Warning - SS Question !

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Warning - SS Question !

Postby TomMcNally » Wed Jan 23, 2008 2:01 pm

I was wondering if any of you guitar guys have any experience
repairing solid state Fender amps? I am fixing one for a friend's kid,
it's a Frontman series with a TDA1514A output IC. It comes on
and just hums loudly - no shorted diodes, and I replaced the
caps just because I had some. Just a loud nasty 60 Hz hum.
I'm going to replace the chip ... but reading a little info on the
web, I see some comments that the transformers in these amps
tend to short windings and cause hum ? I guess I could lift it
from the chassis and see what happens.

Any insight would be appreciated.
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Postby mesherm » Wed Jan 23, 2008 5:08 pm

Pins 6 and 4 of that IC should have like +25 to 30 volts and -25 to 30 DC volts on them respectively. If you have a scope check them for ripple.
Pin 1 and 9 are the non-inverting and inverting input of the power IC so shorting to ground should produce quiet.
If it has a Power Amp jack, check the jack as it contains a contact that connects the preamp section to the internal power amp when no plug inserted. If that is corroded or dirty it might pickup hum, much like an open input.
Mike's N-1 Rule: When looking for N number of components to finish a job, you have a 95% chance of only finding N-1 of them.
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Postby Sal Brisindi » Mon Jan 28, 2008 10:10 pm

Tom,
It will be easier for you to build a PP 6L6 tube amp than to fix that SS amp... :-)

Bring it to the station this Sunday, we'll let the smoke out that thing...

Sal
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Postby TomMcNally » Tue Jan 29, 2008 9:50 pm

Thanks Sal -

I'll let you know - I have the output IC on order, should
be here ... I have a funny feeling the power transformer
has an internal short to ground. I need to isolate it
or sub in another one.

... tom
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Postby Sal Brisindi » Tue Jan 29, 2008 10:19 pm

Tom,
Do you happen to know the specs on the transformer, you never know what I might find in my basement... Yellow_Light_Colorz_PDT_20

Sal
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Check reverb circuit

Postby coman61 » Tue Feb 05, 2008 9:28 am

Tom, Try unplugging the RCA patch cord to the reverb tank and see if the hum goes away.
A phone call to fender will get the schematic.
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Postby EWBrown » Tue Feb 05, 2008 12:58 pm

If you figure out that power trannie and DC voltages, I may have something that would be suitable, I have a lot of LV power iron laying around with not much use for it.

/ed B in NH
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Postby TomMcNally » Tue Feb 05, 2008 4:54 pm

I haven't updated the thread - Mike and I have done a few private
emails ... but here is the progress (well actually lack thereof)

I do have a schematic from the Fender website.
I cut loose the entire preamp section from the
power amp ... which is a 50 watt mono IC running
about 25 watts with +/- 24 volts. I replaced the
filter caps (I happened to have some) and the diodes,
even though they test ok. I still have major hum
on either the speaker or headphones. The +/-
voltages test OK, but the waveform on the scope
shows a nasty saw-tooth looking ripple. I subbed
another center-tapped power transformer, no change.

I did replace the power IC to no avail. Mike thinks the
"bootstrap capacitor" (whatever that is for) could be bad.
It's the only other large value cap ... I haven't spent
much time on this thing ... it really seems like a POS.

... tom
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Postby EWBrown » Wed Feb 06, 2008 7:00 am

I've never had much faith in those power IC chimps, though I did start (stuffed the PC boards) but never completed a "gainclone" chimp-amp to experiment with. It's a very low priority project at this time, I was considering it mostly for some powered subs sometime.

I've got several 22-0-22 VAC power trannies, salvaged from some Simplex fire alarm control panels many years ago, they're good for +/- 24VDC to +/- 28VDC at quite a few amps. These mostly were manufactured by Merrimack Magnetics long ago....

For the chimp-amps I got a couple of hevty 22-0-22VAC toroids, around 160VA capacity.

I'll probably be in my "methusela" stage of life before I ever get these completed Yellow_Light_Colorz_PDT_09 Yellow_Light_Colorz_PDT_04 Yellow_Light_Colorz_PDT_03 Yellow_Light_Colorz_PDT_02

Around 15 years ago, I was given a 50W SS Peavey guitar amp that had been in a house fire, and it had suffered some amount of smoke and water damage. (After my friend got the insurance settlement, he gave away a lot of the damaged goods).

I managed to get it working, but never got around to fully "restoring" it, and have considered just gutting the darn thing and building up a 6BQ5 or 6GK6 "18 watt" tube amp to replace the smoked and watered plastic and sand bits. I figure the cab is worthmore than the guts at this point...

/ed B in NH
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hum in chip-amp.

Postby 4-CHANNEL » Thu Feb 28, 2008 6:35 pm

Have you check the grounding spot, once, I had a chip-amp. uses TDA2030, single 18V DC rectified supply. the NEG. rail didn't connect to chassis, may be you check for the null potential whether is to ground. ( chassis ).

may this helps you. ;) ;)
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