Hi, I have been tasked with fixing this Carlsboro Spring Reverb unit from the 70s. Upon opening it and hooking it up to power, I noticed that the transformer connected to the 240V mains AC was creating sparks at the primary side.
I disconnected the secondary side to see if it was anything to do with the load and it still sparks.
I'm looking for some help understanding what the different legs on this transformer are doing. It looks like the live is connected to a center tap on the primary side, and then in circuit, the secondary side is connected across the full winding (the two outer connections).
The reason that I ask is that it seems like the output of this transformer is quite unstable, and I think this is making the rest of the circuit play up (inconsistent output volume). The transformer was sparking a little near the neutral input and gave a slight burning smell.
I've measured the output voltage with a multimeter and it moves between 9VAC and 13VAC. I'm guessing I want a 240V -> 12V transformer. Does anyone know if that sounds right for a unit from this time.
If anyone else has any more information on this circuit would greatly appreciate it.
I've attached an image of the unit: https://drive.google.com/file/d/1PC6jseZAoFp6y7S0jCCuBVUTVrBcKYZd/view?usp=drive_link
An image of the transformer: https://drive.google.com/file/d/1lWGa4cazlWKj6C2WJpXJeOmWTjdCqA-A/view?usp=drive_link
The transformer in circuit: https://drive.google.com/file/d/1Y2wx8A0PtAs1wylrxuNHHhk5cTzqkA7A/view?usp=drive_link
Thanks a lot
I disconnected the secondary side to see if it was anything to do with the load and it still sparks.
I'm looking for some help understanding what the different legs on this transformer are doing. It looks like the live is connected to a center tap on the primary side, and then in circuit, the secondary side is connected across the full winding (the two outer connections).
The reason that I ask is that it seems like the output of this transformer is quite unstable, and I think this is making the rest of the circuit play up (inconsistent output volume). The transformer was sparking a little near the neutral input and gave a slight burning smell.
I've measured the output voltage with a multimeter and it moves between 9VAC and 13VAC. I'm guessing I want a 240V -> 12V transformer. Does anyone know if that sounds right for a unit from this time.
If anyone else has any more information on this circuit would greatly appreciate it.
I've attached an image of the unit: https://drive.google.com/file/d/1PC6jseZAoFp6y7S0jCCuBVUTVrBcKYZd/view?usp=drive_link
An image of the transformer: https://drive.google.com/file/d/1lWGa4cazlWKj6C2WJpXJeOmWTjdCqA-A/view?usp=drive_link
The transformer in circuit: https://drive.google.com/file/d/1Y2wx8A0PtAs1wylrxuNHHhk5cTzqkA7A/view?usp=drive_link
Thanks a lot
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Thanks for getting back to me. Could you explain why that is the case?
From my understanding, the full wave rectifier and the smoothing capacitors would only be able to generate a max DC of the peak of the AC voltage.
Thanks
From my understanding, the full wave rectifier and the smoothing capacitors would only be able to generate a max DC of the peak of the AC voltage.
Thanks
Ahh I see. I didn't realise that multimeter reads AC values as RMS and not peak, but that makes a lot of sense. Also just learned that UK power is 230VAC meaning RMS and not peak. Thanks for that 🙂
that's good to know!
So for this unknown transformer, knowing that the output voltage is fluctuating quite a lot (between 7 and 13V and sometimes even outside of these values) is there a way to know what DC voltage the circuit is looking for?)
The original smoothing caps were rated at 40V so clearly not more than that. But that's all I have to go on at the moment.
So for this unknown transformer, knowing that the output voltage is fluctuating quite a lot (between 7 and 13V and sometimes even outside of these values) is there a way to know what DC voltage the circuit is looking for?)
The original smoothing caps were rated at 40V so clearly not more than that. But that's all I have to go on at the moment.
The sparking could be from a poor contact, or an internal short, where exactly was the visible spark on the transformer?
It's coming from the neutral connection of the primary side of the transformer. I tried to record but it's difficult to see on camera. If I look closely at the connections on the transformer, I can see a really small amount of the enamelled coil wire running between the connection points on the primary side (where you'd solder a wire). I was wondering if that enamelled wire had overheated, or burn out.
It really looked like sparks were just coming from the soldered joint for the neutral input.
It really looked like sparks were just coming from the soldered joint for the neutral input.
Can you post a very sharp, clear, well lighted photo (not video) of the point in question?
Solder joints, when defective, can heat up or spark. Possibly corrosion as well.
If still uncertain, maybe replacing the transformer is the best choice.
Solder joints, when defective, can heat up or spark. Possibly corrosion as well.
If still uncertain, maybe replacing the transformer is the best choice.
Unfortunately don't have access to the circuit at the moment. Next time I am at my bench I will upload a clear photo. Thank you for your help!
I reckon it is looking for 15 V DC which is about what you would get from a 12 V AC transformer in this circuit after allowing for the voltage drop across the bridge rectifier when the reverb unit is in use.
Choose a transformer with a low current rating - say 300 mA.
rayma has the expertise to keep us right on that!
Hi, I've attached an image of the transformer pointing to where the sparking is coming from.
https://drive.google.com/file/d/1JnevAJ5USCOcoRZ_h6FfjWjinelKuG1H/view?usp=drive_link
At this point, with the output of the transformer fluctuating so much I'm pretty confident a replacement would help. Just looking for an idea of what voltage, as you've said Galu.
rayma, do you agree that around 15V DC is what the reverb might be looking for, I'll go ahead and try replacing the transformer if so.
I've also emailed Carlsbro asking for the service manual, but, with the unit being so old, it's not a guarantee they'd have a copy.
What signs are there that could be used to identify the operating voltage on old audio electronics, beyond the capacitor ratings?
Thanks again
https://drive.google.com/file/d/1JnevAJ5USCOcoRZ_h6FfjWjinelKuG1H/view?usp=drive_link
At this point, with the output of the transformer fluctuating so much I'm pretty confident a replacement would help. Just looking for an idea of what voltage, as you've said Galu.
rayma, do you agree that around 15V DC is what the reverb might be looking for, I'll go ahead and try replacing the transformer if so.
I've also emailed Carlsbro asking for the service manual, but, with the unit being so old, it's not a guarantee they'd have a copy.
What signs are there that could be used to identify the operating voltage on old audio electronics, beyond the capacitor ratings?
Thanks again
This little 12 V/3 VA transformer outputs 13.8 V AC at no load (0mA) and 10 V AC at full load (300 mA):
https://cpc.farnell.com/carel/tra12un100/transformer-240vac-12vac/dp/MC01881
This would translate in the reverb circuit to as high as 18 V DC (0 mA) and as low as 12.7 V DC (300 mA).
What you don't want is too high a voltage that could damage the circuit, so I'd play safe with a 12 V AC transformer like this.
But let's see if @rayma will agree with me - he's the expert after all!
https://cpc.farnell.com/carel/tra12un100/transformer-240vac-12vac/dp/MC01881
This would translate in the reverb circuit to as high as 18 V DC (0 mA) and as low as 12.7 V DC (300 mA).
What you don't want is too high a voltage that could damage the circuit, so I'd play safe with a 12 V AC transformer like this.
But let's see if @rayma will agree with me - he's the expert after all!
Hey, thanks for this @Galu. I've ordered this transformer from RS which I think is quite similar to the one you've linked. I ordered from RS as there's a branch near me so I can get free shipping. It's also a 230 -> 12 VAC transformer with an output around 3VA.
Do you think this looks reasonable?
Will try this once it arrives!
Do you think this looks reasonable?
Will try this once it arrives!
The reverb unit is a low power device so won't draw a large current from the transformer when it's being played.
The small physical size of the original transformer suggests it has a low output current or VA rating.
Did you mean to include a link to the RS transformer? Its physical size would be a good indicator of its suitability.
The small physical size of the original transformer suggests it has a low output current or VA rating.
Did you mean to include a link to the RS transformer? Its physical size would be a good indicator of its suitability.
Ahh sorry, my mistake. Here's the item I ordered: https://uk.rs-online.com/web/p/pcb-transformers/2857215
Looking at the datasheet looks like this specific model is in the region of 20mm x 20mm x 30mm. Quite similar to the existing one so hopeful this will do what we need.
Looking at the datasheet looks like this specific model is in the region of 20mm x 20mm x 30mm. Quite similar to the existing one so hopeful this will do what we need.
You've chosen transformer BV EI 305 2057. So just to ensure we've both got it straight:
Primary winding: 230 V on pins 1 & 5
There are two 12 V secondary windings: Sec. I on pins 6 & 7 and Sec. II on pins 9 & 10.
Each secondary can deliver 125 mA so you would connect both secondaries in parallel to double up to the full 250 mA.
On Connection Scheme 2 shown in the Datasheet, paralleling would entail bridging pins 6 + 9 and bridging pins 7 + 10.
"Inherently short-circuit proof" - that's good!
Please let us know how you get on.
Primary winding: 230 V on pins 1 & 5
There are two 12 V secondary windings: Sec. I on pins 6 & 7 and Sec. II on pins 9 & 10.
Each secondary can deliver 125 mA so you would connect both secondaries in parallel to double up to the full 250 mA.
On Connection Scheme 2 shown in the Datasheet, paralleling would entail bridging pins 6 + 9 and bridging pins 7 + 10.
"Inherently short-circuit proof" - that's good!
Please let us know how you get on.