Cheers anatech. My meter does have Hfe but it's broke. I searched for GFA-565 and nothing comes up. Searching for Adcom brings up dozens. Can you remember when about you posted it?
Thanks for that long explanation, anatech. I'll bear in mid your advice re; a matched pair of input transistors, but in the meantime whilst in my workshop this afternoon, I 'soak tested' the A3i for around 3 hours continuously, playing 3 CDs in succession, with no sign of any problems, and very good quality sound. I'll repeat the tests tomorrow, then check the DC offsets again.
Hi spandrel,
It was a long time ago, but I have a very poor sense of time. Thinking back I should have started a thread about it, but I posted the information in order to help some people while they were working on their amps. It came up a couple of times. Let me have a quick look, it will be in the solid state forum for sure.
Here's one that I have tested, and it works fine: Matching transistors & measuring the results
Here's something I found that should be mine: https://www.diyaudio.com/forums/solid-state/286742-adcom-gfa-565-thread-5.html#post4791822
With any luck, you can get one of these working. I use mine a lot, sorting out matches when I buy a bunch of transistors. Something I need to do again as I use matched pairs often. The more transistors you buy, the more tight matches you will have percentage-wise. I normally buy 50 or 100 from authorized distributors. Digikey for example, Mouser, Newark - you get the idea.
-Chris
It was a long time ago, but I have a very poor sense of time. Thinking back I should have started a thread about it, but I posted the information in order to help some people while they were working on their amps. It came up a couple of times. Let me have a quick look, it will be in the solid state forum for sure.
Here's one that I have tested, and it works fine: Matching transistors & measuring the results
Here's something I found that should be mine: https://www.diyaudio.com/forums/solid-state/286742-adcom-gfa-565-thread-5.html#post4791822
With any luck, you can get one of these working. I use mine a lot, sorting out matches when I buy a bunch of transistors. Something I need to do again as I use matched pairs often. The more transistors you buy, the more tight matches you will have percentage-wise. I normally buy 50 or 100 from authorized distributors. Digikey for example, Mouser, Newark - you get the idea.
-Chris
Let's try the original Diptrace files - darn, I can't rename the file extension (Win7).
I'll have to send them to you directly, or I'll zip them! Got it!
Okay, so these files are for a program called DipTrace. It's a great (free) schematic and PCB design program. From there you can print the files and generate PCB diagrams that you can use the toner transfer method.
I hope this does it for you. If you don't have DipTrace, get it. The staff there has been extremely helpful to me over the years.
-Chris
I'll have to send them to you directly, or I'll zip them! Got it!
Okay, so these files are for a program called DipTrace. It's a great (free) schematic and PCB design program. From there you can print the files and generate PCB diagrams that you can use the toner transfer method.
I hope this does it for you. If you don't have DipTrace, get it. The staff there has been extremely helpful to me over the years.
-Chris
Hi spandrel,
I can't imagine how long it took you to find, but thanks. It's so hard to find that it's almost a secret!
-Chris
I can't imagine how long it took you to find, but thanks. It's so hard to find that it's almost a secret!
-Chris
As of yesterday afternoon, after re-checking the Iq adjustments and running the amp for over 3 hours whilst playing CDs, the DC offset on the RH channel was only 14mV, whilst that on the RH channel was 43mV, so, to me, that's 'job done', and 'all's well that ends well'. Incidentally, if, by input transistors, you are, as I would expect, referring to Q1 & Q2 (Q101/102), then, due to the physical layout of the PCB, they cannot be in physical contact with each other.
The legs of Q1,2 can be extended a bit to get them in thermal contact. It's more important to match the Hfe though, but that's not easy and as anatech said you need a bunch of BC550s.
Anyway well done for getting it running.
Anyway well done for getting it running.
O.K., but as another 3hour test this afternoon didn't show up any problems, albeit that for most of that time the amp was run at medium to low volume, playing CDs in the background, whilst I was busy with other things, after which I re-checked the Iq. with no speakers connected, I'm inclined to leave things as they are, although the DC offset in the RH channel is still, at best 40mV. After all I found and fixed the original faults several days ago (no -15v supply due to bad solder joints, and an o/c BC639(Q10), which I replaced with a 2SC1845 I happened to have in my spares box (lower Ic than a 639, and I didn't check it's Hfe either!-sacrilege I hear you cry!) This could possibly account for the higher offset figure in the L.H. channel
Concerned about low current handling of 2SC1845, especially as anatech recommends replacing drivers with MJE235/245.
What Iq did you decide on in the end?
What Iq did you decide on in the end?
BC639/640 are the VAS transistors. Because the original design was uprated to 60W from 40W by increasing the supply voltage, it runs hotter and it's the heat that kills the BC640, its BC639 current-sink or LED voltage reference. If you read the beginning of the thread, that's the issue Alex (X-pro) addresses several times over. He recommends raising those transistors up to 15 mm off the board as well as the LED. It works, as long as the replacement parts are genuine, full rated spec and not low grade generics as we find everywhere now.
Its may be possible to replace these with electrically identical and same pinout BD139/BD140 These have the same TO126 package as MJE243/253 but you don't need the latter's 4 amp rating which is intended to drive a pair or more of power transistors. I doubt that subbing T0126 transistors is either easy to do or in the case of MJE243/253, you probably would have to alter the compensation and that's not a simple matter. Stay with same spec parts if you want to keep things stable and doable.
Cambridge Audio A3i - Manual - Stereo Integrated Amplifier - HiFi Engine
Its may be possible to replace these with electrically identical and same pinout BD139/BD140 These have the same TO126 package as MJE243/253 but you don't need the latter's 4 amp rating which is intended to drive a pair or more of power transistors. I doubt that subbing T0126 transistors is either easy to do or in the case of MJE243/253, you probably would have to alter the compensation and that's not a simple matter. Stay with same spec parts if you want to keep things stable and doable.
Cambridge Audio A3i - Manual - Stereo Integrated Amplifier - HiFi Engine
Thans, spandrel and Ian for your comments and advice. I do have the manual and have been making good use of it, although PCB layout diagrams, which din't seem to be available anywhere, would be helpful. All the LED's and transistors in the example on which I;m working are already raised off the board, the LED's having sleeved leads. Maybe I ought to replace the BD639/640s in both channels with BD139/140's, although this started out as a 'get it working again' repair for someone else, not a rebuild/upgrade project, though if the changes should improve long-term reliability, no doubt they should be carried out.
Get genuine quality BC639/640 parts first if you can. On Semi's are soon to disappear entirely with only BC639, BC639-16 now on last shipment status. This would be my preference and we know it works well enough. Perhaps the sleeving the leads isn't such a good idea when cooling is at a premium though.
I'm sure Alex's upgrade plan is a good one but it isn't necessary to get the amplifier going reliably. Unless you want to do the upgrade regardless, I'd leave it be but I would be concerned about getting fake or off-spec semis which may work initially but go the same way when the heat is on.
I'm sure Alex's upgrade plan is a good one but it isn't necessary to get the amplifier going reliably. Unless you want to do the upgrade regardless, I'd leave it be but I would be concerned about getting fake or off-spec semis which may work initially but go the same way when the heat is on.
O.K. Ian, I'll look to get BC639 &640. Incidentally (1) the 2SC1845 isn't a fake. it's one of a pack of 5 I bought from the now defunct Willow Vale Company, with whom I used to have an account years ago. Incidentally (2) It's the LED's which are sleeved, and have been that way since the amp. was made. Incidentally (3) I ran the amp again this afternoon for the best part of 3 hours with no sign of any problems.
Replacing BC639/640 (Q8, Q10, Q11, Q12) in A3i with TO126 transistors has benefits:
(1) Q11 and Q12 can be attached to Q13 and Q14 respectively, which improves thermal stability (requires running short wires to where the transistors were originally);
(2) TO126 is better suited for Q8 and Q10 with their dissipation and can even be heatsinked, helping to keep the transistors and the (paper based) PCB cooler and hopefully more reliable in the long run.
x-pro reported that MJEs work better than BD139/140 - see post #19.
I modified two A3i's with MJE243/253 and can confirm they work well and do not require re-compensation - in fact, x-pro recommended slightly de-compensating the modified A3i to add loop gain and decrease distortion at HF. Apparently, there is also room for 2nd order compensation, but I did not try this one.
(1) Q11 and Q12 can be attached to Q13 and Q14 respectively, which improves thermal stability (requires running short wires to where the transistors were originally);
(2) TO126 is better suited for Q8 and Q10 with their dissipation and can even be heatsinked, helping to keep the transistors and the (paper based) PCB cooler and hopefully more reliable in the long run.
x-pro reported that MJEs work better than BD139/140 - see post #19.
I modified two A3i's with MJE243/253 and can confirm they work well and do not require re-compensation - in fact, x-pro recommended slightly de-compensating the modified A3i to add loop gain and decrease distortion at HF. Apparently, there is also room for 2nd order compensation, but I did not try this one.
Alexcp, BD139/140 were indeed specified for use in A3i but as as drivers (Q11,12) for the power output stage (Q13,14). This is what x-pro refers to in posts 13-19 where he is queried about their merit compared to MJE243/253 for that application. They may work well enough in the VAS too but 4 amp drivers with 50pF Cob wouldn't be my choice there in any amplifier.
As I've just ordered BC639 x 2, & BC640 x 2 with a view to replacing Q8, 10, 108 & 110, these are what I shall use. My present inclination, given that the A3i works well for hours at a time as it is, is simply to replace the 2SC1846 I used to get the LH channel working with a BC 639, leaving the other original transistors in place.
x-pro ended up replacing Q8 and Q10 with MJEs as well, see post #93. I see nothing wrong with other choices though.