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Old 31st March 2011, 06:17 PM   #51
Mooly is offline Mooly  United Kingdom
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That sounds promising. Does the amp play OK now ? You may have noticeable distortion at low volume but thats because of the wire link.

If all is OK then switch off and remove the link.
Now turn the bias preset to minimum resistance. In other words so that the preset is a "short" leaving just the two diodes between the transistor bases.

Switch on (with the bulb tester) and make sure the lamp is still dim.

If that is OK then the preset is adjusted as per the manual to give the recommended current.
If you don't know that then I would adjust for around 25ma or so initially and that is best done by just measureing the volt drop across one of the 0.47 ohm resistors on the outputs. Set for around 12 millivolts DC (no audio or speakers).
Keep rechecking as the value will drift with temperature. If all is OK then remove the bulb tester and again recheck the current to make sure it's still around 25ma.
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Old 31st March 2011, 07:27 PM   #52
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OK, I'm getting sound out of both channels, (which I must say has had me dancing for joy in itself).

To confirm before I do this, the bias preset = the quiescent current.

And I can test this value by testing the voltage drop across the 0.47 ohm resistor. According to the circuit diagram I test this by measuring the current into the collector of one of the power transistors.
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Old 1st April 2011, 06:23 AM   #53
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Yes, it is the quiescent current. Minimum resistance of the pot (when it's a short) gives minimum current.

What current does it say to adjust for ?

It is always easier to measure a voltage than a current (which requires breaking the circuit and inserting the meter in series). The collector current flows through both outputs and the two 0.47 ohms.

So calculate the voltage expected by ohms law. V=I * R (units in base values of amps,volts and ohms)

So I said 25ma (as a safe value). Thats V = 0.025 * 0.47 which is 11.75 millivolts. It will never stay put at that... that's normal... aim for approx the correct value when it's hot.

Most meters should have no trouble with millivolts. If it's easier then solder wires to the resistor and connect the meter... then you can watch and adjust without any risk of shorting anything.

Either resistor can be used... it doesn't matter.
I would do the adjustment first with the bulb in place to get a feel for what happens, then repeat the adjustment with full mains applied. Do both channels. Make sure the value isn't to high or creeps up as the amp gets hot.
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Old 1st April 2011, 02:25 PM   #54
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The schematic says adjust for 20mA, so you were close enough. I am having trouble with the channel that was damaged however. I have managed to set the Right channel bias to 9.2 on one resistor and 10mV on the other. However the moment I start to turn up the bias on the right channel the bulb lit up, and knowing what happened last time, I switched off the amp straight away. The reading before I started to turn the bias up was 0.0 on one resistor and 1.5mV.

Either way up the sound is great, back to its old self (or rather new self, what with all the new caps), all except for the Left channel being a little louder than the right.
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Old 1st April 2011, 03:19 PM   #55
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There may not be a problem... as the bulb will begin to light as current is drawn. A filament bulb has a low cold resistance and a high hot resistance.... its very non linear so a small increase in current could well just appear to illuminate the bulb very suddenly. Maybe pull the fuse on the good channel and try again adjusting the other channel.

Without the unit in front of me though, you have to play safe and be sure by basic tests.

While monitering the voltage across the 0.47 ohm does the voltage increase slowly to around the target of 9 mv, or does it suddenly jump ? If the current can be set OK remember to recheck when on full mains voltage. That's very important.

If the voltage jumps up uncontrollably then the pot may be faulty. In fact it's worth putting your meter across it on ohms (amp off) and seeing if it adjusts OK. As its such a low value resistance I would guess it measures OK in circuit.

As to one channel louder than the other... we'll come to that later if you want too.
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Old 2nd April 2011, 09:58 AM   #56
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The voltage did not change on the meter, with the other side of the amplifier unfused the voltage varied around 0 - 1mv. I slowly turned up the bias and nothing happened, the light didn't come on. I pushed it to about where the left channel is at and still no change, so then checked the voltage going to that amp and found that the fuse had blown.

So maybe the pot is damaged but the meter didn't pick up on the spike?
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Old 2nd April 2011, 11:21 AM   #57
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You need to reconfirm the faulty channel is OK by refitting the wire link as in post #48 then start again keeping the bulb in place at all times during testing.

The fact the amp is OK with the link in place suggests a problem with the network of those two series connected diodes and the pot. If you are unsure at all about testing and interpreting the results then I would replace all three components. The diodes are common small signal silicon types and you can use 1N4148 or IN914 available anywhere.

Have you tested the pot on your meter ?

The way the bias circuit works is simple. The two diodes generate a constant volt drop of around 1.4 volts (0.7 each). The pot also generates a variable volt drop that is added to the 1.4 volts of the diodes. This voltage as it is increased by turning the pot starts to turn on the output driver and output transistors.
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Old 2nd April 2011, 11:53 AM   #58
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I'll test the pot out of circuit now.

What I don't understand is the channel functions fine without the link in place now, sounds great in fact. But I can't adjust the bias without it blowing a fuse and it's set to what must be a very high Amplitude, with the voltage being so low across those resistors.
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Old 2nd April 2011, 11:58 AM   #59
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The pot, even if faulty probably does still turn down to "zero" ohms and that is why the amp would run without the link. You have just the two series diodes in circuit and the "shorted" pot.

If there is a crack or burnt area in the carbon track then as soon as that is encountered as the pot is turned, then the bias voltage goes sky high blowing the fuse.

Does that make sense ?
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Old 2nd April 2011, 12:12 PM   #60
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yeh, that makes sense. An there must be a break in the carbon track, 37kr seems like a lot for a 100r trimmer. I think I have a spare lying around, I'll let you know how I get on.
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