303 driver board voltages

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13-Jul-2018 10:54 PM

Ray

Ray

Posts: 16

Hi

I have just done a 303 service with a DaDa kit. I've come to calibrating the voltages and quiescent current and find I have a problem on one driver board.

I can set all the voltages as per instructions on the right channel board. On the left channel I have 67 volts between pins 1 and 9 but I can only get around 14 volts between pins 1 and 5, with RV100 having no affect in any position, and, while I can initially set 11mV between pins 4 and 6 with RV 101, each time I go back and check it has dropped to around 7mV.

 When I got the amp the left channel wasn't working and I hoped the service would cure it. Worth doing anyway of course, but it hasn't fixed it. Can you suggest where I need to look to find the problem?

Many thanks

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14-Jul-2018 05:23 AM

EJP

EJP

Posts: 1537

I would start with R130=150K, which often drifts high, then R110=8K2 or possibly 6k8 in some units, and check the polarity of C104 if you changed it. If no joy then check the voltage between pins 6 and 7. If it is zero, the entire top half is off, so investigate Tr103/5 and then Tr1, and their surrounding resistors.
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14-Jul-2018 09:00 PM

Ray

Ray

Posts: 16

Thanks for your reply EJP.

I have removed and checked the resistors you mention, all measure ok, and removed and tested all the transistors on the board, plus TR1, with a digital meter, again, all test ok. The problem remains.

On the working board I get around 33 volts between pins 6 and 7 but on the faulty one I get around 58 volts.

Any further suggestions would be appreciated.

Many thanks

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15-Jul-2018 03:59 AM

EJP

EJP

Posts: 1537

Well that's a bigger error than you had before, and in the opposite direction, right?

If you've checked all that, see if it goes away when you unplug the input. If so, the input capacitor C100 is leaky. This is a bit subtle. The base of Tr100 is held at about 7.5V by the voltage divider but if the capacitor is leaky it can be pulled down by the output impedance of the input device, and as the 303 inverts, this raises the output DC level.

Otherwise you now need to check the bottom half of the output stage.

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15-Jul-2018 01:10 PM

Ray

Ray

Posts: 16

I haven't previously measured the voltages between pins 6 and 7 so it's not a bigger error as such. I don't know what that measurement is supposed to be. As before, I'm getting 9 to 14 volts between pins 1 and 5 and around 7mV between pins 4 and 6.

I'm not an electronics engineer, and I'm no amplifier expert. I can use a soldering iron and a meter and follow instructions, which is why your kits are so useful, so you start to lose me with terms like 'top half' and 'bottom half'.

All the components from one of your kits have been replaced, including all the capacitors, and I have checked the polarities. All the transistors on the board have been checked and appear ok, as does TR1. Apart from that I'm in the dark.

Should TR2 be the next to be checked?

Thanks again.

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16-Jul-2018 01:27 AM

EJP

EJP

Posts: 1537

I can only further advise you to send it in for servicing.
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