Quad 50D Amplifier Distorting

What is the voltage at the collector of TR1o w.r.t the zero volts line?

Sorry, that ought to say "TR10".

While you are about it, what are the voltages at the bases of TR4 and TR5, and are they controllable with the pots?  (this is how the standing current in the output stage is set).

Hi Occi,

TR10 collector is +5.8

TR4 collector is -10 and base is varying with pot

TR5 as TR4

Hi all

Just been doing some checks and I have a question...

The output pair (RCA 40411) have an hFE of 23  for TR8 and 44 for TR9.

After a search on Google I found data stating min hFE 35 and max hFE 100 for these devices so how much of a problem would TR8 be ? 

Posted by: Marc

Hi Occi,

TR10 collector is +5.8

TR4 collector is -10 and base is varying with pot

TR5 as TR4

I forgot to say that the voltage at the base of TR4 & 5 was 0.6 and variable 0.3 to 0.8

Applying Ohm's Law to the network R14/R15/RV2/RV3/R16 will tell you whether that range of variation is reasonable.

Since RV2 and RV3 set the standing current in the two halves of the output stage, you ought to see the current drawn by the output stage from the +38V supply vary as you adjust the pots.  The value you measured in the other amplifier suggests that the standing current in TR8 and TR9 ought to be about 95mA each.  It might be instructive to see if you can achieve this by adjustment of RV2 and RV3, and,if you can, what the voltage on the +38V line then is.

I also suggest that you change C12 so as to restore the correct value (or, as you will probably have to, 4700 microfarads).

C12 has been replaced with a 6800 uf not ideal but it's all I have to hand and has to be better than 15,000 uf that was fitted. The HT voltage still remains at 59

More checks to come....

Hi all,

Sorry for the large gaps in reply's but I'm having to juggle hospital visits and organising to get my mum back home from there, She manage to fracture her hip but luckily it only required pinning and a short stay.

Back to the amplifier.... I've been searching all over for faulty components on the PCB and found nothing, so just to confirm my suspicions I've swapped the PCB over from amp to amp and the fault has stayed with the original unit whilst the other sings away merrily. So the fault is on the chassis somewhere.

The only chassis mounted components are...

mains TX, which is delivering 45vac.

Output TX....I hope this isn't the fault !

L1 (shows no drop of voltage)

bridge

res cap (newly fitted 6800 uf)

2X 40411 transistors

Plus wiring & connectors 

Just tested the output pair using a Peak DCA55 and one of them is showing leakage of 0.04mA the other zero

Both output TX primaries are 6 ohm, so at least even.

Hi Marc, it looks like the output devices are under scrutiny then....unlikely to be anything else I would dare say.

Hope your mum will soon be leaping and dancing around !   ? 

Posted by: PYE625

Hope your mum will soon be leaping and dancing around !   ? 

Thanks Andrew, I've already booked a piggy back ride from mum though me at 6'1" 16 stone against 5ft and 7 stone might be fun ? 

I'll have to order some transistors by the look of it, RS do some MJ802G which are supposed to be equivalents of the 40411.

I do have some MJ15003 but I'm not sure if they would be suitable, any thoughts ?

Missed the post about your mum, best wishes for her.

Sorry nothing to add for the Quad amp, never worked on one but trying to follow the thread.

Posted by: Nuvistor

Missed the post about your mum, best wishes for her.

Thanks Frank, she's recovering well and quite quickly all being considered.

Marc, I'm more or less certain that the MJ15003 will be perfect, but replaced as a pair.

Thanks for that Andrew, I'll give them a try tomorrow. 

EDIT...just checked the pinout of both devices and they are the same too. ? 

Hi Marc,

Glad your mum's recovering well. Good luck with the output trannies. 

All the best, 

Tony

Posted by: Boingy

Hi Marc,

Glad your mum's recovering well. Good luck with the output trannies. 

All the best, 

Tony

Cheers Tony, I've got my fingers crossed...on both counts.

Well I reckon it's time to bang my head on a brick wall...new output pair fitted and still 59v HT !  ? 

A leaky diode in the bridge?

Reduce the capacitor to something nearer to the correct value, if your 6800uf is on top tolerance it is still well big.