Concentrate on this. We have a basic problem here to address. The volt drop must be identical across both (providing the circuit diagram is accurate and it certainly looks correct circuit configuration wise)
That's good. This shows nothing is being tapped of that junction.
I think next step is to lift those 3k3 resistors and check the value with them isolated. If they happen to be correct then something is appearing in parallel with the one with low voltage across it and there is nothing in the circuit itself that can do that. A short, something conductive... something... the theory is absolute here. Those resistors must see equal volt drop.
I've thrown the circuit into a simulation to see what is going on and it might be clearer to follow than the printed diagram.
Look at R306 and R307. The volt drop across two series connected equal value resistors is always identical provided there is nothing across one or other of them to unbalance or change the value of one or the other. R311 is the only connection to these series resistors and your measurement of no volt drop across this resistor suggests its not related... but to be even more certain you can remove R311 for test purposes and the amp should still work normally for testing.
And these are the normal voltages:
Two series resistors of equal value have the same current flowing through them and that develops the same volt drop across each.
I would also swap their positions and see where the voltage drop stays or follows.
Or take the easy way out and replace them with new ones.
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🙂 Indeed
I'll be very interested to know what they read. It is a real puzzler is this one but this is something really definite we can work with now,
With one end lifted of the 3.3K ohm resistors, each tested as 3.3K ohms out of circuit.
Under magnification, I examined their solder pads at length and one of them belonging to R307 began to look a bit suspicious. Not a "blob". more like a minute smear which was not easy to see. But I thought that it could possibly have created solder bridge. I scraped it clear with a scalpel and re-soldered the ends of R306 and R307.
Both R306 and R307 now measure 3.3K ohms in circuit and the voltage drop across both has changed. Voltage across R306 is now 17.6 volts and voltage drop across R307 is now 17.7 volts.
I'm just about done for today but this sound like real progress 🙂 That is one definite issue fixed if the voltages are now equal across those two.
I'll look in again tomorrow but just take it slowly now and see how it all looks. We need the offset voltage to be close to zero at the speaker output. When that is OK we can try biasing the amp.
I'll look in again tomorrow but just take it slowly now and see how it all looks. We need the offset voltage to be close to zero at the speaker output. When that is OK we can try biasing the amp.
I'll post the .asc file while I remember. You will have to put your own models in and delete the .include C:\discrete models.txt directive
The load is not connected in the attached .asc but is in this run.
The load is not connected in the attached .asc but is in this run.
After 30 minutes at idle:
Left channel offset is around 0.0mV. It wanders up and down from (-)0.7mv to (+)1.5mV.
Right channel offset is more or less a steady 27mV.
I was hoping they would be nearer to each other and the right channel closer to zero.
I have done nothing further.
That all sounds great at this point tbh. Offset can be -/+100mv and historically this was a typical spec.
What you need to do now is carefully see if the bias adjust OK. Keep the bulb in place for now and adjust one channel at a time and make sure it seems OK. Assuming it is then turn that bias back to zero and check the other channel. We do that because the bias current of both channels will pull the rail voltages down with a bulb in place. So we confirm each channel good on its own.
If all that is OK I think we remove the bulb and adjust the bias for real on both channels.
What you need to do now is carefully see if the bias adjust OK. Keep the bulb in place for now and adjust one channel at a time and make sure it seems OK. Assuming it is then turn that bias back to zero and check the other channel. We do that because the bias current of both channels will pull the rail voltages down with a bulb in place. So we confirm each channel good on its own.
If all that is OK I think we remove the bulb and adjust the bias for real on both channels.
Attempting bias adjustment was anticlimactic.
Using the test points and (+) speaker terminals as per the service manual, both channels have a minimum bias of 0.0mV but each could only be brought up to a maximum bias of 3.0mV.
Using the test points and (+) speaker terminals as per the service manual, both channels have a minimum bias of 0.0mV but each could only be brought up to a maximum bias of 3.0mV.
That still sounds good to me as nothing bad happens and it does begin to adjust. All you need to do now is tweak the value of R308 by reducing its value a little. It looks to be a 1k8 (printing is poor) but the parts list say 1.2k
Try adding a 10k across R308 and going lower in value gradually until you get the bias in range. Again do one channel at a time and set the bias back to minimum when you are happy with it.
Remember that the bias may go a bit higher without the bulb because the bulb reduces the supply voltage a little.
When it seems in range and with it on minimum you can then try on full mains with no bulb and adjust both channels for real.
Try adding a 10k across R308 and going lower in value gradually until you get the bias in range. Again do one channel at a time and set the bias back to minimum when you are happy with it.
Remember that the bias may go a bit higher without the bulb because the bulb reduces the supply voltage a little.
When it seems in range and with it on minimum you can then try on full mains with no bulb and adjust both channels for real.
Beginning in post #164, R308 was discussed.
R308 was originally a 1.2K ohm resistor that was swapped out for a 2.2K ohm resistor. A 2.2K ohm is currently at R308 and at R408.
Would re-installing a 1.2K ohm resistor be of benefit here?
Or should I proceed by experimenting, as advised, paralleling R308 with different value resistors beginning with 10K and working downward?
R308 was originally a 1.2K ohm resistor that was swapped out for a 2.2K ohm resistor. A 2.2K ohm is currently at R308 and at R408.
Would re-installing a 1.2K ohm resistor be of benefit here?
Or should I proceed by experimenting, as advised, paralleling R308 with different value resistors beginning with 10K and working downward?
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You can try the original 1.2k on just one channel if you want. Keep the bulb in place and make sure the bias is set to minimum before switching on. So that is replacing the resistor, not paralleling the 2.2k with 1.2k
If the bias is to high (bulb lights) with a 1.2k then we will have to increase its value to a 1.5k or 1.8k.
OR:
Alternatively if you have a 4.7K you then could parallel that with the existing 2.2k as that would give the equivalent if a 1.5k. The advantage of paralleling is you can tweak values without desoldering anything.
If the bias is to high (bulb lights) with a 1.2k then we will have to increase its value to a 1.5k or 1.8k.
OR:
Alternatively if you have a 4.7K you then could parallel that with the existing 2.2k as that would give the equivalent if a 1.5k. The advantage of paralleling is you can tweak values without desoldering anything.
Another advantage is less serious soldering stress on the pads. Adding a resistor under the board is always easier than pulling one out and inserting another one.
I agree that adding a resistor is a tidier way to go about this.
Unfortunately I have no 4.7K, 1/2 watt or higher, resistors on hand. Nor do I have the 1.5K or 1.8K resistors for a swap out.
I wull need to order some 1/2 watt 4.7K ohm resistors. I found an online parallel resistor calculator which said that to achieve 1.8K ohms with the 2.2K ohm resistors in place, I need 10K ohm so I will order those too.
Slightly off topic but back again to the vicinity of post #164, I referred to installing new 1/2 watt 820 ohm replacements for the four original resistors that were cracked from excess heat, You pointed out that 1/2 watt resistors were pretty much at their limit. I have been noticing that the new 1/2 watt resistors I used are throwing off a fair bit of heat. So since I am ordering resistors anyway, I will get 1 watt 820 ohm as well.
The heat given off by any resistor wattage for a given voltage across the resistor is actually the same and is independent of the wattage rating. You just need a rating high enough to handle the wattage in question and you can easily work that out by measuring the voltage across the resistor and then multiply by that voltage by itself. Divide the answer by the resistor value and you get the watts dissipated. Physically small resistors will run hotter than a physically big one and give off the heat better but the actual total heat is the same.
So 12 volt across 330 ohm dissipates (12*12)/300 which is 0.48 watt. For reliability we would want at least a 0.6 watt and likely a 1 watt.
Those resistors in the bias circuit can be 0.25 watt or even 0.125 watt as they see virtually no power.
While you are waiting why not turn the bias down and try the amp for real with speakers. Keep the volume quite low while the bulb is in place. When the bulb starts to flicker with the music you have gone loud enough.
If all is OK then try without the bulb and you can go as loud as you like then.
So 12 volt across 330 ohm dissipates (12*12)/300 which is 0.48 watt. For reliability we would want at least a 0.6 watt and likely a 1 watt.
Those resistors in the bias circuit can be 0.25 watt or even 0.125 watt as they see virtually no power.
While you are waiting why not turn the bias down and try the amp for real with speakers. Keep the volume quite low while the bulb is in place. When the bulb starts to flicker with the music you have gone loud enough.
If all is OK then try without the bulb and you can go as loud as you like then.
The resistors are ordered and have an ETA of Tuesday.
I am going to wait until they get here before hooking up speakers and play something through this amplifier. Reason being, it's entire front is detached and laying face down. The first resistors I install will be those 1W 820 ohm. When those are in, I can bolt up the amplifier's front. If for nothing else, the controls will be in view and accessible.
I am going to wait until they get here before hooking up speakers and play something through this amplifier. Reason being, it's entire front is detached and laying face down. The first resistors I install will be those 1W 820 ohm. When those are in, I can bolt up the amplifier's front. If for nothing else, the controls will be in view and accessible.
During that time you could take one more measurement - measure voltages (against ground) on collector and emitter of Q304 while adjusting the bias. They should change in a similar tempo and end up (at max bias position) around 1,1...1,2V (according to the schematics).
Fair enough 🙂
That's right, and the voltage between C and E ultimately determines the bias current. As you turn the bias up the voltage across the transistor rises until the five transistors in the putput stage are turned on just enough (the Sony is an odd one with Q311 making an odd number of transistors, three drivers and then the output pairs)
With DBT in place, turning bias pot to maximum made voltage at Q304 base increase to (-)1.04mv. Voltage at Q304 collector reached (+)1.09mV.
Is there some sort of problem with your reading skills?
I asked for the EMITTER, not base voltage dynamics.