As I mentioned earlier, the SAP15N &Ps all check out O.K. 'cold'(including the built-in emitter resistors!), as do other components. I am, however, well aware that devices can break down under operating conditions, so may have to replace the SAPs - the later SAP16s are listed by Cricklewood Electronics -'on spec' Point taken about that heatsink, though it is screwed to the metal case of the amplifier, which surely means that the whole surface area of the metal work is also part of the heat sink, so if, as others have said, the amp is (a) only used with speakers of 6 ohms or higher impedance, and (b) not run for hours on end at very high volume levels, it should work reliably. After all, I assume it was designed for normal domestic use, not a s a disco amplifier!
Yeah, at domestic listening level's they're OK. Most of the ones I've repaired have been owned by students who invariably have a house party and crank it up too far.
The case doesn't make much difference to the heat dissipation TBH.
The other thing that tends to go with age on these amps is the cheap Chinese power supply capacitors. Have seen that a few times from people who leave their amp powered up all the time.
The case doesn't make much difference to the heat dissipation TBH.
The other thing that tends to go with age on these amps is the cheap Chinese power supply capacitors. Have seen that a few times from people who leave their amp powered up all the time.
A thought occurred to me re; those Capacitors. Since the total (nominal) capacitance on each (+ve & -ve)rail is 8800uf, could the 8 Capacitors be replaced with two, say, 5600 or 6800uf ? I ask the question because in my parts 'stock' I have a couple of high value electrolytics obtained some time ago from Mouser. Due to their size, they would have to be mounted horizontally and secured with cable ties, but I can't think of a reason why they shouldn't be used in the A500.
I've checked the Capacitors supplied by Mouser. They are 12000uf, 63v working 85 deg. types, made by 'Nippon Chemi-con' Type(?)SMH &Nd0SL46. Don't have any more info. on them, but can maybe find out via the web. They are 'snap-in' types 35mm diameter x 46mm high, intended for use as smoothers in a HiFi amp's PSU, so I may well fit them in the A500. I have a replacement mains transformer on order,,, so will fit the new capacitors when the TX arrives. Should then have a working A500!!
According to an on-line data sheet, they have a ripple current rating of 6.19A
so should be perfect!
Only problem is that there isn't room to fit them! I've ordered some new 2200uf Capacitors from Charles Hyde, although the originals check OK, and there is no sign of any leakage or physical stress, at least in the four I've removed!
I should also add that, in the A500 which I'm checking, there is no connection to the 'Chassis Ground' side of R267/C210, andnever has been I have added a ground wire, since, without it, the center tap of the mains transformer secondary is effectively o/c, as it's connected to the 'Gnd' side of the above network. The speaker common terminals are connected to the '0V' line.
Further to my earlier posts, I have now checked the amplifier with an outboard DC PSU. This only has an o/p of 25v,(single rail)@2A Maximum, but, when connected across the A500's + & - lines, I get readings of -17v. & +8v., with respect to '0v', which suggests that something is loading the +ve side, possibly one or more of the o/p transistors. Re; my post #31, of course the center tap of the transformer is common with the '0v' line. The above test has however been carried out without a transformer connected.
Apart from showing that the output transistors aren't both short circuit its meaningless tbh.BTW, does anyone know from where I can buy STD03n &03P transistors(other than Profusion PLC, with whom I would have to register, setting up yet another user name & password! I have seen them on e-bay, but at over £17 for four.
STD03N &03P are seemingly Sanken's replacements for the SAP15 & SAP16 series, but, unlike the latter types, they don't include internal emitter resistors.
However, since Rapid Electronics still list SAP16s, I may well opt for them instead.
It's not unusual with double insulated and some earthed chassis amplifiers, to float the secondary power supply and so avoid contamination from a "dirty" ground or simply not connect the OV return to an otherwise non-functional ground. The "ground" is made instead, to the input device. It's fine to discuss unearthed equipment but for general DIY use, designs for DIY puposes have to be a no-no according to forum rules.
Frankly, I would leave the earthing details to the manufacturer and not tinker with earthing unless you do actually know the ramifications of what you are doing regarding personal safety and insurance. This is, according to the schematic, a double insulated appliance and that is not the same as an earthed appliance for grounding or safety issues. Do you see an actual earth wire leading to the rear mains connector?
Using the SAP's seems the best option to me if you want a guaranteed replacement that works with no other changes to the circuit.
Remember to power up using a bulb tester when the new devices are fitted.
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