I have a Cambridge A1 Mk 1, the one with the volume control at the far right, and a few days ago the power light went off as it was in use at low volume , and sound stopped completely.
I disconnected the amp from the mains, took the lid off and found that the fuse on the power supply switch board (top left in photo) was blown. A new fuse T2A 250 V blew out immediately the amp was switched on. I then disconnected the amp from the system and carried some continuity/resistance checks. I noted that the rectifier circuit is actually built into the main power amplifier board and I found that the secondary output power leads from the transformer to the rectifier bridge (purple and yellow in the photo) were both zero resistance to earth, even when I disconnected the two fuses. As I read the layout, those wires should not be earthing, as they are feeding power to the rectifier circuit which then feeds the two amplifier circuits.
The red and white wires from the secondary windings are earthed as they are connected to the black earthing wire at the “rail” which is between the two large smoothing capacitors , and the "rail" is achieved by a large trail of solder under the circuit board.
As I don’t have a schematic, I’m basing my knowledge on a circuit diagram of a Crimson Electrik CE170 kit from 1980 (no IC power transistors), and application of the basic arrangement to a few other amps including a NAD3070.
My conclusion is that the secondary windings of the transformer have short-circuited to earth. Can anyone confirm this?
I disconnected the amp from the mains, took the lid off and found that the fuse on the power supply switch board (top left in photo) was blown. A new fuse T2A 250 V blew out immediately the amp was switched on. I then disconnected the amp from the system and carried some continuity/resistance checks. I noted that the rectifier circuit is actually built into the main power amplifier board and I found that the secondary output power leads from the transformer to the rectifier bridge (purple and yellow in the photo) were both zero resistance to earth, even when I disconnected the two fuses. As I read the layout, those wires should not be earthing, as they are feeding power to the rectifier circuit which then feeds the two amplifier circuits.
The red and white wires from the secondary windings are earthed as they are connected to the black earthing wire at the “rail” which is between the two large smoothing capacitors , and the "rail" is achieved by a large trail of solder under the circuit board.
As I don’t have a schematic, I’m basing my knowledge on a circuit diagram of a Crimson Electrik CE170 kit from 1980 (no IC power transistors), and application of the basic arrangement to a few other amps including a NAD3070.
My conclusion is that the secondary windings of the transformer have short-circuited to earth. Can anyone confirm this?
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Did primary fuse blow with both secondary fuses removed? With both secondary fuses removed, is either diode in the bridge shorted to ground?
Definitive test would also remove red and white wires. If there's still short to ground with all secondary leads "floating", then secondary short to ground is confirmed.
Definitive test would also remove red and white wires. If there's still short to ground with all secondary leads "floating", then secondary short to ground is confirmed.
Thanks for the suggestions, I'll give them a try and report back. Just to check, do you mean the red and white (actually light grey wires but my mistake when describing them) secondary winding wires from the transformer to the earth "rail" ?
I was hoping that someone would be able to say " I know exactly what the problem is..!", but that would be a bit boring would it not!
I was hoping that someone would be able to say " I know exactly what the problem is..!", but that would be a bit boring would it not!
It's likely that the red and light grey wires are the ends of two separate windings, joined and grounded to form a "center-tapped" secondary. So the secondary will show continuity to ground. The hope is that the bridge rectifier is shorted---- a much cheaper repair; if this is the case, with the secondary fuses removed, the primary fuse won't blow and you'll find center-tapped AC on the purple and yellow leads secondary leads.
Good luck!
Good luck!
Thank you BSST for your responses and encouragement. I believe that you are correct in that the red and light grey wires are the other ends of two parallel secondary windings, with the yellow and purple wires being the other ends. The colours are all actually sleeves. The red and light grey wires are joined at the earth/ground rail that has a third, black wire which goes to the earthing bolt near the corner of the main board adjacent to the power switch. The black wires from the speaker terminals also go to that earth rail.
I decided to go the whole hog and isolate the secondary windings, so that I could work back to reconnections which caused the main fuse on the PSU supply board to blow rather than blow several 2TA fuses in experimenting. I therefore cut the red and light grey wires, and also removed the fuses adjacent to the yellow and purple wires, this should have isolated the transformer from the main power amplifier board. I then plugged the mains back in, switched on and no PSU board fuse blow...I thought good, something has changed. I then checked for mains voltage I found that the 3A fuse in the mains plug had blown! In a way that was a good indication as it indicated that whatever is happening is not being caused by the power amp board. I checked checked both primary and secondary windings and they appear to have infinite resistance i.e. not grounding, so I am at a bit of a loss as to why the main fuse is blowing, although if the AC current is leaking into the windings all sorts of unusual impedance effects might be occuring. Regardless, it appears that the problem is in the toroidal transformer and it is progressively failing - the previous day the primary windings appeared to have about 20 ohm dc resistance.
So my next step is to try changing the transformer. The Cambridge Transformer is about 9.5cm diameter but has no markings other than 230V and "custom-made high current transformer". Does anyone know the secondary voltages for the A1-1 integrated amplifier ?
I do have another toroidal transformer, from a Hitachi AX-M68D mini-hifi which had about the same power output (25W per channel) . This transformer is about 9cm diameter, same thickness as the Cambridge one and the label says the main power amp output is two white wires +/-18V, with a common black return wire. As it is free I plan to try to install this and see what happens.... I can't think of anything disastrous that could happen and the worst case is it doesn't work at all, and hopefully it does work but might run into clipping a bit earlier. As I won't be running this amp at anywhere near full volume I don't think that will be an issue. If it doesn't I might have to explore the output voltage issue further - there is plenty of info on the A1 Mk3 but that a square shaped (E-I?) transformer and had a different power amp circuit design, different power transistors etc.
I decided to go the whole hog and isolate the secondary windings, so that I could work back to reconnections which caused the main fuse on the PSU supply board to blow rather than blow several 2TA fuses in experimenting. I therefore cut the red and light grey wires, and also removed the fuses adjacent to the yellow and purple wires, this should have isolated the transformer from the main power amplifier board. I then plugged the mains back in, switched on and no PSU board fuse blow...I thought good, something has changed. I then checked for mains voltage I found that the 3A fuse in the mains plug had blown! In a way that was a good indication as it indicated that whatever is happening is not being caused by the power amp board. I checked checked both primary and secondary windings and they appear to have infinite resistance i.e. not grounding, so I am at a bit of a loss as to why the main fuse is blowing, although if the AC current is leaking into the windings all sorts of unusual impedance effects might be occuring. Regardless, it appears that the problem is in the toroidal transformer and it is progressively failing - the previous day the primary windings appeared to have about 20 ohm dc resistance.
So my next step is to try changing the transformer. The Cambridge Transformer is about 9.5cm diameter but has no markings other than 230V and "custom-made high current transformer". Does anyone know the secondary voltages for the A1-1 integrated amplifier ?
I do have another toroidal transformer, from a Hitachi AX-M68D mini-hifi which had about the same power output (25W per channel) . This transformer is about 9cm diameter, same thickness as the Cambridge one and the label says the main power amp output is two white wires +/-18V, with a common black return wire. As it is free I plan to try to install this and see what happens.... I can't think of anything disastrous that could happen and the worst case is it doesn't work at all, and hopefully it does work but might run into clipping a bit earlier. As I won't be running this amp at anywhere near full volume I don't think that will be an issue. If it doesn't I might have to explore the output voltage issue further - there is plenty of info on the A1 Mk3 but that a square shaped (E-I?) transformer and had a different power amp circuit design, different power transistors etc.
I believe you’re on the right track.
There could be shorted tunes within any of any of the windings that could blow the primary fuse. It’s probably academic trying to identify the failure mechanism, but there should be isolation between all windings, the two secondaries should have very similar resistances.
You could inspect once transformer is removed. Might get lucky.
There could be shorted tunes within any of any of the windings that could blow the primary fuse. It’s probably academic trying to identify the failure mechanism, but there should be isolation between all windings, the two secondaries should have very similar resistances.
You could inspect once transformer is removed. Might get lucky.
Amplifier sorted - I Installed the ex-Hitachi AX-68MD toroidal transformer, connected the single black secondary return wire to the red and light grey amplifier return wires on the amplifier board ( the ones connected to the earth rail), checked that neither the plug fuse nor the internal power fuse blew when I switched on and then re-installed the rectifier fuses ( FS501 and FS502). Had to go searching for a RCA to 3mm headphone plug to connect my phone and very chuffed when music came out of the old but re-worked JPW Mini Monitor speakers - all sounds ok to me.
I celebrated with Steely Dan's Deacon Blue, which contains the lines "They've got name for the winners in this world, I want a name when I lose, they call Alabama the Crimson Tide, call me Deacon Blue". I chose Crimsontide for my moniker partly because of my Crimson Electrik power amplifier, and partly for that lyric. Today I'm Crimson Tide! For those who don't follow USA college football, Crimson Tide is the nickname for the University of Alabama team who play in crimson red strip, and have a very long history of success.
Here's a pic - not quite as tidy as the original amp but it all fits, I had to thread a cable tie through one of the holes in the baseplate of the amp to tie the superflous low voltage secondary wires, and I did not attempt to solder direct to any circuit boards.
I celebrated with Steely Dan's Deacon Blue, which contains the lines "They've got name for the winners in this world, I want a name when I lose, they call Alabama the Crimson Tide, call me Deacon Blue". I chose Crimsontide for my moniker partly because of my Crimson Electrik power amplifier, and partly for that lyric. Today I'm Crimson Tide! For those who don't follow USA college football, Crimson Tide is the nickname for the University of Alabama team who play in crimson red strip, and have a very long history of success.
Here's a pic - not quite as tidy as the original amp but it all fits, I had to thread a cable tie through one of the holes in the baseplate of the amp to tie the superflous low voltage secondary wires, and I did not attempt to solder direct to any circuit boards.
