Amplifier repair alternative parts

[cowasaki]

Amplifiers are not my bag but I am trying to repair a Cambridge A500.

Where the line level signal enters the amplifier circuit there is a non polarised 1uF electrolytic capacitor.  Can I use a 1uF DC film capacitor as an alternative? Would this affect the sound?

There are quite a number of MPSA93 & MPSA43 transistors plus a MJE350 which are all TO92 package would changing these for something different affect the sound?  If not what would be ideal?

I was hoping to collect an order at CPC tomorrow hence the question.  If I need to keep the same as I have now I'll have to order them on line.

Thanks

[TimFox]

Ordinarily, a 1 uF polypropylene capacitor is a good substitute for a non-polarized Al electrolytic, but is probably physically larger.  Polyester (Mylar) is a reasonable second choice.  Besides all the nonsense, a film capacitor will have zero DC leakage current.  Large ceramic capacitors may have enough voltage dependence of capacitance to cause distortion at low frequencies where the AC voltage is relatively high.

[David Hess]

Large ceramic capacitors may have enough voltage dependence of capacitance to cause distortion at low frequencies where the AC voltage is relatively high.

The same is also true for lower performance film capacitor dielectrics like polyester (Mylar) but of course to a lesser extent than an electrolytic capacitor.  The normal way to combat this is to use the best dielectrics for filtering capacitors and grossly oversize the capacitance of capacitors which are only used for AC coupling.

1 microfarad strikes me as low for an AC coupled line input but maybe the input resistance is high.  Any film capacitor will make a fine replacement.

There are quite a number of MPSA93 & MPSA43 transistors plus a MJE350 which are all TO92 package would changing these for something different affect the sound?  If not what would be ideal?

I would not mess with them unless I had to.  Getting an improvement almost always requires a different circuit rather than different parts.

[cowasaki]

Large ceramic capacitors may have enough voltage dependence of capacitance to cause distortion at low frequencies where the AC voltage is relatively high.

The same is also true for lower performance film capacitor dielectrics like polyester (Mylar) but of course to a lesser extent than an electrolytic capacitor.  The normal way to combat this is to use the best dielectrics for filtering capacitors and grossly oversize the capacitance of capacitors which are only used for AC coupling.

1 microfarad strikes me as low for an AC coupled line input but maybe the input resistance is high.  Any film capacitor will make a fine replacement.

There are quite a number of MPSA93 & MPSA43 transistors plus a MJE350 which are all TO92 package would changing these for something different affect the sound?  If not what would be ideal?

I would not mess with them unless I had to.  Getting an improvement almost always requires a different circuit rather than different parts.

Thanks.  I was more looking for parity rather than an improvement.  The issue is that I can't get hold of any MPSA43/93 or MJE350 transistors for a few days.  My 8 ohm 100watt resistors arrived today so I can start a more reasoned approach to trouble shooting later.....  I'm hoping it is obvious!  I'm replacing all the electrolytic anyway and have ordered Rubycon or Panasonics

[Gyro]

Have you diagnosed it to the actual fault? I wouldn't have thought that the input coupling caps would have been implicated.

What's the fault with the amp?

[cowasaki]

Have you diagnosed it to the actual fault? I wouldn't have thought that the input coupling caps would have been implicated.

What's the fault with the amp?

I was wanting to replace all the electrolytic caps and the input coupling cap is a 1uF non polarised electrolytic.  One channel is running well and the other is now working but very poorly.  It is very quiet and very crackly.  I have a signal generator, a decent scope and now a couple of 8ohm 100w resistors so I'm going to follow the signal through and hopefully find the actual fault.  So far I have swapped the two final transistor pairs from one channel to the other and the fault remained with the left channel so proving that it's not that. 

[Gyro]

So far I have swapped the two final transistor pairs from one channel to the other and the fault remained with the left channel so proving that it's not that. 

That was brave! Power amps can cascade fail, a duff output transistor taking out its partner and driver stages - glad it worked out.

I wouldn't necessarily jump to a full electrolytic replacement, the caps are very low stressed and no SMPS involved. I'd wait until you've located and fixed the fault and seen how it sounds first.

You can easily see whether the input cap on the LH channel by checking the bias levels at each side. You have a fully working channel for comparison.

From what you say, I'd be looking for dry joints and dodgy switch / relay contacts. Amplifiers tend to either work, become grossly distorted, or smoke. Quiet and crackly doesn't really come under those categories.

I'd go through the stages and do a sanity check -  checking for obvious DC biassing errors compared to the good channel and then scope and trace the signal through as you said. I wouldn't be buying any new caps and transistors at this stage.

P.S. Don't let your probes slip! 

[cowasaki]

So far I have swapped the two final transistor pairs from one channel to the other and the fault remained with the left channel so proving that it's not that. 

That was brave! Power amps can cascade fail, a duff output transistor taking out its partner and driver stages - glad it worked out.

I wouldn't necessarily jump to a full electrolytic replacement, the caps are very low stressed and no SMPS involved. I'd wait until you've located and fixed the fault and seen how it sounds first.

You can easily see whether the input cap on the LH channel by checking the bias levels at each side. You have a fully working channel for comparison.

From what you say, I'd be looking for dry joints and dodgy switch / relay contacts. Amplifiers tend to either work, become grossly distorted, or smoke. Quiet and crackly doesn't really come under those categories.

I'd go through the stages and do a sanity check -  checking for obvious DC biassing errors compared to the good channel and then scope and trace the signal through as you said. I wouldn't be buying any new caps and transistors at this stage.

P.S. Don't let your probes slip! 

Thanks.  Several of the 2200uF caps were leaking so I've actually replaced all the electrolytic caps on the main board.  Now both channels work but one is quite tinny whilst the other is quite boomy.......  I'm going to have a proper good mess tomorrow as I've now made up my 8 ohm loads. 

So I connect an 8ohm load across each speaker out and connect the signal generator with 1v sine wave about 4KHz into the line in on both channels and follow it through the amp? 

This is why I fix computers :-) Analogue scares me....  it's so touchy!

[David Hess]

Thanks.  I was more looking for parity rather than an improvement.  The issue is that I can't get hold of any MPSA43/93 or MJE350 transistors for a few days.  My 8 ohm 100watt resistors arrived today so I can start a more reasoned approach to trouble shooting later.....  I'm hoping it is obvious!  I'm replacing all the electrolytic anyway and have ordered Rubycon or Panasonics

Any similar high voltage transistors will work for now but general purpose parts may lower performance.  For instance the MPSA43/93 are used for high voltage compliance where the capacitance must be minimized like the voltage amplifier stage.  CRT driver transistors are commonly selected for this.  The MPSA43/93 is very common in audio power amplifiers anywhere that a high voltage TO-92 sized part is suitable.

I am not sure about the MJE350.  It depends on exactly where it was used.

[Gyro]

...
So I connect an 8ohm load across each speaker out and connect the signal generator with 1v sine wave about 4KHz into the line in on both channels and follow it through the amp? 

This is why I fix computers :-) Analogue scares me....  it's so touchy!

Kind of...

I'd still check with no input first and compare DC levels on the same nodes between the two channels. Also check that the voltage across the low value emitter resistors on the output transistors are similar (indicating correct output stage idle current) Swapping the output pairs may have made one channel low and the other high if their gains/Vbe are significantly different. Also check that there isn't more than 10mV or so DC offsets at the speaker outputs!

Then follow up with your signal generator. NOTE: Don't drive the amp at full power - most consumer amps won't handle full power continuous tone without potentially overheating. I'd go with 100mV and use the volume control, keeping an eye on load resistor temperature.

If you now have different apparent frequency response on the two channels (assuming that it's not the speakers - you did check the output DC offsets didn't you), then it should show up if you sweep to different frequencies. I suspect that it doesn't have tone controls but if it does, then there might be a problem in that area (dirty pots?).

I'm not quite sure where your low volume and crackling has gone. Hopefully you will find the cause in the process.

Good luck. 

[cowasaki]

...
So I connect an 8ohm load across each speaker out and connect the signal generator with 1v sine wave about 4KHz into the line in on both channels and follow it through the amp? 

This is why I fix computers :-) Analogue scares me....  it's so touchy!

Kind of...

I'd still check with no input first and compare DC levels on the same nodes between the two channels. Also check that the voltage across the low value emitter resistors on the output transistors are similar (indicating correct output stage idle current) Swapping the output pairs may have made one channel low and the other high if their gains/Vbe are significantly different. Also check that there isn't more than 10mV or so DC offsets at the speaker outputs!

Then follow up with your signal generator. NOTE: Don't drive the amp at full power - most consumer amps won't handle full power continuous tone without potentially overheating. I'd go with 100mV and use the volume control, keeping an eye on load resistor temperature.

If you now have different apparent frequency response on the two channels (assuming that it's not the speakers - you did check the output DC offsets didn't you), then it should show up if you sweep to different frequencies. I suspect that it doesn't have tone controls but if it does, then there might be a problem in that area (dirty pots?).

I'm not quite sure where your low volume and crackling has gone. Hopefully you will find the cause in the process.

Good luck. 

Thanks for your help.  I've enjoyed watching hundreds (yes hundreds) of videos on youtube regarding old valve repairs, newer amps etc and I understand and can follow them but actually jumping in and doing it myself is more difficult. 

Thanks for explaining and the 100mV input part.

I had a dodgy connection hence the strange last result and having corrected that I do have one PERFECT channel.  With the new quality caps it sounds superb.  The amp is a Cambridge A500 and when I swapped the output transistor pairs round I re-adjusted both bias voltages using the page in the manual explaining how to do it.  The voltage jumps around a bit rather than staying constant but I've sort of got it around the correct value.  The left channel is now the only issue but I've only just got to that point.  Unfortunately the left and right inputs from the tone section into the main board have different length cables and as such I can't swap them round to see if the issue is prior to the main board.  I will  try the left one in the right though and if it is still good then it shows the inputs to the main board are correct.  After that it's down to trouble shooting.  All the caps on the board are new and the output transistors and bias variable resistors are ok. I've just managed to get another A500 main board with a single working channel due to blow output transistors so I have a pair of output transistors and spares of everything else.  If I can get the parts from the blown channel I can keep the single working channel as a single channel amp for the bench or just keep it for spares.

Will update tomorrow.... shame I can't find the BNC to phone adapters than I know I have somewhere. 

I have a lot of respect for those people who really understand analogue!  Digital is easy in comparison!

[Gyro]

That sounds to be going well, well done.

Everybody needs to do a bit of analogue occasionally, as a break from all that dry digital stuff if nothing else! Fixing audio stuff and analogue test gear is as good an opportunity as any. 

[cowasaki]

I've been checking the speakers and I had a set of 4 Gale 3010 bookshelf speakers 3 of them have blown mid/low drivers and I am not sure if this was caused by the amp or whether they were already blown. 

IF the speakers have been damaged by the amp what is the likely reason?  Is there any way to protect speakers from amps whilst not risking damage to the amp either?

Also can I place two 4 ohm speakers in series to act as one 8 ohm speaker?  (I have some old car speakers that I would rather blow than the decent ones!)

[Gyro]

Oh dear, that's sad. It would certainly account for the tinny sound on one channel though.

If it's the LF driver, then severe DC offset on the amplifier output would be the likely cause. The tweeters are protected by the series capacitors in the crossovers. I'd be surprised if you hadn't noticed that though - you would get a very loud bang as the voice coil bottomed out or over-extended forwards, the voice coils wouldn't burn out instantly.

It's worth checking the DC resistance of the speakers to see if the LF voice coils are actually open circuit though. Another fault that can occur due to physical handling or age, is the glue holding the magnets failing, causing the centre pole piece to move sideways and trap the voice coil. If you can access the front of the drivers then gently move them in and out using centred force and see if they are jammed or making scratching sounds.

Yes, you can place two 4 ohm speakers in series to act as an 8 ohm one. You could also put one of your load resistors in series with one of them to get >8 ohms for testing purposes.

P.S. Over-driving the speakers with a single frequency from your signal generator could also kill the drivers but you would probably be at the threshold of pain by that stage - unless it was set to a very low frequency, when you would see the woofer cones flapping wildly! Pretty hard to miss.

P.P.S. If the speakers have been in storage for a long time, internal corroded spade connectors or rotting cone surrounds are also a possibility.

[cowasaki]

Oh dear, that's sad. It would certainly account for the tinny sound on one channel though.

If it's the LF driver, then severe DC offset on the amplifier output would be the likely cause. The tweeters are protected by the series capacitors in the crossovers. I'd be surprised if you hadn't noticed that though - you would get a very loud bang as the voice coil bottomed out or over-extended forwards, the voice coils wouldn't burn out instantly.

It's worth checking the DC resistance of the speakers to see if the LF voice coils are actually open circuit though. Another fault that can occur due to physical handling or age, is the glue holding the magnets failing, causing the centre pole piece to move sideways and trap the voice coil. If you can access the front of the drivers then gently move them in and out using centred force and see if they are jammed or making scratching sounds.

Yes, you can place two 4 ohm speakers in series to act as an 8 ohm one. You could also put one of your load resistors in series with one of them to get >8 ohms for testing purposes.

P.S. Over-driving the speakers with a single frequency from your signal generator could also kill the drivers but you would probably be at the threshold of pain by that stage - unless it was set to a very low frequency, when you would see the woofer cones flapping wildly! Pretty hard to miss.

P.P.S. If the speakers have been in storage for a long time, internal corroded spade connectors or rotting cone surrounds are also a possibility.

Thanks for taking the time to help.  I am right out of my comfort zone but want to learn.  I have already taken apart the 4 speakers.  I removed the cross-over and dis-connected the mid/bass speaker then tested that on it's own.  All 3 are either open or resistance in Mohms.

I've fired up the amp with the loads in place and the multimeter on DC voltage.  I get nothing at the speakers.  I've not heard any pops and it is possible that I was ripped off when I bought the speakers.  I had a random pari of them sat on top of each other in the kitchen connected to a small hifi so it's possible that I had the good one and another. 

The left channel is still tinny but the other channel sounds superb so.  I have a another board with a working channel so SHOULD have what I need to get it going.  It's a shame that the power supply for the amp is mixed up in the main amp board as it would have made trouble shooting easier if the power supply could be tested separately.  Anyway I know that the output transistors SHOULD be ok so just need to feed the 0.1V sine wave in and follow it through both channels comparing the wave and see where it gets distorted or vanishes.  I need to pick up a couple of BNC to RCA phono jack leads as I'm near CPC tomorrow and that should make it a bit easier.

I'll probably have more questions tomorrow :-)

Thanks

[cowasaki]

Right....

Now that I have removed dodgy speakers from my test setup and got the correct leads to pass my signal generator into the amp correctly I have one perfect channel and one distinctly dodgy one.

The circuit diagram is shown and the bias on the NPN output transistor is 13mV on the right channel.  On the left channel it is 380mV.

All the electrolytic caps are new.  The variable resistor RV201 is ok.  The SAP15N & P should be ok as I swapped them and the fault stayed.

I've tested R225,229,230,228,231,227,216,215 & Q206.  D202,203,204,205,206

R226 is 82 ohms not 39 ohms BUT it is 82 ohms on the other channel and the extra board I have.

The voltages are 42 & -42.  Both LEDs light up

I have had to stop for tonight but that is where I'm up to at the moment.

[Gyro]

Well that's a pretty significant difference! Your Left channel certainly seems to have a biassing issue.

It looks as if you've found the schematic, If it's the same one as I found (attached), there's a page on output current setting which indicates that the voltage across the output stage emitter resistor (approx 216.7mR) should be 13mV (60mA idle current), so your right channel is correct. The 380mV on the Left channel seems to indicate that it is idling at 1.75A, not a situation that is going to sustain itself for very long!

I can't see any particular reason why they would have changed the value of R226, but it is the same on both the boards/channels then it must be an engineering change - R213 should match it in value too (for symmetrical drive). Maybe they've also changed the value of R227 and R212 to reflect the changes in R226 etc.

The first thing to check (ASAP) is to see if you can adjust the idle current of the Left channel down to the correct 13mv/60mA idle point. If not, then there is either a fault in the driver circuit - or maybe a value change in the internal emitter resistor of U201 (not unheard of with these integrated output transistors).

The easiest way to check, rather than trying to measure a very low value resistor with a DMM, is to check the equivalent points on U202 (make sure you check the pinout, also in the PDF). If that looks sensible then it's likely that U201 is faulty. There's no way that U201 can be passing 1.75A unless it's going through U202 too... if it was being diverted into the driver stage then smoke would be billowing.

At the moment it feels like either the idle current is grossly mis-set by RV201 (or C238 shorted or reversed - check!) or a faulty U202 - hopefully not, they look expensive (I know you have a spare). If the supply rails are sitting at +/-42V (schematic +/-45V) then is seems pretty unlikely that there is actually 1.75A being drawn.

I hope this allows you to make some progress. 

P.S. Check the Speaker output offset voltage too (unloaded), that may be another clue.

[cowasaki]

Well that's a pretty significant difference! Your Left channel certainly seems to have a biassing issue.

It looks as if you've found the schematic, If it's the same one as I found (attached), there's a page on output current setting which indicates that the voltage across the output stage emitter resistor (approx 216.7mR) should be 13mV (60mA idle current), so your right channel is correct. The 380mV on the Left channel seems to indicate that it is idling at 1.75A, not a situation that is going to sustain itself for very long!

I can't see any particular reason why they would have changed the value of R226, but it is the same on both the boards/channels then it must be an engineering change - R213 should match it in value too (for symmetrical drive). Maybe they've also changed the value of R227 and R212 to reflect the changes in R226 etc.

The first thing to check (ASAP) is to see if you can adjust the idle current of the Left channel down to the correct 13mv/60mA idle point. If not, then there is either a fault in the driver circuit - or maybe a value change in the internal emitter resistor of U201 (not unheard of with these integrated output transistors).

The easiest way to check, rather than trying to measure a very low value resistor with a DMM, is to check the equivalent points on U202 (make sure you check the pinout, also in the PDF). If that looks sensible then it's likely that U201 is faulty. There's no way that U201 can be passing 1.75A unless it's going through U202 too... if it was being diverted into the driver stage then smoke would be billowing.

At the moment it feels like either the idle current is grossly mis-set by RV201 (or C238 shorted or reversed - check!) or a faulty U202 - hopefully not, they look expensive (I know you have a spare). If the supply rails are sitting at +/-42V (schematic +/-45V) then is seems pretty unlikely that there is actually 1.75A being drawn.

I hope this allows you to make some progress. 

P.S. Check the Speaker output offset voltage too (unloaded), that may be another clue.

Thanks.  I will be looking at it later.  I have the same schematic as you.  I've changed all the caps with them in the same direction as they were originally and matching the spare board.  As the fault was there before and after I would suggest that the caps had no influence (although they seem to have improved the sound of the working channel a bit.  When I originally swapped the output transistors over I was sure I set the bias resistor and both channels have it about half way.  I may well remove the components and check them out of circuit so I can be absolutely sure that they work exactly as they should.  I do have the spare pair of transistors but wanted to avoid fitting them until I was sure that there wasn't an issue which blew the original and was still there.  I have put the multimeter across the output on DC and checked that but.......  What IS a bad amount?

[Gyro]

Just thinking - the only other place the 1.75A could go is into your speaker/dummy load (thinking blown woofers) but as I mentioned previously, I'm sure you would have heard that.

Make sure you check the bias voltage/current with the speaker load disconnected, then there's no confusion about where the current is going.


As for what IS a bad value for speaker output offset voltage - certainly anything approaching 100mV is a bad sign, it should most likely be less than than 10mV.


If the integrated emitter resistor in U202 has failed you ought to be able to measure it in-circuit at those same bias test points. 216mR should look like a nearly-short, if it's gone bad then you should be able to see some significant resistance.

I've never tried it myself, but I don't see any particular reason why you couldn't replace it with an external resistor across those same pins, say, 220mR 3W.

I'm getting ahead of myself here though, let's see what you find (after enjoying the rare bank holiday sunshine!).


EDIT:  I knew I'd seen the problem with the integral emitter resistors somewhere... https://www.eevblog.com/forum/repair/repair-of-audio-amp-using-sanken-sap15n-sap15p-output-transistors-fixed/

[cowasaki]

Just thinking - the only other place the 1.75A could go is into your speaker/dummy load (thinking blown woofers) but as I mentioned previously, I'm sure you would have heard that.

Make sure you check the bias voltage/current with the speaker load disconnected, then there's no confusion about where the current is going.


As for what IS a bad value for speaker output offset voltage - certainly anything approaching 100mV is a bad sign, it should most likely be less than than 10mV.


If the integrated emitter resistor in U202 has failed you ought to be able to measure it in-circuit at those same bias test points. 216mR should look like a nearly-short, if it's gone bad then you should be able to see some significant resistance.

I've never tried it myself, but I don't see any particular reason why you couldn't replace it with an external resistor across those same pins, say, 220mR 3W.

I'm getting ahead of myself here though, let's see what you find (after enjoying the rare bank holiday sunshine!).


EDIT:  I knew I'd seen the problem with the integral emitter resistors somewhere... https://www.eevblog.com/forum/repair/repair-of-audio-amp-using-sanken-sap15n-sap15p-output-transistors-fixed/

I've removed, tested and replaced all the transistors and their values are attached.  Can't see anything obviously wrong.

The voltage reg appears ok

If I plug the left channel line in into the right channel the sound is perfect so it must be on the main board.

Run out of time again but will hopefully get a chance to have another look tomorrow and will have a look at what you have mentioned above and look at the other article.  I really do appreciate your help.  Thanks

[cowasaki]

THANKS TO EVERYONE especially Gyro....

My amp is now working perfectly and I have learnt a lot!

The fault ended up being one of the SAP15 transistors after all.  I'm not sure how it happened but it appears that I mixed up which channel was working originally so when I swapped them round the fault swapped as well but I thought it hadn't.  I have raised it up and it sounds great.  So now I have a fully working amp with all new high quality caps as well.  Total cost was under £30 but the most important thing is that I have learned a lot.

This now means that I have a nearly perfect Cambridge A500 main board with two pairs of blown SAP15 caps and a few other minor faults on one channel.  I might well follow the other thread and fix the board just for the fun of it.  Always handy to have a spare I suppose...  Either that or pick up another faulty one (there are a good few).

Now I need 4 matched 4" 8ohm mid/bass drivers to fix my 4 speakers (one works but better all matched).