Sound Workshop 242 spring reverb supply voltage

[beatnik]

Hi, I have bought this spring reverb in quite bad condition

It did work for a while but then a power supply capacitor exploded

Replaced the capacitors and powered the unit again

I measured the voltage post rectification I get +/- 45V that are then brought to +/- 15V by a couple of transistors

I have found datasheet for the power transformer and the secondary voltage is rated at 56V CT

It seems to me the secondary voltage on the power transformer is too high and that's what made the cap explode in the first place

I am wondering if the problem might be the unit is 110V version ?? I just assumed it was for 220V but the thing is the unit came without the power cord and there is no label on the back stating the input voltage !

I couldn't find schematics for this unit but I checked the pcb mounted transformer and two terminals are tied together, mains power applied  to the other pair. It seems to me the transformer is wired for 220V operation but I really don't know why the voltages post rectification are so high

Hope someone can share light on this so I will avoid another explosion !

 

[Whoops]

Hello,

just to clear things, you didn't tell us in what country you were, I just assume that you are in a 220/240V country.
If you have a transformer primary  for 120V and you feed it 240V the primary will burn and Bye Bye transformer. (It's quite fast this, less than 5seconds)
So your transformer is still working, it means that the primary it's man enough to receive 240v

The Cap Exploded
You should be more specific in your post once again.
What Cap Exploded? Type? Value? Function?

I will assume it's an Electrolytic capacitor that exploded,
normally it's said they will only last 10 years working properly, after 10 years anything can happen, sometimes they will be fine for more 40 years, most of the times not.
This situation can be worse if the equipment is not used and the Electrolytic capacitor is left with being polarized for a long period oof time. How long is too long?  3 to 5 years is Long. Search for" Electrolytic Shelf Life"
So there's an high probability that  the Lytic capacitor was just saying it reached it's time to rest forever.

So for how long was the equipment left without being turned on?
What's the aprox age of the electrolytic capacitors?

I measured the voltage post rectification I get +/- 45V that are then brought to +/- 15V by a couple of transistors

I have found datasheet for the power transformer and the secondary voltage is rated at 56V CT

It seems to me the secondary voltage on the power transformer is too high and that's what made the cap explode in the first place

Why do you think that?
Are there any other secondaries?

What voltages are the original capacitors rated at?

Anyway I'm sure the transformer secondary is not putting out 56V,
it's much less.
If you have 45V after rectification the secondary must be supplying 31 to 32V AC  (45/1.414), measure it with the DMM.

 

[beatnik]

yeah thanks for asking that i am in a 220V/240V country thanks for clarifying that the power transformer would destroy quickly, i was kinda thinking of that but still i don't know what to think about the power transformer secondary voltage, seems kinda high isn't it ?

the capacitor that exploded is the electrolytic for filtering right after the regulators. the original capacitors were rated 25V, and there is +/- 45V in that points. i now put 50V capacitors there

i also think you are correct regarding the item being left powered off for a long time, the unit was left in a basement

i am wondering if might be better to change the power supply circuit. right now there is a pair of to-39 transistors i think as regulators but they drop the voltage from +/-45 to +/- 15 seems a lot of drop

i know it's hard to tell without a schematic but i have searched everywhere. i just would like to make the unit safe to operate and avoid as much other problems in the future.

 

[Whoops]

yeah thanks for asking that i am in a 220V/240V country thanks for clarifying that the power transformer would destroy quickly, i was kinda thinking of that but still i don't know what to think about the power transformer secondary voltage, seems kinda high isn't it ?

Don't really know if it's high or not , it depends on specific circuit and PSU requirements, without a schematic is hard.
what AC voltage do you have in the secondaries before rectification?

[/quote]
the capacitor that exploded is the electrolytic for filtering right after the regulators. the original capacitors were rated 25V, and there is +/- 45V in that points. i now put 50V capacitors there
[/quote]

yes, having 45v feeding  25V capacitors thats high for sure, I normally double the voltage of the PSu for the capacitors, so if it's 45v I would use 100V rated caps, if I dont double I would at leat put 80% more voltage on the capacitor.
[/quote]

[/quote]
i am wondering if might be better to change the power supply circuit. right now there is a pair of to-39 transistors i think as regulators but they drop the voltage from +/-45 to +/- 15 seems a lot of drop
i know it's hard to tell without a schematic but i have searched everywhere. i just would like to make the unit safe to operate and avoid as much other problems in the future.
[/quote]

I was going to suggest that, but to be honest it all depends on the circuit requirements,
what are the requirements? what voltages go from the PSU to the circuit board?
Just +15V and -15v? How about consunption (amps)? 1amp/1.5amp?

Show pictures of the circuit.

Building a PSU is simple enough, I would recommend using this PCB:
https://pcbgrinder.com/index.php?route=product/product&path=59_73&product_id=169

But then you would also need another power transformer,
it's a possibility but first check the requirements

 

[Walrus]

the capacitor that exploded is the electrolytic for filtering right after the regulators. the original capacitors were rated 25V, and there is +/- 45V in that points. i now put 50V capacitors there

45volts *after* the regulator with an original 25 volt rated capacitor there says to me the regulators are toast!

I found a sales leaflet on a website that said 240v was "special order" when new. So your unit may well be set for 110v, hence why the regulator blew.
I have known transformers work for some time on the wrong setting, i.e. much more than 5 seconds, but they *will* eventually smoke.

 

[Whoops]

the capacitor that exploded is the electrolytic for filtering right after the regulators. the original capacitors were rated 25V, and there is +/- 45V in that points. i now put 50V capacitors there

45volts *after* the regulator with an original 25 volt rated capacitor there says to me the regulators are toast!

I found a sales leaflet on a website that said 240v was "special order" when new. So your unit may well be set for 110v, hence why the regulator blew.
I have known transformers work for some time on the wrong setting, i.e. much more than 5 seconds, but they *will* eventually smoke.

He said 45V after the Rectification and 15 volts after the discreet regulator circuit

If the transformer was 110v the primary  would be already burned

 

[Walrus]

Not in the later post that I quoted just now...... 

I beg to differ re the transformer burning, from personal experience of many an assistant plugging 110v gear into 240v and then bringing it to me to fix when the fuse eventually blows. (wrong fuse value usually)

 

[Whoops]

Not in the later post that I quoted just now...... 

I beg to differ re the transformer burning, from personal experience of many an assistant plugging 110v gear into 240v and then bringing it to me to fix when the fuse eventually blows. (wrong fuse value usually)

Nice to know, never happened to me and never seen it, but it's nice to know that.
thanks

 

[beatnik]

I am back with this to clarify a couple of things

45V is after the rectification. The regulators bring that down to 15V

The unit is working fine now, but should I expect it to blow at some point ?

There is no fuse on the mains !

 

[Walrus]

30 volts does seem rather a lot for the regulator to drop and will be very wastefull in heat. Out of interest, what is the voltage rating of the capacitors after the rectifiers and before the regulator?

 

[beatnik]

seems a lot to me too

I have now put 50V  capacitors there

I have recapped the whole circuit but the reverb is damn noisy. The eq control is almost useless, as you turn it up it just brings too much hiss

I am wondering if this might be a consequence of the power supply operating points or it's just normal problem with this model. The amplifier/driver circuit is just a bunch of 4558 chips

Anyone here has experience with these units ?

I might try swapping the power supply with a different circuit and see if there is any improvement

 

[analogguru]

hows about taking some pictures of the innards, the transformer,  the pcb from both sides (component and solder side) ?
So that maybe a schematic could be traced and thereby made it easier to answer all of yor questions.

BTW, if the wire of the primary winding of the transformer is thick enough the transformer will not be broken immediately, but if you play around long enough with the wrong setting/wiring it will be broken finally.  And I suspect strongly, that you are on the wrong setting/wiring.  But without pictures....

 

[Walrus]

seems a lot to me too

I have now put 50V  capacitors there

So to clear up any confusion in my mind, the original 25V capacitors you replaced with 50V ones were *after* the rectifiers and *before* the regulator transistors?
If that is correct, then the transformer is set for 110-120V or has shorted turns on a 240V primary, and will def. burn out at some time.
As analoguru says, some pics would be good.