7.79mA and the power through a 3.4k resistor would be 206mW, if the voltage drop stayed the same at 26.5v.
A bit high in my mind but still well within the 250mW rating.
To my mind putting 292mW through a 250mW rated resistor is asking for problems over time.
The resistor may be able to handle it, but it would be cooking,so why push it.
I have learned quite a lot about audio repair over the last couple of years.
My thanks to all the audio repair and restorers on the AK forums for their great posts.
Recently I found out I could join the discussions without paying the annual fees.
Silly me, I once tried to join and thought the fee was a requirement to join the discussions, so I backed out of the process.
As a bit of an intro I am an old EE, as you may figure out from my AK user name Imanoldee.
I have been restoring and re-capping old audio gear for sometime now.
Recently, I found an error in the SX-1050 protection circuit (AWM-090) that I have not seen discussed before.
While looking at the circuit it struck me that R25, a 2.4k ¼ Watt resistor, had a large voltage drop across it and the resistor value seemed low for that, since it was only a ¼ watt rated resistor.
So I ran a quick set of calculations.
The schematic shows a voltage drop of (60v-33.5v) = 26.5v across that 2.4k resistor.
The current through that resistor would be I=V/R or 26.5v/2400 = 11mA.
Using one of the equations for power, in this case, P= V*I then the power through that 2.4k is 26.5v*11mA = 292mW!
Or you can use this equation for power as well.
P = (V*V)/R which gives you the same 292mW.
I checked how hot this resistor was on one of the SX-1050’s had restored and re-capped.
First I turned on the SX-1050 and let it warm up a minute or two, then I simply put my finger on the R25 resistor in question.
It was very hot, as I would expect if it was dissipating 292mW, on a 250mW rated resistor.
I then pulled the resistor lead and measured it, the meter showed it was indeed 2.4k.
FYI, the band colors showed a 2.4k value as well.
I then checked the other SX-1050 unit I have yet to restore, and it too was uses a 2.4k resistor.
Needless to say I bought some 2.4k, ½ Watt resistors the next time I had an order to Mouser, and switched out the under wattage resistors from the protection circuit.
This seems to be a double error. When I look at the schematic the value shown is 2.4k, on the foil image it is also shown as 2.4k, however on the parts list for the AWM-090 it shows a value of 3.4k (RD ¼ PS342J) which would be a 1/4 watt resistor per their coding scheme.
So, either the original design called out a 3.4k that was later changed to 2.4k or the designers missed 2 errors, the more critical to me would be the power handling of the 2.4k.
The second error would be the parts list showing 3.4k.
A bit of speculation would be that if the original value had been 3.4k then a ¼ watt resistor would have been ok since the current through a 3.4k would be 7.79mA and the power through a 3.4k resistor would be 206mW, if the voltage drop stayed the same at 26.5v.
A bit high in my mind but still well within the 250mW rating.
To my mind putting 292mW through a 250mW rated resistor is asking for problems over time.
The resistor may be able to handle it, but it would be cooking,so why push it.
