Hi Guys,
A few weeks ago a friend brought me his two mono blocs from Metaxas (Solitaire III). A previous tech plugged them in to a 230V mains outlet when they were configured for 110V mains so... lets just say there wasn't much left of the fuse and most of the small signal transistors shattered to pieces.
I changed most of the small transistors, replaced the main caps that vented and all the power transistors (2 were shorted so I replaced the whole bunch with MJW 1302/MJW3281) and drivers (MJE15030 ans MJE15031). All the other transistor are original references. I powered the amp up with a dim bulb, and it puts out a beautiful clean sine wave, voltages are spot on and I measured 1mV DC offset.
There is some service info available but some are contradictory and a suppose they vary between the 3 versions of the amp. One of them says to adjust bias for 50mV across the 0.51ohm emitter resistors which would get about a 100mA bias current per output transistor, does this sound right ??
A few weeks ago a friend brought me his two mono blocs from Metaxas (Solitaire III). A previous tech plugged them in to a 230V mains outlet when they were configured for 110V mains so... lets just say there wasn't much left of the fuse and most of the small signal transistors shattered to pieces.
I changed most of the small transistors, replaced the main caps that vented and all the power transistors (2 were shorted so I replaced the whole bunch with MJW 1302/MJW3281) and drivers (MJE15030 ans MJE15031). All the other transistor are original references. I powered the amp up with a dim bulb, and it puts out a beautiful clean sine wave, voltages are spot on and I measured 1mV DC offset.
There is some service info available but some are contradictory and a suppose they vary between the 3 versions of the amp. One of them says to adjust bias for 50mV across the 0.51ohm emitter resistors which would get about a 100mA bias current per output transistor, does this sound right ??
Anyone ?
Adason, schematics are not available unless for sale on ebay but they are not even complete and from the wrong version so... The part of the service manual that I do have says to adjust for 0.05V on the emitter resistors but that means 100mA and event at 30mA (approximately what it was reading when I first powered it up on the dim bulb tester) it already gets warm, than again it's supposed to be a class A amp for the first 15W.
RM, I really feel comfortable following your advice but won't this affect the working in class A mode ?
RM, I really feel comfortable following your advice but won't this affect the working in class A mode ?
Depaj,
Look for a small cap around 47pF or thereabout between a base and collector of the VAS, which is typically in the centre of the schematic/layout. It will connect directly to the bias generator. Metaxas did not reveal his schematic, but if you can post a detail picture of the pcb it should be possible to identify the lag compensation cap. Often these caps are silver mica, or polystyrene, or even ceramic NPO.
Hugh
Look for a small cap around 47pF or thereabout between a base and collector of the VAS, which is typically in the centre of the schematic/layout. It will connect directly to the bias generator. Metaxas did not reveal his schematic, but if you can post a detail picture of the pcb it should be possible to identify the lag compensation cap. Often these caps are silver mica, or polystyrene, or even ceramic NPO.
Hugh
Sorry I misread, they were talking about the zobel network, I have a translated version and there were some errors in it.
The cap has just written 11 on it so I guess that would be 1 x 10 = 10pF, I can get some 56pF or 68pF 65V 2.5% styroflex caps from a distributor here in france, does this look like a good option ?
I replaced the caps and am testing the amp now with an 8ohm load but still with the dim bulb (200w bulb so it can take a little power). I set bias at 10mV for now just for some testing but I did find some differences between the 2 x 4 outputs in parallel (the bias voltage ranges from 9 to 11mV without signal so some devices are conducting a little more than others). The difference amplifies with output : at 12W into 8ohms I read from 89mV all the way to 101mV on different emitter resistors. As I said I have two mono blocs, one original, the other has the new outputs, emitter resistors and drivers I mentioned before, both have these differences, are they acceptable ?
The "old" ones used to have 2SA1095 & equivalant. These are superior from a heat transfer point of view, which really is of paramount importance considering the piddling, god I love that word, heatsinking on these amps. 2SA1227 & its equivalent are stellar performers, I used more than 3K pairs of these in the late eighties, still have a few left.
But I would still opt for the former given your special circumstance.
I have forgotten, what is the value of the resistor between the emitters of the driver transitors? It should be quite low.
I wouldn`t be surprised if Hugh also agreed on this notion. From our different conversations ages ago I seem to remember we shared many opinions on the Metaxas brand of amps.
R
But I would still opt for the former given your special circumstance.
I have forgotten, what is the value of the resistor between the emitters of the driver transitors? It should be quite low.
I wouldn`t be surprised if Hugh also agreed on this notion. From our different conversations ages ago I seem to remember we shared many opinions on the Metaxas brand of amps.
R
Hello RM,
I think you misunderstood, I have one example that has its original transistors the 2SA1227 and complementary indeed and the other one as I said in my first post has undergone some changes :
The 2sa1227 aren't available any more that is why I went for the on semi.
The drivers have 27ohms between their emitters.
My actual question was : is the difference of bias current between the different paralleled outputs going to be a problem or should I be just fine ?
I think you misunderstood, I have one example that has its original transistors the 2SA1227 and complementary indeed and the other one as I said in my first post has undergone some changes :
The 2sa1227 aren't available any more that is why I went for the on semi.
The drivers have 27ohms between their emitters.
My actual question was : is the difference of bias current between the different paralleled outputs going to be a problem or should I be just fine ?
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