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...
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...
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
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
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
Counterfeit Transistors
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
Stuart
Sorry to disagree, maybe the newer MJE15032 won't cause any problem, but the correct replacement device is the TIP41C!
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.
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.
Sounds like classic output transistor short.
If BJT check CE with an ohmmeter for a short.
If MOSFET check SD with an ohmmeter for a short.
Its also worth checking any associated resistors for going open circuit just in case the short has taken them as well.
You are probably best starting a new thread for this...
The outputs should not be hot under quiescent conditions, if the heatsink is to hot to touch you have a problem, either DC bias issue or instability.
It was the early Quad 405s that had the facility to insert a 1k8 limiting resistor. On later Quad 405s a wire link was inserted.
Maybe that was what you were thinking of?
On the 606, the only form of protection (obviously, other than mains fuses) appears to be a 2A resettable breaker, referred to on page10 of the manual. Shown on the drawing in series with the primary of the mains TX.
Page11 of the manual has a paragraph (a load of woffle, basically) about speaker protection. To sum up; there isn't any!
Maybe that was what you were thinking of?
On the 606, the only form of protection (obviously, other than mains fuses) appears to be a 2A resettable breaker, referred to on page10 of the manual. Shown on the drawing in series with the primary of the mains TX.
Page11 of the manual has a paragraph (a load of woffle, basically) about speaker protection. To sum up; there isn't any!
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