I'm actually thinking to bypass the lot and just connect the two board points together!
ta for bias info i might let it settle at 22mA for a few hours then try 15mA before removing the bulb limiter
Connecting the points on the board would be the best solution (and remove the wires from the board, don't leave them connected as well because they can both pick up interference and also add a capacitive load to the points of the circuit they connect to).
So is the amp working... as in you have it powered ? or are we speaking hypothetically about when you actually test it.
So is the amp working... as in you have it powered ? or are we speaking hypothetically about when you actually test it.
first power up just now. first thing i notice is that R119 and possibly r120 are HOT
the ceramic insulator collars are a little blackened and the board discoloured there too. Wonder what they do and if it's essential 🙂 *peering at schematic with glasses on end of nose*
Those are in the power supply and are specified as 1.6w metal film types. Just quickly looking at the circuit and they will run hot... the ceramic spacers and board discolouration confirming that.
Its an odd bit of the circuit for sure, at least the way they have done it...... as it seems to be just a resistive feed for the power on LED. The LED is run off the AC of the secondary winding of the transformer via these series resistors. The diode across the LED protects it from the negative half cycle and that in turn increases the dissipation yet more in the resistors as they conduct on both half cycles.
Yes, all normal and any failures there would only affect the power LED.
Yes, all normal and any failures there would only affect the power LED.
Attachments
thanks ! So I suppose i could just remove them if I find it's keeping me up at night
The circuit shows the power switch acting on the secondary side of the transformer which means the transformer is permanently powered... that's how it looks from what I see.
The LED should only be on when the switch is on but there will be slight current flowing due to the 4700pF cap across the switch.
I won't go into details of how that works other than to say that with a high efficiency LED it could be possible to still see it illuminated when the switch is off.
Another weird (make that WEIRD) observation is that the secondary voltage per winding will be around 21 volts AC (rms) which means one heck of a current flows in that LED per peak of each half cycle. A modern high efficiency LED could be blindingly bright on 1 milliamp or less, this one sees nearer 100ma peak (or more).
That doesn't sound right to me. Just for curiosity, are they really 100 ohm resistors and not 1k ? Mind you, the wattage rating and heat generation fit. WEIRD.
The LED should only be on when the switch is on but there will be slight current flowing due to the 4700pF cap across the switch.
I won't go into details of how that works other than to say that with a high efficiency LED it could be possible to still see it illuminated when the switch is off.
Another weird (make that WEIRD) observation is that the secondary voltage per winding will be around 21 volts AC (rms) which means one heck of a current flows in that LED per peak of each half cycle. A modern high efficiency LED could be blindingly bright on 1 milliamp or less, this one sees nearer 100ma peak (or more).
That doesn't sound right to me. Just for curiosity, are they really 100 ohm resistors and not 1k ? Mind you, the wattage rating and heat generation fit. WEIRD.
hopefully my last stupid question of the evening ... "where do i put the meter probes to measure the quiescent current ?"
the manual says "connect meter to the bias test point" and that the points are "across the output transistor emitter resistors" (107A left channel 108A right channel.
by 107A do they mean across one of the resistors in my newly cobbled dual resistor ? is 107A the other side? I'm measuring from centre to one side ?
sorry to be so cautious i just really don't want to mess up now I've got things this far !
The current is measured by measuring the voltage across either (it doesn't matter which) of the 0.22 ohm resistors. NO speakers attached. Ohms law is then used to derive the current flowing.
Also confirm the DC voltage on the middle of these resistors (the amplifier output) is within 100mv of zero as measured with respect to ground.
To measure the current safely (as in no twitchy fingers and accidental shorts) it can be helpful to tag a couple of wires to appropriate points and connect these directly to the meter.
If the current recommended is say 20 milliamps then we know V=I*R and so we know to adjust for 0.02*0.22 which is just 4.4 millivolts. Very very small value.
That is another reason why having a definite known good connection to the meter is important. Holding meter leads, watching a meter and turning a preset all at the same time isn't easy.
The LED is just plain weird although old (and this amp is old) LED's of that era were hugely inefficient compared to modern ones... not 100ma + inefficient though.
Also confirm the DC voltage on the middle of these resistors (the amplifier output) is within 100mv of zero as measured with respect to ground.
To measure the current safely (as in no twitchy fingers and accidental shorts) it can be helpful to tag a couple of wires to appropriate points and connect these directly to the meter.
If the current recommended is say 20 milliamps then we know V=I*R and so we know to adjust for 0.02*0.22 which is just 4.4 millivolts. Very very small value.
That is another reason why having a definite known good connection to the meter is important. Holding meter leads, watching a meter and turning a preset all at the same time isn't easy.
The LED is just plain weird although old (and this amp is old) LED's of that era were hugely inefficient compared to modern ones... not 100ma + inefficient though.
bad news update. with the presets turned to max resistance i switched on and the bulb shone brightly and failed to fade
🙁
What is different now to when you noticed those resistors getting hot ? Presumably the bulb wasn't lit brightly then.
they were 1A fuses cos i can't find any 3A in the house. anyway by the time I'd taken a dc voltage reading from 107 to ground both fuses burned out
The fuses burned with a bulb in circuit ?
What sort of bulb is it ?
I'm going to have to disappear in a little while but I'll look in later.
What sort of bulb is it ?
I'm going to have to disappear in a little while but I'll look in later.
clear 100 watt borrowed of neighbour. he's building a pass f5turbo class A
i guess i need to find a low watt incandescent bulb somewhere ...
Its getting toward the wrong end of the day for this 🙂 but here is how I would proceed...
If you have a lower wattage bulb such as a 60W then lets use it but nothing to small. If not we stick with the 100W
The bulb tester is our main safety feature, and it will limit the overall power that the transformer can deliver on the secondary side.
Although the bulb shouldn't be bright (which indicates a problem somewhere) we still need to work with this to see what is going on.
If you haven't the correct the fuses for the secondary side then we can link them out as a temporary measure providing we are always using the bulb tester. The original fuses will probably be marked AT or T which denotes time delay or anti-surge. A little over 1A would be around the level of fault current the bulb would allow, the 3.15A wouldn't have blown.
We also need to go to the next level for forcing zero bias which is to apply a short across the bias transistor... this removes the whole preset and the transistor out of the equation. Shorting the cap across the transistor is perhaps an easier option.
Now we test again. If the bulb is still bright then you need to measure the following.
1/ The DC voltage on fuse 1 that has been replaced/linked.
2/ The DC voltage on fuse 2 that has been replaced/linked.
The above measured with respect to ground.
3/ The DC voltage across either of the 0.22 ohm for left channel.
4/ The DC voltage across either of the 0.22 ohm for right channel.
The voltage across the 0.22 ohm will tell us if that is where the fault current is flowing, and if it is, in which channel/s.
If anything is getting unduly hot in the amplifier section then switch off and terminate the test.
If you have a lower wattage bulb such as a 60W then lets use it but nothing to small. If not we stick with the 100W
The bulb tester is our main safety feature, and it will limit the overall power that the transformer can deliver on the secondary side.
Although the bulb shouldn't be bright (which indicates a problem somewhere) we still need to work with this to see what is going on.
If you haven't the correct the fuses for the secondary side then we can link them out as a temporary measure providing we are always using the bulb tester. The original fuses will probably be marked AT or T which denotes time delay or anti-surge. A little over 1A would be around the level of fault current the bulb would allow, the 3.15A wouldn't have blown.
We also need to go to the next level for forcing zero bias which is to apply a short across the bias transistor... this removes the whole preset and the transistor out of the equation. Shorting the cap across the transistor is perhaps an easier option.
Now we test again. If the bulb is still bright then you need to measure the following.
1/ The DC voltage on fuse 1 that has been replaced/linked.
2/ The DC voltage on fuse 2 that has been replaced/linked.
The above measured with respect to ground.
3/ The DC voltage across either of the 0.22 ohm for left channel.
4/ The DC voltage across either of the 0.22 ohm for right channel.
The voltage across the 0.22 ohm will tell us if that is where the fault current is flowing, and if it is, in which channel/s.
If anything is getting unduly hot in the amplifier section then switch off and terminate the test.
