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Quad 50D Amplifier Distorting

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Marc
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Hi all,

My latest project is yet another 'Quad' distortion problem but this time with one of my 50D mono block amplifiers. I am led to believe that this unit is ex BBC which judging by all the inlet/outlet and mains sockets being 'Cannon XLR' types I wouldn't at all be surprised. Any advice as to where to start with this amp would be very useful, I've also made a request for any service data that may be available too. 

Cheers, Marc.

DSCF4492 (Medium)

 

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Posted : 21/04/2018 8:38 pm
Cathovisor
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The real giveaway that it's an ex-BBC amplifier is the BBC Coded Equipment designation of AM8/11.

It's also been modified in that it has lost the more usual Plessey '159' input/output connectors and had them replaced with XLR connectors, but still in the BBC 'reverse' configuration - the normal convention for an XLR is "signals on pins" but the BBC took exception to that and thus reversed it.

Now, my memory may well be playing tricks on me now but from memory the 50D was designed with 100V line working in mind, which is why it had the '159' connectors so you could pick off the appropriate taps on the output transformer. The low impedance version was the 50E.

I'll ask around if anyone has a diagram for it.

 
Posted : 21/04/2018 8:54 pm
Marc
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Posted by: Cathovisor

The real giveaway that it's an ex-BBC amplifier is the BBC Coded Equipment designation of AM8/11.

I also seem to remember that the serial number plates on Quad's supplied to the BBC were red instead of silver as in this particular case "Red 7414"

 

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Posted : 21/04/2018 9:03 pm
crustytv
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Posted by: Cathovisor

 

I'll ask around if anyone has a diagram for it.

I've responded to Marc's request and just uploaded the data. Doubt it covers BBC mods though

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Posted : 21/04/2018 9:11 pm
PYE625
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Hi, I never worked on a 50D/E, but would suspect like the 303 it will have dried up small electrolytics and possibly intermittent preset pots. Otherwise, the drill will be similar to the 306 by taking measurements and scoping the signal path.   ? 

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Posted : 22/04/2018 7:49 am
Cathovisor
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I can't remember how close a 50 is to a 303 (other than being half of one, with big transformers) but I remember a 303 came into me at work with very low power and it turned out to be an auxiliary voltage rail gone low.

As ever, break the voltmeter out first.

Note of trivia: there were some now much sought-after floor monitoring loudspeakers used by the BBC when I joined. These were the LSU/10 and the LS3/1. The latter were fitted with a BBC valve amp called the AM8/4, loosely based on the Mullard 5-20. Whenever the valve amps developed excessive hum, they were removed, marked for transfer to BBC Redundant Plant and a Quad 50E fitted in their place. I did quite a few of those.

 
Posted : 22/04/2018 9:29 am
Marc
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Hi all,

Andrew, my thoughts were caps too although this amp appears to have had them replaced.

Catho, I've got my Fluke ready for action  ? 

One thing I did notice was a crumbling C10 (220n) tropical fish cap as you can see in the photo below.

DSCF4493 (Medium)
DSCF4494 (Medium)

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Posted : 22/04/2018 3:02 pm
crustytv
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Not familiar with Quads at all but that PCB looks a little  cooked at the bottom of the photo ( Top of the PCB) is that normal for these? Guessing that resistor gets a tad warm, probably dried those black electrolytics out too.

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Posted : 22/04/2018 3:24 pm
Marc
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Hi Chris,

Yep it's certainly a bit on the cooked side where that large resistor is in fact a piece of the PCB has just broken away right where it clips back to the chassis  ☹️ so I'll have to try and figure a way of mending or adapting that now.

I've managed to cobble together a set of leads to put the amp on test so I shall report back later. 

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Posted : 22/04/2018 3:38 pm
PYE625
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Looks like an OC75 in MR2 position, probably ok, but germanium and heat aren't the best of friends.

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Posted : 22/04/2018 4:15 pm
Marc
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Tried the amp out and it's silent and main rail voltage is way too high at +59v (should be+38v) and R32 (the big 180R) is getting very hot

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Posted : 22/04/2018 5:07 pm
Cathovisor
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My immediate thought is that the output devices aren't drawing any current, hence the high HT (which in turn won't half make that 180R resistor sweat (do the maths) - it might be worth checking there's volts on the collectors of the output devices. That should at least exonerate the output transformer. Silence is not a good thing here!

Even in its distressed state, I'd also check that there's 5.6V at the other end of the 180R resistor.

The other thing that's not helping is that a previous owner has fitted an electrolytic way in excess of the designed value (three times the original): this will raise the HT as well as putting a strain on the rectifier.

I'm reminded of an old adage here: Trust no-one's work but your own, and even then check it.

 
Posted : 22/04/2018 6:49 pm
Marc
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Hi Catho,

The collectors of the output transistors are both 59v and I have 5.6v at the other side of the 180R.

I didn't notice the main cap was so high (15000 uf) ! I better find something more suitable for the amps future. 

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Posted : 22/04/2018 7:17 pm
occiput
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I agree that the symptoms are of the output pair doing nothing useful. 

My recollection of these amplifiers is that the half-primaries have straps to the HT line to allow the standing current to be set up (using the two pots on the p.c.b.)

Now, victories don't usually come this easily, in my experience at any rate, but might a previous miscreant have removed the straps and not replaced them, I wonder?

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Posted : 22/04/2018 7:23 pm
occiput
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Posted by: occiput

 

My recollection of these amplifiers is that the half-primaries have straps to the HT line to allow the standing current to be set up (using the two pots on the p.c.b.)

 

As you were - clearly not.

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Posted : 22/04/2018 7:23 pm
Boingy
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Hi Marc,
Is there 20V at the emitter of the regulator transistor TR11? Maybe the excessive HT has damaged that part of the circuit, which would be one way to account for no sound?

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Posted : 22/04/2018 7:34 pm
Marc
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Posted by: Boingy

Hi Marc,
Is there 20V at the emitter of the regulator transistor TR11? Maybe the excessive HT has damaged that part of the circuit, which would be one way to account for no sound?

Tony, you're very close to the answer.... There was a hairline crack in the track to TR11 collector. The amp is now back to life BUT.....the HT rail is still high, I've wound down the Variac to 160v to maintain the correct +38v. So why is the HT so high, could that be the 15,000uf main cap to blame ?

EDIT, just found a NOS 6700uf 50v cap, Not perfect but I wonder if that would help matters

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Posted : 22/04/2018 8:06 pm
Boingy
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Posted by: Marc
Very close to the answer ! There was a hairline crack in the track to TR11 collector. The amp is now back to life BUT.....the HT rail is still high, I've wound down the Variac to 160v to maintain the correct +38v. So why is the HT so high, could that be the 15,000uf main cap to blame ?

What's the AC voltage across the input of bridge?
From what I can remember from basics, for 38V DC, it should be in the region of 27V, i.e. 38V/square root of 2. Also, check the -10V line to see if that's more negative.
If both of the above voltages are greater than they should be, maybe check the mains voltage setting?
If those voltages are correct, then maybe the cap is the problem.
All the best,
Tony

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Posted : 22/04/2018 8:25 pm
Marc
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I can confirm the mains tapping is correct but unfortunately I can't do any further checks tonight but I will check the PSU voltages tomorrow.

However just to fill any gaps in the story these amps were in regular use by me for a few years until this one started distorting and they were shelved until now. It's obvious that someone else has been there before me and since Quad but that said they did work well. I've a good mind to take the voltages from the other amp and compare them, I wonder if that's running excess HT as well ? 

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Posted : 22/04/2018 8:46 pm
Boingy
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Comparing voltages with a known good amp is an excellent idea, Marc 🙂
It would be interesting to see what you find out.

All the best,
Tony

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Posted : 22/04/2018 9:16 pm
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