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September 24, 2022, 10:25:19 PM

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I’ve Really, Really, Done it This Time.

Started by TheBadHombre, September 16, 2022, 10:18:30 PM

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TheBadHombre

Hello smart people.

That designation is appropriate because despite what I'd like to think, I feel I have exited from that category of human being just moments ago.

I'll spare you my lamentations and backstory and give it to you straight — and in return, I only ask you do the same for me; even if it means informing me that we'll have to amputate.

Peavey special 130. Love it. Gigged it. Customized it.
I go to open it up to replace a few RC4558p op-Amps with some burr brown OPA2134PA. All went well, except i had to leave 3 of the 4 original chips in due to weird noise floor issues and another weird sound but whatever. It was cool to open it up, get at least one burr brown in there, and rotate out the chip for the reverb with the one replaced RC4558p as it was humming like Festus Haggen when it was turned up.

Mission success right?

Except I had to try on more thing: plugging in the patch cable I always have going from the fx send to the fx return to engage a completely hypothetical buffer that I heard is sometimes engaged when doing so.

So I did... except I didn't. Here's what I did do:

You cannot view this attachment.

 (I'm hoping the picture will be embedded here, but if it isn't, now would be the appropriate time to gaze upon my illustration.)

What ensued was an electrifying experience to say the least. Behold, I looked, and I saw a great thunderbolt being exchanged between the two sockets I had just plugged into. The sound it made, although I lack exact memory of it, was approximately commensurate with what one would imagine would be accompanied by the aforementioned visual.

Needless to say, I quickly unplugged the jumper.

I was fortunately spared the physical experience of this manifested power, but alas my beloved amp was not. The only sound my love makes now is a ominous and tragic hum; not all too dissimilar from the one you would hear from an hold your finger on the end of a 1/4 cable.

My only hint would be that the current humming sound exactly like the humming I got from plugging in one of the op-amps upside down momentarily, as well as the failed reverb chip. I assume this means that at least one or more of the op amps is fried.

Please, please, help me friends.


willpirkle

#1
1. is power supply OK? check all supply rails for proper voltages

2. if PS is ok, is output fried? The FX return should have a high enough impedance that hopefully the power amp isn't fried. Is there DC on the speaker outputs? Is Vbe >>> 0.7V on power transistors?

3. If amp OK, check the FX return DC coupling cap, which is probably rated at a lower voltage than what you may have fed it. If it is fried, it may have been the best outcome as it would be acting like a sort of fuse. Plug an oscillator into the FX return jack and check to see if that cap is passing the signal with a DVM.

4. If cap is good, check the first op-amp in the FX rx circuit by checking the op-amp output pins - if its fried they'll likely have either one of the supply rail voltages, or a non-zero DC voltage on them, if not the you'll find the oscillator signal there (an oscillator app for your phone will work here).

With luck, it might be as simple as replacing the DC coupling cap, but it is always worth your sanity to check the PS and power amp first.

Hope that helps, Will

tonyharker

More likely is you have blown one or more output transistors. :(

g1

If you don't find DC on the output you may have blown a ground trace.
If you do find DC, probably a blown output transistor as mentioned above.

TheBadHombre

Hello! I do in fact see dc on the speaker output. So if that means it's likely a fried output transistor, my next question would be which ones are the output transistors?: the 4 circular transistors, or the three grey guys bolted directly to the pcb

Then, what exactly should I be seeing from a functional output transistor vs a fried one, and how would I measure that?

My apologies for my lack of basic knowledge with this stuff; I barely know what I'm looking at. I sure appreciate you guys going through the drudgery of explaining the basics to me as y'all are basically my only hope 😭


TheBadHombre

Oh and willpirkle,
 
Thank you so much for going into such detail. Are you able to tell instantly by looking at that photo where the DC coupling cap is? That is just one among many components that you mentioned that I have no idea how to locate.

I have only vague intuition of the basic signal flow throughout the pcb, and I absolutely do not know how to read the circuit diagram for the amp I found on the web.


Tassieviking

#6
Have you got +16V and -16V on the board ? Check all fuses if not.
Measure voltage across C61, C62 for 16V dc   (+C61 to -C62 = 32V dc)
Measure voltage across C55, C56 for 42V dc   (+C55 to -C56 = 84V dc)
What is the DC voltage on the speaker output ? is it + or - voltage ?
A few mV on the speaker output is ok, 40V is very bad.

Here is the schematic for you to check with.
Output transistors are Q7, Q8, Q11, Q12
You cannot view this attachment. 
There are no stupid questions.
There are only stupid mistakes.

Tassieviking

#7
The coupling caps are C41 and C42, they are in parallel to form one capacitor of 0.2 uF
Measure the DC output of the power amp with black probe on gray wire and red probe on yellow wire going to the speaker jacks on the back panel.

IF YOU HAVE DC ON THE SPEAKER JACK, MAKE SURE YOU DO NOT PLUG THE SPEAKER IN.
DC on a speaker can cook the speaker coil, not a good thing at all.
Maybe isolate the power amp from the pre-amp, put a plug into the FXreturn jack, even plug the speaker into the FXreturn as this isolates the power amp from the pre-amp and it makes sure there is no input on the power amp.

Also, running a lead from the FXsend to the FXreturn does nothing, there is no buffer or anything else that magically appears.
If you trace the wires you will see that the fxsend is wired directly to the fxreturn socket, when you plug into the fxreturn this link is broken by the switch in the fxreturn jack.
There are no stupid questions.
There are only stupid mistakes.

Tassieviking

#8
A quick drawing of how most pre-out signals go into poweramp-in jacks, but not always on every amp.You cannot view this attachment.  You cannot view this attachment.
This is the switch that plays up when you lose signal to the power-amp with no jacks inserted, I like to get a small strip of normal printer paper and put a drop of WD40 or contact cleaner on the paper, put between the contacts and run paper up/down to clean the switch contacts.
There is no need to spray lots of anything in there, the paper will rub the dirt away, simple.
There are no stupid questions.
There are only stupid mistakes.

phatt

Hi Hombre,
           Yes as already noted  most likely blown the poweramp.

If you don't know which are the Power transistors then my advice is that you are better to send it to a workshop otherwise you may well struggle to fix it, possibly creating even more damage.
If you do wish to fix it yourself then you will need to take many tests and it will take a lot of time.
If you have high DC voltage (i.e. 40 volts) at the speaker terminals then you have blown major parts and at a guess you may have to replace most of the transistors in the power amp section.

The good news is that the preamp sections are likely ok as you only shorted the power amp.

Yes absolutely no mojo gain of any sorts by jumping the FX.
 BTW it's not an FX loop as such, just a passive break loop between Pre out and Power in which can be used in similar fashion as a Dedicated FX loop.
It's a shame the designer placed the speaker outputs right next to the Preamp/Poweramp loop as there is plenty of space to mount the Spk outputs on the other side of that back panel. :-X
Maybe don't try modifying the amp in future, changing chips in hope of improvements is often not as good as claimed by youtube geeks. ;)
Phil.

willpirkle

Well, looks like the power amp is blown, which sucks. I've seen something similar to this in the past, but the guy plugged one amp's speaker output into another's FX return - amazingly the power amp actually survived in that case, probably due to good protection circuitry.

But I agree with Phil here - if you've never changed a power transistor, especially the T0-3 (round) power transistors, take it to someone qualified. It will save you time, pain (getting shocked with DC supplies sucks), and maybe money too. The collector on the T0-3 BJT is tied to case itself, so you need mica or rubber insulators as well as a lot of heat sink grease (thermal compound) to properly mount and isolate/insulate the collectors from ground (the heat sink aluminum bar), and even the slightest mis-match or offset will light it right back up. In addition you need at least a light-bulb protection circuit to plug into; a variac is better, because you can ramp up the supply voltage while monitoring Vbe on the transistors and catch the collector current ramping, and stop it before they fry. If that all sounds like more strange jargon, take it to a tech.

Oh - and Phil, one of my personal favorite amps, with horrible jack placement, is the GK400RB -- all identical 1/4" jacks in a rectangular block, crammed next to each other on the back...

Will

TheBadHombre

Alright guys

Thank you. You've given me some closure that it won't be some simple fix. My last question would be how would I go about finding someone equipped with the skill and equipment to fix this thing.

Or...

What do you guys think about finding a poweramp that could fit inside the empty chassis? Or maybe there's some other more comprehensive drop in solution you guys know about that I obviously do not know about?

Thank you guys so much for your time.

phatt

An electrical technician repair shop.
Often the music store has connections to repair techs, maybe ask them.

You said you liked the Amp?,, so likely worth fixing as trying to shoehorn another circuit into the chassis might be even more complex. :-\
Phil.

Tassieviking

If you want something that slips into that chassis, start looking around for another Peavey,
You like the sound of the amp so just get another one,

If you get the same amp you can start looking up some electronic lessons on the web, and then start taking measurements on both amps and slowly trace the fault yourself.
You might slowly learn more and more, get the amp fixed eventually.

I bet you would start building your own stomp boxes in no time.
I saw a Peavey Special 130 go for $65 at an auction where I live 2 weeks ago.
Keep looking till you find what you want.
Just don't throw it out, someone else would love to have a go at repairing it.
There are no stupid questions.
There are only stupid mistakes.