Fender Stage 112 repair - how to tackle burnt PCB

[Steve A.]

FWIW the Fender Stage 112 se amps sell used for ~$125-150 in the US. Of course the shipping charges to the UK would be outrageous but someone might want to ship you just the printed circuit board. Just a thought...

I love these 1990s Fender SS amps and have all of them besides the 160W Stage 112 SE. But I do rewire the preamps and tone control circuits...

Steve A.

[J M Fahey]

Avoided adding to the hijack, but since it became personal, let me add a few lines to the mess it already is:

Quote Originally Posted by bob p View Post
Quote Originally Posted by J M Fahey View Post
Quote Originally Posted by bob p View Post
LMAO. Ethyl cyanoacrylate is Crazy Glue.
Crazy Glue is *one* Cyanoacrylate adhesive.
There are many more out there, catering to specific needs
Thanks, Juan. I didn't know that you were a polymer chemist.

Typical despising Bob P answer, so no surprise here.
But Bob P, *you* might be surprised:
1) I very much doubt you are a "polymer chemist" and if you claim to be one, please post Academic degree proving so.
2) surprise surprise, I studied 4 years Industrial Engineering which includes a fair amount of Chemistry, basically centered in Industrial processes (I wonder why ) , before switching to EE (2 more years) and then to Business Administration (1 year), since there is not a *specific* University level career focused on "Musical Instrument Design and Manufacturing".
Although not as much used as other skills, I still keep and use my Chemistry knowledge as needed for Production, a very important part of which is based in knowledge and use of Adhesives ... or else.
And yes, I found different cyanoacrilate based adhesives to be different, specially into what concerns ma and Production.
IMPORTANT parameters (specially in speaker building) are: adhesive strength (varies a lot), flexibility/elasticity (again varies a lot), viscosity (same) and temperature at which it starts losing mechanical properties (again).
So do not dismiss me ironically as an ignorant.

As a final note (surprise surprise) I have successfully run a **Chemical** based Industry , Fotoquímicos Victoria, making Photography oriented products, mainly Developers, Fixers, Fog reducers, wetting agents , etc. .
As a bonus: also ran the parallel plant dedicated to electrolytic recovery of metallic silver from used Photo fixer baths, which we got back from larger Photo Labs, printing Press/graphic arts ones, and mainly Hospital X Ray Labs.
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back to the OP problem.

IF you want to check the presence of oscillation in that amp output (and I guess we still have it) , regular meters wonīt show it or lie about its level, they are simply not designed for that.

Best tool: an oscilloscope.
Will not only show continuous oscillation but way harder to catch oscillation "bursts" which may not exist under idle (no signal) conditions, yet be triggered at certain points of the Audio wave when driving a speaker (which is a *complex* load)

This one shows a not too strong oscillation , wonīt burn and maybe not even overheat the amp or speaker (might slightly brown the Zobel resistor) but makes amp soind dirt, hissy, specially acompanying the "S" letter ... and must be corrected of course.


this one is a horrible waveform produced by a (poorly) homemade LM1875 chipamp:


No plain multimeter will show that.

Second best is to build the old Ham Radio amateurs, TV servicemen and a few others secret weapon: the RF probe.
It turns unreadable RF voltage into very readable DC


You will have a hard time finding a Germanium 1N34 , 1N60 and similar diode, but since we donīt expect *real* RF here but high ultrasonic, a small signal diode such as 1N914 or 1N4148 will do.
Heck, not for precise measurement but to simply show HF oscillation presence, even an 1N4''2 (or higher) should do .

So build a probe, omit the 4M7 series resistor, connect it between speaker out and multimeter set to 20V DC scale.
Blinking when connecting and then zero is whatīs expected; some steady voltage, from , say, 0.5V to full scale indicates a problem.

If you get a reading, first disconnect power amp from preamp but leave its input open, to decide whether itīs internal oscillation or it comes from outside.
Then short power amp input and repeat test.
Results?

[ralphonz]

Ok, thanks for that. I was wondering, does anyone think it might be worth getting one of these? https://www.ebay.co.uk/p/KKMOON-LCD-...m1851&thm=1000

Or would that be a waste of Ģ50? I don't wan't to fork out a lot for an oscilloscope just yet but there are some old analog models floating around for a similar price on eBay.

[g1]

I think you would do much better with a cheap old working analog scope than that unit. Just from a quick read through, the language (poorly translated) indicates there would be problems with documentation and support, both for building and using that unit. And it looks like a very small display as well.

[ralphonz]

Hi Juan,

Sorry I have been very busy, my computer motherboard decided to die!

Anyway, I finally built the probe, with an actual 1N34A (bought some from eBay for a couple of pounds!), and omitted the resistor.

I set my MM to 20V DC, clipped ground of the probe to chassis and test lead to speaker+.

In all 3 scenarios the results of the test were the same. Turn amp on, MM reads 32 V initially which drops all the way to around 7.75V after a minute or less where it stabilises. The first time I carried out the test (with preamp connected) I hear the occasional crackle/pop in the speaker.

[J M Fahey]

In all 3 scenarios the results of the test were the same. Turn amp on, MM reads 32 V initially which drops all the way to around 7.75V after a minute or less where it stabilises. The first time I carried out the test (with preamp connected) I hear the occasional crackle/pop in the speaker.

Then you definitely have oscillation there.
a) I guess you are usin g a speaker load, please confirm.
b)initial jump is normal, you are measuring turn-on Pop, no big deal
c) *staying* at >7V DC is not normal at all, indicates presence of Audio at the speaker out.
And since you hear nothing plus itīs stable, it means you have an ultrasonic oscillation.

Please check and confirm results of:

If you get a reading, first disconnect power amp from preamp but leave its input open, to decide whether itīs internal oscillation or it comes from outside.
Then short power amp input and repeat test.
Results?

[ralphonz]

a) Yes, testing with a speaker connected
b) Good to know
c) Confirmed, I get the same reading >7VDC with pre amp disconnected and power amp input shorted. Double checked readings. Occasionally goes >8VDC but never <7VDC

[Steve A.]

Avoided adding to the hijack, but since it became personal, let me add a few lines to the mess it already is...

I don't think of threads here at MEF being hijacked, Juan... it's more like a side trip in our journey, some of them helpful and productive, but unfortunately some of them not (or they might just go on way too long.)

I'd guess that we all have a bit of libertarianism in our blood and that must be mine... Let threads go where they may and heaven help the OP!

Steve A.

P.S. Are there any oscilloscope programs/devices for a computer or tablet that you (or anyone else here) might recommend? Like something that would plug into a USB 2 or 3 port...
It would have to be safe (no possible damage to computer.) IMO display resolution would not be as important as the display speed since oscillation above the audio frequencies can cause all sorts of problems.

[J M Fahey]

USB scopes can read very high frequencies, "draw" a nice drawing purportedly showing what "you would see if you had a real scope" and display that nice coloured picture on PC screen. Easy peasy.

I published here and elsewhere full instructions and usage examples for a safe signal attenuator which matches whatever you are reading to a PC/Notebook *audio* (not USB) input, whether Line or Mic, usually a 1/8" jack.
Signal is diode clipped above 600/700mVpp so 200mV tops , fullt safe for any PC input.

Processing comes courtesy of the Sound Board so it displays waveforms faithfully with the following limitations:
a) AC only since Audio boards are exactly that.
b) bandwidth limited to somewhat above 20kHz by Audio Boars filtering.

In any case you can usually see HF oscillation, whether continuous or bursts, just you canīt resolve its waveform , only its envelope and "shadow", which are still useful if all you want is to get rid of it.
Not too different to whatīs seen on both screen captures shown on post #62

[ralphonz]

I just managed to pick up an old Philipps oscilloscope (PM3540) on eBay for pretty cheap. Now I just need to figure out how to use it

If you get a reading, first disconnect power amp from preamp but leave its input open, to decide whether itīs internal oscillation or it comes from outside.
Then short power amp input and repeat test.
Results?

Haven't tested with the scope yet but using the probe I read >7VDC both with the input open and with the power amp input shorted.

[glebert]

I just managed to pick up an old Philipps oscilloscope (PM3540) on eBay for pretty cheap. Now I just need to figure out how to use it



Haven't tested with the scope yet but using the probe I read >7VDC both with the input open and with the power amp input shorted.

To get familiar with using your oscilloscope, I recommend getting a function generator app for your phone or tablet and hack up a cheap headset cord to have a "known" safe thing to measure and practice with.