SPL Optimiser

[alexgg]

hi

this is my first post, I am very new to diy , I am just starting to make my own wires + have a basic idea of electronic components + their uses, anyway here goes ..

i have a SPL optimiser EQ , i bought it secondhand + have noticed a degree of 50hz hum (i am in uk)

the EQ is quite complicated + i cannot find schematics online but the board is quite well marked.

the hum does not occur at all in bypass, it seems more aparant when using bass frequencies in parametric or high pass.

do you think i should replace caps, there are many but maybe just PSU related caps ?

here are some photos from :

http://rhythminmind.net/1313/?p=1146

eric also posted in gearslutz "gutz"thread where photos more easily expandable somehow .. http://www.gearslutz.com/board/geekslutz-forum/110401-gutz-22.html

if anyone has any advice or pointers , schematic even it would be appreciated.

alex

 

[tzman]

Have you tried contacting SPL in Germany?
I've had excellent help of their tech support. I'm pretty sure they will send you the schematic as it is a discontinued product...

 

[alexgg]

good idea, thanks, i'll try and email them.

any ideas about where to start looking for hum in such an eq ?

is it most likely caps around psu ?

doesn't hum in bypass + like I say the hum increases when dealing with bass bands even if cutting!?

alex

 

[gyraf]

I would suspect powersupply - most probably worn-down primary reservoir electrolytics.

If not that, a blown power regulator - but this is a rare occurrence

last, it could be some HF-instability, generating out-of-band oscillation and thereby disturbing supply

Jakob E.

 

[Harpo]

If the hum is at 50/60Hz, look out for a broken rectifyer/diode, if hum is dominant at double mains frequency, most likely the reservoir caps Jakob already mentioned.

 

[alexgg]

hi guys

thanks for the advices , its definitely dominant at 50/60 hz but I can't see any diodes in the circuit.

alex

 

[Harpo]

Look at the psu section. If rectification is done with 4 single diodes (most often SPL used 1N4007 in their devices) in a bridging arrangement, one side of each diode connects to the mains transformer secondary wires. If done with a bridge rectifier instead (boxy or round shaped device, containing 4 diodes in a single housing), two of its 4 legs connect to the mains transformer secondaries.

 

[alexgg]

ah yes, thanks harpo.

I can see them now d3 - d6 on PSU board (d1 + d2 on each balanced input I guess not important).

I can make out ..N40.. on the upward facing side but does look most probably to be 1N4007 from size + appearance

so it could likely be any 1 of these 4 that is the problem?

its only now that I realise the cheap secondhand fluke I just bought does not have diode test

I can see they are 16p each at electronic stockist, so maybe best to just replace all 4 and hope this fixes the problem ?

 

[usekgb]

Might as well replace the power supply filter caps while you're in there.  I can't hurt.  Just make sure that you maintain the polarity of the parts you are replacing.

 

[alexgg]

yep that sounds like a good idea.

I'm going to change all the diodes (my cheap fluke does actually have a diode meter on it I think after all)

I also ordered 2x 100uf 50v electrolytic caps that are on psu circuit, 1 of these possibly swelling a tiny amount.

I'm going to check the 4x 4700uf 50v also , but these look good, + replace if they test bad.

is this a general approach to only change caps that possibly look bad ? or would you more laboriously check all all electrolytics (on psu at least) ?

 

[Andreas Pfeiffer]

It might be good to change all for sonic quality

 

[alexgg]

hi

please can anyone suggest what might still be wrong with this eq if ..

I changed psu diodes + some caps.

50hz hum increases as the frequency pot is turned to lower frequencies

it seems to be dropping the audio signal quite a bit , something like 10db , I am using the -10db input selector to boost this back up but then clipping leds lighting on filters so maybe its just output dropping after filters,

I got a schematic from spl support but not getting a lot more information from them at the moment

 

[alexgg]

I just tested the 2 power regulators

both are 7915

both input 24v

1 outputs 15v , the other outputs 9v

this is 15v circuit, so does that mean the 9v output regulator is faulty ?

 

[alexgg]

correction to above

closer inspection, both schematic + circuit

+ve side = 7815 regulator , this outputting 15v

-ve side = 7915  , this outputting 9v only

is 7915 most likely in need of replacement >?

 

[Majestic12]

Yes. Seems as if the regulator has been damaged somehow.

 

[alexgg]

hmm, its weird I changed the 7915 and now same readings as before but I think maybe I made a mistake

the measure from 7915 ground bolt to output is 9.5v

if I measure from  chassis ground to the 7915 out it is -15v as expected
it does also seem that from chassis ground to where the +/-ve rails leave psu that +15v / -15v so power ok

however from chassis ground to the positive 7815 input is 24v
but from chassis ground to negative 7915 input there is 0v (from 7915 ground bolt to input is showing24v)

does this reveal anything about my psu and grounding? , (I still have hum problem + signal drop in output)

anything else I could test here?

 

[alexgg]

I actually think output signal drop is more of a problem here

can anyone please suggest what might be causing both channels to lose around 10db on output ?

I guess this probably psu related because common to both channels.

 

[Harpo]

hmm, its weird I changed the 7915 and now same readings as before but I think maybe I made a mistake
the measure from 7915 ground bolt to output is 9.5v
if I measure from  chassis ground to the 7915 out it is -15v as expected
it does also seem that from chassis ground to where the +/-ve rails leave psu that +15v / -15v so power ok
however from chassis ground to the positive 7815 input is 24v
but from chassis ground to negative 7915 input there is 0v (from 7915 ground bolt to input is showing24v)
does this reveal anything about my psu and grounding? , (I still have hum problem + signal drop in output)
anything else I could test here?

When you look at the datasheets of the positive 78xx and negative 79xx fixed voltage regulators you might notice their different pinout of input, reference and output.

I actually think output signal drop is more of a problem here
can anyone please suggest what might be causing both channels to lose around 10db on output ?
I guess this probably psu related because common to both channels.

If you would lose 6dB instead of around 10dB, this might point to a wrong or shorted balanced/unbalanced connection.

 

[alexgg]

ah right, yeah thanks harpo , that explains the regulator voltages.
basically the 2 power rails are outputting around -14.8v and 14.4v, so voltage seems ok.

not  sure how else to test the psu.

the output is definitely balanced, I just checked on headphones from the xlr out (phones plugged into patchbay, + same on both ears)

but .. output is circa -10db

it really matches up well with the 10db of boost from using low level input, this designed for -10db input levels.
on high level input , this not boosted and comes out -10db.
why this is obviously wrong is because the 10 db boost for low level signals is causing clipping leds to light constantly until I reduce the input by 10db from pc output.

so somehow the signal is being cut by 10db after the filters. + a lot of hum when filter frequencies turned to lows.

attached is the schematic,
would really appreciated any help with this

 

[mjrippe]

When you are losing the ~10dB, are you going in and out via XLR or are you using the 1/4" jacks or a combination of both?