Quad 303 overheating

Jump to Latest
#2
Hi, it's a resistor. Someone who is very intimate with the amp may be able to tell you more, but, I think it would be most useful if you could post some more pictures from a little farther away in order for the rest of us to have a better chance of seeing where it might be in the circuit
 
#3
Last edited:
#4
There must be a lot of heat as your circuit board also has a scorched area on it. You did not twiddle any adjustments I hope. The amplifier is drawing a lot of current which it should not be doing under quiescent conditions. If you don't have hands on experience with electronics Quad have a repair service.
 
#6
My last two repairs by Quad themselves, were far from satisfactory. I am afraid the repair and service seems to have slipped a lot from the best in the business, which it was in the Peter Walker era. If you are in the UK, I can unreservedly recommend CPT Acoustics. Colin Toogood has never done anything but a first class job for me and his prices are reasonable. No connection with him other than being a very satisfied customer. Colin is very familiar with all legacy Quad amplifiers.
 
#7
I would start by buying a Dada kit or replace all the old resistors and capacitors (could be out of spec or dried out). Also the the three pots oxidate and need to be replaced. As far as I remember the resistor you're talking about is known to cause problems (overheating, too low a wattage).
 
#8
wilsonlaidlaw said:
My last two repairs by Quad themselves, were far from satisfactory. I am afraid the repair and service seems to have slipped a lot from the best in the business, which it was in the Peter Walker era.
Click to expand...

I think the same can be said about the majority of what were once great British Hi - Fi companies.
Back in the day when almost everything was done in - house employees were enthusiastic and took pride in their work ,if there was a problem with a product they could literally go 'next door' and ask the designer for advice.
With most of these guys now retired or passed away and the majority of companies now owned by huge conglomerates whose only concern is the bottom line there isn't much left in the way of pride or enthusiasm , nowadays its ' that'll do ' and ' is it time to go home yet ? '
 
#13
You need to check that the regulated supply voltage is correct at 67 volts. If that has risen because of a fault (and/or just the resistor) then the power amp will run to hot.

The procedure is detailed in the service manual.
 

Attachments

  • Capture2.JPG
    Capture2.JPG
    55.3 KB · Views: 261
#16
Looking at the circuit in post 13 Tr200 could be driving TR201 too hard because RV200 trimmer resistance is not set correctly.

There is a significant heat sink on TR201 - The hope is there is a good chance it can withstand up to 1W of dissipation.

As someone pointed out this circuit is a shunt regulator which pulls down on the unregulated supply - so an incorrect trimmer setting can increase the dissipation from TR3 causing further heating and a reduction in the regulated supply voltage which should be 67 V.

I suspect you will find a lower voltage than this - you should measure and note what you have.

If not correct as a first step - with the power off I suggest you measure the resistance between board connection point 12 and the base of TR200 - then mark the point on the black part of trimmer body on RV200 in line with the white adjustment slot for reference. (You can stick a little masking tape to the black body and mark the reference with a ballpoint pen.)

TR200 conducts increasingly when the emitter has a more positive voltage on it than at the base - to reduce the conduction in this circuit the base needs to be made more positive which means making the resistance you just measured lower in value.

You can see what direction of rotation you need to take to do this by moving trimmer however far is necessary and return this to the marked original position.

After replacing R206 measure the voltage at connection point 15 to see if the value has changed. If not then gently turn the trimmer in the direction indicated by the experiment mentioned above while monitoring the voltage at connection 15.
 
You must log in or register to reply here.
Top Bottom