Cambridge Audio P50 Amplifier MK1.

[Station X]

I have a acquired one of these amplifiers and would like to test it, then carry out repairs if necessary. I have the correct circuit diagram.

The power amplifier section is DC coupled with 9 transistors in each section. The output transistors are a TIP3055 and a TIP2955 wired as a complementary pair.

As received the two 2.5A fuses which feed 60VDC to each channel had been removed along with the screw in part of the fuse holders.

I found the fuses and their holders in an envelope inside the unit. Both fuses were intact. Also in the envelope were a number of new transistors as used in the power amp along with a "pulled" TIP2955 which actually tested OK. It was obvious that one of the TIP2955's had been replaced with one from a different manufacturer, but the job was a neat one.

I haven't inserted the fuses, but 60VDC is present at the input to each fuseholder. "Diode" tests of the output transistors show the some junctions are OK, but results for other junctions are inconsistent due to shunting by other components. Results are the same for both channels. The 2500uF output capacitors test OK. I don't do Compulsive Component Changing.

I'm reluctant to fit one fuse, plug the unit in and test that channel for functionality. I'm concerned that one faulty transistor could destroy the others in the PA or that I might damage my speakers.

Are there any useful tests I could perform before applying power?

I'm not too concerned about the preamp section as it's AC coupled and I feel more at home there.

Thanks.

[Ted Kendall]

As David frequently notes, probably the best course of action is to pull all the semiconductors in one power amplifier and test them out of circuit, replacing any that are faulty. Those fuses were removed for a reason, after all, and one undetected dud transistor in a DC coupled amplifier may take the lot out - Murphy's Law clearly states that a fuse inserted to protect a transistor will be protected itself by the transistor blowing...

[stainless]

A useful dodge I have used, is to put a 1k resistor in place of each fuse. Serious current cannot flow. The resistors will get hot.

[Radio Wrangler]

It's anybody's guess what condition the power amps are in, then.

It's a power amp with a single supply rail and an output blocking capacitor. Despite the gorgeous looks of the case, the amplifier circuit is rather crude.

Someone may have already stuck full power on it. If so, you've nothing to lose. Or it may have had parts swapped and be ready to test. Or someone could have been round and round the mulberry bush and just given in.

If you have a lab type power supply with a current limit, you could set the current limit to a couple of hundred mA and bring the voltage up slowly and see if it comes to life... monitoring the centre point DC voltage at the input end of the output blocking capacitor. I don't think there is much added risk.

Testing transistors would be best done at realistic voltages, looking for any with a breakdown problem, but even counting the diodes is better than nothing.

David

[Craig Sawyers]

Like all early Cambridge Audio amps these went through various iterations. The schematic I have is definitely not DC coupled, and the output devices are TIP35B/TIP36B. Two of the transistors (Q15 and Q16 on my schematic) are to do with protection. The fuses are 1.6A.

Anyway, attached.

Craig

[Station X]

Thanks Craig, but that's the wrong diagram for my amp which uses 3055 and 2955 devices. It came with the correct diagram which, as far as I know, is not available on the 'net. I'll post it here later.

[Craig Sawyers]

OK. I'd be very interested in what you have - the history of Cambridge Audio is very interesting for me.

Craig

[Radio Wrangler]

These amps are from a time of rapid changes. Silicon had just replaced Germanium and people were exploring how to best use it.

It's amusing how Cambridge and Leak stuck rather different circuitry in similar looking boxes, while a couple of decades later the industry trend became to change the box, keep the circuitry and release it as a new product.

Are people in Oxford even allowed to mention Cambridge (and vice-versa)?

David

[Craig Sawyers]

I used to work in Cambridge, at PA Technology and then at Scientific Generics. In both places I worked for my good friend and mentor Gordon Edge (RIP). Gordon was the founder of Cambridge Audio, so I have no problem at all living now near Oxford and mentioning Cambridge

And yes - there were great changes in semiconductors - certainly power transistors - and also in power amp topology at that time, and Cambridge Audio was not immune to that.

In honour of Gordon's role in founding the company fifty years ago, Cambridge Audio have released the Edge series of high end products. They have even put Gordon's barely legible signature (Gordon's brain, always on creative overdrive, always worked faster than his pen!) on the circuit boards https://www.cambridgeaudio.com/gbr/en/products/edge

Craig

[Station X]

Here's the full circuit along with an extract showing one Power Amplifier channel. Apologies for the poor quality, but at the moment I'm reduced to taking pictures using a smart phone. I'll get the circuit scanned at high res.

As far as I know this circuit is not available on the net. If that's indeed the case I'll submit a decent copy to the HiFi Engine website.

I have no notes to go with the circuit, but there are some on the net which appear to match the circuit, but using different component designations. Better than nothing though.

Q14 and Q15 are a flip-flop forming a protection circuit. Under fault conditions they remove the supply voltage from the output transistors.

Originally Posted by Radio Wrangler

If you have a lab type power supply with a current limit, you could set the current limit to a couple of hundred mA and bring the voltage up slowly and see if it comes to life... monitoring the centre point DC voltage at the input end of the output blocking capacitor. I don't think there is much added risk.

David

I'll try that, but my PSU is limited to 100mA. It did occur to me though that I could power the amp via a Variac, perhaps with a current limiting resistor in place of one of the DC fuses.

First thing I have to do is make up some test leads. I never have the right ones.

[Radio Wrangler]

There seems to be a lot of similarity... the flip flop overcurrent shut-down. AC coupled output, one fewer amplifier stage in the DC feedback loop than in the AC feedback loop.

It looks like they have done a bit more work on bias control than in the earlier diagram I found. It uses a group of diodes to set the voltage reference of a constant current source. This should help stabilise bias current and help it come up gracefully if you progressively increase the supply voltage.

It's worth trying with that 100mA supply even if it doesn't go up to the full normal supply voltage.

There's no unobtainium floating around in this amp, just straight forward bipolar transistors which have plenty of opportunities for substitute types.

David

[TrevorG3VLF]

100mA should be ample if you keep the signal low and do not connect a speaker.
I have used a lamp limiter on valve radios and transistor amplifiers, at least it saves the fuses.

[Station X]

I powered the unmolested LH channel from my current limited PSU. As the voltage was increased the current also increased until the 100mA current limit came into operation at 22.85V. The mid point voltage was 9V.

As soon as I connected the PSU to the molested RH channel the 100mA current limit came into operation before I even increased the voltage.

I did a visual inspection and found that the original MJE2955 and MJE3055 transistors had a pin out which did not match the data sheet. If you imagine holding the device with its lettering towards you and the leads hanging down, then from L to R they are E C B. These transistors do not have a mounting tab. The fixing screw goes right through them and I don't know which lead is connected to the heat sink.

The replacement MJE2955 has the leads in the order B C E with C connected the mounting tab as per the data sheet. It has been wired correctly, but there is no insulating sheet between the mounting tab and the chassis. Hence the collector is connected directly to the chassis, putting a short circuit across the supply which explains the instant current rise to 100mA.

You can guess what tomorrow's job will be!

EDIT.

A bit of research showed that the original devices are in TO-127 packages for which the E B C pin out is correct. The new device is in a TO-220 package for which the B C E pin out is correct.

[Herald1360]

An original Motorola datasheet shows MJE2955/3055 in TO127 but MJE2955T/3055T in TO220AB. Pinouts as already noted.

[Radio Wrangler]

It sounds like the last person in there wasn't up enough on transistor circuitry to check pinouts or to know about mica washers. 9v at the midpoint on low supply doesn't seem too bad. It may have been designed with high-ish quiescent current to keep it in class-A at low signal levels.

I've just spotted one giant whoopsie in the circuit design.

Power to the top power transistor collector is fed through a variable resistor. this has to carry almost the entire output current of the amp. if turned while on, any bounce on the slider disconnects power to that transistor, and the driver tries to provide all the output current via the b-e junction of that output device. Yuk! The current limit flip flop then gets a heck of a lot of drive into Q14 which it hopefully survives, and the power amp is shut down. Probably not quickly enough to be sure of not damaging transistors.

Similarly any slider bounce on the quiescent current trimmer makes the quiescent setting jump to max.

This is a cute looking amplifier and historically interesting, but not a good candidate for a regularly used one. It contains some things amp designers learned to not do.

David

[Station X]

Thanks for all your replies, they've been most helpful.

I have both good and bad news to report.

Originally Posted by Radio Wrangler

I've just spotted one giant whoopsie in the circuit design.

Power to the top power transistor collector is fed through a variable resistor. This has to carry almost the entire output current of the amp.

David

Those pots don't seem to be fitted to this amplifier. The 60VDC supply is taken directly to the output transistors.

Originally Posted by Radio Wrangler

It sounds like the last person in there wasn't up enough on transistor circuitry to check pinouts or to know about mica washers.

I think the previous repairer did understand transistor lead outs as he'd wired the replacement transistor correctly, crossing over wires and lengthening the emitter to emitter bridge wire.

However he fell down when it came to insulating the new transistor's mounting tab/collector from the chassis. The existing transistors had a circular mica sheet under them with a hole in the centre. This was perfectly good for transistors with a TO-127 package, but when this was replaced with a TO-220 package the mounting tab/collector overlapped the mica sheet and contacted the chassis. The previous repairer also neglected to fit a plastic "top hat" in the mounting hole to insulate the tab from the mounting screw.

I salvaged a correct mica sheet and top hat from the PCB which I'd removed from my scrapped Beko CRT TV and fitted them to the replacement transistor, using heatsink compound. A check with a meter on ohms range showed that the tab/collector was now insulated from the chassis.

With the repair complete it was time for some testing:-

1. My bench power supply was connected to each channel of the amp in turn. On both channels the 100mA current limit came in at about 23VDC and the midpoint voltage was about 9VDC

2. The 2.5A DC fuse was fitted to each channel of the amp in turn which applied the full 60VDC to that channel. On both channels the midpoint voltage was about 24VDC.

3. A check was made to ensure that there was no DC voltage across the speaker terminals.

4. An AF signal generator was connected to the AUX input and a power meter to the speaker terminals of each channel in turn. With a 1kHz input 10 Watts output (the limit of the power meter) was achieved from both channels.

5. The power meter was replaced with an 8 ohm speaker and the 1kHz tone was audible from both channels with no distortion or mains hum.

-------------

It was time for some listening tests, so I connected a CD Player and a pair of speakers. I would describe the sound as good and undistorted. I don't do flowery language to describe sound.

Unfortunately after about half an hour of playing CD's there was a loud "crack", I don't know where from, and the amp died.

I performed an immediate post mortem. The AC and DC fuses of the amp were intact, but the 3A fuse in the mains plug had ruptured. With the lid off there was a smell of transformer burning, but the toroidal mains transformer showed no visible signs of distress. All four output transistors were intact. There was no visible damage to any components.

The amp is full of tants which I know have a habit of going short-circuit and sometimes blowing up. None of them tested low resistance though.

I replaced the fuse in the mains plug and with the DC fuses removed brought the amp up using a Variac. There were no signs of distress and, once full mains voltage was reached, 60VDC was present at the output of the PSU, off load of course.

I then repeated the tests described above with the same results. The mains transformer stayed cool throughout.

Time for some listening tests again. This time I left the lid off and monitored the 60VDC supply, ready to switch off if it showed signs of dropping. I also felt the mains transformer and smoothing cap after each track, but even after an hours playing they were cool to touch.

I have an unexplained problem which so far hasn't repeated itself. I'll do some more listening tomorrow.

I did wonder whether one of the mains transformer's windings had suffered an insulation breakdown to the chassis. I don't think either the amp or the CD Player has three core mains flexes though.

Any further advice gratefully received.

[Radio Wrangler]

It looks like you've got a result on the amplifier stages, but with the nasty feeling that something unknown is floating around. For the transformer, you could try the highpot test from a PAT tester just to check primary insulation.

Good job that suicide pot isn't there. I looked at it and went "They can't have!" I suppose it didn't last long.

David

[turretslug]

I wonder if that preset current sense wasn't a conventional type with a rotating wiper etc. but something more like a tweakable shunt as in multimeters and ammeters- a short length of wire with a slideable screw terminal set by the proverbial "qualified personell" at manufacture on a jig? I can't quite make out the value from the diagram, but assume from the general circuit spec. that it would certainly be a very low resistance, perhaps the sort of thing that would be difficult/unusual/expensive to both procure and get past the accountants. I certainly concur with the point that it would be a weak link and liability as anything other than a firmly screwed-down connection type of preset.

It's good to hear that it seems to be back in action again- dependent on circumstance/mood/potential expense, my attitude to high-current DC-coupled amps varies between "weary anticipation" to "dread" and I can't be alone in that! Fingers crossed that the fuse blowing was a freak one-off, but I absolutely concur that it's difficult to place trust in something that does that out of the blue. I daresay that concealed Rifas have been ruled out, could the toroid have suffered insulation damage by dropping/overtightening, or a tiny copper whisker/solder drop be inducing a time-bomb short somewhere on the primary side?

Colin

[Radio Wrangler]

Are the amplifier fuses timed or slow-blow?

Plug top fuses are quite quick

David

[Station X]

SORTED!!!

The clue was in the "crack".

Originally Posted by Station X

Unfortunately after about half an hour of playing CD's there was a loud "crack", I don't know where from, and the amp died.

turretslug wins the prize for remote fault finding.

Originally Posted by turretslug

I daresay that concealed Rifas have been ruled out?

Colin

Wired across the incoming mains supply was a 0.1uF Durolit VIMA capacitor in series with a 47 ohm resistor. The capacitor had gone short-circuit putting the full mains voltage across the resistor. Being unable to dissipate around a kilowatt it vaporized, I could find no trace of the remains, giving rise to the loud crack and the smell. The components were positioned in the circuit before the internal mains fuse, which is why only the fuse in the mains plug ruptured.

I fitted a new Class X2 capacitor and new resistor.

The primary of the mains transformer was Meggered to chassis and found to be OK.

The unit is on soak test at the moment with the lid off. If it's still running OK at midnight I'll declare the job done and refit the lid.