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Old 23rd March 2011, 03:00 PM   #11
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Quote:
Originally Posted by underwurlde View Post
Weird, I've NEVER 'cranked' my amp. God know's why it popped and why BOTH channels exhibit the same failure mode.
Seen it all now!
OK, Q6 appears fine! (just buzzed it only though, lost my damn transistor tester).
This means from my view unwanted oscillation (checking about oscilloscope).
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Old 23rd March 2011, 05:18 PM   #12
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^ Nope, nothing oscillating on either channel... everything seems sucked down to the negative rail.

Andy.
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Old 23rd March 2011, 05:36 PM   #13
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Originally Posted by jaycee View Post
ostripper, they got it from the A60 which was designed to a tight budget, in the very early days when everything was expensive Despite the crudeness they are usually quite reliable, and usually only fail when "cranked"
"Fail when cranked" , what a bummer .... That's the first thing I do on a new design , until the cops come.

OS
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Old 23rd March 2011, 06:57 PM   #14
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the only way is the DC condition testing of each follow sections independently:
1) Power supply without amplifier sections (transformer, rectifier, caps) - cut therefore the pos. and neg. rail and connect parallel to each power cap resistors between 22K and 33K (for discharging after switch off).
2) front end, i. e. diffential LTP amp and VAS stage (disconnect power buffer stage and short the Vbe multiplier, additional connect the NFB-Resistor at the short bridge wire)
3) buffer without short protect and without load (create a new independend Vbe multiplier therefore and check idle current stability)
4) like number three but now with short protect
5) like number four but now with commonly load
6) completly amplifier include front end
Attention !!! If you are beginner, I don't recommend this steps

Last edited by tiefbassuebertr; 23rd March 2011 at 06:59 PM.
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Old 23rd March 2011, 07:12 PM   #15
jaycee is offline jaycee  United Kingdom
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My other guess would be the VAS transistor itself, Q8. I'd imagine this could run fairly hot and die. MPSA56 are still available fortunately. I'd recommend replacing with Onsemi's MPSW56 if you can, it's a TO-92L package and will dissipate heat better.

It might be a good idea to measure the voltages at the indicated test points relative to ground. This would give a good idea whats going on. Also measure on the collectors of Q2, Q3 and Q4.
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Old 23rd March 2011, 07:13 PM   #16
jaycee is offline jaycee  United Kingdom
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Quote:
Originally Posted by ostripper View Post
"Fail when cranked" , what a bummer .... That's the first thing I do on a new design , until the cops come.

OS
This was a mid 70's budget design, as I recall reading (I wasn't alive then!). For the domestic use they would've seen, they worked well
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Old 28th March 2011, 12:20 PM   #17
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Looking at Left channel to begin with~:

R40 getting hot. Q12 (FST240), upon removal, appears to be OK. D2 OK too.

Will work it back now, if I can get my head around this stuff.

R30 has 1.8V across it... should only be 0.7ish >> Q6 must be at fault... where's that damn transistor tester!

Last edited by underwurlde; 28th March 2011 at 12:27 PM.
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Old 28th March 2011, 07:25 PM   #18
jaycee is offline jaycee  United Kingdom
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R40 getting hot is expected as Q12 will be hard on (hurr hurr). It might be worth pulling the VI transistors Q9 and Q10, testing them, and seeing how the amp behaves without them (use a bulb limiter in the mains just in case!)

It is also worth noting that on the Alpha III, Arcam replaced Q7 with a BD139, though that amp does also have a higher rail voltage to deal with.
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Old 29th March 2011, 08:16 AM   #19
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Right, borrowed an AVO meter. Now we'll see...

Cold checked all the semi's and no problem found with leakage, O/C junctions etc... still the power transistors heated up.

Turned the quiescent pot down to zero, this time no heating!

Slowly turned up the current to 7mV across 0R22 and all fine. Power switch turned off then on: immediately power transistors started heating again! Now there was over 300mV.

Long thought....

Checking other channel demonstrated the same problem.

After a while considered the amplifiers were probably going unstable, the 'earth' connection is referenced back through very long track loops. SO disconnected the 'GND' link to C12, C13, R22 and R24 (part of the bias circuit) and referenced it directly to the earth at the centre of D3/D4 C210/C211, (also essential to have a 22k resistor wired from Q2 base to this new temporary ground). Discovered the pot had to be set mechanically higher to reobtain 7mv across 0R22 resistors (?) but more importantly no longer goes into parasitic oscillation when first powered on!!!!!

A classic ground loop problem and this is the original Arcam design too! To be honest it's poor as the quiescent current is affected by whatever signal currents are on the GND line.

Now I'm wondering what I've done to the amp to suddenly 'create' this problem, where it didn't exist before!

Thanks again,

Andy
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Old 29th March 2011, 08:37 AM   #20
jaycee is offline jaycee  United Kingdom
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Hmm so you have oscillation now? How old is the unit? It could be up for capacitor replacement simply due to age, and this is causing oscillation. Being stuck at the negative rail does suggest a fault though.
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