Good to hear that you are (finally) making progress.
Not surprised by your diode result BTW. My recommendation was to update it to the Vbe multiplier solution but it would mean finding space somewhere for the bias pot and components.
If you do not get a good result with the 1N4448 the only other option I can suggest is BAV21.
That's got a 1V max at 100mA. 1S920 is 1.2V at 200mA.
1N4448 hopefully will work - spec is Vmin 0.62 Vmax 0.72 at 5mA.
Can't find any datasheets giving Vf:If for 1S920. Too old now.
Not that manudfacturers published useful graphs like a log plot. Only linear ones where you can't read the "small print".
Not surprised by your diode result BTW. My recommendation was to update it to the Vbe multiplier solution but it would mean finding space somewhere for the bias pot and components.
If you do not get a good result with the 1N4448 the only other option I can suggest is BAV21.
That's got a 1V max at 100mA. 1S920 is 1.2V at 200mA.
1N4448 hopefully will work - spec is Vmin 0.62 Vmax 0.72 at 5mA.
Can't find any datasheets giving Vf:If for 1S920. Too old now.
Not that manudfacturers published useful graphs like a log plot. Only linear ones where you can't read the "small print".
Well this is interesting......
Built the R board up last night with exactly the same components as the L board, unfortunately our friends at Farnell seem to have lost my 1N4448s in the post - so I though 'what the hell' I'll put 1N4148s in instead.
Amplifier works perfectly!!!
I'm going to check (later) the forward voltage of the 1N4148s I used in both boards to see if they differ substantially - they came from the same tape so I would expect them to be within certain parameters.
Built the R board up last night with exactly the same components as the L board, unfortunately our friends at Farnell seem to have lost my 1N4448s in the post - so I though 'what the hell' I'll put 1N4148s in instead.
Amplifier works perfectly!!!
I'm going to check (later) the forward voltage of the 1N4148s I used in both boards to see if they differ substantially - they came from the same tape so I would expect them to be within certain parameters.
It may be obvious in hindsight but your 1N4148's may not have a high enough forward voltage. ejp didn't explain why 4148's didn't work.
Simulations show that the base to base voltage of the first driver stage is about 1.39V. That needs two diodes at 0.695V which has to be at 3mA. That is around the top end of the 1N4448 spec. (0.72V at 5mA). So I may have been wrong to have suggested 4448's. Sorry! (And if I were a manufacturer I'd probably be making 4148's and 4448's on the same line, suggests they may not be very different, but 4148's will have 4448 rejects)
Trouble is exact Vbe matching is a challenge at the best of times since manufacturing lines and manufacturers' processes are movable objects. So the Vbe's of the drivers are an uncertain quantity. Then that is added to because of the 100 ohm emitter resistors and 1k base resistors on the next pair of transistors, adding maybe 180mV to the proverbial 1.2V, which is close to the simulated result.
If need be you can increase the bias voltage with a small resistor (physically and electrically). The bias current range with a 22k pot in the original 303 is about 1-3.5mA which is about 30mV per diode (or maybe up to 60 depending on the gold doping used. Last time I measured a 4148 the (non)ideality factor was about 1.5) so to bump up the voltage in 60mV jumps needs resistors of about 56 ohms per step. I'd try smaller resistors (22 ohms say) as I'd want the VAS current to be closer to 3mA than 1. (This resistor is in series with the diodes, obvs.)
You can measure the base-to-base voltages between the two boards to check this, I think.
Simulations show that the base to base voltage of the first driver stage is about 1.39V. That needs two diodes at 0.695V which has to be at 3mA. That is around the top end of the 1N4448 spec. (0.72V at 5mA). So I may have been wrong to have suggested 4448's. Sorry! (And if I were a manufacturer I'd probably be making 4148's and 4448's on the same line, suggests they may not be very different, but 4148's will have 4448 rejects)
Trouble is exact Vbe matching is a challenge at the best of times since manufacturing lines and manufacturers' processes are movable objects. So the Vbe's of the drivers are an uncertain quantity. Then that is added to because of the 100 ohm emitter resistors and 1k base resistors on the next pair of transistors, adding maybe 180mV to the proverbial 1.2V, which is close to the simulated result.
If need be you can increase the bias voltage with a small resistor (physically and electrically). The bias current range with a 22k pot in the original 303 is about 1-3.5mA which is about 30mV per diode (or maybe up to 60 depending on the gold doping used. Last time I measured a 4148 the (non)ideality factor was about 1.5) so to bump up the voltage in 60mV jumps needs resistors of about 56 ohms per step. I'd try smaller resistors (22 ohms say) as I'd want the VAS current to be closer to 3mA than 1. (This resistor is in series with the diodes, obvs.)
You can measure the base-to-base voltages between the two boards to check this, I think.
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Good to hear you are getting there but its all still a bit of an unknown because we talked about the critical bias voltage (developed by these diodes) as early as post #3 and I don't think we got anywhere 🙂
Well.... 😉
I'm pleased it is all working now, its just all a bit inconclusive. There may have been multiple issues and so a rebuild of the board has corrected unseen problems.
I'm pleased it is all working now, its just all a bit inconclusive. There may have been multiple issues and so a rebuild of the board has corrected unseen problems.
Indeed 😕- However, what intrigues me is the fact that 4148s will work in 1 board and not the other - even though both boards are otherwise identical.
I've no answer to that one I'm afraid. I would need to measure myself what was going on with both sets of different parts fitted.
Those diodes are run at about 3.5 milliamps and so you could if you wanted try running two in series from a 9 volt battery via a series 1k8 or 2k2 resistor and compare forward voltages.
Use one of each diode type in the series chain. That way you know the currents are identical and so any difference in voltage is down to the diode.
Those diodes are run at about 3.5 milliamps and so you could if you wanted try running two in series from a 9 volt battery via a series 1k8 or 2k2 resistor and compare forward voltages.
Use one of each diode type in the series chain. That way you know the currents are identical and so any difference in voltage is down to the diode.
It can really only be variations in Vf. Just measured a couple at 5mA and got 710mV and 707mV. Depending on the (non)ideality that may drop 10-15mV at 3mA. But this is in the 1N4448 spec. And the voltages seem marginal. Suspect you will need one of those resistors to push the volts up a bit.
Hi, I thought I would add my experience with similar issue. There are 2:
1) few years back I got a unit, it was the older style board with 4 diodes in a row. Could not set the bias. Eventually I bought new boards and components and all worked. But I still didnt like the fact I didnt find the cause
2) I have been fixing/upgrading a unit for a friend last and this week. Before doing any upgrades I teated transistors, plugged it in and could set all volatges except for bias on one channel. It would be close to 0mV, then jump to 60-90 mV with slightest touch. Tjis time I had the amp from point 1 to compare. I changed small transistors to bc. Electrolytics and Diodes as well. Still the same. What was left were resistors and small caps. Changed the caps from the new unit one by one. Obviously it was the last one. C109 was 68pF, where it should be 0.1 uF. Found 0.1 50V (there is max 35 volts in that section) and all perfect. Measured the 'good' board and it was out too, but not as much and still doing its function then (0.02 uF). Swapped it too obviously. I still have the old boards that I could not get working and so I measured those caps and they were the same as the bad board around 70pF. So it was the reason I could not set bias previously.
So check the C109 whem working on quad 303 to save yourself a lot of hassle🙂
1) few years back I got a unit, it was the older style board with 4 diodes in a row. Could not set the bias. Eventually I bought new boards and components and all worked. But I still didnt like the fact I didnt find the cause
2) I have been fixing/upgrading a unit for a friend last and this week. Before doing any upgrades I teated transistors, plugged it in and could set all volatges except for bias on one channel. It would be close to 0mV, then jump to 60-90 mV with slightest touch. Tjis time I had the amp from point 1 to compare. I changed small transistors to bc. Electrolytics and Diodes as well. Still the same. What was left were resistors and small caps. Changed the caps from the new unit one by one. Obviously it was the last one. C109 was 68pF, where it should be 0.1 uF. Found 0.1 50V (there is max 35 volts in that section) and all perfect. Measured the 'good' board and it was out too, but not as much and still doing its function then (0.02 uF). Swapped it too obviously. I still have the old boards that I could not get working and so I measured those caps and they were the same as the bad board around 70pF. So it was the reason I could not set bias previously.
So check the C109 whem working on quad 303 to save yourself a lot of hassle🙂
C109 is not 0.1uF, it is 1000pF, with less than 9V across it, and has nothing to do with the bias whatsoever.
Do you mean C108? In the Zobel network? Hard to see what that has to do with bias either, unless it was DC-leaky or the amplifier was oscillating.
Do you mean C108? In the Zobel network? Hard to see what that has to do with bias either, unless it was DC-leaky or the amplifier was oscillating.
Sorry, wrote it in bed without the schematics and my memory was misleading me. Yes it was c108. I don't know the theory behind, but when the original which measured
68pF was in, I could not set the bias, with a correct value cap in I could. And the cap was out of value on all 4 boards I worked on (one of them not critically)
Hello,
I have a strange problem. The left driver board is okay, but I can't set the quiescent current on the right driver board.
If I put a 1kHz sine-wave on the input and look on a scope on the load, 8,2 Ohms resistors, I notice that the positive rise of the sine has been disappeared ( on the faulty board )...
I have replaced all capacitors on both driver boards, tested several transistors by changing them from one board to the other, but no luck so far.
Is there anyone who has experienced the same problem and managed to fix it? Or anyone who can give me a clue where to look?
Thank you in advance!
I have a strange problem. The left driver board is okay, but I can't set the quiescent current on the right driver board.
If I put a 1kHz sine-wave on the input and look on a scope on the load, 8,2 Ohms resistors, I notice that the positive rise of the sine has been disappeared ( on the faulty board )...
I have replaced all capacitors on both driver boards, tested several transistors by changing them from one board to the other, but no luck so far.
Is there anyone who has experienced the same problem and managed to fix it? Or anyone who can give me a clue where to look?
Thank you in advance!
The reason you can't set the bias current is because you have some other fault. Please do not swap parts between channels as that is a recipe for disaster 🙂 and you can easily end up with two faulty boards.
I think you will find this by careful voltage measurement. Check the midpoint voltage at the output and take it from there. A problem with the upper 'triple' (the upper output transistor and drivers) could do this. Make sure the 0.3 ohm resistors are OK, if one is open then its likely the relevant output transistor is short.
As always though, voltage measurement is key and a check all transistor nodes will I'm sure point to the problem pretty quickly.
I think you will find this by careful voltage measurement. Check the midpoint voltage at the output and take it from there. A problem with the upper 'triple' (the upper output transistor and drivers) could do this. Make sure the 0.3 ohm resistors are OK, if one is open then its likely the relevant output transistor is short.
As always though, voltage measurement is key and a check all transistor nodes will I'm sure point to the problem pretty quickly.
I've seen output stages where one of the drivers has gone open circuit, but the power device survived. If parts of the triple which is suspect still work, the bias voltages may appear reasonable, but you can compare directly with the channel which is working. As Mooly says, don't swap devices between channels. Hopefully your right channel still functions correctly.
If that does not work, disconnecting all transistors and testing individually is generally the best option. But you'd need to check for breakdown voltage (simple curve tracing may work with old 2N3055'sbut some types of device may show abnormally low breakdown. (I've seen awful behaviour with RCA epitaxial MJ2955's). Proper testing involves pulse testing with a small inductor rather than winding the voltage up statically, so it is not trivial.
If transistors are out of the way you may be able to check resistor values more easily too.
If there has been a short you may also want to check the PCB tracks in case one has burnt out.
If that does not work, disconnecting all transistors and testing individually is generally the best option. But you'd need to check for breakdown voltage (simple curve tracing may work with old 2N3055'sbut some types of device may show abnormally low breakdown. (I've seen awful behaviour with RCA epitaxial MJ2955's). Proper testing involves pulse testing with a small inductor rather than winding the voltage up statically, so it is not trivial.
If transistors are out of the way you may be able to check resistor values more easily too.
If there has been a short you may also want to check the PCB tracks in case one has burnt out.
Thank you both for your replies. I took your advice Mooly, didn't swap again and measured the 0.3 ohm resistors, they are ok.
The resistors among the upper three transistors are ok. The left channel is still ok, the right channel isn't.
The value between the nodes 3 and 4 is 0.5V DC on the channel which is good. On the faulty channel I can't seem to get a steady value...
Another thing, in the circuit diagram I see an connection of the 67V between the two channels on the output transistors. But on my 303 it's not.
Should I connect?
You notice I am not that handy ( and very slow ) with repairing, be gentle I am still learning...
The resistors among the upper three transistors are ok. The left channel is still ok, the right channel isn't.
The value between the nodes 3 and 4 is 0.5V DC on the channel which is good. On the faulty channel I can't seem to get a steady value...
Another thing, in the circuit diagram I see an connection of the 67V between the two channels on the output transistors. But on my 303 it's not.
Should I connect?
You notice I am not that handy ( and very slow ) with repairing, be gentle I am still learning...
The connection is created by pins 1 and 2 of the amplifier board being connected together. If your output transistors weren’t powered nothing would work. You are certainly barking up the wrong tree here. I would check the driver transistors.
You are going to have to take several voltage checks to get an idea of what might be happening here. Report back with what you measure:
Measuring from ground can you measure the voltage on node 5. This is a vital clue.
Measure the voltage on node 1
Measure the voltage on node 2
This shows some key voltages on the 303
Measuring from ground can you measure the voltage on node 5. This is a vital clue.
Measure the voltage on node 1
Measure the voltage on node 2
This shows some key voltages on the 303
Thank you Mooly for your advise and effort!
I have systematically measured all the pins 1 to 9, on the left and right channel. No load, no input.
Surprisingly enough, pin 7 seems to be where it all goes wrong... I think that TR2R ( the 2N3055 ) is defective, that base to emitter is short circuited. Do you agree?
Seems to me, I was betting on the wrong horse..?!?
I have systematically measured all the pins 1 to 9, on the left and right channel. No load, no input.
| Pin | Left Channel (V) | Right Channel (defective) (V) |
| 1 | 66.2 | 66.2 |
| 2 | 66.4 | 66.4 |
| 3 | 33.6 | 33.2 |
| 4 | 33.2 | 33.4 |
| 5 | 33.3 | 33.4 |
| 6 | 33.3 | 33.4 |
| 7 | 4 | 0 |
| 8 | 0 | 0 |
| 9 | 0 | 0 |
Surprisingly enough, pin 7 seems to be where it all goes wrong... I think that TR2R ( the 2N3055 ) is defective, that base to emitter is short circuited. Do you agree?
Seems to me, I was betting on the wrong horse..?!?