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Old 6th July 2015, 06:35 PM   #1
bozx is offline bozx  United Kingdom
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Default Cambridge Audio A1mk3SE repair

My old A1mk3SE recently died, and I'm looking to repair it, if it's economically viable.

(Disclaimer: My electronics knowledge is rusty, so please forgive any stupid questions/ideas. My degree was joint electronics and computing, and I've done some repair and circuit design, but all that's a long time ago.)

Short version:
I think the transformer is dead, and that the secondary winding has short circuited (resistance is 0.5 ohms according to my multimeter, vs significantly higher for the primary). That's not something I've seen before, so I'm looking for a second opinion. Is that a common failure mode for transformers? Are there any readings I can usefully take to further check it?

Long version:

Symptoms/background:
- The amp hadn't been used for several years, but before that was working fine
- When connected and powered up, there was very little sound, which quickly dwindled to nothing, and the power light went out.
- The fuse in the mains plug had blown
- After replacing the mains fuse, the built-in fuse also blew.
- Fuse now consistently blows immediately on powering up.

Physical inspection:
There is no obvious sign of physical damage upon visual inspection (no sign of heat damage/charring/burnt out components, no loose wires or components) No sign of obvious dry joints or shorts. I've checked all circuit boards, component side and copper-side.

Other findings:
The amp consists of three main boards (schematic refers to them as "Power", "Tone board", and "IPT board")

Since the power supply and power amp are on a single board, I disconnected the other boards, to attempt to isolate the problem. On powering up, the fuse went again. To me, this indicates that there is (at least) a problem with this board.

The power supply consists of a simple transformer/bridge rectifier with the usual smoothing capacitors. Conveniently, the transformer is isolated from the rectifier via two 4 amp fuses. I popped those out, to isolate the transformer. Powering on like this, the (main) fuse blows (again)

A resistance check across the secondary of the transformer shows less than 1 ohm, even with the rest of the circuit disconnected. There's no sign of short circuit where the leads are soldered to the board, but I've not (yet) unsoldered them to fully isolate the transformer.

Any help or advice gratefully received.
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Old 6th July 2015, 07:32 PM   #2
Mooly is offline Mooly  United Kingdom
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Depending on the transformer va rating (how big it is), a low dc resistance of both primary and secondary is normal. At 50/60Hz the impedance is considerably higher due to the large inductance presented by the windings.

Unless you can locate a short on the secondary side, then you need to isolate the secondary leads to prove the tranny is good (99% sure it will be).
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Old 6th July 2015, 07:57 PM   #3
Mike P is offline Mike P  United Kingdom
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I've got the schematic if you need it.

Looking at the schematic at the secondary side of the transformer yellow and red are one winding and grey and purple are another winding. Red and grey are connected to make 0v.
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Old 6th July 2015, 08:11 PM   #4
Mooly is offline Mooly  United Kingdom
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Thanks I think I've got it tbh.

Yes, the 4 amp fuses totally isolate the secondary. If the primary fuse is blowing with the 4 amps fuses removed, then make sure that you have an 'anti surge' or time delay fuse fitted for F1which is the mains input fuse.

I would advise you to fit a bulb tester (100 watt mains filament bulb) in place of F1 for testing. That will limit current and simply illuminate in the event of a fault.

Favourite faults would be failed electrolytic caps in the power supply and perhaps failure of the TDA1514 output IC's.
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Old 7th July 2015, 11:29 AM   #5
bozx is offline bozx  United Kingdom
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Quote:
Originally Posted by Mike P View Post
I've got the schematic if you need it.

Looking at the schematic at the secondary side of the transformer yellow and red are one winding and grey and purple are another winding. Red and grey are connected to make 0v.
I also have the schematic if anyone needs it. I just wasn't sure about the rules on posting such (potentially copyrighted) material on the forums here. It can be found fairly easily online.
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Old 7th July 2015, 11:32 AM   #6
bozx is offline bozx  United Kingdom
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Quote:
Originally Posted by Mooly View Post
Depending on the transformer va rating (how big it is), a low dc resistance of both primary and secondary is normal. At 50/60Hz the impedance is considerably higher due to the large inductance presented by the windings.

Unless you can locate a short on the secondary side, then you need to isolate the secondary leads to prove the tranny is good (99% sure it will be).
I was expecting a low DC resistance, but the secondary is significantly lower than the primary (less than 1 ohm vs a few ohms for the primary)

What would be the best way to check the transformer?
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Old 7th July 2015, 11:35 AM   #7
bozx is offline bozx  United Kingdom
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Thank you for your responses.

Fitting a bulb in place of the fuse is a nice idea, I had been looking for a good way to avoid burning through so many fuses. I'll find a suitable bulb & holder for use in further testing.

I'm fairly sure I'm using a time delay fuse for F1, (but I'll double check that tonight) and removing the 4 amp fuses seems to rule out a problem with the caps in the power supply.

Any idea what else it could be? (I've checked for shorts around the power switch and the transformer primary, which is about all that's left in circuit after removing the 4 amp fuses)

What would be the best way to check for a faulty cap in the power supply? The DC resistance across the caps seems suitably high, suggesting none have gone completely short-circuit.
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Old 7th July 2015, 01:12 PM   #8
Mooly is offline Mooly  United Kingdom
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The best way to check the tranny is just with the bulb tester and the 4 amp fuses removed. The bulb should flash brightly as power is initially applied and then essentially go out as the current required by the tranny itself is small. If you wished, you could then measure the AC voltage on the secondary's.

If it fails the bulb test then the tranny is faulty (but that would be really unusual).

Caps aren't easy to test as they could be failing under voltage. Again, the bulb tester is the way and by isolating various sections of the circuitry you can quickly determine where the fault lies.

Favourite would be the output IC's to have failed in some way... but we need to get to that conclusion methodically.
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Old 7th July 2015, 01:18 PM   #9
bozx is offline bozx  United Kingdom
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Thanks. I just bought a bulb holder from Maplin, so I should be able to try that out tonight. (time and work commitments permitting)

I'll post back once I have more information.
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Old 7th July 2015, 09:05 PM   #10
bozx is offline bozx  United Kingdom
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(For all of the checks listed below, the two 4amp fuses were removed)

As expected, bulb lit at first, then went out. (I could only find a 60w at home, so I'm limiting the current even further, but I don't think that should matter for testing the transformer.)

When I tried powering off/on again, I no longer got a flash from the bulb. Not sure what that means.

DVM reading across the primary was a steady 230v, but the secondary only reads ~150mv which is far too low.

I also double checked the DC resistance of the transformer again (I was going off memory for my post yesterday)

The primary reads ~15megaohms, the secondary reads ~0.7ohms. I recall the primary being significantly lower than that when I looked at this before.

Any suggestions for a next step in diagnosing this? To me, the transformer still looks suspect. But (as you say) transformer faults are much more rare than (eg) capacitor or output IC faults.
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