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Old 25th July 2015, 12:25 PM   #1
Skooz is offline Skooz  United Kingdom
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Default Arcam Alpha 2 Help/Advice

A while ago my Arcam Alpha 2 had a short across the right channel output, damaging several components.

After reading a couple of other threads about "[HELP] Repair Arcam Alpha 2 - 240v" and "Arcam Alpha 3 Left Channel broken..." I managed to find the faults, then replaced all the transistors, smaller capacitors and a couple of charred resistors in the power amp section.

Using the advice in the above threads, the BC547/BC557 were replaced directly, the FST240/239 were replaced with ZTX753/653, and the TIP3055/2955 with MJE2955T/3055T. And finally the MPS56 with 2N5401.

The amp works in all respects (bias adjusted fine, sound output is fine, all controls are working (balance is a bit iffy at extremes, but fine when centred))

- The 2N5401 (Q8/Q108) is running very hot on both channels - too hot to touch for more than about half a second.
- Is this normal (is it worth getting heatsinks for them) are these underpowered for general use?
- Also, the case has no venting, would adding some holes to the top panel be worth the trouble?

Any advice appreciated, thanks.
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Old 26th July 2015, 06:17 PM   #2
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I can understand substitutions for FST parts but surely there was no need to fit TO220 size substitute output transistors. It's not like TIP3055/2955 or even obsolete MPSA56/06 are hard to find. Q8 is the Voltage Amplifier and being only TO92 size, will get hot but it should not be over 75C even in a closed case. Some manufacturers, Arcam included, seem hell-bent on pushing components to their thermal limits and DIYs often fret over this, when they discover the truth about many power amplifiers designs

Both channel Q8s being the same temperature suggests there's probably nothing wrong and changing parts won't change the heat dissipated there. Id simply replace with the original type MPSA56 because they are more likely to be genuine and better quality parts if you buy locally. 2N5551/5401 are made everywhere now - mostly unbranded and doubtful quality copies though.

You should not need to drill holes but you could form a little flag heatsink by cutting a strip of thin auminium and rolling it smoothly, once around a smaller diameter rod such as a drill bit. It only has to hold in place with even contact and grip so that it doesn't fall off and short anything. The pic is just a similar item and the hole is not required.

Click the image to open in full size.
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Old 26th July 2015, 08:54 PM   #3
Skooz is offline Skooz  United Kingdom
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This is the first time I've tried repairing anything like this, (at worst, a dead amp remains dead) so I followed the advice in the above threads and assumed where it said 'xx can also be used' that they were interchangeable.

- The MPSA056 were missed when I ordered all the required parts (online from Farnell) and 2N5401 were suggested as a suitable replacement stocked locally (There's a Maplin near my work - very likely to be genuine) so I just bought a couple on my lunch break.

The amp now seems to work fine, and sounds fantastic - exceeding expectations.
I'll try the small wraparound heat sinks for those transistors, if only for piece of mind.

Thanks for the advice!
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Old Yesterday, 02:35 AM   #4
Bare is offline Bare  Canada
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Good for you. My experiences are far less stellar.
My Arcams D290 Inty and power ( Bought New.. grrr) proved Hopelessly Outclassed by a fairly inexpensive LM 3886 Amp.
Pissed and ecstatic.
Pissed at All the $$$ wasted on the poor sounding Arcam, yet delighted by the whole new world of Sound quality in my system as result of fitting a lowly lm 3886 based amp in place of the dual arcams.
Food for thought.. My Friend
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Old Yesterday, 03:06 AM   #5
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Genuine parts are a big worry for repairers of electronic gear but even at this distance I've had no problem with NOS parts from NIKKO-Dalbani. They will be worth a try since Farnell will only supply currently manufactured products. Maplin, I understand, are no longer the paragon of electronics supplies they once were. Search results for: 'MPSA56'

I'm surprised about the output transistors though. MJE3055/2955 (T suffix means TO220 size) are way smaller than TIP3055/2955 (TO218 or TO247 size). Are we speaking of the same parts?

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Old Yesterday, 12:17 PM   #6
Skooz is offline Skooz  United Kingdom
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The replacement output transistors are definitely smaller - splayed the legs slightly to fit though. The heat sink they're clamped onto barely gets warm until an hour or so of use, and then never feels hot.

I've tried the temperature sensor on my multimeter and it said 60C but don't know how trustworthy these are - I could borrow a proper infra-red measure, is it worth checking, or better to just assume dubious quality and replace with specified part anyway?

And finally, I used resin dipped ceramic (C0R) capacitors for the 10pF C14 (Maplin again - RA33), which were originally polystyrene. It 'works', but is it good enough for a permanent repair, or will this cause problems?

Thanks.
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Old Yesterday, 02:32 PM   #7
Skooz is offline Skooz  United Kingdom
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(All substitutions were made based on information on this, or similar models found on threads on the diyaudio forum)
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Old Today, 05:11 AM   #8
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Being a DIY design/build forum, replies and advice on repairs come from members with a very wide range of competence. Advice may be good, bad, questioned by others or simply ignored but in hindsight, you may have needed to probe further to get answers to the questions you didn't even know existed

Amplifiers such as your Alpha 2 have a few "tweaks" to the frequency compensation to get the sound just right in the designer's view and this was common practice in UK products back in the day. It would be advisable to use the original or similar parts where they can influence the sound quality and general performance. That's relative to your own preferences of course but I've heard a few "fixed" and "modded" amplifiers that were little short of excruciating to listen to.

The 10pF cap. is also a part of the frequency compensation but the multilayer COG ceramic type replacement won't cause problems. I'm surprised that the polystyrene foil type needed replacement actually - good work finding that problem. Purists might say polystyrene foil types are better for sound quality but it wouldn't hurt to bring the cap type in line with the other channel, as with all but the mundane component replacements in any stereo amplifier.

The new output transistors won't match the dissipation and power rating of the larger parts. The chip is also smaller usually and the amplifier should properly be run with lower voltage supply rails to suit the reduced safe operating area (SOA) of the MJE parts. This is another case of saying something will work but knowing full well it cannot work with the same margins as the original. If the manufacturer could have used the smaller and cheaper parts, it's certain they would have done so. Naim NAP90 and Nait models also used TO220 size parts but they eventually reduced the power rating from 30W, which is appropriate since modern speakers tend to be lower sensitivity and impedance and this becomes a harder ask for the output transistors.

There is only a fuse and fixed basic VI current limiting for the amplifier here and we should be aware that the protection offered by these is directly related to the output stage - specifically the SOA ratings and margins of the semis they are designed to protect. Also, the amplifier is rated up to 60W into 4Ω loads - quite a lot for a nominal 30W rated amplifier. Fit the correct TIP3055/2955 (it won't make any difference if they are newer TO247 outline) and rest easier with better protected and matched channels.

When I can't match replacement types and brands adequately between the 2 channels, I usually suggest the replacement be done in both channels but that will be your decision. TIP and MJE parts are also multi-source and with that comes quality issues like Hfe (gain) rating. Check these are similar or higher than originals where possible. On-Semi or ST Micro types are best quality but buying extras for the other channel is a good idea in this situation.
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