Fixing a Quad 606

Lee in Montreal · 9th January 2010, 03:05 PM

Question from a guy who got an electro technician degree... 30 years ago, but never really practiced as being a creative in advertizing was more groovy.

Being very happy with my two upgraded Quad 405s, I decided to buy a Quad 606 with one bad channel, for cheap.

- When the amp boards are disconnected and I power it on, no fuse problem.

- When I connect the amp boards, it burns one of the 3 Amp fuses. If I switch the amp boards side-to-side, the burned fuse follows. Meaning the problem is with one of the amp boards not with power supply (no hum either fro the transformer).

- None of the resistors or caps on the board show a sign of being burned. Maybe one of the power transistors is shorting. Can they be tested while still on the board, and how? Would a simple Ohm-meter be enough? BTW I have two dozens of brand new Motorola MJ15003G transistors just in case...

gootee · 9th January 2010, 04:21 PM

Good news: Since you have two identical boards, with one working and one not, you should be able to compare some type of measurements made between corresponding points on each of the two boards, and find the problem.

I would probably start with resistance measurements, made while the boards are completely disconnected.

I don't know what voltage your meter applies, to measure resistance. So I have to suppose that it is possible that it is high-enough to "turn on" a PN semiconductor junction. But if there is a short in one amp board where there is still a PN semiconductor junction in the other amp board, I think you should be able to verify it by measuring the resistance in both directions in each case.

Tom

Lee in Montreal · 9th January 2010, 04:58 PM

Thanks for the reply. after some research on the all mighty internet, I started doubting the "Motorola" transistors on the boards. I may be paranoiac here, but the welds on the back of the amp board indicates the transistors are not original. As the above picture shows, transistors may have been replaced at some time with Motorola units. Knowing there was a lot of fake Motorola transistors, it is possible the ones installed are fake and not up to the task, which could indicate a short somewhere.

Counterfeit Transistors

405man · 11th January 2010, 07:46 AM

I would remove the 6 suspect output transistors and then fit 2 known good ones, you can then power the amplifier up with no load connected and check that the DC output voltage is zero, if it is connect a signal to the input and measure the AC output and you should be able to see the output signal if everything is well you can listen to the amplifier with only one pair of output transistors, you then have in effect a low power 606 very similar to a 306 you can then replace the other output transistors.

Stuart

jaycee · 11th January 2010, 08:22 AM

You might also want to check the driver transistors on that channel - those are the TO-220 devices that have the "Harris" logo on them. MJE15032/3 should be a suitable replacement if they are damaged.

Mooly · 11th January 2010, 08:35 AM

And always power up with a 100 watt bulb in series with the mains to stop it blowing outputs etc until you are sure it's fixed.

Aldotin · 11th January 2010, 03:54 PM

Quote:

Originally Posted by jaycee

You might also want to check the driver transistors on that channel - those are the TO-220 devices that have the "Harris" logo on them. MJE15032/3 should be a suitable replacement if they are damaged.

Sorry to disagree, maybe the newer MJE15032 won't cause any problem, but the correct replacement device is the TIP41C!

stainless · 13th January 2010, 09:49 AM

schematic can be fond at

Quad 405 Information Page

(bottom of page)

Lee in Montreal · 13th January 2010, 08:31 PM

Problem solved.

The driver transistors (T7/T8 - 408729.99) were burned and took the first two output transistors with them. T7-T8 were replaced by better/faster units. I replaced all output transistors with new ON-MJ15003.

As the new drivers were faster than the originals, I replaced them on both boards.

Otherwise, there were no burn sign on the PCBs and no burnt resistance.

The power amp now works great and it may receive an update in the Spring. Next step is to replace the four 6,8k µF power caps with four 15kµF units, as well as changing a few old caps on the boards. Maybe changing the original op amp with a BB 604.

Iain McNeill · 13th January 2010, 08:36 PM

watch the inrush current when doubling the input caps and check rating on rectifier.

9th January 2010, 03:05 PM	#1
Lee in Montreal diyAudio Member Join Date: Oct 2009	Fixing a Quad 606 Question from a guy who got an electro technician degree... 30 years ago, but never really practiced as being a creative in advertizing was more groovy. Being very happy with my two upgraded Quad 405s, I decided to buy a Quad 606 with one bad channel, for cheap. - When the amp boards are disconnected and I power it on, no fuse problem. - When I connect the amp boards, it burns one of the 3 Amp fuses. If I switch the amp boards side-to-side, the burned fuse follows. Meaning the problem is with one of the amp boards not with power supply (no hum either fro the transformer). - None of the resistors or caps on the board show a sign of being burned. Maybe one of the power transistors is shorting. Can they be tested while still on the board, and how? Would a simple Ohm-meter be enough? BTW I have two dozens of brand new Motorola MJ15003G transistors just in case...

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9th January 2010, 04:21 PM	#2
gootee diyAudio Member Join Date: Nov 2006 Location: Indiana Blog Entries: 1	Good news: Since you have two identical boards, with one working and one not, you should be able to compare some type of measurements made between corresponding points on each of the two boards, and find the problem. I would probably start with resistance measurements, made while the boards are completely disconnected. I don't know what voltage your meter applies, to measure resistance. So I have to suppose that it is possible that it is high-enough to "turn on" a PN semiconductor junction. But if there is a short in one amp board where there is still a PN semiconductor junction in the other amp board, I think you should be able to verify it by measuring the resistance in both directions in each case. Tom

9th January 2010, 04:58 PM	#3
Lee in Montreal diyAudio Member Join Date: Oct 2009	Thanks for the reply. after some research on the all mighty internet, I started doubting the "Motorola" transistors on the boards. I may be paranoiac here, but the welds on the back of the amp board indicates the transistors are not original. As the above picture shows, transistors may have been replaced at some time with Motorola units. Knowing there was a lot of fake Motorola transistors, it is possible the ones installed are fake and not up to the task, which could indicate a short somewhere. Counterfeit Transistors

11th January 2010, 07:46 AM	#4
405man diyAudio Member Join Date: Dec 2003 Location: Near Glasgow Scotland	I would remove the 6 suspect output transistors and then fit 2 known good ones, you can then power the amplifier up with no load connected and check that the DC output voltage is zero, if it is connect a signal to the input and measure the AC output and you should be able to see the output signal if everything is well you can listen to the amplifier with only one pair of output transistors, you then have in effect a low power 606 very similar to a 306 you can then replace the other output transistors. Stuart

11th January 2010, 08:22 AM	#5
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	You might also want to check the driver transistors on that channel - those are the TO-220 devices that have the "Harris" logo on them. MJE15032/3 should be a suitable replacement if they are damaged.

11th January 2010, 08:35 AM	#6
Mooly diyAudio Member Join Date: Sep 2007	And always power up with a 100 watt bulb in series with the mains to stop it blowing outputs etc until you are sure it's fixed.

13th January 2010, 09:49 AM	#8
stainless diyAudio Member Join Date: Feb 2007 Location: western aust	schematic can be fond at Quad 405 Information Page (bottom of page)

13th January 2010, 08:31 PM	#9
Lee in Montreal diyAudio Member Join Date: Oct 2009	Problem solved. The driver transistors (T7/T8 - 408729.99) were burned and took the first two output transistors with them. T7-T8 were replaced by better/faster units. I replaced all output transistors with new ON-MJ15003. As the new drivers were faster than the originals, I replaced them on both boards. Otherwise, there were no burn sign on the PCBs and no burnt resistance. The power amp now works great and it may receive an update in the Spring. Next step is to replace the four 6,8k µF power caps with four 15kµF units, as well as changing a few old caps on the boards. Maybe changing the original op amp with a BB 604.

13th January 2010, 08:36 PM	#10
Iain McNeill diyAudio Moderator Join Date: Oct 2007 Location: Santa Cruz, California	watch the inrush current when doubling the input caps and check rating on rectifier.

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