HK Audio Actor / Lucas power amps , generic problem?
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From: N_Cook on 6 Jan 2010 05:20 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?
From: N_Cook on 6 Jan 2010 05:41 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
From: Arfa Daily on 6 Jan 2010 09:28 Those failed caps 47uF, 63V - in similar HK amp > they were rated 40V 47uF , what is the problem in that area of the > circuit? > > > 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
From: Gareth Magennis on 6 Jan 2010 15:19 "Ron" <ron(a)lunevalleyaudio.com> wrote in message news:FMWdnar5p5rYhd7WnZ2dnUVZ8nGdnZ2d(a)bt.com... > On 05/01/2010 09:59, Arfa Daily wrote: >> "N_Cook"<diverse(a)tcp.co.uk> wrote in message >> news:hhv006$kqa$1(a)news.eternal-september.org... >>> They have an otherwise original assembly method of the final contact to >>> the >>> pins of TO3 power transistors by 2 zero ohm "resistor" links, so 2 >>> current >>> paths to the pcb traces. Anyone else observed bad solder joints to these >>> links on the pcb? and cause? believed conventional solder not PbF >>> >>> >>> >>> >> >> 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. > > 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) 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.
From: Arfa Daily on 6 Jan 2010 21:46
"N_Cook" <diverse(a)tcp.co.uk> wrote in message news:hi1o7d$fic$1(a)news.eternal-september.org... > > 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? > > > 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 |