I have a reverb unit (Demeter RV-1, 1st version) that keeps blowing mains fuses as soon as it is powered up. I disconnected the power supply from the main board and it still blows fuses. I then, perhaps stupidly, bypassed the fuse, and powered it up - this was followed by a spark (and small bang/pop) at the toroidal transformer, where the connections to transformer primary are (I guess suggesting a short between live and neutral?)
However, I then put the fuse back in the circuit and powered it up, and the fuse now does not blow. I thought it wasn't working at first but here are voltage readings (with description of what is being measured as far as I can discern), it appears to use a toroidal transformer wired for 240v in series or 120v in parallel:
From transformer primary first (live side) connection to ground - 240v
Across the transformer primary (first to last connection) - 240v
From transformer primary first connection to transformer centre - 100v
(0v between all the other primary connections)
From transformer primary centre connection to ground - 15v
From transformer primary last connection to ground - 15v (although would need recheck this one)
From transformer secondary first, centre and last connections to ground - 13.8v
(0v between any two of the three secondary connections).
The website for the new version demeter rv1 says the power supply is rated at +/- 18v, so 13.8v AC going to the circuit board in this one doesn't seem that far away from that.
Do these voltage readings seem normal? It is tempting to reconnect the power supply to the main circuit board and see if it works, but I feel I must have got something wrong. Do you have any ideas why it stopped blowing fuses? Is miraculously fixed (temporarily?) or have I just caused a further problem?
Thanks
However, I then put the fuse back in the circuit and powered it up, and the fuse now does not blow. I thought it wasn't working at first but here are voltage readings (with description of what is being measured as far as I can discern), it appears to use a toroidal transformer wired for 240v in series or 120v in parallel:
From transformer primary first (live side) connection to ground - 240v
Across the transformer primary (first to last connection) - 240v
From transformer primary first connection to transformer centre - 100v
(0v between all the other primary connections)
From transformer primary centre connection to ground - 15v
From transformer primary last connection to ground - 15v (although would need recheck this one)
From transformer secondary first, centre and last connections to ground - 13.8v
(0v between any two of the three secondary connections).
The website for the new version demeter rv1 says the power supply is rated at +/- 18v, so 13.8v AC going to the circuit board in this one doesn't seem that far away from that.
Do these voltage readings seem normal? It is tempting to reconnect the power supply to the main circuit board and see if it works, but I feel I must have got something wrong. Do you have any ideas why it stopped blowing fuses? Is miraculously fixed (temporarily?) or have I just caused a further problem?
Thanks
Impossible to say without being there and seeing the sparks fly.
You can use a mains filament bulb in place of the fuse as a test, perhaps a 40 or 60 watt as this looks low power stuff. That will save major explosions
If you actually/definitely/conclusively/no doubt whatsoever saw the transformer itself spark then it has to be 100% assumed it is compromised in some way and as such must be replaced.
You can use a mains filament bulb in place of the fuse as a test, perhaps a 40 or 60 watt as this looks low power stuff. That will save major explosions
If you actually/definitely/conclusively/no doubt whatsoever saw the transformer itself spark then it has to be 100% assumed it is compromised in some way and as such must be replaced.
Sounds like the transformer wire has melted inside.
Its needs rewinding or replacing.
If its a small transformer then sometimes they have embedded fuses inside them.
Ok, I definitely see now why the bulb trick is used
I suppose the spark/bang from the transformer suggests that it was shorted and therefore blowing fuses - so I gather that either the short has blown out or the wire melted somehow after bypassing the fuse, so now the fuse now does not blow when put back in the circuit? - should I assume the transformer is now unusable? - the spark definitely occurred right on the primary connections.
If it is unusable, is rewinding a toroidal transformer something that can be done DIY?
If not, then I have a couple more questions if I may -
1) There aren't many details on the transformer (it seems to be custom), if I pick a similar sized dual primary (120/240v) toroidal transformer with a secondary that can produce 15-18v, and a decent VA, say 30 or 60 - is this a very bad idea - do I need to be certain that it needs 18v or do the readings suggest it should be more like 14-15v despite what the website says (I am not sure if the readings I have taken are suggesting that, however, as it's 0v across any of the secondary connections)? The alternative is to source the original part but that could be over $100 incl. shipping to the UK. I could do that, but if possible to identify an alternative that would be very helpful.
2) Once I find a suitable transformer and put it in the circuit, how can I be sure before connecting it to the main board that something on the main board did not cause the problem in the first place? I know how to check the rectifier diodes, but beyond that do I need check everything on the board? Or, do the sparks from the transformer suggest that the transformer itself was indeed the original problem and therefore just replace that?
Thanks again
I suppose the spark/bang from the transformer suggests that it was shorted and therefore blowing fuses - so I gather that either the short has blown out or the wire melted somehow after bypassing the fuse, so now the fuse now does not blow when put back in the circuit? - should I assume the transformer is now unusable? - the spark definitely occurred right on the primary connections.
If it is unusable, is rewinding a toroidal transformer something that can be done DIY?
If not, then I have a couple more questions if I may -
1) There aren't many details on the transformer (it seems to be custom), if I pick a similar sized dual primary (120/240v) toroidal transformer with a secondary that can produce 15-18v, and a decent VA, say 30 or 60 - is this a very bad idea - do I need to be certain that it needs 18v or do the readings suggest it should be more like 14-15v despite what the website says (I am not sure if the readings I have taken are suggesting that, however, as it's 0v across any of the secondary connections)? The alternative is to source the original part but that could be over $100 incl. shipping to the UK. I could do that, but if possible to identify an alternative that would be very helpful.
2) Once I find a suitable transformer and put it in the circuit, how can I be sure before connecting it to the main board that something on the main board did not cause the problem in the first place? I know how to check the rectifier diodes, but beyond that do I need check everything on the board? Or, do the sparks from the transformer suggest that the transformer itself was indeed the original problem and therefore just replace that?
Thanks again
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Have you?
Something like that, depends, if you can find the short you may be able to make it safe/usable
As well as sourcing a replacement, you have the possibility that there could be other problems with the unit. Transformers are normally extremely reliable and so you have to ask 'did it jump, or was it pushed' meaning did something else fail, perhaps overheating the device and causing failure.
-/+18 volts DC (be sure it is DC) is a typical figure for small signal circuitry. This requires around 13 volts AC (13-0-13) because the DC voltage will be the AC value multiplied by root 2.
You need to do more research I think before deciding. It is vitally important to get the voltage correct.
-/+18 volts DC (be sure it is DC) is a typical figure for small signal circuitry. This requires around 13 volts AC (13-0-13) because the DC voltage will be the AC value multiplied by root 2.
You need to do more research I think before deciding. It is vitally important to get the voltage correct.
Thanks, I will do a bit more digging - I have emailed the manufacturer to see if they can give a few more details on the transformer specs, but from replies so far I suspect they might be more interested in selling me a new transformer.
Scott, I checked where the sparks came from at the opening of the transformer on the primary side (the secondary connections are on the opposite side of this transformer), there is a fairly large slit where the wires to the primary go, I suppose it's possible that the copper from the primary could have been pushed away at this point, but I'm not sure. It is difficult to see without opening it up more.
I can't get a better image in the dark but here it is, I've cut apart some of the black covering:
Imgur: The most awesome images on the Internet
Scott, I checked where the sparks came from at the opening of the transformer on the primary side (the secondary connections are on the opposite side of this transformer), there is a fairly large slit where the wires to the primary go, I suppose it's possible that the copper from the primary could have been pushed away at this point, but I'm not sure. It is difficult to see without opening it up more.
I can't get a better image in the dark but here it is, I've cut apart some of the black covering:
Imgur: The most awesome images on the Internet
Appears to be +/-18V DC Regulated. Add 3+V for regulator loss, you need over +/-20V DC. So 15V AC windings (21V peak) are quite reasonable. Thin margin, but pennies matter.
I can not find a power consumption (can you??). For what it does, it may be just a few Watts. A 10VA 15V+15V transformer may be fine.
You do not want to re-wind this toroid. Sure it can be done. Asian factories will sell and ship one new, with some warranty, cheaper than the value of your time.
I can not find a power consumption (can you??). For what it does, it may be just a few Watts. A 10VA 15V+15V transformer may be fine.
You do not want to re-wind this toroid. Sure it can be done. Asian factories will sell and ship one new, with some warranty, cheaper than the value of your time.
Thanks, I'll have a look for a new one. The manufacturer was really helpful in the end, they said:
The output is 46 volts AC center tapped for the final output of +/-18V regulated. 30 watts total power. 2 times 23V AC. You can use a 48V to 38V center taped and they will all work as the power supply is regulated.
Can I also get a transformer that is not centre tapped and wire together the two centre connections? I read that 30 VA is near enough 30 watts for hobbyist purposes, so anything above 30 VA should be fine?
Also, I thought that AC voltage gets higher when converted to DC, as it gets closer to the peak as the filter capacitor stores electrical energy - if so, any ideas on how 23V A becomes +/-18V DC (+ 3V for regulator loss)?
The output is 46 volts AC center tapped for the final output of +/-18V regulated. 30 watts total power. 2 times 23V AC. You can use a 48V to 38V center taped and they will all work as the power supply is regulated.
Can I also get a transformer that is not centre tapped and wire together the two centre connections? I read that 30 VA is near enough 30 watts for hobbyist purposes, so anything above 30 VA should be fine?
Also, I thought that AC voltage gets higher when converted to DC, as it gets closer to the peak as the filter capacitor stores electrical energy - if so, any ideas on how 23V A becomes +/-18V DC (+ 3V for regulator loss)?
The AC voltage from a winding will be 1.4 times higher when 'converted' to DC.
23 volts AC will give 33 volts DC.
The regulators always deliver the same constant 18 volt output as long as the input is higher than the regulators minimum designed input voltage. An 18 volt regulator typically needs at least 22 volts or more to work properly, but not so much that the regulator specification for maximum voltage is exceeded (typically 36 volts).
Here is a typical regulator:
Panasonic - datasheet pdf
The negative version is 7918.
You must use either a centre tapped transformer or one that has two separate secondary's that can be joined together. If the later then make sure you join them the correct way around... which is easy.
23 volts AC will give 33 volts DC.
The regulators always deliver the same constant 18 volt output as long as the input is higher than the regulators minimum designed input voltage. An 18 volt regulator typically needs at least 22 volts or more to work properly, but not so much that the regulator specification for maximum voltage is exceeded (typically 36 volts).
Here is a typical regulator:
Panasonic - datasheet pdf
The negative version is 7918.
You must use either a centre tapped transformer or one that has two separate secondary's that can be joined together. If the later then make sure you join them the correct way around... which is easy.
Thanks all, really helpful advice, much appreciated.
I think I found a couple of suitable replacements. Both with a 2 x 24V secondary that can connected in series or parallel, and a dual 115Vac 50/60Hz primary that I can hook up as it is currently with the voltage selector switch for parallel/series operation (115v/230v).
Do these look suitable?
2 Output Toroidal Transformer, 60VA, 2 x 24V ac
RKD 30/2x24 | 2 Output Toroidal Transformer, 30VA, 2 x 24V ac | Block
2 Output Toroidal Transformer, 30VA, 2 x 24V ac
RKD 60/2x24 | 2 Output Toroidal Transformer, 60VA, 2 x 24V ac | Block
Is the 30VA one OK? It seems similar sized to the one currently in the unit, and the guy said 30 watts so seems about right.
Thanks again
I think I found a couple of suitable replacements. Both with a 2 x 24V secondary that can connected in series or parallel, and a dual 115Vac 50/60Hz primary that I can hook up as it is currently with the voltage selector switch for parallel/series operation (115v/230v).
Do these look suitable?
2 Output Toroidal Transformer, 60VA, 2 x 24V ac
RKD 30/2x24 | 2 Output Toroidal Transformer, 30VA, 2 x 24V ac | Block
2 Output Toroidal Transformer, 30VA, 2 x 24V ac
RKD 60/2x24 | 2 Output Toroidal Transformer, 60VA, 2 x 24V ac | Block
Is the 30VA one OK? It seems similar sized to the one currently in the unit, and the guy said 30 watts so seems about right.
Thanks again
Measuring the weight can be a good guide to VA ratings.
Based on the information we have then those should be suitable. The 30va will be good for around 600 milliamps per winding (less of course once rectified and smoothed) but I would think still more than adequate.
Measure the dimensions and weigh the original if possible. That's the best clue to VA ratings.
Based on the information we have then those should be suitable. The 30va will be good for around 600 milliamps per winding (less of course once rectified and smoothed) but I would think still more than adequate.
Measure the dimensions and weigh the original if possible. That's the best clue to VA ratings.
Ok, great, I will weigh and measure it.
When I get the new transformer and connect it up, I am still worried that something else might have caused the transformer to fail. Is there anything in particular I should check?
I could measure the current draw of the main circuit board, or check the rectifier diodes, the filter capacitors, etc. but I am not sure what the general process would be following transformer failure to ascertain that it's not going to fail again.
When I get the new transformer and connect it up, I am still worried that something else might have caused the transformer to fail. Is there anything in particular I should check?
I could measure the current draw of the main circuit board, or check the rectifier diodes, the filter capacitors, etc. but I am not sure what the general process would be following transformer failure to ascertain that it's not going to fail again.
The only basic static checks you can do are really seeing that the bridge rectifier isn't short circuit.
When the new transformer is fitted be sure to use the bulb tester initially.
Assuming it works OK you would then check for anything getting hot as being a sign that something was amiss.
Transformers are incredibly reliable as a rule, and also able to withstand massive abuse. Hopefully all should be OK.
To identify the new windings (mains and secondary) and to be sure you connect them correctly use a bulb tester to power just the transformer and confirm the voltages.
Post #10 tells you how to do that:
Need help for power supply
When the new transformer is fitted be sure to use the bulb tester initially.
Assuming it works OK you would then check for anything getting hot as being a sign that something was amiss.
Transformers are incredibly reliable as a rule, and also able to withstand massive abuse. Hopefully all should be OK.
To identify the new windings (mains and secondary) and to be sure you connect them correctly use a bulb tester to power just the transformer and confirm the voltages.
Post #10 tells you how to do that:
Need help for power supply
Thanks,
Really great info.
The transformer weighs 0.6 kg and is 75x28mm so going to go for this 40VA one of similar size I think: RKD 40/2x24 | 2 Output Toroidal Transformer, 40VA, 2 x 24V ac | Block
For the dim bulb tester, what is the simplest way to make one of these? Can I just solder two wires to a bulb holder in series just after the IEC socket in the circuit?
I guess I will need bulbs of 15W, 20W, 30W, or similar, which seems low compared to normal usage of a dim bulb tester, but I guess that's OK - on the back panel it says 0.125A/250v for the fuse, which would be 30A (less with power factor) - a 15W bulb would be 50% of that draw. Have I got this right?
Thanks,
Jon.
Really great info.
The transformer weighs 0.6 kg and is 75x28mm so going to go for this 40VA one of similar size I think: RKD 40/2x24 | 2 Output Toroidal Transformer, 40VA, 2 x 24V ac | Block
For the dim bulb tester, what is the simplest way to make one of these? Can I just solder two wires to a bulb holder in series just after the IEC socket in the circuit?
I guess I will need bulbs of 15W, 20W, 30W, or similar, which seems low compared to normal usage of a dim bulb tester, but I guess that's OK - on the back panel it says 0.125A/250v for the fuse, which would be 30A (less with power factor) - a 15W bulb would be 50% of that draw. Have I got this right?
Thanks,
Jon.
The transformer looks OK and is within the voltage range the manufacturer recommended to you.
As long as you wire the bulb safely then you can use whatever seems appropriate. If there is a fuse within the appliance (on the mains side of course) then you could remove that and tag the bulb across there. It is only for an initial test so it doesn't have to look pretty.
Don't sweat the details of what rating to use, if you have a 60 watt bulb, fine, if its a 30 then that is OK as well. Ideally you want a traditional old fashioned GLS type rather than a modern halogen equivalent.
You can predict what current will flow simply because the resistance of the filament vs temperature (caused by the actual current) is unknown.
Measure the resistance of a cold bulb and then calculate the current that will flow at 340 volts (the peak value of the AC cycle). When that filament is even slightly hot, the ohmage will increase dramatically
As long as you wire the bulb safely then you can use whatever seems appropriate. If there is a fuse within the appliance (on the mains side of course) then you could remove that and tag the bulb across there. It is only for an initial test so it doesn't have to look pretty.
Don't sweat the details of what rating to use, if you have a 60 watt bulb, fine, if its a 30 then that is OK as well. Ideally you want a traditional old fashioned GLS type rather than a modern halogen equivalent.
You can predict what current will flow simply because the resistance of the filament vs temperature (caused by the actual current) is unknown.
Measure the resistance of a cold bulb and then calculate the current that will flow at 340 volts (the peak value of the AC cycle). When that filament is even slightly hot, the ohmage will increase dramatically
Halogen is fine: they are just hotter plain-old-incandescents. A "100W equiv" gives light like an old 100W bulb with "only" 85W of electricity; not a big difference.
CFL and LED are NOT suitable. Run down to the store and grab hot-filament lamps while you still can. In the US, Halogens qualify as "high efficiency" (they suck 15% less than old-style incandescents) and are legally fine. But going out of style as LEDs come down in price and 5X the light per Watt of any filament lamp. 150W plain-old lamps are non-household and readily available at the end of the aisle-- I still light my garage this way (hours too small to care about the bill, and low first cost).
As a wild-guide: the actual (not equiv) Watts should be some larger than the device you are testing but much less than the rating of your wall-circuit and line-cord. In the US, all line-cord is rated more than the biggest single lamp you can buy, so that's not an issue. But you can't even begin to test a 300 Watt SVT with a 45W lamp-- big amps need big lamps.
CFL and LED are NOT suitable. Run down to the store and grab hot-filament lamps while you still can. In the US, Halogens qualify as "high efficiency" (they suck 15% less than old-style incandescents) and are legally fine. But going out of style as LEDs come down in price and 5X the light per Watt of any filament lamp. 150W plain-old lamps are non-household and readily available at the end of the aisle-- I still light my garage this way (hours too small to care about the bill, and low first cost).
As a wild-guide: the actual (not equiv) Watts should be some larger than the device you are testing but much less than the rating of your wall-circuit and line-cord. In the US, all line-cord is rated more than the biggest single lamp you can buy, so that's not an issue. But you can't even begin to test a 300 Watt SVT with a 45W lamp-- big amps need big lamps.
...Measure the resistance of a cold bulb and then calculate the current that will flow at 340 volts (the peak value of the AC cycle). When that filament is even slightly hot, the ohmage will increase dramatically
One 60 Watt 120V incandescent I had was 16 Ohms dead-cold.
Obviously when full-hot it was 120V and 0.5A thus 240 Ohms.
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