Yes... that makes total sense, Mooly.
I must admit, gain, I wasn't best pleased when the amp took out the speaker, and a few choice words were emitted! But I took comfort from the fact that it is, after all, over 15 years old and I was starting to get to the point of considering replacement of both amp and speakers anyway... so while it's been a little more costly than I'd originally hoped
it's not the end of the world, and this has been a useful re-introduction to the world of analogue electronics.
I must say, Mooly, many thanks for your patience and persistence with helping me on this too. Very much appreciated.
PS. I did consider wiring up a small buzzer - but the local Maplin didn't have any in stock. What I might do tonight is to remove the light bulb again, so that more current is flowing through the amp and may re-cause the fault again - although I think a buzzer would be best for this so that I can quickly attend the amp, instead of leaving it to melt something if I'm in another room watching the TV!
I must admit, gain, I wasn't best pleased when the amp took out the speaker, and a few choice words were emitted! But I took comfort from the fact that it is, after all, over 15 years old and I was starting to get to the point of considering replacement of both amp and speakers anyway... so while it's been a little more costly than I'd originally hoped
it's not the end of the world, and this has been a useful re-introduction to the world of analogue electronics.I must say, Mooly, many thanks for your patience and persistence with helping me on this too. Very much appreciated.
PS. I did consider wiring up a small buzzer - but the local Maplin didn't have any in stock. What I might do tonight is to remove the light bulb again, so that more current is flowing through the amp and may re-cause the fault again - although I think a buzzer would be best for this so that I can quickly attend the amp, instead of leaving it to melt something if I'm in another room watching the TV!
Update please Is it fixed yet. Mentioned earlier replacing Q11/12 with 2N5401/5551. If Maplin have them, get them, they will work to test but are a little underated current wise for the drivers, particularly if you are running low impedance speakers at high level. Jaycee suggested using ZTX replacements I think for these. I have not used Rapid, I tend to use www.cpc.co.uk as they are fairly local in an "emergency" and delivery is always next day by post.
If the fault has become so intermittent, and I hate doing this, is it worth doing a blanket change of the "silicon" as we have I think ruled DJ's and the like. What do you think ? Or have you found something ?
Is it fixed yet. Mentioned earlier replacing Q11/12 with 2N5401/5551. If Maplin have them, get them, they will work to test but are a little underated current wise for the drivers, particularly if you are running low impedance speakers at high level. Jaycee suggested using ZTX replacements I think for these. I have not used Rapid, I tend to use www.cpc.co.uk as they are fairly local in an "emergency" and delivery is always next day by post.If the fault has become so intermittent, and I hate doing this, is it worth doing a blanket change of the "silicon" as we have I think ruled DJ's and the like. What do you think ? Or have you found something ?
Unfortunately, no update. It was switched on from around 9am in the morning until about 11pm last night, with no bright LED shining whenever I checked it. I'm going to get hold of a buzzer and zener diode, I think, and leave that on the output for a day to see what happens.
Also, I think I have (buried in a box somewhere) an old speaker from a dismantled radio or something lying around. I'll stick that across the outputs as that will obviously draw more current than leaving is open circuit (or higher resistance with the LED and bridge). That might produce a different effect, especially if I leave it playing some music through the output.
I've switched it on again this morning, but nothing yet. I'm actually away all day today aswell, so I won't be able to do much with it today.
I've used CPC before, but not Rapid. CPC are generally fairly expensive for small orders simply due to the postage charges. No idea about Rapid yet.
I've just been reading through the older messages in the thread, and the voltage readings I gave in post #16, I thought were a little odd at the time, in the fact that TP17 voltage was more negative than the speaker output. This does, however, correlate with your suspicion about Q8.
Like you, I'd prefer to have it nailed down so that I *know* the problem has been fixed (e.g. if a resistor is breaking down, and not the silicon, then I might not know about it until another speaker is blown!).
I'm in no hurry to fix this now that I have a replacement amp, so I'm quite happy to let it do it's thing until it shows the problem again, and I can catch it in it's bad ways.
I'll keep you updated as soon as I have some news.
Also, I think I have (buried in a box somewhere) an old speaker from a dismantled radio or something lying around. I'll stick that across the outputs as that will obviously draw more current than leaving is open circuit (or higher resistance with the LED and bridge). That might produce a different effect, especially if I leave it playing some music through the output.
I've switched it on again this morning, but nothing yet. I'm actually away all day today aswell, so I won't be able to do much with it today.
I've used CPC before, but not Rapid. CPC are generally fairly expensive for small orders simply due to the postage charges. No idea about Rapid yet.
I've just been reading through the older messages in the thread, and the voltage readings I gave in post #16, I thought were a little odd at the time, in the fact that TP17 voltage was more negative than the speaker output. This does, however, correlate with your suspicion about Q8.
Like you, I'd prefer to have it nailed down so that I *know* the problem has been fixed (e.g. if a resistor is breaking down, and not the silicon, then I might not know about it until another speaker is blown!).
I'm in no hurry to fix this now that I have a replacement amp, so I'm quite happy to let it do it's thing until it shows the problem again, and I can catch it in it's bad ways.
I'll keep you updated as soon as I have some news.
Final thought, two ways to see if it misbehaves while you are not in it's presence. The technical way, rig up a flip/flop + LED to trigger when fault happens however briefly. The easy way, get a 4.7/6.8/8.2 or similar wirewound, anything 2watt 5 watt ,and connect across the output in place of speaker BUT have a series fuse 100ma or so quick blow in series. Fault occurs, fuse blows, actually it has output fuses, swap for 100ma and connect resistor.
have a series fuse 100ma or so quick blow in series. Fault occurs, fuse blows, actually it has output fuses, swap for 100ma and connect resistor.Yes, this would detect that it's still going wrong, but what I really need is to be able to check the voltages while it's in the faulty state - so in that respect, the buzzer should do the trick - I'll hear it, and have a few minutes to probe the voltages around the board to confirm the fault while it's gone.
So we are not admitting defeat yet then. Good Did you try the iron heating Q8 etc.
About the buzzer, the LED will have to come out, it clamps the voltage to around 2 volts on the bridge. Replace it with the zener, and the series resistor may need tweaking if there is not enough current to work buzzer. Around 820 ohm I would guess at.
Did you try the iron heating Q8 etc. About the buzzer, the LED will have to come out, it clamps the voltage to around 2 volts on the bridge. Replace it with the zener, and the series resistor may need tweaking if there is not enough current to work buzzer. Around 820 ohm I would guess at.
Nope... not defeated yet.
I did try using the iron (holding it close, then on the legs, and also on the top of Q8). I very quickly stopped that when I decided the iron was actually too hot (I don't have a temperature sensor on it, and the smell of plastic gave it away. There's a small indent on the top of Q8 now!
Yes, I was going to replace the LED with zener/buzzer, and I did wonder whether the resistor would need tweaking. Figured it would given ohms law and all that!
I did try using the iron (holding it close, then on the legs, and also on the top of Q8). I very quickly stopped that when I decided the iron was actually too hot (I don't have a temperature sensor on it, and the smell of plastic gave it away. There's a small indent on the top of Q8 now!
Yes, I was going to replace the LED with zener/buzzer, and I did wonder whether the resistor would need tweaking. Figured it would given ohms law and all that!
An update... or not as the case seems to be...
Nothing to report yet, I'm afraid - I've been busy with work this week, so haven't had much of a chance to play. When I have had the chance, I've had it switched on with the lamp in circuit, waiting for the LED to show up nice and bright... nothing.
So to push it more towards a real working situation... I've taken the lamp out from the mains input... still no LED lighting up bright...
And I've dug out an old speaker now (from an old dismantled radio or something) and plugged that across the outputs (aswell as the LED) with some music from my MP3 player playing. Seems to be happily playing music and the problem doesn't want to reappear.
Waiting on the local Maplin to get a buzzer in stock so that I can leave it pretty much unattended waiting for the buzzer, and when it goes off, I can probe the suspect areas. I'd be hesitant about leaving the speaker on there while probing, as that'd probably go the way of my main one and take out the fuse too. At least with the buzzer (and zener), it won't matter how long it's at -36V on the output.
Nothing to report yet, I'm afraid - I've been busy with work this week, so haven't had much of a chance to play. When I have had the chance, I've had it switched on with the lamp in circuit, waiting for the LED to show up nice and bright... nothing.
So to push it more towards a real working situation... I've taken the lamp out from the mains input... still no LED lighting up bright...
And I've dug out an old speaker now (from an old dismantled radio or something) and plugged that across the outputs (aswell as the LED) with some music from my MP3 player playing. Seems to be happily playing music and the problem doesn't want to reappear.
Waiting on the local Maplin to get a buzzer in stock so that I can leave it pretty much unattended waiting for the buzzer, and when it goes off, I can probe the suspect areas. I'd be hesitant about leaving the speaker on there while probing, as that'd probably go the way of my main one and take out the fuse too. At least with the buzzer (and zener), it won't matter how long it's at -36V on the output.
I think we all wondered where you had got to and I think all who have followed this thread want to know the outcome.
I got so interested in this I dropped Arcam a line, and full marks to Arcam for responding, most manufacturers would not want to know. They admitted it "sounds like a tricky one" and the only "common" problem they say are dry joints on Q7 the BD 139 because it runs " a bit on the warm side". If you can't replicate the fault, for what it costs, why not replace the semiconductors, set it up, and start again. £10 to £15 should cover it, that's both channels.
But we don't like doing that do we, we want to know
and I think all who have followed this thread want to know the outcome. I got so interested in this I dropped Arcam a line, and full marks to Arcam for responding, most manufacturers would not want to know. They admitted it "sounds like a tricky one" and the only "common" problem they say are dry joints on Q7 the BD 139 because it runs " a bit on the warm side". If you can't replicate the fault, for what it costs, why not replace the semiconductors, set it up, and start again. £10 to £15 should cover it, that's both channels.
But we don't like doing that do we, we want to know
That's the problem, really... while a blanket change of semiconductors may indeed fix the problem, I'm concerned that something else may still be the issue (R27 faulty, for example), and I don't particularly fancy losing another speaker!
I'd like to know which particular bit is having the issue, so I can rest comfortably in the knowledge that I have actually fixed it.
Be assured that I will keep you updated as and when there's news to report.
Q7 does indeed run a little warm, as does Q8 - in my opinion, I'd say Q8 is actually a little warmer... but that's not empirical - just a guesstimate based on how long I can hold a finger on there! Once this is all sorted out, I might actually attach a small heatsink to Q7 - it has a SOT32 case which is heatsink mountable, and have a think about something for Q8.
And I agree about Arcam. They responded to my original e-mail for the service manual/schematics within 24 hours without hesitation.
I'd like to know which particular bit is having the issue, so I can rest comfortably in the knowledge that I have actually fixed it.
Be assured that I will keep you updated as and when there's news to report.
Q7 does indeed run a little warm, as does Q8 - in my opinion, I'd say Q8 is actually a little warmer... but that's not empirical - just a guesstimate based on how long I can hold a finger on there! Once this is all sorted out, I might actually attach a small heatsink to Q7 - it has a SOT32 case which is heatsink mountable, and have a think about something for Q8.
And I agree about Arcam. They responded to my original e-mail for the service manual/schematics within 24 hours without hesitation.
Hi, I know it's going over old ground again but imagine drawing a line through the center of the circuit. Discounting Q2 and Q3, anything going O/C on the top half allows the output to be pulled hard negative as Q7 is on. If anything went O/C on the lower half Q7/12 etc the output would be pulled hard to the positive rail. A bit simplified but thats about the long and short of it. Resistors don't normally fail intermitant O/C , they either go open circuit or go "high" in value as a permanent change. Adding a heatsink on Q7, may not be a good idea, two reasons, we know they have this slightly weird thermal compensation scheme using Q6 operating on Q7 to reduce the bias as it heats up. It's a bit obscure but adding a heatsink may change the characteristics of this and add "thermal inertia". Secondly and much more important is the fact that adding a heatsink will increase the "capacitance" seen at Q7 collector and could have implications for stability. The collector of Q7 is the metal tab. Even with an insulating washer the effect is there. You used to be able to get TO92 clip on heatsinks (Q8) but it should'nt really need it. Even at 100 degrees C it's still well within limits.
Sometimes you know, you just have to make an educated guess, when all else fails and the fault "heals up".
Sometimes you know, you just have to make an educated guess, when all else fails and the fault "heals up".
Hi Mooly,
Good points regarding the heatsinks - perhaps that wasn't as good an idea as I first thought...
I know educated guesses are sometimes required, and in the worst case, I'll probably end up changing Q7-Q12 inclusive, Q13-Q14 too maybe - for both channels and consider it 'fixed'. It kind of frustrates me that it won't let me play while broken again - that'll be down to me being of the scientist ilk and wanting to know things for certain.
Based on previous voltage readings when it was in the faulty state, where the collector of Q8 was at -37V, from what we've worked out (well, what you're worked out is probably more precise ), it 'feels' like Q8 is the culprit.
I'm going to leave it running as much as I can in the hope the fault rears it's head again, probably for as much of the next week as I can, before doing a silicon change. I'll be changing RV1 and RV101 too - I know variable resistors are prone to getting worn - the main volume control is a little scratchy now aswell, so I'll probably replace that aswell while I've got the soldering iron fired up!
And then perhaps give IC1/IC101 a change to something which sounds better too
Good points regarding the heatsinks - perhaps that wasn't as good an idea as I first thought...
I know educated guesses are sometimes required, and in the worst case, I'll probably end up changing Q7-Q12 inclusive, Q13-Q14 too maybe - for both channels and consider it 'fixed'. It kind of frustrates me that it won't let me play while broken again - that'll be down to me being of the scientist ilk and wanting to know things for certain.
Based on previous voltage readings when it was in the faulty state, where the collector of Q8 was at -37V, from what we've worked out (well, what you're worked out is probably more precise
), it 'feels' like Q8 is the culprit.I'm going to leave it running as much as I can in the hope the fault rears it's head again, probably for as much of the next week as I can, before doing a silicon change. I'll be changing RV1 and RV101 too - I know variable resistors are prone to getting worn - the main volume control is a little scratchy now aswell, so I'll probably replace that aswell while I've got the soldering iron fired up!
And then perhaps give IC1/IC101 a change to something which sounds better too
Whohoo... well, ish...
The amp went into it's fault state again today - I happened to be in the kitchen with the amp at the time, and sure enough, it's taken out the crappy speaker ripped from an old tranny radio (I know that due to the accompanying smoke coming from it!)
Anyway... started taking voltage measurements back from the speaker outputs towards Q8 (these measurements were with an MP3 player playing through it - I was trying to be as quick as possible, so didn't think/have time to disconnect it - and volume about half):
Speaker output: -37.7V
Q14 base: -39.2V
Q7 collector: -39V
Q8 collector: -38.8V
Now while the voltages aren't quite dropping by the 0.6v over each base-emitter junction as would be expected, some of this could be down to RMS voltage differences due to the music coming through the amp.
Now here's the interesting thing... the moment I touched either the base or emitter of Q8 with the red probe (black one at ground) to measure the voltage, the bright green LED light immediately dimmed down and the amp went back to normal. For a while, it seemed that holding the red probe on the colletor of Q8 and wiggling it would put it into the fault state, but as soon as I touched the probe onto either the base or emitter (as gently as I could) back it went to normal running.
So it does appear that something's wrong with Q8, but after managing to get it into the fault state about 3 times and trying to take base/emitter voltages, no amount of waggling Q8 body or any of the legs will put it into the faulty state.
So it does sound to be related to Q8, but I didn't manage to get the voltage drop across R27 to confirm.
Perhaps my wiggling was coincidental with it randomly going faulty, but there does seem to be something here - driving it with something across the outputs does (after a few hours) seem to make the amp want to go faulty again. I don't have anything at the moment to put across the outputs as the little speaker went up in smoke (literally) and I haven't been able to get hold of any 4 or 8 Ohm wirewound resistors yet.
The amp went into it's fault state again today - I happened to be in the kitchen with the amp at the time, and sure enough, it's taken out the crappy speaker ripped from an old tranny radio (I know that due to the accompanying smoke coming from it!)
Anyway... started taking voltage measurements back from the speaker outputs towards Q8 (these measurements were with an MP3 player playing through it - I was trying to be as quick as possible, so didn't think/have time to disconnect it - and volume about half):
Speaker output: -37.7V
Q14 base: -39.2V
Q7 collector: -39V
Q8 collector: -38.8V
Now while the voltages aren't quite dropping by the 0.6v over each base-emitter junction as would be expected, some of this could be down to RMS voltage differences due to the music coming through the amp.
Now here's the interesting thing... the moment I touched either the base or emitter of Q8 with the red probe (black one at ground) to measure the voltage, the bright green LED light immediately dimmed down and the amp went back to normal. For a while, it seemed that holding the red probe on the colletor of Q8 and wiggling it would put it into the fault state, but as soon as I touched the probe onto either the base or emitter (as gently as I could) back it went to normal running.
So it does appear that something's wrong with Q8, but after managing to get it into the fault state about 3 times and trying to take base/emitter voltages, no amount of waggling Q8 body or any of the legs will put it into the faulty state.
So it does sound to be related to Q8, but I didn't manage to get the voltage drop across R27 to confirm.
Perhaps my wiggling was coincidental with it randomly going faulty, but there does seem to be something here - driving it with something across the outputs does (after a few hours) seem to make the amp want to go faulty again. I don't have anything at the moment to put across the outputs as the little speaker went up in smoke (literally) and I haven't been able to get hold of any 4 or 8 Ohm wirewound resistors yet.
I think we have to say Q8, but it's worth changing them all for the cost involved. Do you have anywhere lined up to get them, or I can send you the C.P.C. part numbers, think they were all in stock last time I checked. As to the volume control, if it's PCB mounted you obviously need the correct one to fit properly. Its worth a squirt of WD40 first on the track if you can get at it.. Get a few IC sockets, and perhaps some OpAmps to play with.
A bit nearer then,
Regards Karl
A bit nearer then,
Regards Karl
I don't have anywhere lined up yet to get the bits - either CPC (I have a catalogue here for them already) or rapidonline, as otherwise suggested.
I think I do intend to change all the relevant bits, although given that the problem does appears to be back towards Q8, I'll probably leave Q13 and Q14 for now as they are bolted to the heatsinks, so a bit of extra effort involved!
What I do find interesting is that the fault appears to show up after the amp has been running with a load attached. Once the load has been removed, the amp appears to go back to normal working.
Presumably, something is getting extra hot when the load is attached (extra current being drawn through the outputs) and that's causing it to go intermittent. Once the load was removed, and the component cooled down enough, the fault appears to disappear and not want to recur.
In fact, thinking about it, the mere touching of the probe onto Q8 could have worked like a heatsink and drained the heat from it enough to revert back to working state. Once the amp was running for a while without a load, it's going to be working less and cooler... which is why I can't get it to go faulty again with no load?
Maybe, anyway?
Yes, the main volume control is mounted on the PCB... I know I need a bit with the correct pin spacings, etc. It's not an urgent change, that one, however. It's mainly scratchy when adjusting the volume at lower levels - not across the full range.
I think I do intend to change all the relevant bits, although given that the problem does appears to be back towards Q8, I'll probably leave Q13 and Q14 for now as they are bolted to the heatsinks, so a bit of extra effort involved!
What I do find interesting is that the fault appears to show up after the amp has been running with a load attached. Once the load has been removed, the amp appears to go back to normal working.
Presumably, something is getting extra hot when the load is attached (extra current being drawn through the outputs) and that's causing it to go intermittent. Once the load was removed, and the component cooled down enough, the fault appears to disappear and not want to recur.
In fact, thinking about it, the mere touching of the probe onto Q8 could have worked like a heatsink and drained the heat from it enough to revert back to working state. Once the amp was running for a while without a load, it's going to be working less and cooler... which is why I can't get it to go faulty again with no load?
Maybe, anyway?
Yes, the main volume control is mounted on the PCB... I know I need a bit with the correct pin spacings, etc. It's not an urgent change, that one, however. It's mainly scratchy when adjusting the volume at lower levels - not across the full range.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.