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October 05, 2022, 05:14:20 AM

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#21
Amplifier Discussion / Re: Vox Pathfinder 10 as a mod...
Last post by dimkasta - September 18, 2022, 01:45:09 PM
To be fair, the 18W are peak to peak. The RMS rating should be closer to 10W, and is probably further limited by the small transformer, the small power bank and the distance/inductance from the power bank to the chip itself.

TBH, by just removing the leds and adding an external 12" cab, you already have a decent amp  if you have a good set of pedals to complement and keep your expectations realistic.

For gigs I would probably consider it only if you are very budget constrained, and then I would mic it to avoid overheating the chip.

Unfortunately I cannot provide samples since my p10 is now just an empty chassis for experimenting with other circuits.
#22
Amplifier Discussion / Re: Vox Pathfinder 10 as a mod...
Last post by Puguglybonehead - September 18, 2022, 12:20:34 PM
Great thread! Thank you for sharing your progress. I hadn't realized that the Pathfinder 10 was such a simple circuit and  all thru-hole to boot. Wishing I'd kept mine now. Eliminating that ridiculous 'Boost' section is a great idea. I never found it to be realistically useable. I'd thought about re-homing mine in a new cabinet with a decent speaker (8"or 10") but never got around to it.

At 18 watts you've got the perfect amp for small club gigs. Currently, that's mostly what there is around here. (besides some occasional busking) Hipsters in our neighborhood want wallpaper music to hang out to so little amps like this are really useful. I hope you'll have some sound samples for us in the near future.
#23
Amplifier Discussion / Re: I’ve Really, Really, Done ...
Last post by willpirkle - September 18, 2022, 09:28:08 AM
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
#24
Amplifier Discussion / Re: I’ve Really, Really, Done ...
Last post by phatt - September 18, 2022, 07:39:05 AM
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.
#25
Amplifier Discussion / Re: I’ve Really, Really, Done ...
Last post by Tassieviking - September 18, 2022, 03:18:58 AM
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.
#26
Amplifier Discussion / Re: I’ve Really, Really, Done ...
Last post by Tassieviking - September 18, 2022, 01:06:44 AM
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.
#27
Amplifier Discussion / Re: I’ve Really, Really, Done ...
Last post by Tassieviking - September 17, 2022, 10:14:24 PM
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. 
#28
Amplifier Discussion / Re: I’ve Really, Really, Done ...
Last post by TheBadHombre - September 17, 2022, 09:50:55 PM
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.

#29
Amplifier Discussion / Re: I’ve Really, Really, Done ...
Last post by TheBadHombre - September 17, 2022, 07:54:52 PM
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 😭

#30
Amplifier Discussion / Re: I’ve Really, Really, Done ...
Last post by g1 - September 17, 2022, 12:32:26 PM
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.