Stage center reverb debugging help please

[chaliapin]

Hello all,

thanks for help received so far, I thought it would be simper to begin a new debugging post. I built the GGG stage center reverb, schematic here

http://www.generalguitargadgets.com/diagrams/stage_center_reverb_sc.gif?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

I have effectively no electronics knowledge or skills but I like having a go at something. I thought if I copied the PCB design and put it onto a holeboard it  would work, I copied this (or tried to)

http://www.generalguitargadgets.com/diagrams/stage_center_reverb_lo2.gif?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

Then I made a big mistake and built it effectively left-to-right reversed, I thought a PCB picture was looking from below and seeing the components 'through' the board. Unfortunately it's the other way round. Looked long and hard and before starting again I'm seeing if it will work, turned the IC round so the polarity was now correct (also replaced it) I am now going to man up and post the pics of my random and scruffy work, in my (sort of) defence, my soldering iron is unbelievably rubbish and I need a proper one.

https://picasaweb.google.com/parlemichael/24February2013?authkey=Gv1sRgCI7M0NjTzf_vVw

I have been trying to learn  how to use my little digital multi meter.
The resistors are correct and all the grounds appear to connect.
Where the power connects to the board are around +8v and -8v relative to the ground.
The only voltages at the chip seem to be and the middle pins of the IC, +7.6 and -7.6, can't find and other voltages on the circuit which strikes me as bed news.
Sound from the circuit is very distorted and massively reverby and there seems to be no clean signal coming through.

Can't think of any other info, I know I should buy a PCB etching kit, make one and re-build but thought I'd give this a go first,

thanks all,
Mike

[PRR]

Indeed all the non-power-pin voltages on the chip in this plan *should* be "zero" (you may find 3 milliVolts of DC error, which is normal).

Put guitar in Input, patch "To Reverb Input" to your guitar amp. This should be VERY LOUD, and probably distorted (I'm dubious on this design's cleanliness).

Next try guitar into "From Reverb Output", and patch "Output" to your amp. This also will be VERY LOUD, probably distorted, but you can turn-down with Dwell knob.

If both those work, the only thing could be wrong with the Dry signal is Mix pot and R3.

[chaliapin]

Thanks for the advice post.

post test were as they should be. interestingly, after having manhandled the circuit it seemed to work a little better. Must be some bad connections, I've bought a proper amtex iron and will go over it. I am now getting a not great but ok sound with the dwell right down and the clean signal is now coming through, I think I didn't notice the clean signal as with the dwell up the reverb signal is so loud and distorted the clean is hardly present. I am going to check for loose connections but something seems to be wrong with the reverb signal as the sound is still like mud and turn the dwell up at all and it's very distorted and still very muddy. Any ideas on next steps? Thanks all,

Mike

[chaliapin]

gotta tell you I'm struggling guys, got rid of dodgy connections with my new 25w amtex which actually really works.

My layout swapped C3 and C5 around and somehow I'd ended up with the wrong capacitor value at C5. Unfortunately, putting the correct one in did sod all for the terrible noise this is making.

I read another post about changing the value of R7 but it seems to me that I have plenty more wrong than that as the output is still really loud, muddy and distorted. Should i try this?

I do have the right spring unit I hope, I used accutronics 8db2c1d as the listed impedance is 310 ohms and this is what is specified on the bill of materials.

Is it just time to start again? The off-board wiring keeps breaking as i'm moving the circuit around constantly and I'm getting fed up!

[PRR]

I too think those gains are extreme. Try R7=100K R9=470K.

> The off-board wiring keeps breaking

Then do it up right. Probably good stranded wire.

Wiring that breaks on any repair attempt is the bane of many guitar amps.

[chaliapin]

Thanks Paul. I've replaced the resistors as described, sound is somewhat better but still low quality, when I take all the reverb from the mix the 'clean' sound is still distorted, louder than the bypass tone and with a thin, slightly fizzy sound. I still feel like there something not right going on or this is a poor design. As I turn the dwell up the treble disappears and the sound turns into mud. Basically the whole circuit is making my great sounding boutique 10w amp sound like a cheap old practice amp with rubbish reverb. Any other ideas anyone? I really want to make a great sounding spring reverb!

[PRR]

Try changing R3 to 27K, then tossing the "Mix" pot and jumpering the A and B points together. With spring disconnected, that should give a super clean dry sound at "unity gain" (actually about 20% boost). I don't understand why it has gain of 3 on the dry signal.

I'd like to know if anybody else has built this, either from Craig's original article(?) or from the GGG kits/plans.

[chaliapin]

I've only got 22k but I'm about to try it. Thanks again for the help.

[chaliapin]

ok, that worked, some noise/hum from the unconnected spring tank leads but fairly clear, slightly boosted clean sound.

I've been working from the PCB drawing which has the mix and dwell labelled the other way around which is confusing although I assume not the actual source of the problem.

Been reading other people's posts on this circuit, they don't seem very impressed with it but no-one seems to have produced such filthy sounding results as me!

[chaliapin]

Looks like no-one else wants to help us out on this, has anyone built this and got it to sound good or is it the fact that most settings sound muddy and fizzy? Help please!

[PRR]

What spring-tank do you have?

Decode the part-number. We really want to know the impedances.
http://www.accutronicsreverb.com/main/?skin=sub01_02.html
http://www.amplifiedparts.com/tech_corner/accutronics_products_and_specifications

Craig's plan used a 250 ohm input, 2,250 ohm output tank 4DB2C1D.

Using a DC Ohm meter, you should see *about* 30 ohms input, 300 ohms output. (The audio impedance is higher, variable, and not easily determined with a meter.)

[chaliapin]

Hi Paul, you're a good man. I have 8DB2C1D, I saw that it was stated as designed for the 4DB2C1D but also said it was designed for a tank with a 310 impedance, which th 8DB... was listed as so I went with the spec rather than the model if you see what I mean. It is a brand new tank and has 3 rows of springs. DMM says 36 input 220 output, is that way off?

[PRR]

Yeah, that's the right coils. The DC ohms are reasonable. "8" is I think a higher grade of tank, not a problem.

Wanted to be sure you didn't have a 8/10 ohm tank used in older Fenders.

Back to basics.

> swapped C3 and C5 ...wrong capacitor value at C5.

If there's one mistake, there are others (Murphy's Law). I'm thinking a wrong-part or a bad connection in the reverb send/return stages. Because while there are no rave reviews on this thing, I can't find any gross complaints either.

[chaliapin]

ok, I'll get back to the checking, leave R3 as is?

[chaliapin]

SUCCESS! Ducking sanguinaceous sucess! I'd made an invisible bridge somehow between D2 and R3 and my chip was also not behaving as it should. Some scraping of a razor blade and chip replacement and it bleddy well works!

No Idea how I'm gonna fit it in the box, Would I need an 18v power supply to plug it in and do away with batteries and charge pump?

Gonna replace all the external wire with better thicker stuff as I've melted the covering of quite a bit of it and it generally looks a mess.

Sounds decent, the wet signal is a bit muffled sounding and the whole thing fairly noisy but is definitely doing what it should be! Paul, you are a kind and patient man how do I buy you an internet drink?