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Post subject: 1993 solid-state Bronco input jack
Posted: Mon Jan 07, 2019 6:39 pm
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Hi:

A friend has a 1993 solid-state Bronco amp with a noisy input jack he described as 'plastic'.

I offered to help clean or replace the jack, but haven't seen it yet (and therefore haven't opened it yet).

Does anyone know whether it mounts on a circuit board (may just need solder work)?

I suppose a lot of questions will be answered by looking inside, but I'm just researching/poking around a bit first.

If it's board-mount, finding an identical footprint jack is probably best.

Thanks

Murray


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Post subject: Re: 1993 solid-state Bronco input jack
Posted: Sat Jan 12, 2019 4:30 pm
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It's like the Blues and HotRod series amps of that era.
It is a PCB-mounted jack that is insulated from the chassis.
Replacing the jack with a conventional jack is a common mod for the Blues Jr, and I see a Bronco on Reverb (that sold for $250) that looks to have had the same mod done.
It's not a difficult mod if the PCB traces are in good shape and you can get a good connection for the jumpers to the jack... and the problems he is encountering may in fact be due to the traces being stressed and breaking, meaning the mod would be needed.

You will need a switching jack that grounds the input when the cable is not plugged in, but you'll also need to isolate the jack from the chassis. Not hard with a couple of nylon washers and a spacer to center the jack.
The mod was on the BillM site at one time, but I can't find it now. They do still offer the jack for sale with the isolation washers for $12: http://billmaudio.com/wp/?page_id=65
Here is a source for the original type if the traces are still good: https://amprepairparts.com/fenderjacks.htm

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-- Rich


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Post subject: Re: 1993 solid-state Bronco input jack
Posted: Sat Jan 12, 2019 5:01 pm
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+1
What Rich said.....


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Post subject: Re: 1993 solid-state Bronco input jack
Posted: Tue Jan 22, 2019 2:05 pm
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Thank you.

I went out of town for a week & stopped at 2 guitar stores...one had a standard replacement jack & I got one...just read the mod info today so I guess I'll not plan on that until I finish excavation.

I got the amp 'chassis' out & got interrupted.

The owner shared two more complaints...thinks the ext. speaker & headphone jacks are flakey too. At first glance they look a lot like some Rean TRS jacks I have from a dbx normalling/half-normalling (IIRC that correctly)...TBD...
The other complaint makes no sense to me...says the drive select push button seems to cause the tone controls to work on opposite 'options' (I chose that word instead of 'channels'.

No schematic needed for the input jack change, but to make a response to the 'opposite effect' statement I'll have to really know what the circuit looks like.

I did not find any 'PR258' technical details online and see most people consider the service charge as a show-stopper.

I'm just helping a friend so the labor is 'free' (insert eye roll emoji here if I knew how).

I see most of the hardware has to come off to get the pcb out, so I need to allocate enough of that 'free' time.
He's already installed a Warehouse speaker & likes it, so we keep moving forward...

I'll have to look the whole board over because anything that's broken could cause a symptom that doesn't make sense...even when I have a schematic I have to remind myself that failures are NOT on the schematic...& don't think too hard about an 'impossible' symptom when reading the schematic literally. The circuit 'temporarily' is not-to-print'...let alone finding a drawing revision that is not the same as the hardware.

If there is a question here, I guess it's 'does a PR258 schematic exist'?

Thank you


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Post subject: Re: 1993 solid-state Bronco input jack
Posted: Tue Jan 22, 2019 6:49 pm
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I'd try treating the connections (jacks) with a contact cleaner, like this CAIG DeoxIT D5S-6 Spray, Contact Cleaner, people use, (as long as the input jack solder connections are intact). Note, from the block diagram the output signal routes thru the ext-spk and the headphone jack. Those jack connections must be intact (good) too.
I also think CB's idea of replacing the input-jack is a good idea too.

Yes, the drive button selects which channel your on. There is only one tone control for both channels. So if you change the tone controls for the drive channel, when you switch back to the normal channel, the tone control will be different. If thats the issue the user has.

See block diagram on the last page of the mfg. manual.
https://www.fmicassets.com/Damroot/Original/10001/OM_leg_gtramp_93_Bronco.pdf


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Post subject: Re: 1993 solid-state Bronco input jack
Posted: Wed Jan 23, 2019 2:05 am
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You need a jack with a shorting ( normaly close ) contact to ground, not a guitar output jack .
Most guitar store do not stock amps input jack.
Show a picture of what you buy

Some original Fender input jack here ;

https://www.amprepairparts.com/fenderjacks.htm
Owner manual ;

https://www.fmicassets.com/Damroot/Orig ... Bronco.pdf


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Post subject: Re: 1993 solid-state Bronco input jack
Posted: Wed Jan 23, 2019 7:50 pm
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Four things stand out on this schematic from Electrotanya.
The drive preamp has extensive filtering on U2b to smooth overdrive tone.
The second is the tone control circuitry. It abandons the parallel Fender tone stack for a series hi fi type circuit. It doesn't introduce the phase shift distortion that is characteristic of the tone stack circuit.
It is a series tone control, mid, then treble, then bass. It would be creatively interactive, but won't distort.
Third is the NFB feedback loop from the speaker to the output ic. This corrects speaker error for better tone from the output stage. It also reduces gain for using headphones.
Fourth is the phase compensated u4 output ic. The upc1188 pins 5&6 accomplish a task that is difficult to do with discrete SS components. There is a time delay for sound to get through an amp and low frequencies take longer to get through. This circuit helps keeps the music in phase to retain correct placement of harmonic content. Properly phased harmonics can modify the simple sine wave into a sweet sawtooth wave like a violin.
It also prevents overshoot and ringing of square waves. This renders more accurate overdrive tones that depend on accurate square waves.
The volume controls are still feedback controls, so the amp gets punchier as you turn it up. This is difficult to manage onstage. But at just 22 watts, it needs miking for live gigs so you can find your sound and then control the volume after the mike.


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Post subject: Re: 1993 solid-state Bronco input jack
Posted: Sat Jan 26, 2019 1:54 pm
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Thank you all for the good info.

Got the amp board out...more work than the circuit board work.

Local guitar store owner is good; he had J1, J2 & J3...

I debated replacing the main electrolytics because they are 25 years old, but don't want to wait to mailorder them...they look ok...
Will probably just replace all three jacks and hook power switch & speaker back up listen for him & pot rotation noise...then decide whether to go further.

Must have been some moisture entry - 2 jack nuts were rusty & one pot had rust dust on the plastic shaft. Wiped off with cotton swabs...pots all looked ok (no rust on their covers but I am assuming they are steel and not aluminum).
I do have some Caig D5N but don't want to read all the superstitious HiFi forum stuff about what phase the moon has to be in when applying it vs. removing it. Been there, deaf in one ear now so I can do heretic things now.

TMI, sorry...those are my middle initials...

Forgot about Electrotanya...found old Philips tube amp manuals there...will go look.

When I'm done with this amp I should finish some of my own dust collectors...

Thanks again-


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Post subject: Re: 1993 solid-state Bronco input jack
Posted: Sat Jan 26, 2019 2:00 pm
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Location: Province de Québec, Canada
25 years old 'lytic are almost new. They don't need to be replaced.

I have a lots of gear, and electronic stuff with more the 40 years with original 'lytic.
It is so easy to me and cheap to replace them buy they don't need it.

IMO it is a case by case.


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