Cambridge Audio A500 Amplifier problems!

[Old Sparky]

Today I tested the amplifier, using my bench PSU, which has a maximum o/p of 25 volts.(single rail) What I found, measuring the DC voltages with respect to ground was that they were - 17 & +8v approximately, which leads me to suspect one or more of the o/p transistors. The test, incidentally, was carried out with the two 4A fuses in series with the AC side of the rectifier diodes removed, though this may not have been strictly necessary! Just to correct an error in my previous post - the Ground side of R267/c210 is common with the '0V' line(well it was late and I'm typing this at home, whilst the amplifier is in my workshop some 3/4 mile away!!!)

[Old Sparky]

A summary of the results of my tests so far follows.
With no mains transformer connected and the two 4A(T)secondary fuses removed, I applied power from my single rail PSU(I can actually wind it up to 27v!) across the +ve & -ve lines. With respect to '0'v I measured +12 and -17v. Disconnecting the collectors of the SAP15Ns made no difference ( I assumed something was loading the +ve rail), but disconnecting the collectors of the SAP15Ps restored the equilibrium. I now measure +13.5 & -13.5v WRT 0v. Quiescent Current consumption at these voltages is <20mA regardless of whether the o/p transistor collectors are connected or not. Would I be right in suspecting the SAP15N &/or SAP15P devices from the above tests, admittedly being carried out at 30% of the supply voltage normally used? I am still puzzled by the fact that none of the 4Amp fuses had failed, and that no component I tested(o/p devices and smoothing capacitors mainly) checked faulty.

[Mark Hennessy]

No, the unequal rails just tell you that the current draw on each is unequal, which is not unexpected. Powering an amp with split supplies from a single PSU is asking from trouble. If you only have a single PSU, then you need to make/acquire another one. Or, make a supply divider - just a few transistors (I can show you a suitable design - I made one for work). Quad did this in many of their amplifiers using just a couple of transistors - look up the 306 schematic for an example...

Based on what you've said, the output transistors are probably OK. But I'd want to power the unit with at least 25V per rail first. And for testing with a replacement mains transformer, I'd replace the 4A fuses with suitably rated light bulbs, or use a lamp limiter on the mains side.

Situations like this prove that one can never have too many bench power supplies! A lab PSU should have fully adjustable current limiting and good metering - something like the Thurlby-Thandar PL320 should be the minimum in every lab.

This prompted me to search for the PL320, and I posted details of bargain new units here: http://golbornevintageradio.org/forum/sh...p?tid=3419

All the best,

Mark

[Old Sparky]

I have now run the main amplifier board from a +25/0/-25v DC supply. Admittedly this is still only a little more than half the voltage it's designed to run from, but, on the basis of short-term tests with this supply and signals from an Audio Generator, the amplifier is working perfectly from this. I have, as a precaution, replaced all 8 2200uf 50v Electrolytics with new ones, but I'm rapidly coming to the conclusion that the only fault was the o/c primary of the original mains transformer. There are quite a few threads about the A500 on 'diyaudio.com', where the consensus of opinion seems to be that it's a decent design let down by indifferent build quality and inadequate heat-sinking, but generally OK if 8 ohm speakers are used, and it's not played for many hours at high volume, or into too low a load impedance. The speaker terminals on these are simply wired in parallel, which means that, if 4 speakers are connected, ideally they should each be 15 or 16 ohm types. In addition no cooling fan is fitted. Since posting, I have run the amplifier from a 25-0-25v AC supply, giving +35/0-35vDC, and it is working OK, at least from a test signal at this voltage. I don't know the history of this particular A500. It was brought to me with no screws in the outer cover, and, when I came to put the fuses back, I noticed that the two from the secondary windings of the transformer to the rectifier were 6.3A (T) each, not the specified 4A(T). Enough said?!!
(Naturally I shall replace them with the correct types!)

[Old Sparky]

Job done!! I have reassembled the A500 and fitted a new 160VA Toroidal Transformer. The amplifier has now been run for several hours, producing music from records played on a BSR BDS95, using an outboard magnetic cartridge preamplifier(The internal Cambridge Audio preamp wasn't fitted, and is no longer available)

Attached are a couple of pictures I took after fitting the new mains transformer. The factory fitted mains earth connection is clearly visible in the first one.
The second photo includes what, to me, is irrelevant information about speaker connections. The two outputs for each channel are internally paralleled by the makers, so it is of no consequence which pair are used for normal stereo operation.
Each speaker should be 6 ohms impedance or greater. This is more important when 4 speakers(2 per channel) are connected.

[Skywave]

So, 'normal service' restored: excellent. But why did the transformer primary go O/C in the first place? I ask this since I have had that fault on the occasional toroidal type of transformer myself - but I have never conclusively discovered the reason why. Could it simply be a question of inadequate number of turns per volt? I've found that small non-toroidal transformers seem to fall into that category - and seem to fail the same way, too.

Al. / June 5 //

[boiss]

I had an 80VA 9V toroid and the primary went O/C, one day it was OK the next Duff.
Removing the secondary was of little help as the break was underneath the top layers.
Bin Job !!!
Mike

[camallison]

You're probably aware that toroidal transformers can have a large inrush current on power up. The fuses quoted on the chassis will probably have been slow blow to overcome this. The previous owner may have substituted higher rated standard fuses in the absence of the correct slow blow.

Colin

[Skywave]

You're probably aware that toroidal transformers can have a large inrush current on power up.
Colin

No, that fact I didn't know. Presumably, you're comparing that to the in-rush current for a conventional transformer of the same VA rating. Why is that current so much larger for a toroid - and do you have any idea of the ratio of those two currents?

Just curious - and keen to learn.
Thank you.

Al. / June 5 //

[Old Sparky]

I either wasn't aware, or had forgotten, that toroidal transformers have a bigger inrush current than a conventional one of the same rating, and i certainly don't know why this is. Although I have worked with electronics for over 40 years, more than half that time was spent repairing car audio equipment, and consequently seldom dealing with mains powered gear. Any that i did repair, and the few mains powered items I built, used conventional transformers. I hope the new TX I fitted to the A500 doesn't fail. It's almost forty quid's worth for one thing!