Quad 50D Amplifier Distorting

Oh dear Marc!

May I suggest that you make a voltage map of of the bad board versus the good board, to see where the differences lie? There's not that many components to measure and it might well reveal the area where the trouble lie's.

All the best ,

Tony

Posted by: Boater Sam

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.

I actually substituted the bridge out of pure desperation this afternoon with no effect on the HT. I don't have another cap to hand apart from the 6800uf but that said both the 15000 & 6800 work perfectly fine in the good unit so why not in this amp ? It's all very odd indeed.

Posted by: Boingy

Oh dear Marc!

May I suggest that you make a voltage map of of the bad board versus the good board, to see where the differences lie? There's not that many components to measure and it might well reveal the area where the trouble lie's.

All the best ,

Tony

 

Hi Tony,

The fault doesn't lie within the board, I swapped the PCB's around but the fault stays firmly with the bad chassis. I have compared voltages of both amps and all match with the exception of the 59v HT in the bad chassis.

I also checked the secondary windings on the output TX and they read 2.1 and 3.2 over the 8 windings which would appear correct.

I've even looked at insulation of the output pair to make sure the heatsink isn't in some way conducting and all that is fine.

This amp is getting beyond a joke,  If I wasn't seeing this with my own eyes I would have trouble believing it ! I even stepped away from it this afternoon just to save my sanity.  ?  

Oh no Marc, I was really hoping that replacing the output devices would solve all. What a pain, and how strange? It don't make sense. The capacitance of 6800uf will be fine, this is not the issue. Just wish I could offer some useful help. Is the cabling all intact?

I think I'm heading for last chance saloon here, the only thing I can think of that hasn't been absolutely proven or swapped is the output TX. Now despite taking readings that suggest the TX is healthy I think I'm going to have to bite the bullet and swap it from the other amp ans see what happens. The only other thing left that hasn't been exchanged is the wiring harness ! 

EDIT.... just seen your post Andrew, I see you're thinking along the lines of the harness too. It looks good and clean but what's it like once it's untied and split to singles ?....   

I DON'T HALF PICK THEM DON'T I  ? 

I was kind of wondering if there was a break somewhere in the loom, just worth checking continuity?

Posted by: PYE625

I was kind of wondering if there was a break somewhere in the loom, just worth checking continuity?

I already have (when I did the PCB swap) and no problem but I'm wondering if there is any possibility that the insulation has gone through at some point causing a short with another lead or chassis.

Have you actually tried performing the set-up procedure for the output pair, Marc?

Posted by: Cathovisor

Have you actually tried performing the set-up procedure for the output pair, Marc?

Yes, followed the book.

Posted by: Marc

Posted by: Cathovisor

Have you actually tried performing the set-up procedure for the output pair, Marc?

Yes, followed the book.

And I assume you can get the correct value of collector currents, and matched?

Set to 35mA (half way of suggested 30 to 40)

Just nipping out to carry on banging my head against the wall.... ? 

Hi Marc,

I know you're fed up with it, but I've a couple more thoughts for what they're worth.

Cathovisor said about the BBC wiring XLR connectors in their own way.  I can see that the output is wired pins 2 and 3, instead of the more usual pins 1 and 2. If there's a "standard" wired XLR plugged into that socket, the only load presented to the amp would basically be the speaker connected via one of the tiny capacitors on the XLR socket, so no load and no current drain on the output trannies, so high HT.

If you've connected up the load to pins 2 and 3, then please feel free to tell me to shut up 😉

Another thought is that the feedback circuit is fed off the output transformer and enters the board at pins 4 and 5. With no signal to these pins, it would simulate a shorted speaker and the feedback circuit would cut off drive to the rest of the amp, so again, HT would rise.

Last one, I promise.

Thermal stability is provided by thermistor R18, with is connected via pins 6 and 7 on the board. Maybe something there?

Hope this helps Marc,

All the best,

Tony

Any help or suggestions are very welcome Tony and yours and others time helping is appreciated.

The XLR's are wired pin 2 & 3 and the plugs to sockets are matched, the speaker leads were used for years with these amps. I actually disconnected the caps on the XLR chassis socket to rule out any problem there.

Just to point out that the amp does have sound and can certainly give some good volume despite the HT fault.

I will look at the feedback circuit as you mentioned above and check it through but as the amp has drive this could be doubtful.....but with this amp who knows !

As for the thermistor again no, I disconnected it and used a 10K directly to the PCB pins 6 & 7 just to check.

I think my teeth are getting smaller and I'm sure I can hear a grinding sound too !  ? 

Hi all,

I have now checked the feedback circuit as Tony suggested and all is well compared to the good amp.

I have revisited the set up procedure and again all is well, RV1 = 5.5v,   RV2=35mA,  RV3=35mA. 

I know the PCB works fine by substitution from amp to amp and the output pair are now new.

The mains TX is producing 45V AC which is correct, the +20, +5.6 & -10 rails are fine but that dreaded HT is still 59v instead of the correct 38v

I really can not see how this HT fault is possible unless caused by a faulty output TX despite it's good readings or the wiring loom has a short within in it....Other than that I'm totally lost !

Is this HT fault buried within the loom ?

Get this.....I have found the fault !!!!!  ?   ?  ?  ?  ?  ?  ?   

L1 is the flipping (polite phrase) culprit but it tests fine. It measures 1.7 ohm across it's winding which is exactly the same as the choke from the good amp, but swapping chokes moves the fault to the other amp.

Talk about "headless chickens" this amp has sent me right up the garden path, round the bend and back again many times. L1 was checked very early on in this thread and yet I've ended up right back there again !

So, new problem....where the heck do I get a replacement choke from ?  

Posted by: Marc

So, new problem....where the heck do I get a replacement choke from ?  

As its faulty is it worth investigating as to why, you've nothing to lose by deconstructing. Have you tried ringing it? Depending on what you find, rewind it. Or I suspect If you have a word with Ed he'll do it for you, I'd be tempted to have a go myself, can't be that hard. Just need the correct wire and the ability to lay the turns and count them. Everything is there just need a micrometer and reverse engineer.

You must be relieved to have finally nailed the culprit, I hate faults likes this, all they do is sap your confidence, I've had a few of those lately.

Hi Chris,

I'm wondering if the choke failure was caused by the 15,000 res cap or perhaps as this is an ex BBC amp it's just simply had a hard time and given up.

I was looking through eBay for a replacement but I don't know the value of this choke and it isn't mentioned in the data either, anyone any ideas ?

Rewinding is probably the way to go if a replacement isn't available.

And yes Chris you're bang on right about the relief I now feel, this amp really had me questioning my confidence.

Posted by: Chris

 I hate faults likes this, all they do is sap your confidence, I've had a few of those lately.

They are however very educational.

Posted by: Cathovisor

 

Posted by: Chris

 I hate faults likes this, all they do is sap your confidence, I've had a few of those lately.

They are however very educational.

 

And good for hair loss too !!!

Blimey, what a pig?  Of all the things you would least suspect......