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Old 2nd December 2007, 05:46 AM   #1
steve_v is offline steve_v  New Zealand
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Default A&R Cambridge A60 help

Have replaced blown output transistors, placed 15R resistors across rail fuses as per service manual but the quiescent current is far too high.
Seems the culprit is still there...
Anyone wanna take alook at the schematic and give me some pointers?

http://www.retrohifi.co.uk/a60_amp_service_manual.pdf

Cheers,

Steve.
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Old 2nd December 2007, 10:11 AM   #2
jaycee is offline jaycee  United Kingdom
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If the outputs blew it's likely that the driver transistors are damaged too. Check these - Q18/19 on left, Q118/119 on right.

You will need to adjust the quiescent current whenever you fit new transistors. This is done by adjusting RV2. Honestly, I don't like the way the bias circuit works on this amp, but thats how it works as stock!

If you're prepared to do a little tweaking, this page is full of useful goodies: http://myweb.tiscali.co.uk/nuukspot/decdun/a60mods.html

If it were my amp, I'd definitely do the mod of replacing R50 with the "amplified diode" (VBE multiplier).
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Old 2nd December 2007, 11:35 AM   #3
steve_v is offline steve_v  New Zealand
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Thanks mate, will have a look tomorrow.
Any ideas on a more readily available replacement in case they are toast? I cant find ANYTHING on the 'net regards an FST 240 or FST 239 transistor.
I see someone has replaced Q15 (orig FST 239) with a BC337...

Cheers.

Steve.
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Old 2nd December 2007, 12:00 PM   #4
jaycee is offline jaycee  United Kingdom
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Wow, you have a really old one!

See the schematic for the later versions. Arcam switched to the Zetex ZTX653/753 transistors. Looking at parts, BC639/640 may also be suitable, but the pinout is different.

I would replace with the Zetex transistors. You can get these from Rapid Online in the UK. Farnell also stock them.

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Old 4th December 2007, 09:05 AM   #5
steve_v is offline steve_v  New Zealand
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Hah, SICOM NZ has them. Surprise of the year, just waiting for them to arrive.
I figure I might as well replace all of them as they are a lot cheaper than I thought they would be, will also do the A&R Cambridge upgrade from the service manual. Toying with the amplified diode mod idea... but it's not my amp, and my mate will probably just blow it up again in short order.
The rubber feet on the bottom are partially melted so I would say it gets a fair thrashing. Might have to scrounge a bigger heatsink from somewhere...


Cheers,
Steve.
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Old 4th December 2007, 10:04 AM   #6
jaycee is offline jaycee  United Kingdom
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Ahh, this isn't really an amp designed to take a thrashing!

At this point, the power supply capacitors could probably do with being replaced too, but if he's going to thrash it, its not worth the expense :/
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Old 7th December 2007, 02:53 AM   #7
steve_v is offline steve_v  New Zealand
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Bugger, replaced drivers (actually replaced q13,15,18,19) I still have ~800mA Idle current :-(

Any Ideas?

Cheers,

Steve.
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Old 7th December 2007, 07:11 AM   #8
steve_v is offline steve_v  New Zealand
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Done some testing, voltages as marked in red...
Something is biasing the outputs on but I don't know enough about amplifier design to see what.

Any ideas?

Cheers,

Steve.
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Old 7th December 2007, 07:54 AM   #9
Mooly is offline Mooly  United Kingdom
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Hi steve
Place a short across R50, the 220 ohm resistor and see if current drops right down. Is the output at zero volts? Are the main P.S.U. reservoir caps O.K. not open circuit or anything.
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Old 7th December 2007, 09:38 AM   #10
AndrewT is online now AndrewT  Scotland
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Hi,
what is the DC voltage on each of the supply rails when you take these measurements?
Can your DMM measure AC voltage when also measuring across a high DC voltage?
I can set my DMM to 200.0mVac, when reading across 50Vdc and get an approximation of the ripple (average) on the DC rail.
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regards Andrew T.
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