QUAD303

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21-Feb-2020 06:37 PM

LOWTHERMAN62

LOWTHERMAN62

Posts: 83

I have bought an old in good condition 303 with series 5 boards and have revised.

1 On switch on I am getting  heat and smoke from the area around the choke and the two o.3 ohm wirewound resistors on the left channel driver board, one of these is heating quickly.also the transformer makes a noise. needless to say I switched off immediately, Fuse is not blown. I have limited my checks so far to the driver board and all resistors diodes transistors seem to read ok.

I may consider replacing with 2 x series 10 boards ,Does anyone have a couple in working order?

Can anyone give me a starting point to start diagnosing the problem on the left channel driver board. I am getting a tranformer noise on switch on.

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24-Feb-2020 02:49 AM

EJP

EJP

Posts: 1742

Shorted power transistors, probably both of them. You can check them at the board via the wires at the back: reading from right to left, pins 2,3,4 are C,B,E, and so are pins 6,7,8. Both transistors are NPN. Replace with MJ15003G. You should do the same on the PSU board, pins 2,3,4: replace this one with MJ802.
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24-Feb-2020 08:17 PM

LOWTHERMAN62

LOWTHERMAN62

Posts: 83

Hi EJP. 

Thanks for the reply, You were on the money . 2 x RCA38494 short circuit. 

Will order 4 no mj15003 and 1 no mj802 and fit. Will keep you posted.

Regards S

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24-Feb-2020 10:00 PM

LOWTHERMAN62

LOWTHERMAN62

Posts: 83

Further to checking this 303 serial no 567. I notice that there is no capacitor between the output of the transformer as in later models, My questions are :-

What is the purpose?

Do I need ti install one?

What value and type is best here?

Looking at more recent diagrams it looks like a 0.03uF disc (ceramic type) (C4) 

Regards S




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25-Feb-2020 02:10 AM

EJP

EJP

Posts: 1742

It's called a snubber, and it is there to suppress rectifier spikes. It must be of type X2 and very generously rated for voltage, I think 1000V.
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