Hi everyone,
I recently refurbished a faulty Quad 405 amplifier. I replaced all transistors, electrolytic caps, diodes, zeners , out of spec resistors.
However, one of the board sounded heavily distorted and weak while the other one sounded ok. I swapped the boards and found the same board still distorted and weak when shifted to the other channel.
I carried out an exhaustive check and sis not find anything abnormal with regard to pcb tracks or components. Nevertheless I replaced once again the diodes, zeners, transistors and also swapped the inductors with the other board. I also swapped the ceramic capacitors
But the problem was found to persist.
Can anyone please advise me in the matter
Thanks
AL
I recently refurbished a faulty Quad 405 amplifier. I replaced all transistors, electrolytic caps, diodes, zeners , out of spec resistors.
However, one of the board sounded heavily distorted and weak while the other one sounded ok. I swapped the boards and found the same board still distorted and weak when shifted to the other channel.
I carried out an exhaustive check and sis not find anything abnormal with regard to pcb tracks or components. Nevertheless I replaced once again the diodes, zeners, transistors and also swapped the inductors with the other board. I also swapped the ceramic capacitors
But the problem was found to persist.
Can anyone please advise me in the matter
Thanks
AL
Attachments
There are so many possibilities here... you really need to be using a scope to see exactly what this distortion looks like.
Two basic checks. Is the DC offset zero at the speaker terminals and are R30 and R31 OK and do they have approx 25 volts DC across each one. They should also be hot.
The 405 will give up to 1 watt in Class A without the dumpers (the 2N3773's) even being fitted.
Two basic checks. Is the DC offset zero at the speaker terminals and are R30 and R31 OK and do they have approx 25 volts DC across each one. They should also be hot.
The 405 will give up to 1 watt in Class A without the dumpers (the 2N3773's) even being fitted.
Hi
Thank you for your recommendations.
D3 is in the right direction.
R30 and R31 measure 560 ohms exact and they both heat.
In fact regarding the distortion, same is present below 1Watt.. so I think the issue may be around the op amp. However I have tried many op amps TL071 and LM301 and in both cases the issue is same.
Regarding the Zeners, they are 12 volts and new.
I do not have a scope I have to find out
Regards
Assil
Thank you for your recommendations.
D3 is in the right direction.
R30 and R31 measure 560 ohms exact and they both heat.
In fact regarding the distortion, same is present below 1Watt.. so I think the issue may be around the op amp. However I have tried many op amps TL071 and LM301 and in both cases the issue is same.
Regarding the Zeners, they are 12 volts and new.
I do not have a scope I have to find out
Regards
Assil
Are the voltages on the Zeners correct?
Also is R13 OK. It looks discoloured and would give highly distorted audio if it were high or open.
Also is R13 OK. It looks discoloured and would give highly distorted audio if it were high or open.
Hi Mooly
Thank you very much.
R13 is ok
I’ll measure the voltage and let you know
I am also attaching a photograph of the track side of the pcb
Regards
Assil
Thank you very much.
R13 is ok
I’ll measure the voltage and let you know
I am also attaching a photograph of the track side of the pcb
Regards
Assil
Attachments
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Hi Brian
It’s a Quad 405 which belonged to a person now deceased. It was out of service for around 18 years due to unsuccessful attempts by some technicians to repair. I received it with blown output transistors, leaky drivers etc . The Aforesaid technicians had also damaged some locations on the pcb. I had no other option that to use small links as you can find in the photos I sent earlier.
After checks and remplacement of components, one of the board played good while the other has a distorted sound even at very low volume. I have swapped components such as the inductors with another healthy Quad board. I replaced the diodes, transistors. I also swapped most of the ceramic /Styron capacitors
Problem still same
It’s a Quad 405 which belonged to a person now deceased. It was out of service for around 18 years due to unsuccessful attempts by some technicians to repair. I received it with blown output transistors, leaky drivers etc . The Aforesaid technicians had also damaged some locations on the pcb. I had no other option that to use small links as you can find in the photos I sent earlier.
After checks and remplacement of components, one of the board played good while the other has a distorted sound even at very low volume. I have swapped components such as the inductors with another healthy Quad board. I replaced the diodes, transistors. I also swapped most of the ceramic /Styron capacitors
Problem still same
If all the DC conditions seem correct then you really do need to see what is happening on a scope.
There have been threads on the 405 in the past and apparently stability issues can occur when using modern and faster output and driver devices in those output triples. There was one such thread where the amp was returned to someone specialising in 405 repair and all the problems were down to incorrect substitutions.
This is why you need a scope now, just to see what is happening.
There have been threads on the 405 in the past and apparently stability issues can occur when using modern and faster output and driver devices in those output triples. There was one such thread where the amp was returned to someone specialising in 405 repair and all the problems were down to incorrect substitutions.
This is why you need a scope now, just to see what is happening.
Thanks I’ll endeavour to get one
The transistor substitutions are as follows :
TR1, TR5 and TR6: BC560C (KEC)
TR2: BC546B (Philips)
TR3, TR4: BC556B (Philips)
TR7, TR8: MJE15033 (on)
TR9, TR10: 2N3773 (Motorola)
The drivers and power transistors are not faster than the originals
The transistor substitutions are as follows :
TR1, TR5 and TR6: BC560C (KEC)
TR2: BC546B (Philips)
TR3, TR4: BC556B (Philips)
TR7, TR8: MJE15033 (on)
TR9, TR10: 2N3773 (Motorola)
The drivers and power transistors are not faster than the originals
Hi
The 2N3773 were Motorola’s bought from Singapore in 2014
They are in my opinion originals.
Nevertheless I have Genuine MJ15024 also
Can try them
The 2N3773 were Motorola’s bought from Singapore in 2014
They are in my opinion originals.
Nevertheless I have Genuine MJ15024 also
Can try them
Could you post an image of the track side of the PCB on the good board? With the same orientation.
Where did you source the 2N3773 from?
Where did you source the 2N3773 from?
Hi
Thanks Mooly. I read the thread
I need to highlight something
In fact I used BD244c initially as driver transistors along with the 2N3773. Later 1 replaced both the 2N3773 with same number and used MJE15033 to replace e the driver.
There was no difference. Moreover, it is my understanding that In case of high frequency oscillation, the transistors would overheat.. however such is not the case
Thanks Mooly. I read the thread
I need to highlight something
In fact I used BD244c initially as driver transistors along with the 2N3773. Later 1 replaced both the 2N3773 with same number and used MJE15033 to replace e the driver.
There was no difference. Moreover, it is my understanding that In case of high frequency oscillation, the transistors would overheat.. however such is not the case
Attachments
Looks like you have a solder bridge in the centre of the board.
Cleaning the excess flux would help.
.
Cleaning the excess flux would help.
.
