I still havent shorted TR09/10 yet,Ive fixed the left channel brightening the DBT,it was a broken track at the edge of TR09s emitter. Now the DBT is glowing just above the orange color into white on the right channel and the transformer is making a humming sound? If I pull one of the two output plugs on the right channel, it stops the hum. Ive gone over this board Ad Nauseum checking this and checking that. One thing I noticed is on R01 and R02 470K ohm resistors,if I measure them with my DMM one will read OK and the other wont lock on the DMM? But if I apply the soldering iron to the solder joints it will read OK but the other side wont until I reflow the solder then that one will read OK and the other one wont lock on to the DMM? Weird! Although it might be an idiosyncrisy of the DMM its a Fluke 87V? I will drop one leg of all the diodes and check them before I go any further because I havent done it to all of them yet!
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OK. Without a scope being available there are two things I would do.
1/ Eliminate that offset adjust preset as a test. You do that by simply shorting the wiper to ground. The DC offset will not be zero, but it should be close, and it should be stable.
2/ If the rise and fall is still apparent then the amplifier may be unstable... scope job
You could try as test adding a small cap across the NFB resistor. Try something really tiny like 10pf (pico) and see if the offset becomes more stable.
1/ Eliminate that offset adjust preset as a test. You do that by simply shorting the wiper to ground. The DC offset will not be zero, but it should be close, and it should be stable.
2/ If the rise and fall is still apparent then the amplifier may be unstable... scope job
You could try as test adding a small cap across the NFB resistor. Try something really tiny like 10pf (pico) and see if the offset becomes more stable.
The offset was 120mv on the left channel when I turned the amp on,I grounded the wiper to chassis ground and it went to 20mv but I still couldnt adjust it. Then I put a 12pico ceramic cap across R23, 100K ohm NFB resistor the 20mv remained unchanged,it only rose and fell 2 or 3 mv but still unresponsive to setting with the trimmer.
So 20 millivolts with the wiper grounded, and that 20mv is pretty constant. Yes ?
That is suggesting that there is a problem with either the preset and/or the two diodes that connect to it. The offset won't adjust with the wiper grounded, although 20 mv is actually good enough in itself. In other words you could happily leave it like that.
That is suggesting that there is a problem with either the preset and/or the two diodes that connect to it. The offset won't adjust with the wiper grounded, although 20 mv is actually good enough in itself. In other words you could happily leave it like that.
Thanks Mooly! Ill replace the trimmer and both diodes then and just hope that fixes the bloody thing.If not Ill hardwire the whole board to replace the crappy tracks on it.The previous owner has made a horrible mess of it by using a red hot soldering iron.Its 3.15am so thanks again mate, Good night!
I found out why the DBT was lighting up! A guy on AK advised me to turn the bias right down. So I did and turned the bias trimmers fully anticlockwise to zero ohms.That was the problem I was supposed to turn it right up because it works backward for some reason? I turned both bias trimmers fully clockwise and now the bulb is barely glowing.
This is why I kept banging on about shorting those transistors. It forces the bias to zero, and so if the amp all worked OK after doing that then we know its a simple problem around those transistors (including wrong adjustment ).
Turning up or down, left or right, CW, CCW. It all depends which way you look at it. Always look at the circuit and figure it out from there. Those two transistors need to be fully ON for minimum current, and that happens when the bias trimmer is at MAX or highest resistance.
So it looks like its a runner
).Turning up or down, left or right, CW, CCW. It all depends which way you look at it. Always look at the circuit and figure it out from there. Those two transistors need to be fully ON for minimum current, and that happens when the bias trimmer is at MAX or highest resistance.
So it looks like its a runner
No its not a runner yet! When I said it was sitting on 20mv it was but then I noticed it was only for a while,if I leave it for a minute the mv will climb or fall from pos 20mv to 40mv then 30,20,10 zeromv, then onto neg up to -40mv and back through the whole sequence again to pos 40mv? Thats with the TR09 shorted across C to E and the 12 pico cap across the NFB resistor R23. One thing Ive noticed is that the PNP driver transistors on the left channel ,TR18/TR13/TR07 are all reading lower forward voltages than their complimentarys on the right channel both on the collectors and emitters? Its like a cap is charging and discharging but how and which one? Im going to have to get another oscilloscope! Im starting to miss my music now! Its been a while since the old girl has been going.
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A scope is pretty much mandatory for this kind of work.
OK, so the offset is hunting through -/+40mv, and although that suggests a problem, it isn't in itself a large voltage that would do damage.
Why not try the amp at low volume and see if it actually plays OK or whether there is gross distortion of any kind. It might give a clue as to whether there was some stability issue. You could connect the speaker via a low value 0.25 watt resistor as a safeguard. Keep the bulb tester in place and keep the volume lowish (because the DBT will try and limit the current).
OK, so the offset is hunting through -/+40mv, and although that suggests a problem, it isn't in itself a large voltage that would do damage.
Why not try the amp at low volume and see if it actually plays OK or whether there is gross distortion of any kind. It might give a clue as to whether there was some stability issue. You could connect the speaker via a low value 0.25 watt resistor as a safeguard. Keep the bulb tester in place and keep the volume lowish (because the DBT will try and limit the current).
I started to bias the good right channel that I had set the DC offset on but it got to 18mv and wouldnt go any further,in fact its going backwards ,dropping to 15mv and still dropping? The left one I turned the trimmer all the way until it clicked,the DMM didnt register anything? The right channel went back up to 22mv and is starting to go back down again!
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I was just checking the voltages from the protection and regulator board where the pins connect to the driver power supply board and some of the voltages are all over the place. Ive had a look at the regulator/protection board and it looks OK but its where the pins connect that the problem is I suspect because of the variable voltages plus I heard a slight ticking sound from there when I turned the amp on? I had much the same problem when I first got this amp so I ordered a NOS F-2567 driver power supply because I had to destroy the old one to salvage the regulator/protection board,the previous owner had soldered the reg/protect and driver pwr board pins together but it didnt work anyway? I may have to coat the pins with solder to make them fit firmly or wedge some offcut leads down there to get a good connection somehow?
18mv across 0.33 ohm is around 55ma. Is that a long way short of the recommended value ? The bias current see sawing around might be 'normal' in the short term as the thermal time constants come into play. The vbe multiplier TR09/10 can't heat up and track the outputs instantaneously, everything has to warm and settle. If its a long way short then we can tweak the resistor values around the preset, and the reason it might be unable to reach the correct value is simply that modern semiconductors use slightly different doping techniques and consequently have slightly different (higher) turn on voltages.
The channel that does nothing... first see if the voltages across TR09/10 are similar in their range as you adjust the presets.
Sorry Mooly, I wrote mv instead of ma in my previous post? The right channel TR10 reads 4.56v across from C to E. The left channel TR09 reads 2.470v across C to E and doesnt change when I adjust the DC offset trimmer.It read 24v on the collector and 12v on the emitter to chassis.There is also 13.40v on the output resistors on the left channel.I found that if I measure the outputs with the left channel driver connections plugged in the emitter will read shorted,078v! on the PNP side and .083v for NPN on the left channel? Ive fixed the reg/protect board pins and the voltages arent jumping around now and the protection circuit is working as it should.Ive checked the left side of the driver board but I cant find anything wrong,all the transistors are OK they just have different forward voltages But I still cant find why? Ive measured the outputs through the driver plug connections and both channels come up OK.The problem is on the driver board somewhere but Ive done continuity on all the tracks tested and retested all the components. Its always something simple,its just finding it thats not simple.lol
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No problem we just need to be very clear on it all because the devil is in the detail.
Firstly, the voltage should of course change when the bias current trimmer is adjusted. You are mentioning the DC offset trimmer ! That will not alter anything on TRO09/10
If you have 24 volts on the collector and 12 volts on the emitter then that should give a reading of approximately 12 volts across the device and yet you mention 2.47 across C and E.
we just need to be very clear on it all because the devil is in the detail.Firstly, the voltage should of course change when the bias current trimmer is adjusted. You are mentioning the DC offset trimmer ! That will not alter anything on TRO09/10
If you have 24 volts on the collector and 12 volts on the emitter then that should give a reading of approximately 12 volts across the device and yet you mention 2.47 across C and E.
After 10 minutes to warm up TR09 collector reads 14.86v.TR09 reads 2.5v across C to E. Emitter is 12.40v.The protection circuit is working and the amp is in protection now! Every now and then the green light and relay will click on and off? TR09 voltage across C to E moves up when I turn up the bias trimmer.I got to 3.7v and the DBT started to glow a bit brighter. All the transistors on the driver board have been transfered over from the NOS F-2583 board,so theyre all original except for the MJ21193/94 outputs.
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