Hi,
Looking for help diagnosing problems with an A and R A60. This amp arrived with quite a few burntout resistors and transistors. In essence it seems new transistors had been installed without insulating pads so as well as taking out the transistors and drivers it had taken out all the over-current limiting circuit. I recapped it and installed new transistors Q13-21 and the righthand equivalents. Also any resitor with signs of scorching or out of tolerance was replaced so around 80% of total. However, I did not install the over-current limiting components Diodes D1 and D2, Transistors Q16 and Q17, Resistors R54,R55,R56,R57,R58 and R59.
Basically I wanted to get the amp up and running first before re-installing those components.
On switching on, the Dc offset on the output is within tolerance at around 30mv per channel. However, the quiescent current is way too high on both channels. On one channel the minimum adjustment gives me around 112ma across R63, 64. The other channel measures 1.5v across R163,164. Most of the other voltages are as per the attached schematic except on the worse channel the emitted /base voltages on Q18, 19 ar around 1v.
David
Any help to help me diagnose the problem is very much appreciated!
Looking for help diagnosing problems with an A and R A60. This amp arrived with quite a few burntout resistors and transistors. In essence it seems new transistors had been installed without insulating pads so as well as taking out the transistors and drivers it had taken out all the over-current limiting circuit. I recapped it and installed new transistors Q13-21 and the righthand equivalents. Also any resitor with signs of scorching or out of tolerance was replaced so around 80% of total. However, I did not install the over-current limiting components Diodes D1 and D2, Transistors Q16 and Q17, Resistors R54,R55,R56,R57,R58 and R59.
Basically I wanted to get the amp up and running first before re-installing those components.
On switching on, the Dc offset on the output is within tolerance at around 30mv per channel. However, the quiescent current is way too high on both channels. On one channel the minimum adjustment gives me around 112ma across R63, 64. The other channel measures 1.5v across R163,164. Most of the other voltages are as per the attached schematic except on the worse channel the emitted /base voltages on Q18, 19 ar around 1v.
David
Any help to help me diagnose the problem is very much appreciated!
The way thermal compensation works on this amp is rather bad. It's worth making the "amplified diode" mod described on this page:
A comprehensive guide to upgrading the Arcam A60 amplifier.
edit: It's probably worth looking at the drops across R48, R50, R52 in both channels. If they're off, the bias will be higher. R52 should be 1V to give 10mA through the VAS - this is important as it will affect stability and the "sound" of the amplifier.
A comprehensive guide to upgrading the Arcam A60 amplifier.
edit: It's probably worth looking at the drops across R48, R50, R52 in both channels. If they're off, the bias will be higher. R52 should be 1V to give 10mA through the VAS - this is important as it will affect stability and the "sound" of the amplifier.
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Thanks for your reply. The thermal compensation is a technical bodge on these amps but on the other hand many thousands have run reliably and sounded great for many years, so it worked in the field. I did try the amplified diode mod on another A60 and despite it being built and installed correctly it immediately took out the output transistors on the channel tried. As the designer was no longer contactable and the Decibel dungeon author had no technical advice I didn't pursue this and achieved great sound quality with a high quality decent temp coefficient R50.
Voltages across R48 are 0.68v both channels
" R50 1.9v and 1.95v
" R52 0.85v and 0.85v
So a little off spec but not too bad. They also match pretty well whereas the quiescent current differs by a factor of 10........
David
Voltages across R48 are 0.68v both channels
" R50 1.9v and 1.95v
" R52 0.85v and 0.85v
So a little off spec but not too bad. They also match pretty well whereas the quiescent current differs by a factor of 10........
David
What is the voltage across R63 should be 8-9mV?
Connect a meter across R63 and adjust RV2 until you get 8-9mV.
Connect a meter across R63 and adjust RV2 until you get 8-9mV.
'However, the quiescent current is way too high on both channels. On one channel the minimum adjustment gives me around 112ma across R63, 64. The other channel measures 1.5v across R163,164.'
This is the problem!
This is the problem!
What revision of the circuit are we dealing with, and what have you used for replacement transistors instead of the FST239/240 ?
The circuit is big standard apart from removal of the current limiting components detailed in my first post, which shouldn't affect correct operation of the amplifier part.
I realise my attachment picked up all the circuit diagrams.my model is the power amp circuit on Sheet 5 which used ZXT653 and 753 transistors and those are what I have used too
I realise my attachment picked up all the circuit diagrams.my model is the power amp circuit on Sheet 5 which used ZXT653 and 753 transistors and those are what I have used too
Is it possible there's oscillation involved - do you have a scope to check this ? If not, is R66 getting hot?
OK. R66 on the worst channel is getting warm (the channel with 1.5v quiescent current). There's also quite a strong oscillation on that channel at 100khz. The other channel-with 120mv quiescent current-shows no signs of oscillation and R60 is cool to the touch
David
David
That would explain the drastically higher current being read on that channel then. I would check the compensation components next - R71/C36 and R51/C27. Make sure those 470pF capacitors are replaced with NP0/C0G dielectric type ceramics if you replace them. It's worth checking R51/C27 have their connection to ground.
Cheers fort that. However, my circuit-Page 5 of the attachment-serial nos 3981-19999-only goes to R69 so I can't identify R71 /C36
David
David
The originals wouldve been polystyrene.. however thats now difficult to get. If you REALLY want to go nuts, polypropylene eg Wima FKP2 are a good choice. I've personally never had any problems with good NPO ceramics
One problem with quality film caps is their size and susceptibility to noise and heat. Even small value polystyrene caps tend to be bulky and melt all too easily with the heat of soldering unless you're quick - no problem for techs who are aware and know how to tackle them but DIYs tend to do the damage before seeking help.
I can only agree with Jaycee that NPO or COG type ceramic caps are the better option these days, unless you're into the sonic flavour of different dielectrics which will be anybody's guess and game.
Common MLCC types we often see now, are also COG or NPO type but check catalogues to make sure before buying any old style Chinese product of X7R or similar ceramic material that really is the wrong type for stable frequency compensation and hence low distortion. (the signal voltage modulates the capacitance and that's a real problem for Hifi.
Here's a suitable example but there are other leaded shapes and also scads of SMD variations: https://au.mouser.com/ProductDetail...re0BjpKknw7zFbpc-fOWuXKkRSEBBNmhoCyUkQAvD_BwE
I can only agree with Jaycee that NPO or COG type ceramic caps are the better option these days, unless you're into the sonic flavour of different dielectrics which will be anybody's guess and game.
Common MLCC types we often see now, are also COG or NPO type but check catalogues to make sure before buying any old style Chinese product of X7R or similar ceramic material that really is the wrong type for stable frequency compensation and hence low distortion. (the signal voltage modulates the capacitance and that's a real problem for Hifi.
Here's a suitable example but there are other leaded shapes and also scads of SMD variations: https://au.mouser.com/ProductDetail...re0BjpKknw7zFbpc-fOWuXKkRSEBBNmhoCyUkQAvD_BwE