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Output stage fault? Advice please
Output stage fault? Advice please
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Old 8th August 2017, 09:59 AM   #1
vlv81 is offline vlv81  New Zealand
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Default Output stage fault? Advice please

Hi all, first post in the forum so please be kind lol

I have attached a couple of photos of a power amp I have. Today the right channel gave a constant hum from the right channel. In the photos, if you are able to enlarge them, you can see at least four of the resistors appear to have gotten very very hot. Using the same loud speakers for over the last decade or so, but i tried them on another amp and they still seem to be fine so not sure what has caused this failure in this amp. The volume at the time was just above 1/4. I don't know the actual output power of this power amp but they drive a pair of Mission 734 floor standers and the output of the amp at just under 1/2 is enough to reach the excursion limit of these drivers. 99% of the time I listen at about 1/3 volume so not sure what has happened.

I was about to solder in some new resistors and thought I should ask if anyone could suggest a possible cause or something I should look at as a cause for this sort of thing.

Thanks in advance
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Old 8th August 2017, 11:23 AM   #2
Bone is offline Bone  United Kingdom
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Its likely that the output transistors have failed. In which case the transistors coloured red and black probably have also failed. I assume that the resistors in question are those at the top of the board. If these are getting hot then this also indicates that the transistors have failed. You dont say which amplifier you have, do you have a schematic?
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Old 8th August 2017, 06:53 PM   #3
vlv81 is offline vlv81  New Zealand
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Hi Bone, sorry no schematic. It is an Aucom APA60 power amplifier made in Christchurch New Zealand. I have had no success finding an online manual, or any information for that matter. I contacted Aucom but haven't had a reply. The amp and pre-amp are both pretty old. I got them in 1992 second hand, and they were old then coming form a family member. I have had a few amplifiers in my life but keep returning to this, plenty of head room and with my speakers sound nice to my ear. There will be much better products available but the only other set up I really liked was a Rogers valve amp and some Tannoy's, just lacked the volume. Every other set-up I have had I eventually sold. Probably sentimental value more than anything else, buy something new, put it away for a few months and then drag it out again.
I'm not unfamiliar with electronics, made several Megasquirt ECU set-ups on cars from scratch but I have only ever built one amplifier, a Hood, when I was a teenager that a teacher from school oversaw as a project.

Thanks again, I guess I have a lot of testing to do...
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Old 8th August 2017, 07:28 PM   #4
indianajo is offline indianajo  United States
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Stressed power transistors can pass a double diode DVM test at 2v test voltage, but still be leaky.
You need to isolate the base (usually by removing) and do a Icbo leakage test. 12 v was enough to make mine fail, from a car battery charger plus parallel capacitor. Transistor should hold off nearly 2/3 of the rated Vceo voltage but leaky ones won't. Put a DVM current scale in series with the emitter of the transistor, plus maybe 470 ohms to keep the meter fuse from blowing. Then rated voltage on the collector (plus for NPN, minus for PNP) and read the current. Should be microamps; tens of milliamps shows a big problem.
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Old 8th August 2017, 07:43 PM   #5
vlv81 is offline vlv81  New Zealand
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Thanks indianajo. I will have to get stuck in this weekend I guess. You're testing method is a great idea, rather than the usual dmm on diode test mode. Doing the 12v battery charger etc... isn't something I have done before on an amplifier. I will let you know how I get on!
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Old 8th August 2017, 08:36 PM   #6
anatech is offline anatech  Canada
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Output stage fault? Advice please
Hi vlv81,
Test with the diode check first to avoid dramatics in case one is shorted. If it fails the diode test, it is bad. If you find a bad output, change the other in the same channel as well as the driver transistors. Can you see the part number on the outputs and drivers?

Do not buy replacements off Ebay! Most are fake / renumbered and therefore useless to you. They often fail under normal conditions as well. We will probably cross those to currently available transistors if possible. Finally, don't use replacement brand devices, that would include NTE / ECG / SK and other packaged replacement brand components. This is only if the outputs are damaged.

Your speaker load is reasonable, although your listening levels are high-ish. The important thing about how you run an amplifier is how hot it gets. Most amplifiers need 4~6" of free air above them at the very minimum. The hotter something runs, the more prone to failure it will be. The failure rate doubles for every 10C rise in temperature.

As far as testing goes, the higher voltage leakage test is a good idea but it helps to use equipment designed for this purpose. If you have access to a technician who can help you out, that would be the best.

Good luck! -Chris
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Old 9th August 2017, 02:32 AM   #7
nigelwright7557 is online now nigelwright7557  United Kingdom
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I think its just a matter of going around checking output transistors and driver stage for bad components. Use a mains lamp in series with amp when you come to power it up again. I would check all components in the amp. I have a transistor tester that also tests breakdown voltage.

These sort of problems can be frustrating as sometimes just replacing output transistors ends in them blowing up again on first power up.
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Old 9th August 2017, 04:15 AM   #8
kct is offline kct  Canada
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Output stage fault? Advice please
As per the image you posted, you might have to remove the circuit board to get easy access. Definitely you will have to replaced the burned resistors, it may be necessary to see what color code the have on the other channel and find suitable replacements.
I would also double check other resistors that seem to look fine but could also be damaged. This is most easy to do when the transistors are removed. The removed transistors can then be checked easy too.
It is advisable when taking the repaired channel back into operation to put a lamp in series with the mains to reduce the chance of damaging components again in case something got overlooked. Also a variac comes in handy at that time.

With the circuit board removed, you could take two more pictures for us, perhaps the circuit can be quickly reverse engineered in case you can not get hold of the schematic anymore.
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Old 12th August 2017, 08:34 PM   #9
vlv81 is offline vlv81  New Zealand
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Thanks so much to all of you!! kct, nigelwright7557, anatech, indianjo and Bone. I took an annual leave day from work on Friday and got to work on this "project". I believe this power amp is approx 30 - 40 years old. An Aucom Apa60. Long story short, I eventually removed the other channel from the amp and started testing it as well using methods listed by you etc... I have a long list of components that I believe will need replacement without even opening or looking at the Pre-Amp, the worst of which are transistors that have been painted from the factory making it impossible to read the part numbers, some don't even seem to have numbers on them. Aucom now make motor speed controllers etc... and any information on these amplifiers seems to be long since gone when queried by phone.
A quick look online for the items I need from places such as Mouser, Digikey etc... and a look around the forum has put the idea up that I could possibly build something much better for a similar or lower price and scavenging items from this amp, such as the case and heat-sinks. The biggest question is what to build? I'm thinking a First Watt F-5T, purchasing the boards from the forum store. Pretty sure this will outshine anything I have previously built or owned. I'm hoping not to get caught up in the upgrade cycle again (speakers, source units, cables etc...) as I have other priorities these days with home, work and automotive projects. Is there anywhere in particular people would recommend buying other components such as transistors, toroids etc.. from? For my automotive stuff I generally get things locally or I import from Digikey or known e-Bay sellers.

Thanks a bunch
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