advertisement


Quad 306 distorting on one channel

T

tmf

Active Member
Hi I just got a 306. It has had a service with upgraded reservoir caps and changed input sensitivity, and everything looks fine inside.
The problem is that it distorts on one side (left) after a while and at reasonable levels. Turning it off and on solves it for a while and I went through a period of thinking that fiddling with the speaker connection helped as well, but not so sure about his any more.
Any thoughts? I'd be grateful for suggestions!
 
Fixed!
I've fixed the amp and am just leaving the details here in case they are at all useful for anyone searching in the future.
The distortion started happening more quickly after the amp was turned on, so I started to think a perhaps a component was on its last legs. I measured all the voltages on the circuit diagram for both channels and they were all about right apart from the -111mv output from the opamp which was about -16mv on the rhs and -46 on the lhs. Swapped the opamps round, but the readings were the same and the distortion was still there. I read somewhere someone suggesting prodding components to try to identify any that might be failing, so I got a wooden chopstick out and had a go. Applying pressure to T8 seemed to have an effect, so I bought two 40872 replacements from the wonderful Cricklewood electronics (they are labeled 1C04), put them in and problem solved!
One question: is there anything to be gained by fitting 'better' transistors? or just leave it alone. Thanks!
 
To be honest, the 306 is a sweet sounding little amp, and leaving it pretty much as it is would be the best bet.

It will also help maximise resale value if and when the time arises.

Another tip to identify intermittent component faults is aerosol freezer spray, with a long nozzle.
 
Great. Thanks for the tip and the advice. After feeling pleased with myself for fixing it, there's a temptation to keep fidling, but I think you are right - leave it and play some music.
 
Yeah agree with the above the 306 is a very good and transparent amp on its own so if its working well leave it alone

Alan
 
IIRC Rob stuck slightly larger PSU caps in mine, which doesn't alter the sound but does give it a bit more dynamic headroom (not that I need it with such efficient speakers). It's a very nice little amp IMO. A real sleeper in the marketplace.
 
Alan, I don't know you were a tuner expert too! Now I've got this lovely Pioneer Tx-9800...

Was it just a power supply problem or similar with the Quad?
 
Well Im no expert but it was just the 47nf X2 Rifa Cap accross the mains had bulged and cracked and failed short popping the fuse, common problem and easy to spot,
I had a spare X2 cap and fuse so its working fine now except that the memory back up battery is leaky so needs changing, another common problem with the FM4 so I have removed it and the new one has just arrived from ebay so will be fitted tonight

Alan
 
I also have a Quad 306 that is distorted on the left channel.

Can tmf tell me if he changed T8 on both channels or T7 and T8 on the left channel.

Do any Quad 306 users ( might include 405, 606 and 909 too ) know if this is a common problem. Is it some issue with the 40872 or thermal management?
 
I decided to further investigate the distortion by looking at the back of the PCB with a scope. Imagine my surprise when I discovered that it worked perfectly when upside down.!!!

Further investigation showed that the base and emitter lead of T8 were broken and that when upside down the weight of the transformer remade the connections!

My 306 was previously used for exhibitions, so my guess is with rough handling the weight of the transformer flexes the PCB and in time this breaks the leads. T8 left is closest to the transformer.

A replacement transistor from Cricklewood Electronics has fixed the problem, according to them the 1C04 is just a renumbered 40862 , can anyone confirm this??
 
johnroe said:
I decided to further investigate the distortion by looking at the back of the PCB with a scope. Imagine my surprise when I discovered that it worked perfectly when upside down.!!!

Further investigation showed that the base and emitter lead of T8 were broken and that when upside down the weight of the transformer remade the connections!

My 306 was previously used for exhibitions, so my guess is with rough handling the weight of the transformer flexes the PCB and in time this breaks the leads. T8 left is closest to the transformer.

A replacement transistor from Cricklewood Electronics has fixed the problem, according to them the 1C04 is just a renumbered 40862 , can anyone confirm this??
Click to expand...

Nice one, good work :)
 
davidsrsb said:
So you are saying the actual transistor leads break, not the solder joint?
Click to expand...

Yes, It happened on mine too. I was getting a crackling sound In the left channel when I stamped on the floor. There was a hairline crack through the transistor leg (the one nearest the transformer) so I just went over It with some solder.
 
fatmarley said:
Yes, It happened on mine too. I was getting a crackling sound In the left channel when I stamped on the floor. There was a hairline crack through the transistor leg (the one nearest the transformer) so I just went over It with some solder.
Click to expand...

Wow...that must have taken some detective work finding break!!!
 
You must log in or register to reply here.


advertisement


Back
Top