Cambridge Audio A500 Amplifier problems!

[Old Sparky]

I've been asked to repair one of the above which 'won't switch on' The reason for that was soon obvious - mains transformer primary open circuit!
There being no burn marks or any other sign of distress on or around the transformer, plus the fact that the internal primary and secondary fuses are intact, as is the secondary winding, makes me feel that a replacement transformer will get the amp. working again. However because the TX is likely to be expensive, I decided to 'cold' check the main amplifier PCB. First thing
I discovered was that, unlike in the majority of amplifiers the speaker common negative terminals are not earthed. Checking the components and PC Tracks with a meter revealed no obvious problems, nor is there any sign of leaky capacitors, dry joints, etc. The output transistors(?) are a type I've not encountered before. Each has 5 terminals. They are SAP15s, which, from what I've seen on the 'net, can be troublesome, and may have been superseded by other devices. Should I obtain replacements for these as a
precaution? (That is, assuming the amp's owner wants to fork out the price of a new mains transformer!)

[Skywave]

This unit has a few 4-amp. 'T' fuses in its PSU. Have they blown? If they have then the SAP15s will almost be S/C and that is a problem since they are now obsolete. Worth checking for O/C resistors in the emitters of the SAP15s and defective caps. in the PSU.

But the most important Q. is this, of course: why has the mains tx. gone O/C on its primary? Usually this is caused by a sustained S/C in the PSU or the 'wattful' parts of the amp.: viz. the O/P transistors.
Which neatly brings me back to the previous paragraph. Me? I'd tread carefully with this before committing any money on it and / or making 'promising noises' to the customer.

HTH,
Skywave.

[Mark Hennessy]

I remember when these transistors came out. A fascinating idea, and a valiant attempt to improve thermal tracking, which of course is most important with class B amplifiers. But sadly, including the emitter resistor was a step too far, as this often fails before the transistor. Sanken dropped the family a while back - but do similar products that are sans the resistor...

The first amp that I came across with these in was the Arcam A85. This was a lovely thing, and would be well-worth fixing.

Then there is this one

As Al wisely says, walk away. This amp was made to a price - a pretty low one - and has woefully inadequate heat sinking.

In case you haven't see it, the service manual is attached. The negative terminals are earthed - it's a conventional single-ended design. Of course, signal earth and chassis might be two separate things. It seems that they are 47 ohms apart, thanks to R267.

The motorised pot might be worth rescuing though...

[tiv48]

http://www.ebay.co.uk/itm/SAP15-pair-of-...4171fb9155
rob t

[Old Sparky]

Thanks to both of you for your advice, and, Mark, for the circuit diagram. I shall certainly make some further checks, especially of those o/p transistors, before advising the owner further about a possible repair. As previously noted, none of the internal fuses have failed, so the cause of the TX failure is, for the moment, puzzling. I believe there are devices that, possibly with modifications, can replace the SAP15n & 15P Darlingtons, so will check on that. BTW, Mark, R267 is 47K, not 47 ohms( a 'typo' no doubt!)

[Mark Hennessy]

I saw those. Just over £30 for the transistors, and probably another £30-40 quid for a transformer. Plus about the same for labour. The amp's just not worth that. It's bargain-basement stuff that was never great when new. Nothing like the Arcam I mentioned

[Skywave]

The repair of things like this with a fault as described means that you have to tread carefully - and not simply 'dive in', as is often the temptation, fuelled by enthusiasm.
The one and critical factor that determines the feasibility of a repair of this amp. is to determine if a replacement transformer - and nothing else - will successfully repair it. In my case, I would do the following.

I'd try to find a suitable mains-powered PSU (or two) that will deliver ±45 volts at about 1 amp per output. (I am fortunate enough to have a reasonable stock of test equipment). I'd use that (or those) to by-pass the on-board rectifier in order to test the rest of the PSU + amplifier. If I couldn't find such kit, I know I can lay my hands on a couple of 30 v. 1 amp. (secondary) mains transformers, and by connecting those windings appropriately, make a temporary 'out-of-the-case' lash-up to test the amp. + rectifier + PSU. In other words, a temporary transformer substitute for basic testing.
Either route should enable me to determine if the O/P transistors are O.K. and test for any other major faults: that is all; it's simple a 'go' / 'no - go'; 'do I / don't I' test. I would not be interested in 'performance criteria' at this stage.
At that point, I could then make an informed decision - then talk to the owner of the item.

HTH,
Al. / April 19 //

[Mark Hennessy]

As previously noted, none of the internal fuses have failed, so the cause of the TX failure is, for the moment, puzzling.

I suppose the transformer might have failed by itself. It can happen - I've had a couple of random failures (good torroidal transformers, bought from Farnell, and not overloaded at any time). Of course, here is where a dual current-limited bench PSU is handy. You can never have enough power supplies - I've made loads over the years, and collected a fair few as well. But I digress...

BTW, Mark, R267 is 47K, not 47 ohms( a 'typo' no doubt!)

That's a worry. Makes you wonder what other mistakes could be in that manual?

I'll be interested to hear how you get on. Good luck

Mark

[Skywave]

Re: Mark's last post: transformers can, occasionally, go O/C primary for no apparent reason: yes, I've had that trouble too. Not often, but yes, it can happen. But more to the point: is this transformer one of the types that has a built-in thermal fuse? If so . . . . .

Just a thought.

Al. / April 19 //

[Old Sparky]

Not sure if it has a thermal fuse, Al. If it does, the T.F. is inside the windings of the Toroidal TX.