Some of us though do have experience with electronics and soldering.................
And I like my cambridge amp, it's cheap and good for the price a little tweak if someone could advise and it could be VERY good for the price.
Not all of us have deep pockets for expensive HiFi and upgrade kits, that's why we use forums like this to help up make the most of what we have.
Now I would like to bypass the tone controls ("Tone Defeat") and if possible upgrade some components if possible, I don't have a service manual for it so that would be good to.
Your help would be very much appreciated I promise.
Tom
And I like my cambridge amp, it's cheap and good for the price a little tweak if someone could advise and it could be VERY good for the price.
Not all of us have deep pockets for expensive HiFi and upgrade kits, that's why we use forums like this to help up make the most of what we have.
Now I would like to bypass the tone controls ("Tone Defeat") and if possible upgrade some components if possible, I don't have a service manual for it so that would be good to.
Your help would be very much appreciated I promise.
Tom
One member of this thread can send you all schematics. The schema from power amp section you will find about post #1. If you disconnect the connection between r1/c1 and the tone control so as the output of volume control to the tone control and connect the output of volume control to r1/c1 you get a shortest possible way for your signal.
Additional I would replace follow:
- LED1/LED2 through new types, if the old types are solder in without distance to the PCB surface
- Q8/Q10 (BC640/639) through BD139/BD140 and heatsinks
- R12/R17 through 5 watt versions (low inductance)
- C8 (100uF Elko) through 2x10uF at least, WIMA MKS4
WIMA
- C1 (10 uF Elko) through 2uF at least, WIMA MKS4
- power supply Elkos through 4700-10000uF, 63V version e. g.
Evox-Rifa | Passives | Capacitors | Aluminium | Clamp-fixing - ALS30, B43564 85°C |PEH200MA4470M
or
Evox-Rifa | Passives | Capacitors | Aluminium | Clamp-fixing - ALS30, B43564 85°C |PEH200MD5100M
4700uF/63V is always to prefer to a 10.000uF/35-40V, if space room to small for 10.000uF/63V. Prefer also screw-terminals (snap-in legs still higher ESR-values).
Additional you can make a bypass across your input source select switches (connection between one of the input sockets and the input of volume control). But now you need an external input select switch box with rotary switch therefore - e g. one model of
http://www.diyaudio.com/forums/soli...nny-giles-tkd-vishay-more-whats-your-fav.html
Don't forget the disass'y of your relay for speaker DC protect for contact polish.
Last edited:
Hi tiefbassuebertr,
-Chris
I strongly disagree with you on this. There are times when the manufacturer may have skimped on these, but then if you look, the supply voltage sag is what has been saving that amplifier design from going boom. Besides, large increases in filter capacitors have down sides. Before blindly charging ahead, it would be extremely wise to fully understand the circuit and know what effect the changes (no matter what they are) may have. No matter how good your intentions may be, the rules of physics apply all the time. A hobbyist who crashes on ahead with changes is foolish. A service person who charges ahead in this same manner is completely irresponsible as far as I'm concerned. Following popular trends as published on internet sites is not knowledge. That is "monkey see, monkey do". This tends to be one of the biggest challenges that really good technicians face these days. I hold service people who do this type of work without actually knowing exactly what they are doing in contempt. This should not be allowed, or at least the customer should have an accurate assessment of the technician's competence, and also what the true effects of whatever changes will be. After all, everyone knows that every change removed "a veil" from between the music and the listener. Insert other descriptive terms as you feel moved to do so. Note that I did not claim these people have bad intentions, although these types of technicians and hobbyists do exist. They create untold damage for profit.
Now in the real world, what are the actual differences, and under what conditions where they measured under? I can assure you that other connections will add more impedance than the terminal type difference is. That and you are now hacking a the PCB, which may throw a carefully designed grounding system out the window. Making any changes in the PCB for a dubious "upgrade" like this is pure folly. This goes back to some people not understanding the circuit fully before even suggesting any changes. It's really only a call for basic competence, nothing more.
Ahhhh, no. At this point you really would be further ahead building your own from scratch. Then you can create everything exactly the way you want it, right from the word go.
-Chris
Hello,
I have the amp a3i but i have a problem, when the volume is high the protection cut the output .( 3/4 volume approx sometimes less )
have you any idea of what is gong on?
which component is in default ?
is anyone has the electronic schema, it could help ?
Thank you for your response
Best regards
Philippe.
I have the amp a3i but i have a problem, when the volume is high the protection cut the output .( 3/4 volume approx sometimes less )
have you any idea of what is gong on?
which component is in default ?
is anyone has the electronic schema, it could help ?
Thank you for your response
Best regards
Philippe.
Thanks for the reply, do A3 and A1 amps have same/similar circuits?
Regards
Tom
Some A3i hints
Gentlemen,
thanks to x-pro for sharing his expertise.
Please let me take the privilege to add some comments
from my own repair experience of the power amp section.
A3i amps arriving in the workshop today suffer from various
forms of burnout due to local overheating and poor parts quality.
I have an original diagram in front of me, it corresponds well
with the one in post #1. All designators of parts of the second
channel have the same number plus 100, parts shared by
both channels start with 200 plus xx on the board and in the
diagram. Obviously all modifications apply to both channels.
This diagram is said to apply to "A3 (A2) amplifier".
The polarity issue of C216, 217 has already been discussed.
Note also that the designator C14 in diagram of post #1 occurs
at the output also, it should be C9 in said position.
Learn about the LED problem in earlier posts. But even with
new LEDs in my opinion the LED current is too high and so I
usually replace R16 and R18 with 10k in both channels. The
standard resistors of 3k3 are deteriorated most times and are
in no way 1 watt parts as seen on the board. Check R8 also.
Q11 and Q12 have no thermal tracking with output transistors
Q13, Q14 and sensor Q9. But Q9 senses all four Ube drops
and so the bias current will rise in an unhealthy manner when
Q11, Q12 heat up. I regularly add small heatsinks to these in
order to combat this problem.
Anyway, great care should be taken in adjusting the standing
current. The action of P1 is reversed (applies to both channels)
and it should only be operated cautiously when simultaneously
watching the voltage drop over R22 and R23 without signal.
I adjust to less than 10 millivolts and this is near the mechanical
centre of the adjustment pot.
I recommend using 4 amps quick acting fuses instead of the
standard fuses. When repairing or modifying the amp I insert
resistors instead of the fuses in order to limit unwanted current
due to failure. 15 to 30 ohms and at least 5 watts work fine and
dispense you from the usual habit of posessing a variac. Re-
member that a variac can supply disastrous currents any time
when not turned down to zero.
I also exchange C16 in both channels by an electrolytic cap of
say at least 470 uF in order to limit inrush current which I noted
to be unduly high. The cap has to bear a small voltage only, so
a tiny 6V3 type can be conveniently used.
In the case of failure of mosfet Q9 [January 2024 edit: This should read Q7] which is unavailable here I
substituted a p-channel j-fet successfully. If you encounter harsh
noise after some form of input overload have a look at Q1, Q2
and Q4.
A3i No. 1059-0998-0212 is now finished on my workbench and
I hope it will last some time again.
Gentlemen,
thanks to x-pro for sharing his expertise.
Please let me take the privilege to add some comments
from my own repair experience of the power amp section.
A3i amps arriving in the workshop today suffer from various
forms of burnout due to local overheating and poor parts quality.
I have an original diagram in front of me, it corresponds well
with the one in post #1. All designators of parts of the second
channel have the same number plus 100, parts shared by
both channels start with 200 plus xx on the board and in the
diagram. Obviously all modifications apply to both channels.
This diagram is said to apply to "A3 (A2) amplifier".
The polarity issue of C216, 217 has already been discussed.
Note also that the designator C14 in diagram of post #1 occurs
at the output also, it should be C9 in said position.
Learn about the LED problem in earlier posts. But even with
new LEDs in my opinion the LED current is too high and so I
usually replace R16 and R18 with 10k in both channels. The
standard resistors of 3k3 are deteriorated most times and are
in no way 1 watt parts as seen on the board. Check R8 also.
Q11 and Q12 have no thermal tracking with output transistors
Q13, Q14 and sensor Q9. But Q9 senses all four Ube drops
and so the bias current will rise in an unhealthy manner when
Q11, Q12 heat up. I regularly add small heatsinks to these in
order to combat this problem.
Anyway, great care should be taken in adjusting the standing
current. The action of P1 is reversed (applies to both channels)
and it should only be operated cautiously when simultaneously
watching the voltage drop over R22 and R23 without signal.
I adjust to less than 10 millivolts and this is near the mechanical
centre of the adjustment pot.
I recommend using 4 amps quick acting fuses instead of the
standard fuses. When repairing or modifying the amp I insert
resistors instead of the fuses in order to limit unwanted current
due to failure. 15 to 30 ohms and at least 5 watts work fine and
dispense you from the usual habit of posessing a variac. Re-
member that a variac can supply disastrous currents any time
when not turned down to zero.
I also exchange C16 in both channels by an electrolytic cap of
say at least 470 uF in order to limit inrush current which I noted
to be unduly high. The cap has to bear a small voltage only, so
a tiny 6V3 type can be conveniently used.
In the case of failure of mosfet Q9 [January 2024 edit: This should read Q7] which is unavailable here I
substituted a p-channel j-fet successfully. If you encounter harsh
noise after some form of input overload have a look at Q1, Q2
and Q4.
A3i No. 1059-0998-0212 is now finished on my workbench and
I hope it will last some time again.
Last edited by a moderator:
Hi ,
I'm working on one this week. Q8+108 changed, LED2 + 102 changed .All LEDs glow.
but the volts on bases of Q13/14 is almost +Vcc
Would this be caused by Q9 or Q11??
I know this is 3 yrs down-the-road but any help would be.......helpful.
Thanks
It is a gate protection for Q7.
I will use this opportunity to make a full list of modifications for the A3i.
For both performance and reliability:
1) All 4 LEDs replaced with a high temperature type green or yellow colour, lifted at least 15 mm from the PCB.
2) Q10 (BC639) replaced with MJE243.
3) Q8 (BC640) replaced with MJE253.
4) Q11 (BD139) replaced with MJE243, mounted on top of Q13 (under one of its screws).
5) Q12 (BD140) replaced with MJE253, mounted on top of Q14 (under one of its screws).
Q11 and Q12 connected to the PCB by wires (either twisted or a bit of a "computer" flat cable)
6) D210 and D211 replaced with 10 Ohm 1W resistors
7) The idle current is set for 85-90 mA (approx. 40 mV between emitters of Q13 and Q14) after a warm-up.
8) Resistors R8, R16, R18 should be at least 1W power. If these are only 0.5W - they have to be replaced.
For performance only:
9) Power supply rectifier diodes replaced with 100V 10A Schottky and capacitors in parallel with these diodes removed.
10) Electrolytic capacitors replaced with better types. The most important are C216 and C217. I've used Panasonic FC there with good results.
11) Protection supply rectifier diodes replaced with Shottky 1A 100V, with added 30 Ohm 0.5W resistor in series with each diode.
12) The input differential pair Q1/Q2 replaced with a matched gain pair of BC550 (B,C) - this will substantially reduce the output DC offset.
Alex
A3i
Hi everybody
I'm not very active in this forum but i read a lot...
I just got hold of 3 CA A3i (faulty) for peanuts in a car boot sale and have some plans for them but for that i would need a service manual or at least the schematic.
My plan is to move the transformer closer to the back of the case, remove the supply caps from the amp board and make a CRC supply on a separate board
I'm also planning doing a new board for the amp with bigger sinks (I have many cpu heatsinks) and with the changes given by X-PRO
Question to Alex (X-PRO): what was the original transformer voltage before they decided to jack the power up to 60W?
Ric
Hi everybody
I'm not very active in this forum but i read a lot...
I just got hold of 3 CA A3i (faulty) for peanuts in a car boot sale and have some plans for them but for that i would need a service manual or at least the schematic.
My plan is to move the transformer closer to the back of the case, remove the supply caps from the amp board and make a CRC supply on a separate board
I'm also planning doing a new board for the amp with bigger sinks (I have many cpu heatsinks) and with the changes given by X-PRO
Question to Alex (X-PRO): what was the original transformer voltage before they decided to jack the power up to 60W?
Ric
A diagram of the power amp is in post 1, see also 90 and especially 93 for advice .
Work on other parts of the amp is not necessary. I do not recommend doig much of
modification and additional effort apart from repairing, don't think it is worthwile.
By the way somebody has a diagram of Cambridge Audio A70 type power amp ?
Thanks.
Thank you very much for this explicit explanation of the modification steps. Nevertheless for me there are additional questions:
1) which other behaviour is to expect by the use of high temperature LED's ?
7) such Idle current are also to observe by Linn amps like LK2-80, Powertek or Intek. Advantage is pure class A mode at low levels (medium levels by high efficiency loudspeakers). But the internal temperatur is much more larger about ambient than by typical values between 20 and 30 mA through the SANKEN MT-100 output devices. Are an additional fan unit under the amp enough as a countermeasure ?
9) Anythere here on diyaudio there is a thread, where are to see measurements between the use of normal diodes and schottky diodes with parallel switched caps. Who knows this thread URL?
12) What happens, when I use unmatched pairs for different Ube voltages and additional devices for offset adjust, but matched pairs for same hfe factor?
Last edited:
1) Better reliability, that's all.
7) I have A3i running 24x7 in my system with this idle current. It runs a bit warm but not excessively so. With 85-90 mA idle the 7th harmonic is minimal and the sound is better IMHO. Just don't close the air path for the heatsink, that should be enough.
12) Vbe matching is not important, only the current gain (hfe).
Cheers
Alex