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.
(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.
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).
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).
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.
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.
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.
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?
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.
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.
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.
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.
(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.
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.
As it is a single primary winding and I assume you are measuring directly across the lead terminations, there's not much room for measuring error. The transformer may have a thermal fuse wrapped between the windings somewhere. It may not be obvious, other than by a small lump around the wire exit point and easily confused with flex connections and sleeving there. I could not find a web reference to the model ED 7574R1 but maybe MEI-AH suffix means something.
I doubt the windings would burn and still make occasional contact but some thermal fuses are/were resettable, even if you might say more by accident than design.
I doubt the windings would burn and still make occasional contact but some thermal fuses are/were resettable, even if you might say more by accident than design.
The primary reading 15 meg seems to show its now open circuit. As Ian mentions, are you reading that directly across the windings ? and not the mains plug.
All large transformers draw a massive initial current pulse when switched on. The reason is that the magnetic field within the core has to establish and that takes a few milliseconds. That's one reason a large time delay fuse is needed on the primary, it has to survive that initial surge.
All large transformers draw a massive initial current pulse when switched on. The reason is that the magnetic field within the core has to establish and that takes a few milliseconds. That's one reason a large time delay fuse is needed on the primary, it has to survive that initial surge.
It does indeed sound to be open circuit. Its very strange because any thermal fuse (if it has one) wouldn't fail intermittently, they fail open and that's it... and yet the bulb flashed showing continuity for an instant.
No varnish or lacquer on the pads, you are getting a true reading ?
If you do an image search for 'transformer thermal fuse' you will see what they typically look like.
Its odd because a fault should blow the fuses, and 4 amp fuses are big enough to blow most faults up... if the output IC fails short it would simply crack and blow itself apart.
Transformers can stand massive overloads, the only thing that kills them is heat melting the insulation of the wire.
No varnish or lacquer on the pads, you are getting a true reading ?
If you do an image search for 'transformer thermal fuse' you will see what they typically look like.
Its odd because a fault should blow the fuses, and 4 amp fuses are big enough to blow most faults up... if the output IC fails short it would simply crack and blow itself apart.
Transformers can stand massive overloads, the only thing that kills them is heat melting the insulation of the wire.
No varnish or lacquer on the pads. I'm fairly confident that the reading is good. (I double-checked several times, because it didn't match what I'd seen previously)
Is it possible that the insulation of the windings could have deteriorated over time? This amp had been out of service for several years and is about 17 years old. Seems unlikely to me, but I'm stuck for any other explanation.
The transformer was cool to the touch throughout my tests last night, and shows no obvious physical sign of damage. (Although it's a sealed toroidal, so it's hard to see anything much really)
Is it possible that the insulation of the windings could have deteriorated over time? This amp had been out of service for several years and is about 17 years old. Seems unlikely to me, but I'm stuck for any other explanation.
The transformer was cool to the touch throughout my tests last night, and shows no obvious physical sign of damage. (Although it's a sealed toroidal, so it's hard to see anything much really)
Whatever is amiss is something internal and presumably not getatable. It can only be a fractured wire (which would have to be a manufacturing defect), some poor connection internally (but there normally aren't any... the wires you see are just the ends of the winding) or some bizarre failure of an internal fuse.
Clutching at straws here... have you tried switching ranges on the meter, or tried another meter ? Just vague hazy thoughts that maybe some DVM's (auto ranging ?) don't like the inductance of the tranny and give weird readings.
Clutching at straws here... have you tried switching ranges on the meter, or tried another meter ? Just vague hazy thoughts that maybe some DVM's (auto ranging ?) don't like the inductance of the tranny and give weird readings.
Given that a voltage reading across the secondary shows only a few mv, and the resistance across the primary is extremely high, it seems very likely that the transformer is faulty.
Are there things I could/should check to determine if there are other problems elsewhere before just replacing the transformer? I had considered attaching an external DC power supply to test the rest of the circuit. (Although I'm not sure I have a suitable power supply to use)
Are there things I could/should check to determine if there are other problems elsewhere before just replacing the transformer? I had considered attaching an external DC power supply to test the rest of the circuit. (Although I'm not sure I have a suitable power supply to use)
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I agree it does look like the tranny is faulty. As you mention the mv reading on the secondary then the only test would be to confirm that 240 vac is present across the primary winding when its switched on.
Replacement toroids shouldn't be overly expensive. Have a look at CPC, they do many, both encapsulated and standard. Whatever has happened to this one wont happen to new replacement, even if there were a short somewhere... it would just zap the fuse. And we test with a bulb initially anyway.
Rigging a PSU could be difficult because its a split supply... so you need two supplies in series with the junction being 'ground'.
Replacement toroids shouldn't be overly expensive. Have a look at CPC, they do many, both encapsulated and standard. Whatever has happened to this one wont happen to new replacement, even if there were a short somewhere... it would just zap the fuse. And we test with a bulb initially anyway.
Rigging a PSU could be difficult because its a split supply... so you need two supplies in series with the junction being 'ground'.
I confirmed a steady 230V AC across the primary last night, so I've ordered a replacement transformer from Maplin (amazingly, they were cheaper than CPC)
I guess we'll see what happened when that turns up and I test it out. Delivery is 3-5 days, so I'll post back some time after that to let you know how I get on.
Thanks again for all the help.
I guess we'll see what happened when that turns up and I test it out. Delivery is 3-5 days, so I'll post back some time after that to let you know how I get on.
Thanks again for all the help.
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