Arcam Alpha mk1 gave me a fright

[gridrunner]

Hello all,

I've had an original Arcam Alpha amp chugging away for a few years now. The only thing I've done to it in the last 5 years was to replace a couple of capacitors and the mains flex. Recently I'd started to notice a bit of a small crackle and disturbance at low volume with the amp at rest. While listening to some music this suddenly escalated into extremely loud popping and crackling - loud enough to make me fear for my speakers and switch off. I found the service manual elsewhere on the forum but would appreciate any pointers from a troubleshooting perspective. It's been a good workhorse and sounds good enough for what I need so I'd love to keep it going.

thanks all

Stu

[Mooly]

Prime suspects are the two transistors that run hot, the VAS (Q8) and the current sink (Q7). Be sure to use a DBT (Dim Bulb Tester aka Lamp Limiter) when fault finding and it would probably be good practice to replace at least these two transistors and the drivers. Output devices never seem to go intermittent in my experience. Also do check for dries on these hot running parts.

Turn the bias down before powering up with replacement semiconductors fitted.

I covered a seemingly identical fault here and I hope no one minds me posting a link. It is a very long and detailed thread that begins with almost your exact words:

It started off with a very quiet scratchy/crackly noise on the left channel (the one that's blown), which was annoying so I switched the amp off for a while.

https://www.diyaudio.com/forums/soli...el-broken.html

[majoconz]

I have a locally produced amplifier - a Fountain AF2550 - with an almost identical problem. An initial cleanup after purchase from our local website, lube and switch cleaner applied to the pots, replaced a dodgy power switch, reset the output stage bias to what I thought would be a reasonable 15mA and it all sounded good on the bench without cabinet.

Then the problem began - after about 4 hours of use in the cabinet it suddenly burst into 25 watts of snap, crackle and pop - in the left hand channel only and not affected by the input or the preamp controls.
Reflowed all the soldered joints on the left hand power amp board, tested the electros for ESR, including the big ones and all good. This appears to be a typical quasi-complementary design with 2 x 2N3055's in each channel, a couple of driver BD139s and BC140s and a diode totem pole, 40volt single rail power supply. Unfortunately no info found on the web's usual places so it's all guesswork.
Put it all back together and...exactly the same! Put it back on the shelf in disgust!
This was intended to be an amplifier/small speaker setup for a recently purchased LCD slim telly with those pitiful tiny tinny speakers. One day when the weather's bad and swmbo's to-do list is pretty short I'll give it another go. Fortunately the right hand channel doesn't act up so I'll start transposing electro's one at a time and warming things up with the hot air gun to bring the fault on - hopefully. But how do you know when you've fixed an intermittant fault - how long is a piece of string?

[gridrunner]

Thanks for the all the advice so far. The Arcam is in the naughty pile while it thinks about what it tried to do to my ageing wharfedale speakers!
I'm going to start working though it based on the advice offered. Can someone recommend a preferred way to protect any speakers used for testing? I'm going to use a different set of bookshelf jobs for this, but they are still quite precious to me. I did see different advice on the post about light bulbs and capacitors, but is there a definitive way of putting some inline protection between the speaker outputs and the speakers while testing?

thanks all.

Stu

[Refugee]

Use a tapped dummy load.
Something like 6.2/3.9 ohms or if it is really bad and the test speaker is small 9.1/1.0 ohms.

[Mooly]

Couple the plus speaker output via a 220uF or 470uF electrolytic of 100vdc working to the speaker and connect the speaker return to the main negative rail.

The amp will behave like an AC coupled design, the speaker is safe from DC and the cap can not become reverse biased.

You could also just use two back to back electrolytics to form a bi polar cap and just place that in series with the speaker.

If the noises when faulty are to loud then add a series resistor as well.

[SiriusHardware]

Series capacitors won't stop big, sharp pulses which could still theoretically damage speakers. I would be inclined to use resistors as substitute speakers and monitor the output audio and any DC offset on a scope.

When the fault reappears, maybe attacking suspect components with freezer spray might narrow things down.

I had a Kenwood MIDI system which was behaving this way but it was because it was chock-full of bad electrolytics, many visibly leaking. The more obviously faulty caps I replaced, the longer it took for the fault to come on from switch-on. To fix it for good, I had to completely recap it because I didn't have enough hours left in my life to change one or two suspect capacitors and then wait for up to half a day for the fault to come back on, repeat.

[HECTOR63]

In years gone by didn't Boston Speakers(or Acoustics) use to employ a festoon bulb for there protection? We tried this years ago with car speakers and proved very helpful if and when we came across issues such as this.

[HECTOR63]

https://www.simplyspeakers.com/jbl-s...-bulb-sk3.html

Just an example.

[gridrunner]

Hi folks,

I've made a start by socketing all the Op-Amps and I'll a take a look at those suspect transistors and report back.

stu

[gridrunner]

Hi everyone,

I'm just reporting back with news of a successful repair. The low volume rustle has gone and the sound is sweet again with no explosive static.

It was something of a shotgun repair in the end, but these were the parts replaced:

All 4 Op-Amps fitted with sockets and replaced.

IC201, 202, 1 - NE5532
IC101 - TL071

Transistors:

Q7,Q107 - ZTX653
Q8,Q108 - ZTX753

Driver transistors x2 TIP2955 x2 TIP3055

[Mooly]

Excellent, well done. I'd actually bet on the opamps being OK, they are super reliable as a generalisation but those hot running transistors can and do fail.