DBX 118 PSU
« on: January 29, 2019, 12:23:30 PM »
Hey Guys


So many info on mod's of these old unites but nowhere I can find information on replacement PSU.

Could you please advise what replacement PSU would fit in this ?

All I have gathered according to the schema is that at the end of the power stage we need 12VDC +/- but to get there, we need to start with the right PSU voltage at the starting point which are....?

Please share your thoughts.

Thank you !!   :)


scott2000

Re: DBX 118 PSU
« Reply #1 on: January 29, 2019, 03:07:53 PM »
I saw this....

https://www.gearslutz.com/board/geekslutz-forum/167506-help-me-fix-my-power-supply.html

maybe helpful to what you are asking???

Unless you are asking about building your own supply from scratch???


Re: DBX 118 PSU
« Reply #2 on: February 03, 2019, 10:11:43 AM »
hey..thanks for that. I just wanted to find out a replacement transfomer and the type really as the schematic won't display anything and I can not find anything related to this on the web, even knowing the serial number or the product number from these old DBX transformers...no one is really mentioning it..cheers

scott2000

Re: DBX 118 PSU
« Reply #3 on: February 03, 2019, 11:12:29 AM »
The thread mentions 20vac from secondaries so, I'd guess any suitable 120/20vac would be ok???

I wonder if this even would work??

https://www.fullcompass.com/prod/231422-dbx-26-0268-power-transformer-for-166xs?gclid=EAIaIQobChMIu6e65faf4AIVTkwNCh2QnAZSEAQYASABEgKPMPD_BwE

secondaries in parallel would get you close but I'm just throwing stuff out there to bump this so someone else could chime in..... :)

or I'm sure Hammond has a selection for cheap/er .......


Re: DBX 118 PSU
« Reply #4 on: February 03, 2019, 11:14:25 AM »
Thank you Scott for your help, really appreciate it. Will check that link out... :)

scott2000

Re: DBX 118 PSU
« Reply #5 on: February 03, 2019, 11:26:54 AM »
No problem....

Hopefully someone can chime in who knows better about this...
.... I'm not familiar enough with that circuit  to understand if what that thread is saying is legit in regards to the 20v  ........ I'll look at the schematic in a bit to see if I can decipher what they are talking about...

« Last Edit: February 03, 2019, 05:17:47 PM by scott2000 »

pvision

Re: DBX 118 PSU
« Reply #6 on: February 03, 2019, 05:10:57 PM »
With a full-wave rectifier the DC voltage out is approx 1.4 x AC voltage so 20 VAC would give about 28 VDC. The dbx circuit has a half-wave rectifier so the DC voltage will be less than 28

You're wanting 24 VDC (±12) so you might need a bit more than 20 VAC from the transformer

I don't believe the output voltage is critical so you could use a 24 VAC transformer or a 12-0-12 and use the secondaries in series

You should be able to buy a 110 / 12-0-12 transformer pretty much anywhere

Nick Froome

mjrippe

Re: DBX 118 PSU
« Reply #7 on: February 03, 2019, 11:14:15 PM »
Just to be clear - is your power supply broken or are you looking to replace it as a mod as mentioned in your first post?

Re: DBX 118 PSU
« Reply #8 on: April 16, 2019, 08:08:57 AM »
With a full-wave rectifier the DC voltage out is approx 1.4 x AC voltage so 20 VAC would give about 28 VDC. The dbx circuit has a half-wave rectifier so the DC voltage will be less than 28

You're wanting 24 VDC (±12) so you might need a bit more than 20 VAC from the transformer

I don't believe the output voltage is critical so you could use a 24 VAC transformer or a 12-0-12 and use the secondaries in series

You should be able to buy a 110 / 12-0-12 transformer pretty much anywhere

Nick Froome


Hi Nick,

Thanks for your previous note on this...still no luck finding out the issue with my DBX. I have put it away and now cam back to it again...

would you help me please confirming whether this PSU would be a suitable replacement?

https://hu.mouser.com/datasheet/2/410/media-1067685.pdf

I think it would be....but nit sure this PSU has 12x2 V? its 24VAC so I assume the two lead on the secondary is 2x 12V?

I had some funny measurements on my current PSU still in place in the DBX...its 32,8  on one lead to ground and 22,4 on the other lead to ground..

I am also suspecting that maybe the power regulator gone also bad...

thanks for your advise!

scott2000

Re: DBX 118 PSU
« Reply #9 on: April 16, 2019, 10:09:46 AM »
Pretty sure you need to be looking for an 18vac ish transformer.....

That seems to be what is the general stuff I've seen around when reading on this.....Would make sense and give you 20vac unloaded......

But pvision may know more about these and the 24vac might be fine......the one you posted is only 12vac......

I think that circuit is in fact some kind of doubler but, I'm not familiar with how all of the regulation works after.......

Maybe you could post in the lab with some questions regarding that???

have you tried measuring the transformer out of circuit??
« Last Edit: April 16, 2019, 03:21:52 PM by scott2000 »


Re: DBX 118 PSU
« Reply #10 on: April 17, 2019, 01:49:35 AM »
Thanks guys for the suggestions!

Really appreciate it... it is a bit of a headache...as now I am getting +25V and -25V at the check point on the  C4 me without changing anything...

I have 2 DMM and both telling me the same...


Not sure whats happening.... saw in other threads that it is advised to change those blue tantal cap's which I havent done...but they all reads fine within spec's...  i think the problem here is more sinister..cheers

Walrus

Re: DBX 118 PSU
« Reply #11 on: April 17, 2019, 05:26:47 AM »
Are you definitely measuring that +-25v with respect Pin1 on OA2?
Kevin.

Harpo

Re: DBX 118 PSU
« Reply #12 on: April 17, 2019, 06:00:46 AM »
... The dbx circuit has a half-wave rectifier so the DC voltage will be less than 28
You're wanting 24 VDC (±12) so you might need a bit more than 20 VAC from the transformer
No. The psu in the dbx circuit is a delon or greinacher voltage doubler. Unloaded voltage is doubled peak of AC voltage. The 723 voltage regulator has an abs.max of 40V input, so transformer secondary voltage should be less than 40/1.1 (for allowing 10% high mains)/1.414 (for the peak value)/2 (for the doubler and ignoring the 0.6 voltage drop per diode)=12.86VAC. I'd pick a 12VAC secondary transformer. YMMV. If you tried it with a higher voltage feed, I'd expect some parts to be blown by now.
A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.  -Douglas Adams

Re: DBX 118 PSU
« Reply #13 on: April 17, 2019, 06:08:51 AM »
No. The psu in the dbx circuit is a delon or greinacher voltage doubler. Unloaded voltage is doubled peak of AC voltage. The 723 voltage regulator has an abs.max of 40V input, so transformer secondary voltage should be less than 40/1.1 (for allowing 10% high mains)/1.414 (for the peak value)/2 (for the doubler and ignoring the 0.6 voltage drop per diode)=12.86VAC. I'd pick a 12VAC secondary transformer. YMMV. If you tried it with a higher voltage feed, I'd expect some parts to be blown by now.


wooooo thanks

I have not changed the transformer itself but the measurements are correct of 25VDC where I should measure 12 VDC...so each time I switched the unit on to do the testing, I could damage some components ..? oooh man...

So  should definitely get a new PSU for start ? or shall I desolder it and see what voltage the transformer is giving me ...

would this be a good replacement? https://hu.mouser.com/datasheet/2/410/media-1067868.pdf

thank!  :)

Walrus

Re: DBX 118 PSU
« Reply #14 on: April 17, 2019, 06:45:17 AM »
Quote:
I have not changed the transformer itself but the measurements are correct of 25VDC where I should measure 12 VDC...so each time I switched the unit on to do the testing, I could damage some components ..? oooh man...

So you are saying that if you put your meter *ACROSS* C4, you will measure 50V DC????
That capacitor is rated at 35volts (if it has not been changed) so I'm surprised it hasn't exploded yet. And as for the IC's which all expect about 15-18 volts max.......
Kevin.

Re: DBX 118 PSU
« Reply #15 on: April 17, 2019, 06:50:29 AM »
Quote:
I have not changed the transformer itself but the measurements are correct of 25VDC where I should measure 12 VDC...so each time I switched the unit on to do the testing, I could damage some components ..? oooh man...

So you are saying that if you put your meter *ACROSS* C4, you will measure 50V DC????
That capacitor is rated at 35volts (if it has not been changed) so I'm surprised it hasn't exploded yet. And as for the IC's which all expect about 15-18 volts max.......

hi Kevin

I measure 25VDC on each side.... if I measure the -ve side of that capacitor, I only can measure DC if I swap my DMM pins...so the  red probe goes to the IC pin number 1( ground) and the black probe goes to the -ve side of the capacitor C4.

There is display on my DMM for negative voltages, so far only display +ve voltages with the probes swapped as above, on each side of the cap 25VDC.


Walrus

Re: DBX 118 PSU
« Reply #16 on: April 17, 2019, 07:00:55 AM »
....and if you measure *across* C4 red probe on +ve and black probe on -ve??
Kevin.

Re: DBX 118 PSU
« Reply #17 on: April 17, 2019, 07:02:15 AM »
....and if you measure *across* C4 red probe on +ve and black probe on -ve??

also 25 VDC :)

Walrus

Re: DBX 118 PSU
« Reply #18 on: April 17, 2019, 07:31:43 AM »
also 25 VDC :)

That means you transformer and the 723 voltage regulator are working correctly.
I'm a bit confused by what you wrote regarding your multimeter. Can it not display a negative voltage if the leads are round the wrong way?
If you put the black probe to pin 1 of OA2 and the red to the positive of C4, you should read +12.5 ish volts.
Then swapping the leads round so that the red probe is on pin 1 of OA2 and the black is on the positive of C4, then again you should read +12.5 volts.
If this is not the case, then it would appear OA2 is broken.

[Post edited, I mistakenly wrote OA4 not OA2]
« Last Edit: April 17, 2019, 10:18:57 AM by Walrus »
Kevin.

Re: DBX 118 PSU
« Reply #19 on: April 17, 2019, 07:34:29 AM »
That means you transformer and the 723 voltage regulator are working correctly.
I'm a bit confused by what you wrote regarding your multimeter. Can it not display a negative voltage if the leads are round the wrong way?
If you put the black probe to pin 1 of OA4 and the red to the positive of C4, you should read +12.5 ish volts.
Then swapping the leads round so that the red probe is on pin 1 of OA4 and the black is on the positive of C4, then again you should read +12.5 volts.
If this is not the case, then it would appear OA4 is broken.

Sorry for confusion, it is the case how you described it exactly  but I am measuring 25V each side not 12.5V.  ( not 12.5 times 2) it is 25 on each side.



 

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