Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[probodger]

Hello all!

I've recently bought a Fender Frontman practice amp off ebay. it was fine for a few hours use but now all it does is buzz. It's doesn't stop buzzing/humming when the strings on the guitar oare touched/earthed either.

I'd like to fix it but don't know where to start. I've opened it up and the internal fuse is ok so I'm taking a wild guess it's a capictor or something that needs replacing.
Hopefully not a speaker issue??

Any suggestions/ideas welcome.

its  15watt Mexican made one, Model PR241 I think??

[J M Fahey]

Is it "mosquito" buZZZZZZ or deep down huMMMMM ?
Itīs not the same.
Does it vary with any control or potentiometer?
What happens with everything on "0" ?

[RIS]

I have that same amp ,I use it for testing d.i.y. boxes, speakers, cords etc., I can pop the back off to help you if needed. More info 1st, please. In the hours you used it, what volume levels, effects, etc. Did you try other speakers, if so, 8 ohms? Accidentally lowering the ohms will be "louder" for a bit, but.... Any how, try your guitar and cord on another amp if you can, to eliminate the problem there. Try gently moving the plug around in the input jack, possibly you have a short or bad connection there? Start simple and eliminate the easy stuff. You mentioned capacitors, be careful there. Even un-plugged, electricity can bite back. Now go have a peek, and get back. Part of the fun is the fixin, just be safe.

[probodger]

I'm at work so can't currently do a "live" test to answer the questions but from memory and what I tried at the time.

It's a low down buzz, kind a like "mains frequency" buzz.

Pretty sure turning any of the pots did anything, just constant "buzz/humm"

Guitar & leads work fine in other amps.

I was playing through it at low volume (its a brutal little amp even at low volume!!) and with headphones plugged in when it let go. probably had a fuzz pedal or two in the chain.

Never plugged it in to an extension cab or any other speakers - other than using headphones.

I'm aware about the dangers of capacitors etc - I've made a few of my own effect pedals and modified guitar circuits quite a bit but never had a go at attempting a repair on an amp before.

[tonyharker]

Sounds like you've blown the power IC a TDA2050. Youll probably have to replace it. Could have been caused by plugging/unplugging the headphones? - Switching transient? TDA2050s are a bit fragile in my experience, I've blown a few!

[probodger]

Sounds like you've blown the power IC a TDA2050. Youll probably have to replace it. Could have been caused by plugging/unplugging the headphones? - Switching transient? TDA2050s are a bit fragile in my experience, I've blown a few!

Is there anyway of testing the voltages on that IC? Or is that a bit dangerous 

Actually what you have said makes a lot of sense, If I think about it, I've blown ICs in pedals using this power outlet and extension 4way... think I need to use a different one.

[J M Fahey]

If the power chip (TDA2050?) blew, itīs probably passing DC straight from the Power supply into the speaker.
*Very* bad for its health.
Disconnect the cables at the speaker terminals, and measure its ends with the multimeter in the 200VDC scale, you will probably find around 20VDC there (with the amp ON).
If so, unplug it from the mains, *carefully* desolder the Power IC and replace it.
Be very careful with the PCB which is easy to damage (google some desoldering tutorial and practice on some old useless board first), replace heat transfer grease and the mica, if used.
It might also use a small nylon nipple to insulate the mounting screw.
After finishing the replacement, turn it on with every control on "0" without connecting the speaker and re-measure at the speaker cable, you should find less than 100mV DC.
If OK, re-connect the speaker.
Look carefully and under good light around the headphone connection, maybe there is some problem there which in some way shorts the amp, damaging it.

[probodger]

Thanks!

I'm at work today and tomorrow so I'll be getting the multimeter out on Tuesday and will report back with my findings 

[probodger]

Ok, just had a chance to try out the amp.

• Altering the pots, does nothing to the "hum"  - it's constant.
• Checking the speaker leads I get 0.5V on the 20V scale of my multimeter.
• Also noticed the "power on" indicator LED on the front of the amp, hardly lights up

[J M Fahey]

Read Enzoīs answer to the humming Fender Stage 160, sounds similar to your problem, the same solution might apply 

[joecool85]

Read Enzoīs answer to the humming Fender Stage 160, sounds similar to your problem, the same solution might apply 

Link

[Jack1962]

It's a Bad TDA2050 and or 1 or more filter caps 

[probodger]

Yeah I read that thread too, although I get no sound from the guitar just hum/buzz from amp.

I ahve yet to try "wiggling" those big capacitors to see if the humm/buzz changes.

And I'm guessing the TDA2050 IC is the one that is stuck on the back of that big aluminium heatsink thing as it gets hot.

[J M Fahey]

Ok.
Confirm itīs a TDA2050 (or whatever), itīs labelled so.
Download its datasheet, youīll find what each pin is.
You should have the positive rail (around +20Volts) on one of them, the negative one (-20V) on another, and 0 volts on the other 3.
Check it.
Also, with the amp unplugged from the wall, you can replace the TDA2050.
Use a good solder sucker, heat each pad only to melt the solder, donīt overdo it.
Cleaning the last remains with solder wick is good too.
You should be able to practically pull the IC with very little effort.
Donīt wiggle it, pamper the board, avoind cracking or pulling the solder pads, which are fragile, and a PITA if torn.
Replace a new one, using proper mica and heat grease/paste as the original one.
They are cheap, order a couple, plus micas and some grease, youīll pay more for Mailing than the parts themselves anyway.
Itīs reasonable to think it will work.
If not, roll up your sleeves and will have to keep searching.
Good luck.