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Quad 33/303 erratic behavior

I wonder if anyone has any insight into this problem: my Quad has the most wonderful sound when I turn it on, but then the sound inexplicably becomes soft and filled with static. If I unplug the power lead from the back of the amp and then plug it in again the problem is corrected for a short time, but recurs many times. After a while this misbehavior seems to stop. I have had the unit at a local Quad "expert" several times, with no success in resolving the problem, even though he states that he replaced all connectors.
Any help will be greatly appreciated.

Chriquet's picture

Quad 33/303

The most common problen I have encountered with an elderly 33 is tarnished switches. Unreplaceable and difficult to get at but generous hosing down with Caig DN5 can sometimes do the trick. These switches are often unused, esp the Tape Monitor but the signal still goes through them. Give them regular exercise. The pots would probably benefit from a squirt as well. Replace all the coupling caps on the daughter boards as a matter of course. Clean the sockets and board fingers... ditto on the tape board. If using a disc player, attenuate the input with a divider. The 33 is way too sensitive on its radio inputs and is easilly overloaded. (only runs on a pissy 12v supply) Actually, its not a particularly good pre-amp so another solution would be to bin it. The 303 won't mind.
Re 303. Again replace caps. Most common fault is the bootstrap cap (C106) Replace with 100uf. Front-end caps also fail (C101/104) Don't get too anal about using the same values, just fit the next standard value up. Check the pwr supply regulator is still functioning. Should have 67V across outermost pins on the pcb. Twiddle the pot on the reg board to check that this voltage actually adjusts.


Correction to the post about erratic quad behavior. Should read "unplug the preamplifier...." not the amplifier.


Hello Colin,

This sounds like it wants a service. Did you test the preamp on another amp ? And did you test the 303 with another
source ? If not, please do so to find out wether the problem is in your pre-amp or main-amp.
but .... This really sounds like that elcos had their best time. Did you ever had it serviced with new capacitors ?
But ... It could also be corrossion. To this category belongs solder joints, connectors that may become bad.
Please first find out which one is giving the problems.

With kind regards,