Thanks Bonsai!
Q9 is BC546 and Q11 is BC556
Q11 is blown.. I should point out that when I first hooked the ammeter and powered up the amp I discovered that the meter had a blown fuse.. I have several meters from past jobs so fuse could have been blown from the start.. SO when I powered the amp up, V+ supply would not have been present at the board.. could this cause an issue..
Now I'm working on the other channel and have a good ammeter.. No smoking.. Not getting 50ma at V+, a little inrush current then drops, no change while turning the pot..
I'll play around more and try to come up with something to report..
Hi Joe I think it will be more easy to remove all transistors first then cross check all resistor values. If all values are perfect then need to solder all transistors well I think you may ordered extra numbers use only new transistors .This will be very easy to trouble shoot also time saver.
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Its not a good thing to remove all the transistors.
Attached is a circuit diagram. Can you measure all the voltages and write them on the circuit and then post it up - that way we can help to debug it and get your amps going (This is how Terry did it - its a good approach).
Attached is a circuit diagram. Can you measure all the voltages and write them on the circuit and then post it up - that way we can help to debug it and get your amps going (This is how Terry did it - its a good approach).
Yes Bonsai you are correct not a good idea but sometime helpful to save time but may go worst in my case that helped for other diy build, but I think your are replying very fast so I hope Joe can solve his issue soon
Hi Andrew,
I don't understand the voltages on your schematic. Shouldn't Q9 and Q11 have .6v vbe? Your schematic shows .1V.
I don't understand the voltages on your schematic. Shouldn't Q9 and Q11 have .6v vbe? Your schematic shows .1V.
The 150 and 140 mV is across the resistors - I need to correct that in the document. Thanks for pointing that out!
Joe's front end voltages look ok.
It looks like the OPS devices are off.
Joe, can you measure across Q20 collector and emitter. As you decrease the adjusted resistance of R6, the voltage across Q20 should increase, likewise across the 0.33 Ohms output pair.
When R6 is set to max -I.e 1k- the output devices can be turned right off. It may be your amp is ok - you just need to adjust R6.
If Adjusting R6 you get no change, then we need to look at the Spreader circuit or driver stage.
Joe's front end voltages look ok.
It looks like the OPS devices are off.
Joe, can you measure across Q20 collector and emitter. As you decrease the adjusted resistance of R6, the voltage across Q20 should increase, likewise across the 0.33 Ohms output pair.
When R6 is set to max -I.e 1k- the output devices can be turned right off. It may be your amp is ok - you just need to adjust R6.
If Adjusting R6 you get no change, then we need to look at the Spreader circuit or driver stage.
Assuming your amp is ok:
Do not attach an output load.
Try to adjust R6 for about 100 mA total quiescent current as measured at the fuse and then adjust R3 for 0V at the output.
Recheck the current at the fuse - should still be about the same as you had before.
Connect a load and drive the amplifier with an input signal until the Heatsinks are hot.
Remove the input signal.
Now adjust R6 for thr correct setting of 300 mA total current at the fuse.
Measure the output voltage again - you may have to trim R3 for 0 V on the output.
Attach your load. Recheck the output voltage - shoul be 0 V
Amp should be ready to go.
Do not attach an output load.
Try to adjust R6 for about 100 mA total quiescent current as measured at the fuse and then adjust R3 for 0V at the output.
Recheck the current at the fuse - should still be about the same as you had before.
Connect a load and drive the amplifier with an input signal until the Heatsinks are hot.
Remove the input signal.
Now adjust R6 for thr correct setting of 300 mA total current at the fuse.
Measure the output voltage again - you may have to trim R3 for 0 V on the output.
Attach your load. Recheck the output voltage - shoul be 0 V
Amp should be ready to go.
Excellent support! Thanks Bonsai
I get 140mv across R35 and 36 and 9.5 v across R34 and 35
170mv across R29 and 160mv across R28
Q20 C to E = 1.23V - Yes it increases when adjusting R6
When I adjusted R6 to about 50% voltage started increasing across ,33 ohm pair and current started to raise too.. Looked like I had control while slowly watching current rise above 50ma.. Didn't get to 300ma and then went back to zero again..
I'll try it again in the morning before I say it was a failure..
Thanks!
I did notice something today: R41, R42 Zero ohm jumpers on my boards are zero ohms but print shows 33.
I get 140mv across R35 and 36 and 9.5 v across R34 and 35
170mv across R29 and 160mv across R28
Q20 C to E = 1.23V - Yes it increases when adjusting R6
When I adjusted R6 to about 50% voltage started increasing across ,33 ohm pair and current started to raise too.. Looked like I had control while slowly watching current rise above 50ma.. Didn't get to 300ma and then went back to zero again..
I'll try it again in the morning before I say it was a failure..
Thanks!
I did notice something today: R41, R42 Zero ohm jumpers on my boards are zero ohms but print shows 33.
No - they should be jumpered out.
Can you check the voltage across R5. Should be about 1.2 to 1.3 volts.
Measure the voltage at the base of all the OPS devices. Should be 0.6 wrt ground (non devices +0.6 and pnp -0.6V
If one of the VAS transistors was blown open you might get the type of behavior but then I would expect the output to have a very large DC offset.
Check the 4.7 Ohm base resistors as well.
Are your driver transistors ok?
Can you check the voltage across R5. Should be about 1.2 to 1.3 volts.
Measure the voltage at the base of all the OPS devices. Should be 0.6 wrt ground (non devices +0.6 and pnp -0.6V
If one of the VAS transistors was blown open you might get the type of behavior but then I would expect the output to have a very large DC offset.
Check the 4.7 Ohm base resistors as well.
Are your driver transistors ok?
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Ok - I need to make that clearer on the diagram.
That should not cause the issue you have seen unless the 33 ohm resistor has gone high resistance. Check your SMT resistor soldering. Often I solder only one side and forget the other.
That should not cause the issue you have seen unless the 33 ohm resistor has gone high resistance. Check your SMT resistor soldering. Often I solder only one side and forget the other.
Mmmm
It seems some of my notes on the schematic get lost when I conver it to pdf. That's why the measurement curly braces and note to link the 33 ohms are missing.
It seems some of my notes on the schematic get lost when I conver it to pdf. That's why the measurement curly braces and note to link the 33 ohms are missing.
hi Bonasi its nice to see in your site for the below driver transistor for replacement of 1381 and 1302 with same gain grade which is very difficult to get it now.
KSA1220A (this is a PNP)
KSC2690A (NPN)
in replacement for 1381 and 1302
I have just ordered it
Thanks for the input
KSA1220A (this is a PNP)
KSC2690A (NPN)
in replacement for 1381 and 1302
I have just ordered it
Thanks for the input
1A fuse was blown from last night while adjusting R6 - now replaced
Q1,Q2,Q4,Q5 all have -0.6V from base to ground.. all of them are negative V -Why all Negative V
R5 = 0.3V
No shorts or opens with drivers
all 4.7 ohm base resistors are good
I'm having more issues with the ammeter - seems fuse blew on this one but last night its 15a so doesn't add up..
I put 1a fuse in positive clip to measure voltages above - have to figure out this ammeter problem..
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Make sure you have the leads plugged into the proper jacks for what you are doing. My meter uses different positive jacks for ammeter and resistance. If you plug into the ammeter jacks you have a short from lead to lead. That will blow stuff up if connected across components.
when the amp -fuse blew last night, the .5a fuse in the meter blew as well..
so now I have a good amp reading but turning r6 blows the -rail fuse every time..
a closer check with Q1,2,4 and 5: Base to ground on all of these, the voltage is unstable and slowly fluctuates between -0.6v and +0.6v
Thanks, Joel
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The attached limiting resistance, R20, has too low value at 33R, replace it with a 270R or 220R.
Also the offset correction is limited to +-75mV (x the DC gain) given the 15R feedback resistance, in some cases it would be necessarry to reduce R1 to 5.6k-6.8k.
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You do not need to change any resistor values. The output offset adjustment range is adequate
About 75 sets of these amplifiers (so 150 channels along with 75 PSU +Protect boards) have been sold and most of them built without issues. The only issue was with the first 4 or 5 builds where the degeneration resistors were too low at 15 Ohms.
Please check that both the Vbe spreader transistors are ok, the 33 Ohm base stopper resistors are linked out. If you measure wrt 0V either side of the Vbe spreader you should get +1.2V on the collector of Q20 and about -1.2V on the emitter if Q20. If you adjust R6 from maximum down, you should see the voltage across Q20 increase from about half way through the adjustment range.. Can you confirm that the Vbe spreader adjustment resistor is 1k. If you had feor example a 10k in there, the adjustment would be very course - from no output stage bias to far too much and very difficult to adjust it. Is you R6 ok?
Can you confirm the Voltge drop across the 0.33 Ohm resistors?
Suggest while you debug, you insert a 10 Ohm 5W resistor into each of the fuse holders.
Can you take a good quality picture of the top of the board and post it up?
About 75 sets of these amplifiers (so 150 channels along with 75 PSU +Protect boards) have been sold and most of them built without issues. The only issue was with the first 4 or 5 builds where the degeneration resistors were too low at 15 Ohms.
Please check that both the Vbe spreader transistors are ok, the 33 Ohm base stopper resistors are linked out. If you measure wrt 0V either side of the Vbe spreader you should get +1.2V on the collector of Q20 and about -1.2V on the emitter if Q20. If you adjust R6 from maximum down, you should see the voltage across Q20 increase from about half way through the adjustment range.. Can you confirm that the Vbe spreader adjustment resistor is 1k. If you had feor example a 10k in there, the adjustment would be very course - from no output stage bias to far too much and very difficult to adjust it. Is you R6 ok?
Can you confirm the Voltge drop across the 0.33 Ohm resistors?
Suggest while you debug, you insert a 10 Ohm 5W resistor into each of the fuse holders.
Can you take a good quality picture of the top of the board and post it up?
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