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Old 11th Mar 2019, 9:33 pm   #1
rockinmark
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Default Cambridge Audio A500

Hi I was given this amplifier. The person that gave it to me found it in loft. Before I put any power on it I thought I would look inside. Looking at it one of the sap15ny transistor is burnt out. What I would like to know, is do you think it is worth doing anything with I see on eBay that I could get sap15ny for about 10. Not sure why it burnt out. Or should I just keep it for spares or bin it
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Old 11th Mar 2019, 9:51 pm   #2
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Default Re: Cambridge audio a500

The chances are other component(s) have also failed, particularly other transistors in this DC Coupled circuit. so it may be time consuming to locate & rectify all the faults. Also when replacing the SAP15N, it's companion, the SAP16N, must also be replaced. The circuit may be available from HiFi Engine, so, if you don't already have an account with them, it will be worth your while to register (it's free) with them, and download the data.
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Old 11th Mar 2019, 9:53 pm   #3
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Default Re: Cambridge audio a500

Well worth repairing in my view but then I'm a bit of a fan of Cambridge Audio. However, it's worth trying to establish why the darlington pair unit blew otherwise its replacement could go the same way - very frustrating.

Alan
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Old 11th Mar 2019, 10:04 pm   #4
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Default Re: Cambridge audio a500

One output transistor failing should ring an alarm bell. It did not die by itself -- something killed it, and that something will still be there to kill the replacement transistor if you don't fix it.

You'll have to unsolder and test every transistor in the faulty channel. Start with the other output transistor. If it measures short C-E, you have a fighting chance. Both output transistors short C-E (or blown to bits; that can follow a C-E short fault) is often caused by a short circuit on the speaker wiring and if you are lucky, they will be the only components damaged. If and only if the other output transistor is short C-E, fit any matched pair of reasonably-powerful transistors (BD138/139, TIP41/42 or similar) and power the unit up (via a lamp limiter at first won't hurt, if you have one). If the speaker output is at 0V DC, try connecting a test speaker (not one of your best ones) and a CD player or other audio source. You should get some sort of sound through it, but do not turn it up too loud as the temporary output transistors probably will not be good for as much power as the originals. If the original transistors were especially high gain types, you may get less output and distortion set in sooner. If the smoke stays in for an hour or so, you can breathe a sigh of relief and order yourself the exact replacement parts.

If the other output transistor tests OK, or any other fault than short-circuit collector to emitter, then it will be pointless trying temporary transistors yet. You will just have to unsolder and test every transistor to find which one(s) are faulty. While you are at it, take the opportunity to check for any overheated resistors, bulging capacitors, dry solder joints and so forth.
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Old 12th Mar 2019, 12:11 am   #5
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Default Re: Cambridge audio a500

Would all the sap15 have to be changed
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Old 12th Mar 2019, 12:48 am   #6
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Default Re: Cambridge Audio A500

Only the ones in the faulty channel if the other is working OK.
Here's a previous thread with a similar? fault and a service manual: https://www.vintage-radio.net/forum/...d.php?p=657162
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Old 12th Mar 2019, 1:40 am   #7
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Default Re: Cambridge Audio A500

For the avoidance of confusion here it's worth studying the manual carefully, particularly in relation to the power transistors in each channel. These are both SAP15 devices (suffixes N & P respectively) and each package effectively encloses a pair transistors (see Post #3). All rather unusual.

Alan
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Old 12th Mar 2019, 4:10 am   #8
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Default Re: Cambridge Audio A500

As said before, it's unlikely that the visibly disrupted power device is the only fault. In dying, it likely connected its base to the power rail at least momentarily. Fault condition currents don't obediently follow expected signal flow paths and spread the destruction.

You have to find and replace ALL damaged parts before you apply power to it. Otherwise your new parts get destroyed by the remaining bad ones. You then think "Ah, there must be another bad transistor" and go checking elsewhere, believing that the part you just replaced must be good because it's new. In this way you get led round in circles needing to replace some parts again and again. This characteristic of transistor amp faults has convinced many repairers to never touch the things again! At 10 a pop, it soon gets very expensive.

Test all the transistors and diodes, check all the resistors. An ESR check of the electrolytics won't go amiss. Some commercial repairers had a policy of just fitting a full new set of semiconductors to a power amp channel. As the rest of the devices are a lot cheaper than the output devices and it saves a lot of time and uncertainty it made sense.

Are those two Rifa smoke bombs on the mains PCB?

David
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Old 12th Mar 2019, 8:54 am   #9
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Default Re: Cambridge Audio A500

FWIW the schematic is here http://bee.mif.pg.gda.pl/ciasteczkow...A500%20amp.pdf

There are a number of merits to the overall design, not least of which is that all the other transistors are current parts.

The output devices are complementary Darlingtons with built in temperature compensation, so the design does not need the normal Vbe multiplier and removes thermal tracking issues. Sanken parts, alas discontinued.

The design seem to come from about 1999/2000.

Craig
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Old 12th Mar 2019, 10:13 am   #10
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Default Re: Cambridge Audio A500

Julie, as others have said, the SAP15/16 devices are Darlington Pairs in one package, whereas the two types you mention are each single transistors, so, whilst not impossible, it would be difficult to replace the SAP devices with BD138/139 or TIP41/42. As a matter of interest I currently have an A3i in for repair. That does use individual transistors - a BD139/2SC 2922pair(NPN) and a BD140/2SA1216 pair (PNP) in a similar circuit, so it's at least theoretically possible to replace the SAPs with the above or similar separate transistors, but a re-design would be necessary, and not to be recommended.
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Old 12th Mar 2019, 12:44 pm   #11
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Default Re: Cambridge Audio A500

I have a new SAP16N and an SAP16P purchased from Profusion a couple of years ago - I bought two of each but only used one pair. I think that these are the recommended replacement for SAP15s. If you would like to buy them, please send me a PM.

Ron
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Old 12th Mar 2019, 1:34 pm   #12
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Default Re: Cambridge Audio A500

PM sent ron
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Old 14th Mar 2019, 6:57 pm   #13
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Default Re: Cambridge Audio A500

Hi thanks, for everyone's help and to Ron for getting me the sap16.
I have now taken the boards out. looking at it I can see that the pot to that burnt sap15 side is a bit melted on top. If I am reading it right it is RV202. Would this be a repair already, looking at the other channel RV201 it has a different type, when built would they use 2 different types. what should I replace this with. Please see photos
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Old 14th Mar 2019, 8:56 pm   #14
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Default Re: Cambridge Audio A500

You really do have to test EVERYTHING on that board, or else it will take out your new transistors when power is applied. You don't so much repair transistor audio amps as reconstruct them.

If that pot has been hot it suggests significant current has been coming out of the power darlington's bases. This does not bode well for other devices on the board.

David
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Old 14th Mar 2019, 9:42 pm   #15
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Default Re: Cambridge Audio A500

I am wondering why it has 2 diffarant bias pots makes me think that it has been touched before. I still think that i may leave this amp alone, im thinking its a too low end for the time and effort
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Old 14th Mar 2019, 10:56 pm   #16
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Default Re: Cambridge Audio A500

Given my recent experience with an A3i, I'd second David's remarks re reconstructing transistor amplifiers. In that case I found one pre-driver transistor completely o/c, plus dry joints & bad printed track connections to the tone amplifier I.C. leaving no negative supply to the op-amp in question. This resulted in a DC voltage which varied with it's setting, on the volume control and the function switches. These faults in turn caused the speaker protection relay to trip in and out, which puzzled me for a while, and I've been working with transistor & valve circuits for almost 50 years!
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Old 14th Mar 2019, 11:31 pm   #17
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Default Re: Cambridge Audio A500

On another forum, reference is made to VAS, and, despite many years of repairing electronics, I've been trying to work out what that means. V, I guess, means Voltage, A, presumably Amperes, but S
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Old 14th Mar 2019, 11:36 pm   #18
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Default Re: Cambridge Audio A500

Voltage Amplifier Stage. Any power amplifier can be divided into three parts. The input stage, the voltage amplifier stage (VAS) and the output stage.

Most of the open loop gain of a power amp is in the VAS.

Craig

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Old 15th Mar 2019, 7:20 am   #19
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Default Re: Cambridge Audio A500

Quote:
Originally Posted by rockinmark View Post
I still think that I may leave this amp alone, I'm thinking it's too low end for the time and effort
It might not be the best amp on the planet but you'll learn a lot trying to fix it, though lock the good OP tranny in a deep vault guarded by trolls with big sticks till your 100% sure it's safe to fit it in. Oh and run it on a current limited PSU first power up.

Andy.
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Old 15th Mar 2019, 8:37 am   #20
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Default Re: Cambridge Audio A500

Quote:
Originally Posted by rockinmark View Post
I am wondering why it has 2 diffarant bias pots makes me think that it has been touched before.
I agree.
I have never worked on one and only have the internet for pictures, but the blue trimmer looks like a replacement. All the internet pictures show black pots... (By the way, both output Darlingtons are blown, if you look carefully at the picture the other is cracked.)

I'm not even sure it would be a good 'learning' experience to fix it, you really do have to just re-build it part by part and then hope. (We've all been there.) And then would you trust it with your speakers?

Personally, I would sell it as 'faulty for spares or repairs' along with the other faulty Cambridge amplifiers.

Sorry for the negatives, Alan
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