Hi,
Looking for some help with a late model Quad 405-2 which after recent repair is running significantly hotter on the right channel than the left.
Several weeks ago the amp failed on the right channel and was sent to a recommended audio engineer who replaced both output trannies (TR9 & 10) and both class A drivers (TR7 & 8) and the amp had 2 channels once more. However the amp now runs much hotter on the right (looking from the front) of the heatsink but sounds as good as ever with the output level/quality equal in both channels. There is little point contacting the "repairer" after he tried charging 4 hours labour (£180) plus parts for the above work which I thought was way too much and after questioning the cost he "spat his dummy" ! He did say at the time that the right channel failure was caused by an overtightened MJ15003 over time shorting to ground. The amp had been refurbished by a previous owner with new electrolytics, p/s caps, MJ15003`s, opamps etc.
So following advice I replaced the heatsink compound on both boards (issue 7) after cleaning and refacing them, checked the torque of all 4 output devices and retightened the screws securing the board mounting plate to the front heatsink all to no avail as the problem persists. The right side of the heatsink is virtually untouchable after 20 minutes of moderate volume while the left is its usual warmish temperature. The amp has also been tested in a friends system with the same result. Also checked the DC offset which is 27mv in each channel.
I`m now at a loss what else can be done and see no other option but to return the amp to Quad (which in hindsight is what I should have done when the right channel failed but was deterred by their 4 week lead time) unless anyone can suggest a possible cause of the problem. Unfortunately I don`t have access to a signal gen` or a `scope but can solder to a good standard, use a DMM and replace faulty components. My best guess is that when the MJ15003 shorted it also damaged another component but I`m unsure which one.
Has anyone any thoughts as to the possible cause of the heating imbalance ?
Many thanks in anticipation.
Looking for some help with a late model Quad 405-2 which after recent repair is running significantly hotter on the right channel than the left.
Several weeks ago the amp failed on the right channel and was sent to a recommended audio engineer who replaced both output trannies (TR9 & 10) and both class A drivers (TR7 & 8) and the amp had 2 channels once more. However the amp now runs much hotter on the right (looking from the front) of the heatsink but sounds as good as ever with the output level/quality equal in both channels. There is little point contacting the "repairer" after he tried charging 4 hours labour (£180) plus parts for the above work which I thought was way too much and after questioning the cost he "spat his dummy" ! He did say at the time that the right channel failure was caused by an overtightened MJ15003 over time shorting to ground. The amp had been refurbished by a previous owner with new electrolytics, p/s caps, MJ15003`s, opamps etc.
So following advice I replaced the heatsink compound on both boards (issue 7) after cleaning and refacing them, checked the torque of all 4 output devices and retightened the screws securing the board mounting plate to the front heatsink all to no avail as the problem persists. The right side of the heatsink is virtually untouchable after 20 minutes of moderate volume while the left is its usual warmish temperature. The amp has also been tested in a friends system with the same result. Also checked the DC offset which is 27mv in each channel.
I`m now at a loss what else can be done and see no other option but to return the amp to Quad (which in hindsight is what I should have done when the right channel failed but was deterred by their 4 week lead time) unless anyone can suggest a possible cause of the problem. Unfortunately I don`t have access to a signal gen` or a `scope but can solder to a good standard, use a DMM and replace faulty components. My best guess is that when the MJ15003 shorted it also damaged another component but I`m unsure which one.
Has anyone any thoughts as to the possible cause of the heating imbalance ?
Many thanks in anticipation.
This sounds an interesting problem...
The 405 runs with zero bias and there are no adjustments and so there are only a few possibilities as to the excess heat generation.
1/ Although the amp runs with zero bias, there is the possibility of a problem in the network that sets the 'turn on volts' for the driver and output stage. It should be running just below the point at which the output transistors draw current, however if it is not then there is a possibility of the amp not running in class b but in class ab which would generate heat. That should be an easy one to prove...
2/ There is a DC offset at the speaker terminals which is forcing current through the speaker all the time. Result... overheating. Dead easy to check. There should be zero volts DC at the speaker terminals. Edit... which I see you have done.
3/ The amplifier is oscillating. This is a distinct possibility given the fact that transistors have been replaced for modern versions which will differ in characteristics compared to the originals. The opamp swap needs looking at. What is now fitted ?
The 405 runs with zero bias and there are no adjustments and so there are only a few possibilities as to the excess heat generation.
1/ Although the amp runs with zero bias, there is the possibility of a problem in the network that sets the 'turn on volts' for the driver and output stage. It should be running just below the point at which the output transistors draw current, however if it is not then there is a possibility of the amp not running in class b but in class ab which would generate heat. That should be an easy one to prove...
2/ There is a DC offset at the speaker terminals which is forcing current through the speaker all the time. Result... overheating. Dead easy to check. There should be zero volts DC at the speaker terminals. Edit... which I see you have done.
3/ The amplifier is oscillating. This is a distinct possibility given the fact that transistors have been replaced for modern versions which will differ in characteristics compared to the originals. The opamp swap needs looking at. What is now fitted ?
I had that problem wit ha 1980's Maplin amplifier I bought.
It was short of the output transistors so I bought some new ones from RS Components.
After fitting there was oscillation on the output.
I had to increase VAS capacitor a little to stop the oscillation.
I measured the transistors on a semiconductor analyser and the gain was much higher than the original spec for that transistor.
Thanks for the suggestions on a possible cause.
Oscillation of the two newly replaced MJ15003`s could offer an explanation for the overheating, although the new pair that have been fitted are the same type as fitted before the right channel failed and only differ in batch number (they also say "mex" on them exactly the same as the originals). Could the new pair be faulty in some way causing them to overheat ? Yet the amp ran perfectly with a balanced heatsink temperature for the year I`ve owned it with four MJ15003`s fitted.
Incidently when the amp was returned from the repair the two class A driver transistors (TR7 & 8) had also been changed to RCA 1C04`s which I have since replaced with MJ15031`s to match the left channel thinking they might have caused the overheating but this made no difference.
The opamps currently fitted to the amp are AD825`s which I much prefer to Burr Brown 604`s & 627`s which I also tried upon initial purchase of the Quad. I`ve also recently tried the amp with a pair of 604`s in place to confirm the overheating wasn`t being caused by the super fast AD825`s and it made no difference. If one of the low value caps which are fitted across the Zeners were to fail could that cause a possible oscillation problem which manifests itself as overheating ? The caps installed do appear to be fairly old ITT polypropylene box type which iirc are 100nf.
Oscillation of the two newly replaced MJ15003`s could offer an explanation for the overheating, although the new pair that have been fitted are the same type as fitted before the right channel failed and only differ in batch number (they also say "mex" on them exactly the same as the originals). Could the new pair be faulty in some way causing them to overheat ? Yet the amp ran perfectly with a balanced heatsink temperature for the year I`ve owned it with four MJ15003`s fitted.
Incidently when the amp was returned from the repair the two class A driver transistors (TR7 & 8) had also been changed to RCA 1C04`s which I have since replaced with MJ15031`s to match the left channel thinking they might have caused the overheating but this made no difference.
The opamps currently fitted to the amp are AD825`s which I much prefer to Burr Brown 604`s & 627`s which I also tried upon initial purchase of the Quad. I`ve also recently tried the amp with a pair of 604`s in place to confirm the overheating wasn`t being caused by the super fast AD825`s and it made no difference. If one of the low value caps which are fitted across the Zeners were to fail could that cause a possible oscillation problem which manifests itself as overheating ? The caps installed do appear to be fairly old ITT polypropylene box type which iirc are 100nf.
After researching MJ15003 I`ve discovered this is the most popular transistor to fake with the fake ones usually being 2N3055`s with MJ15003 written on them instead. There is mention of this counterfeiting on this and many other audio forums including Dada`s, so possibly the repairer of the amp has unwittingly installed fake/inferior MJ15003`s possibly sourced from Ebay ?
This may explain why the amp is barely warm in either channel while playing spoken voice on Radio 4 at very low volume yet as the volume is turned up higher when playing music the heating of the right channel becomes logarithmic, in the same way as a volume pot works, presumably as the weaker/fake MJ15003`s struggles to handle what`s being asked of them and overheat ?
Or am I clutching at straws ?
This may explain why the amp is barely warm in either channel while playing spoken voice on Radio 4 at very low volume yet as the volume is turned up higher when playing music the heating of the right channel becomes logarithmic, in the same way as a volume pot works, presumably as the weaker/fake MJ15003`s struggles to handle what`s being asked of them and overheat ?
Or am I clutching at straws ?
The fact it runs cool when not playing loudly suggests the basic DC conditions are correct. Increasing temperatures as the volume increases can in some cases be a sign of instability (hf oscillation).
We'll come back to that.
The 405 appeared in several variations... do you know which yours is ?
Specifically, the output stage of the later ones had a diode between the base of the upper output transistor and the base of the lower driver. This diode brings the output stage closer to the point of conduction to soften the class B discontinuity. Earlier versions omitted this.
If yours has the diode then you could try as a test shorting it out. That indirectly might give a clue.
The opamp in almost all versions is shown with no decoupling cap across the zener supplies... odd in todays thinking. I doubt there is an issue there but it is easy to eliminate as problem by simply tagging a single small cap (say 1uF) across the supply pins of the opamp. Watch the polarity and working voltage.
The output stage has a 10 ohm and 0.1uF (zobel network). Make sure the 10 ohm is OK and that the cap is not open circuit. Check each end of the network goes to where it is supposed to i.e. across the speaker output to ground.
Fakes... this has to be a major possibility and is one that is impossible to diagnose without simply replacing the suspect parts with known good ones. All semiconductors should be obtained from reputable outlets and so if there is doubt then it might be worth getting a new pair, if only to eliminate them.
We'll come back to that.
The 405 appeared in several variations... do you know which yours is ?
Specifically, the output stage of the later ones had a diode between the base of the upper output transistor and the base of the lower driver. This diode brings the output stage closer to the point of conduction to soften the class B discontinuity. Earlier versions omitted this.
If yours has the diode then you could try as a test shorting it out. That indirectly might give a clue.
The opamp in almost all versions is shown with no decoupling cap across the zener supplies... odd in todays thinking. I doubt there is an issue there but it is easy to eliminate as problem by simply tagging a single small cap (say 1uF) across the supply pins of the opamp. Watch the polarity and working voltage.
The output stage has a 10 ohm and 0.1uF (zobel network). Make sure the 10 ohm is OK and that the cap is not open circuit. Check each end of the network goes to where it is supposed to i.e. across the speaker output to ground.
Fakes... this has to be a major possibility and is one that is impossible to diagnose without simply replacing the suspect parts with known good ones. All semiconductors should be obtained from reputable outlets and so if there is doubt then it might be worth getting a new pair, if only to eliminate them.
Attachments
It`s a late model 405-2 (serial# 88xxx) fitted with M12368 issue 7 boards.
I`ll check the Diode as you suggest and test the 10Ohm and 0.1uf (R39 & C12) to confirm they are ok. Is it possible these components have been damaged when the original MJ15003 shorted to ground ? There are existing low value caps (0.1uf) across the Zeners and I`ll also check those in case one has failed, but as you say it is doubtful this is the cause of the issue anyway.
If the above components test ok I`ll source some genuine MJ15003`s from RS as has been suggested and replace the currently installed ones. I`m kind of hoping the ones fitted by the original repairer are fakes as that would explain the overheating issue and these components are the only remaining parts fitted at the time of repair.
Many thanks for your suggestions to the cause of the overheating issue it`s much appreciated
I`ll check the Diode as you suggest and test the 10Ohm and 0.1uf (R39 & C12) to confirm they are ok. Is it possible these components have been damaged when the original MJ15003 shorted to ground ? There are existing low value caps (0.1uf) across the Zeners and I`ll also check those in case one has failed, but as you say it is doubtful this is the cause of the issue anyway.
If the above components test ok I`ll source some genuine MJ15003`s from RS as has been suggested and replace the currently installed ones. I`m kind of hoping the ones fitted by the original repairer are fakes as that would explain the overheating issue and these components are the only remaining parts fitted at the time of repair.
Many thanks for your suggestions to the cause of the overheating issue it`s much appreciated
The usual initial cause of a zobel network failing would be a pre existing massive high frequency instability that simply burns out the 10 ohm.
No other fault condition can stress these parts although I suppose a small cap can fail open circuit but it would be unusual. An open or missing network can be a cause of instability.
Its a bit odd because instability usually makes its presense felt in other ways beside overheating... distortion, distortion on transients, odd hissing sounds and so on.
(The Quad manual I have doesn't seem to list that version of board, but all variants are basically the same with very minor detail changes and additional protection modules fitted to the later production)
Check the zobel, check the diode and then I think it has to be a case of fitting replacements outputs... and take it from there.
Have a really close look over the board and see if any other work seems to have been done, and/or any parts replaced with different values.
Another thought... just for curiosity really... would be to add a small resistor in series with the speaker (added at the amplifier end) of say 1 or 2 ohms. That would drastically alter the load the amplifier sees (isolating it from any capacitive effects of the cables) and might prove if it were a stability issue or not.
No other fault condition can stress these parts although I suppose a small cap can fail open circuit but it would be unusual. An open or missing network can be a cause of instability.
Its a bit odd because instability usually makes its presense felt in other ways beside overheating... distortion, distortion on transients, odd hissing sounds and so on.
(The Quad manual I have doesn't seem to list that version of board, but all variants are basically the same with very minor detail changes and additional protection modules fitted to the later production)
Check the zobel, check the diode and then I think it has to be a case of fitting replacements outputs... and take it from there.
Have a really close look over the board and see if any other work seems to have been done, and/or any parts replaced with different values.
Another thought... just for curiosity really... would be to add a small resistor in series with the speaker (added at the amplifier end) of say 1 or 2 ohms. That would drastically alter the load the amplifier sees (isolating it from any capacitive effects of the cables) and might prove if it were a stability issue or not.
Have now checked D6, C12 & R39 all of which test within tolerance and also gone over the board with a magnifier and found no obvious evidence of poor connections or bad workmanship and all components match on both boards.
So it looks like the final option is to replace the pair of MJ15003`s, but having tried RS components who have no stock until next month I`ve found a guy on Ebay selling genuine NOS Motorola`s (item# 192344226718) who assures me they are the real article supplied in batch matched pairs. They should arrive on Tuesday and once installed I`ll report back with the result.
I gave you incorrect numbers for the boards, they are M12565 - 7 although as you say there`s little difference between the later boards.
So it looks like the final option is to replace the pair of MJ15003`s, but having tried RS components who have no stock until next month I`ve found a guy on Ebay selling genuine NOS Motorola`s (item# 192344226718) who assures me they are the real article supplied in batch matched pairs. They should arrive on Tuesday and once installed I`ll report back with the result.
I gave you incorrect numbers for the boards, they are M12565 - 7 although as you say there`s little difference between the later boards.
To save duplication; I thought the OP was sending this amp to Quad ?
Quad 405-2 Overheating on right channel. - UK Vintage Radio Repair and Restoration Discussion Forum
Quad 405-2 Overheating on right channel. - UK Vintage Radio Repair and Restoration Discussion Forum
Re - I thought the OP was sending the amp to Quad.
My intention was to send the amp back to Quad and if the replacement MJ15003`s fail to rectify the overheating issue that`s exactly where it`s going. The responses on DIY Audio forum offered further scope for possible causes and as couriers don`t collect at weekends I thought I`d at least try what`s kindly been suggested.
But thankyou anyway.
My intention was to send the amp back to Quad and if the replacement MJ15003`s fail to rectify the overheating issue that`s exactly where it`s going. The responses on DIY Audio forum offered further scope for possible causes and as couriers don`t collect at weekends I thought I`d at least try what`s kindly been suggested.
But thankyou anyway.
on the replacements. Let us see what that brings.Earlier today the Motorola MJ15003`s arrived and wasting no time they were duly installed in the Quad. It was then left playing "BuckinghamNicks" at moderate volume as this recording has a full bass and soon warms up most amplifiers within 10 to 15 minutes. Upon my return after 20 minutes I have to report the right channel was considerably hotter than the left.
With this in mind I think it`s time to admit defeat and send the amp to Quad.
Many thanks for everyone`s input with possible solutions to the problem and if there is a lesson to be learned from all this it`s the amp should have gone back to Quad directly once the right channel failed and not allow unknown, unsatisfactory and overpriced non-Quad individuals to attempt repairs.
There seems little wonder Quad have a 4 week lead time on repairs/servicing as it would appear the majority of Quad owners prefer to trust Quad, additionally their prices are very reasonable too.
Once the amp is repaired I`ll report the findings but for now it`s time to dig out an old amp and "slum it" for a few weeks
With this in mind I think it`s time to admit defeat and send the amp to Quad.
Many thanks for everyone`s input with possible solutions to the problem and if there is a lesson to be learned from all this it`s the amp should have gone back to Quad directly once the right channel failed and not allow unknown, unsatisfactory and overpriced non-Quad individuals to attempt repairs.
There seems little wonder Quad have a 4 week lead time on repairs/servicing as it would appear the majority of Quad owners prefer to trust Quad, additionally their prices are very reasonable too.
Once the amp is repaired I`ll report the findings but for now it`s time to dig out an old amp and "slum it" for a few weeks
Well, I would be curious to know the current flowing in a couple of key parts of the circuit, and particularly comparing good channel to bad.
There are two very low value resistors connected to each output transistor collector, actual value depends on the particular variant. (those resistors would be stressed in the event of an output failure, at least until the fuse blew and so would be worth a check to make sure they had not gone high in value)
The voltage dropped across each resistor is a direct indicator of what is flowing where. We just use ohms law to calculate the current from the measured voltage.
Under no signal conditions I would expect the upper resistor to see somewhere around 65 milliamps flowing and the lower around 14 milliamps which would be around 11 millivolts and just 2.5 millivolts respectively. That is DC voltage measured under no signal conditions and they are very tiny voltages.
And then the same again when playing and the heat is being generated.
There are two very low value resistors connected to each output transistor collector, actual value depends on the particular variant. (those resistors would be stressed in the event of an output failure, at least until the fuse blew and so would be worth a check to make sure they had not gone high in value)
The voltage dropped across each resistor is a direct indicator of what is flowing where. We just use ohms law to calculate the current from the measured voltage.
Under no signal conditions I would expect the upper resistor to see somewhere around 65 milliamps flowing and the lower around 14 milliamps which would be around 11 millivolts and just 2.5 millivolts respectively. That is DC voltage measured under no signal conditions and they are very tiny voltages.
And then the same again when playing and the heat is being generated.
Would I be correct that the two resistors that require checking are R35 & R36 which are both 0.08 Ohms on my board version ? Does the testing have to be while the amp is live or could the resistors be dead tested (one leg lifted off the board) ? If not, do I place the negative probe to earth and the positive to either side of the resistor in situ` to find the Millivolt reading ?
Also, do I need speakers connected to live test ?
Apologies for any daft questions, but I`m at the limit of my electronics knowledge !
Also, do I need speakers connected to live test ?
Apologies for any daft questions, but I`m at the limit of my electronics knowledge !
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