HK Audio Actor / Lucas power amps , generic problem?

I repair many of these, and can honestly say that I have *never* observed any bad joints at the place you mention. By far the commonest problem is failed output transistors. This occurs because the heatsinks are only just about adequate with proper cooling. As soon as the fan air intake vents start to clog up with fluff, the outputs start to run too hot. Over a period of time, this dries out the heatsink paste to a powder, leading ultimately to transistor failure.

Whenever I get one, I always remove the other pair of transistors as well, clean down their heatsinks, and re-paste them, not forgetting the flatpak transistor that's in contact with the underside of one heatsink on each channel.

The manufacturers recommend that when the outputs are replaced, two of the BC546Bs nearby are replaced as well (T7 / 8 on one channel, 10 / 11 on the other). Check also C3 and C21 to make sure that they are not bulging.

Other than this, these amps are very well behaved, and new outputs and fuses will, in 99.9% of cases, effect a complete cure. Note, however, that they have proper differential inputs, so are not that easy to drive correctly, unless you have a proper balanced XLR source, and that they don't like earthed test equipment connected to their outputs / inputs simultaneously. I usually hook a completely isolated speaker to them for final check, as the music shop which sends these to me for repair, often remove the amp chassis from the cab, to ease the transport, and save me having to strip it all out. He now tells customers when they collect the repaired unit, that they should brush out the air vents at three monthly intervals.

Arfa

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Looks as though repaired 2 years ago,in UK, with (see logo thread ) with MJ11015/16 power Darlingtons that I suspected could be pirates as could not find logo.

2 of the TO3 devices shorted C-E on one side of the "H" 1 of those 2 are original colour but other three have a dirty brown/grey colouration of cap. I cannot figure out the failure mechanism. Will have to remove the other good pair to check under and will heatsink the 0R links while doing so. I doubt I disrupted the pcb solder when desoldering the B & E pins with the failed ones.

I agree with Arfa, I`ve repaired dozens of these and always found the solderwork to be excellent. Perhaps someone else has been in there before you. There was a mod for early units, different output transistors and the addition of a couple of 1N7007 on the print side of the board.

Ron(UK)

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Amp seems 2004 but replacement devices of matching dates on each pair of

2006 and 2007, looks wll enough done repair, he would have seen any associated duff solder points in the process, surely. I don't like the closeness of the +/- live vanes of the h/s on a board that can easily flex with heat.

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Thanks for that - both of those caps are bulging, I had not noticed up to now. I will replace those and the TO92s. A tip in reply for these and other apparatus with that grey interconnect ribbon. Before removing any such boards I run some hot-melt glue down the join of the cables at and to the board. So any bending, you have to do, is then only in the cable run which consists of multistrand but soldered wire, so will easily break where not supported by the grey plastic sheathing, ie at the ends, where it otherwise would choose to bend. I would have thought the ducted air design was about ideal for a given fan size, but I will bend inwards the outer fingers between the +/- dc carrying adjascent heatsinks. Perhaps the airstream takes the path of least resistance and does not pass through the 44 vane version of this type

staggered fingers of those square format heatsinks and goes around outside instead. Perhaps bending inwards all 20 internal run fingers of each h/s would be a sensible mod, less vortexing/blocking, more open, would make less of a dust trap as well

The sort of wide pitched grey ribbon that has end stripped and conductors soldered directly to pcb, rather than headers

Having a great deal of experience of these, I feel they are a fundamentally well-designed and constructed amp - both electronically and mechanically, and I don't think that I would contemplate altering any aspect away from the way it was designed, including the arrangement of the heatsink fins, and the airflow paths. The heatsinking and cooling of the heatsinks *is* adequate as designed, but becomes a bit marginal if the units are thrashed hard on a regular basis, whilst the air intake vents are allowed to clog. If the transistors / heatsinks of both channels are cleaned down and repasted during the repair process, and the owner is alerted to the importance of just scrubbing a soft paintbrush around the vents once in a while, they are unlikely to give further trouble. I've never had any repeat failure issues with them, anyway.

I take your point about the inner 'row' of fingers, but I don't think I would feel inclined to bend them in more than a few degrees, otherwise, they might actually block convection off the transistor cases ??

Arfa

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The good pair had good solder points under. All white goo was in oily state, One of those electros was o/c the other high ESR and both had more obvious bulge at the lead end

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I have to differ on the aerodynamics. This sort of 4mm wide double wall, staggered finger design, is for convection not passage of air through. Walls of 4mm gap blocked by 4mm finger repeated four times each side of the fan (2 h/s in line each side) is pretty effective wind break. Then 2 "gaps down the sides of 3mm wide x 35mm high and 70 mm long is similarly "resistive". Most cooling would be the 2 outer edges of 12 only of the 44 vanes each h/s where there is unobstructed passage outside the heatsinks and out to the surrounding duct

20 degrees is about the most or you block off access to the TO3s. Maybe only tinkering at the heat build up problem. Most air must go through the 25mm or so wide gap in the ps direction (over the tips of the vanes), rather than the 2 off 5mm gaps to the duct let alone through all those vanes. I would have thought there should be some shuttering to block off most of that large gap along the ps side , to force most air round and through the heatsinks.

They were assembled with enough white goo to keep a goth happy for a week. So much that she filled the B & E holes so pins pushed through it, not cleaned off, and some white goo actually under the solder joints (not actually failures there). Those failed caps 47uF, 63V - in similar HK amp they were rated 40V 47uF , what is the problem in that area of the circuit?

If make is relevant these blown caps, 2 types so I don't know which is original, both dark and light blue

47uF , 63V SG and logo of an eye, type? R4D 85deg C (M)

probably original one as amp is 2004 and previous repair (if only 1) 2007

47uF, 63V YAGEO 08/03 (date Aug 2003? ) SK85 deg C

Those failed caps 47uF, 63V - in similar HK amp

I don't know of one in particular. On about 50% of the ones that I see, there are issues with those caps. They just get replaced as a matter of course, as part of the repair. Output transistors. Little TO92s. Fuses. Caps. That's all that goes wrong with them, and that's as deep as I bother getting in. There are not repeat failure issues, if those parts are all replaced, and these days, all I am interested in is the commercial angle, and spending as little of my time on a repair as is required, whilst making sure that it is actually fixed, and any manufacturer-specified remedial work has been carried out .

Arfa

Well, I agree with Arfa and Ron. I've repaired lots too, since the cooling is also IMHO inadequate, which is quickly made worse by fluff around the fan vents. Never had any dry joints, but seen plenty of the bulging caps Ron mentioned. I replace them with 105 degree types regardless of apparent condition.

One warning - it is easy to accidentally short the solder tags the big diodes are soldered to, to the zero ohm links as you tighten the nuts. If you power up fully in this condition with 4 amp fuses you will blow the Darlingtons again. You have probably noticed both speaker output tags have amplifiers on them, there is no grounded speaker connection, though each output is referenced to ground.

Gareth.

On reflection, I don't think that these are actually designed with 'forced air' cooling in mind. Rather, I think that that type of heatsink is intended to suck the heat off the transistors, and then radiate it to free air, and that the purpose of the fan is to shift new cool air through the chassis / cabinet, to help the radiation take place. When the air intakes get clogged, the internal ambient temperature goes up, stopping the radiation from taking place efficiently, with a consequent large rise in the case temperature of the transistor, made worse by the dried out thermal paste.

I don't quite agree with Gareth that the heatsinking is inadequate. I think it is just about adequate, as long as the designed level of airflow around the chassis can be maintained. If it can't by virtue of the intakes being clogged, then it becomes very marginal.

However, I suppose it could be argued that it is 'inadequate' in that there is little or no margin built in for compromised airflow.

Arfa

That was my definition of inadequate. The fact that 3 of us here have seen large quantities of the same units with the exact same problems bear this out. I'm sure we've all seen plenty of amplifiers with a whole ton of dust inside that have been working hard for years.

I had a theory that because once repaired they tend not to come back, this may point to either a manufacturing issue or dodgy batch of Darlingtons (?)

Gareth.

Oh, BTW, can you confirm which way the fans run? Sucking air out the cabinet or blowing it in?

Gareth.

I suspect usage has a bit to do with it also. The Eliot/Lucas were never intended for 'Heavy Metal' or bass heavy dance music. Prolonged thrashing is probably what heats them up. Another thing, they don't like having the satellite speaker plugged and unplugged while drive is applied, I`ve fixed more than one where the owner admitted that`s what happened.

Ron

Maybe

This one is outwards

4,000 sq mm of purpose built inlet holes and 3,300 sq mm through fan vent I would have thought was quite adequate if the air was directed over the heatsinks and not wind-breaked. This one permanent mounted high up away from floor dust. Compared with Laney CD850 , totally inadequate constriction on outlet over fan . 3 of those I've cut away the "vent" holes over the fan, fared off and covered with a wire grille and they've not bounced back (each was cutting out thermally after extended but normal use) As the Laney original design just 14 small slots with a combined area of only 1.5 sq inches, let alone vortexing, so overloading the 3 inch diameter fan with through ducted area of about 6 sq ins , ridiculous aerodynamics

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I see no circuitwise reason for those repeated cap failures. The Darlington heatsinks are elevated off the pcb (trackside) with spacers and although the minor component side of the pa pcb is un-fanned I would not have thought pcb and working caps would get to 85 deg C Will make airflow mods and RTV-stick non-resettable thermochromic dots around various items , for if it bounces back.