Bought Altec Lancing and Sugden Monoblocks £100

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Paul Barker
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#46

Post by Paul Barker »

better go to my 2pm job.
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#47

Post by jack »

Any good, low-ESR PSU cap which is a similar rating or higher will do - remember you've got those nice clips on the bases holding the caps down - it'd be nice to get replacements that fit those...

You can read my Quad rebuild at the link above, or at http://www.desmith.net/NMdS/Electronics ... rades.html - the PSU is discussed there a bit.

The caps I used (also 10000uF @ 63V) were BHC ALS30A103DE063 http://uk.farnell.com/jsp/displayProduc ... ku=1572893 - BHC is the same company as Evox RIFA, EPCOS, Cornell Dubilier & Kemet, i.e. absolutely the DB's for audio caps. They are almost exactly the same price as the Mundorfs, except that they have proper screw terminals and there is no doubt about the quality you are getting from a very well known & respected brand...

I found a simpler/earlier better-quality (it's legible for a start!) P51 schematic that seems to correctly reflect the component list I posted earlier - see below. This one doesn't have the output protection relay... Still has the over-current protection etc.

That little board isn't a buffer as I first said - its an NPN common-emitter phase inverter used to invert the right-hand channel when in mono mode so that the two channels can be bridged (LH is in phase, RH in anti-phase) between the two RED speaker posts - there should only be one of these boards.
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Another (simple/earlier) P51 schematic
Another (simple/earlier) P51 schematic
Last edited by jack on Wed May 28, 2014 3:43 pm, edited 6 times in total.
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shane
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#48

Post by shane »

This is quite a find, Paul! Back in 1976 at the height of the Linn/Naim lunacy we had a P51/C51 combo in stock. I don't think we managed to sell it because it simply wasn't de rigeur in those days, but we never managed to explain why it sounded so bloody good either. It's a gem.
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#49

Post by Ali Tait »

Bring them to Owston Paul, be interesting to hear.
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Paul Barker
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#50

Post by Paul Barker »

Shall Ali,

Greatful thanks Nick,

Good to know shane.
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#51

Post by Paul Barker »

nickds1 wrote: I found a simpler/earlier better-quality (it's legible for a start!) P51 schematic that seems to correctly reflect the component list I posted earlier - see below.
Nick I can't make the download from your link legible any way you can get higher resolution to me? My email is in my profile.
nickds1 wrote: That little board isn't a buffer as I first said - its an NPN common-emitter phase inverter used to invert the right-hand channel when in mono mode so that the two channels can be bridged (LH is in phase, RH in anti-phase) between the two RED speaker posts - there should only be one of these boards.
How does this work? Are the left and right outputs supposed to both be connected to the speakers? I haven't done that, so am I just using one channel of each monoblock? Or when I hit the mono switch are they combined internally so only one set of outputs used? Also therefore is one about to blow up? Help?
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#52

Post by Paul Barker »

OK now I am scared. Not sure I been bridging it right. So until I know it's a single stereo amp.

Managed to get one to work as a single stereo amp. So no damage done to that one. Have to report other yet to start up again. About to try it again. But there are relays which kick in after power up, may just take those time. Report in a mo.

Yes I have broken one channel of the other amp. Hope it's a fuse. Now have to look inside for fuses.

good news a fuse is blown and it is on the correct side. I think I did that!

OK from that experience I figured out how it's done. You connect speaker to LH black and RH red and you don't link the other two.

So once I find out fuse value and buy a replacement I can safely monoblock. Phew!

What I was doing inadvertently (no to be fair ignorantly) was connecting to just one amplifier of each monoblock so the other amp had no load. they don't like that do they?

OK it's 32mm 2 amp. I might have a 20mm one so can solder that on the back of the 20mm dud.
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#53

Post by pre65 »

I wonder what it/they would sound like with a valve pre amp.

Perhaps we can try at Owston ? :)
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#54

Post by jack »

Paul Barker wrote:OK now I am scared. Not sure I been bridging it right. So until I know it's a single stereo amp.

Managed to get one to work as a single stereo amp. So no damage done to that one. Have to report other yet to start up again. About to try it again. But there are relays which kick in after power up, may just take those time. Report in a mo.

Yes I have broken one channel of the other amp. Hope it's a fuse. Now have to look inside for fuses.

good news a fuse is blown and it is on the correct side. I think I did that!

OK from that experience I figured out how it's done. You connect speaker to LH black and RH red and you don't link the other two.

So once I find out fuse value and buy a replacement I can safely monoblock. Phew!

What I was doing inadvertently (no to be fair ignorantly) was connecting to just one amplifier of each monoblock so the other amp had no load. they don't like that do they?.
DO NOT replace the fuse. Yet.

If your systems has a relay that clicks a few seconds after turning on, its the later version with the schematic I posted first, not the second one I posted (which was the original). The relay should only turn on if the HT rail get up to voltage and disconnects it on turn off - this stops the annoying "pops" at power on/off. It is most emphatically not DC protection for the speaker.

The annoying thing is that the non-relay version has over-current protection on the output pair, whereas the second schematic has two sets of paralleled output transistors and NO over current protection. Go figure.

It blew for a reason - running the amp with no load shouldn't have any effect at all on it - if the fuse blew, it may be that the channel is dead. Normally this is caused by one of the output transistors dying, though seeing that incinerated 1N5401, the bridge may have failed as well...

When running in bridged mode (pressing the MONO button), the right channel has its input signal inverted and the un-inverted (original input) signal is fed to the left channel.

This makes the RED terminal of the LEFT HAND channel the speaker positive, and the RED terminal of the RIGHT HAND channel the speaker negative.

Its a fairly nasty design, but typical of the period.

When running in normal, stereo, mode, the output of the amp at the junction of VT9 & VT10 swings between about +76V and 0V, but because its not a split power supply and the speaker negative is normally connected to 0V, there is a nominal DC voltage of 38V on the output, so there is a 10,000uF blocking capacitor to keep the DC from destroying your speakers, making the output swing effectively +/-38V.

In bridge mode, when one channel is at +38V, the other will be at -38V and vice versa, giving a swing of +/-76V.

As this is an AC coupled amp, I'd be really careful of your speakers as the output cap may have dried out and died, possibly letting DC onto the speakers. Replacing voice coils is a pain.
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Paul Barker
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#55

Post by Paul Barker »

OK got that.

Before reading what you put
I changed fuse and it blew in a nanosecond.

So one channel of one amplifier dead.

It is rather shocking that I am listening through a 10,000µF Electrolytic capacitor.

All my prejudices now shot to bits.

Regarding who killed it? Well it worked before I started messing about with bridging. So I can't blame anyone else.

Suppose I could order all the caps for this one and a new pair of output transistors, new power diodes. Put 'em in cross fingers and hope for best.
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#56

Post by jack »

Paul Barker wrote:It is rather shocking that I am listening through a 10,000µF Electrolytic capacitor.
In bridged mode, its two actually! They won't have been thinking too much about the sound, more that the impedance of the cap at audio frequencies would be stupidly low (0R8 at 20Hz, under a milli-ohm at 20kHz).

Hope your speakers are ok :shock:

Have to confess that I use huge dummy loads when first testing amps, just in case...
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#57

Post by Paul Barker »

Speakers unaffected.
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#58

Post by Paul Barker »

Out with the electrolytics.

These are one channel of one amplifier.

Image

Marked the board with rating value and polarity.

Image

I'll check the mustard caps and the values of the resistors. I happen to know Mullard Mustard caps sound great. I am shure those resistors are Alen Bradly's, which are as good as I can afford, so if they are in value they are staying, if out of value probably replace with same.

Have ordered two pairs of output transistors. Will change this pair automatically, keep the other pair as spares.

All the other transistors, would be hard for me to test, could get them tested. Is there much risk just hooking it up and hoping they are OK? Wandering what was damaged which caused fuse to blow. Any thoughts on transistors other than output?

BJT's are really not my bag.

Can't help noticing how clean inside the amp is, considering it was probably made in the late 1970's at least 34 years old. Very little dust stuck to the boards. The house it came from was clean they probably hoovered every day.

Second revelation how small and simple the main power amp circuit is.

Easy to scratch build a pair of monoblocks, assuming genuin transistors available. I think I bought the genuine article for the outputs. Don't know if all the others available. Get two power transformers specially made, even make each the size of this one. You would have a killer monoblock. The only thing in here not duplicated is the power transformer.

Damn might have to dwell on that myself. :idea:
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#59

Post by Paul Barker »

nickds1 wrote:
You can read my Quad rebuild at the link above, or at http://www.desmith.net/NMdS/Electronics ... rades.html
Had a look through that.

I am not worthy. I need to just pretext this "repair" rather than entire rework like yours, with the context I am not going to make everything look new, not going to try and "improve" the circuit because I haven't got a clue about BJT amplifiers, and if this one was a reference in it's era it will do for me.

But the internet can always do with more examples of projects by imperfect amateurs like me.

If you want to know about repairing your central heating boiler I am a professional. If you want to know how to repair state of the art 1970's solid state gear, am a dabler who is guessing.

Oh, and I don't have enough workshop bling. I need more gadjets that light up.

The old analogue scope I referred to elsewhere is not to be used. I brought it up carefully but so many faults it's beyond repair. Good I have an excuse to sell the valves inside for twice it's value as a scope.

To restore it you would have to renew every switch every capacitor to even make a start. End result, 1950's oscilloscope performance. No thanks. I'll live with my digital one.

Oh and it's about the cubic volume of a stack made of four Tesco's microwave oven's, so when it's on the desk there's about 6 inches square to put the work on.
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#60

Post by Paul Barker »

Trying to keep it simple and use the RS account I have. For Power supply diodes searched "fast and soft recovery" They just had one and it is just 3a which is all the old diodes are and adequate for voltage.

200v 3A Schottky Fast Soft-Recovery Power diode.

Wrong package, sm rather than axial but I'll live with that prefer the soft recovery to the esoterics. Might be a ball ache to install. Learning curve on it's way.

OK here comes a gamble. The large capacitors. Originals are 40v so I included 40v in my search. clal me a fool but an excellent result shone out from the page but over capacity. It is 15,000 rather than 10,000 40v and has lower ESR than most 14 other factors typical of a £12 product, but at nearly half the price. Has screw terminals.

EPCOS 15,000 40v Power Capacitor.

I shall have to rig up an autoformer to make sure my mains of 250 + volts is at 230v. The power transformer in the amp is connected up for 240v, but with amp running I was measuring 40v DC on the large power caps. We'll keep that below 40v. Will my diodes burst open because of extra capacitance? hope not, if they do I can't use the soft recovery version from RS and will have to look elsewhere. Lets risk this one and see what happens.

OK all new capacitors (lowest ESR available in every case) all additional power supply 100nf and 470nf capacitors which Nick used in his Quad referb and the Fast-Soft recovery diodes for both stereo amplifiers.

£164.63

This includes one duplication of one cap size, the lowest ESR not due for delivery until late June so I ordered an alternative next lowest ESR as a time filler. Adds about £7 to order being fernikity about ESR.

So the plan is do one power amplifier, evaluate it against other which is original condition but worn out and ready to pop!

Only fear is what took out the fuse?
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