Help this newbie with a Fender Frontman 15R

[reelguitars]

Hey guys, I'm brand new at trying to fix these things but have been playing guitar longer than I care to remember. I have a little Frontman 15R that took a little extra voltage through the input jack when one of the plugs of my One Spot pedal power unit came into contact with one of the metal cases on one of my pedals. Blew the fuse. Replaced the fuse and now, turn it on, it blows the fuse. I'm do work for the phone company so I know there's either a short or something's grounded. I wanted to isolate the power supply circuit, the preamp circuit, and the amp circuit to see where it was. So far, not having much luck. Someone want to offer an old dog a new trick? Thanks, David

 

[OlPhart]

I'll presume it's solid state: pull all of the output transistors and see if it still blows fuse.

If ok, find some way to test, find and replace the bad ones (probably in pairs).
If still pops fuse, you'll probably need the schematic to keep removing powered stages.

 

[tunedwolf]

First things first. Disconnect the secondaries of the transformer from the board (P5, 6 & 7) and power it up and measure the secondaries, you are looking for around 16V-0V-16V. If all is ok, then measure Diodes CR7-10 and then 15V Zener Diodes CR11 & 12, if those are ok, check for a short to ground on pins 3 & 5 of U4 (TDA2050) and Diodes CR5 & 6. These are the most likely culprits. Good hunting

 

[reelguitars]

I have schematic

I have the entire schematic for the Frontman. I have a wide format printer so the schematic is about 3' wide, easy to read, but not so easy for me to understand. The problem is, if the amp keeps blowing fuses every time I turn it on, how do I check it? I've read about using a light bulb in the place of a fuse and I made a little rig that used a bulb from a night light. It comes on and allows me to test, but I think the small filament in the bulb drops the voltage...is that right? Anyway, what I wanted to do was to use a razor knife to break the path of the circuits for the power supply, then the preamp, and finally the power amp so I could isolate and test each one individually. I don't mind doing a little point to point wiring to reassemble, I just haven't learned proper troubleshooting skills for this type of stuff yet. Bear with me guys, I'm not dumb, just inexperienced.

 

[Nigel Goodwin]

Most likely cause is the output IC, remove that and see if the fuse still blows.

 

[Tann-44]

careful of the mixed voltage's in there....240v's will give a nice shock! back to your fault...the o/p ic is a tda2030 mounted on a small heat sink not the best has it over heats and knackers out...replacement is the best out come. here's the thing to work on your amp..

remove the tda2030. it's ok to power up with out the ic in..now with your test meter place the black lead to the metal case and set the dc range on the meter to 200vdc..now check the small ic's pins going left to right pins are(left top is 1 2 3 4 followed by right had side top 8 7 6 5..pin 4is the -15vdc and pin 8 is 15vdc+.. the main filter caps will be say 2200uf about 35volt dc this feeds the main power ic tda2030 ic. try the voltage checks... order a replacement tda2030 chip and fit it in, next do not hook the speaker up yet but place your meter on the spade connections of the speaker out keeping them part..meter range 200vdc, if there large dc of more than 1volts then there's still a fault.. because your inexperienced in this area of repair work be careful of the voltages.

 

[Tann-44]

reelguitars, These amps small chip type units are straight forwards to repair...fault's ranging from blown Tda2030(tda2040/50) that sometimes burns the speaker up also shorted or open mains transformer to distorted signal broken off level front controls.

 

[HiTech]

Just dump the unit and buy a Marshall or HiWatt !!

 

[Tann-44]

Hey guys, I'm brand new at trying to fix these things but have been playing guitar longer than I care to remember. I have a little Frontman 15R that took a little extra voltage through the input jack when one of the plugs of my One Spot pedal power unit came into contact with one of the metal cases on one of my pedals. Blew the fuse. Replaced the fuse and now, turn it on, it blows the fuse. I'm do work for the phone company so I know there's either a short or something's grounded. I wanted to isolate the power supply circuit, the preamp circuit, and the amp circuit to see where it was. So far, not having much luck. Someone want to offer an old dog a new trick? Thanks, David

post it on here.. I'll have a look over it.. but just remove the o/p tda---and power it back up..

 

[Tann-44]

just found the schematic in me files....replace the tda2050 ic...job done! reelguitars if you want it louder change r39 from 100k to 22k!

 

[unclejed613]

just found the schematic in me files....replace the tda2050 ic...job done! reelguitars if you want it louder change r39 from 100k to 22k!

that's the feedback resistor, and lowering the value would lower the gain...... there's no need to mess with the gain of the power amp IC.... 15 watts is 15 watts, and you can only get so much sound out of it...

 

[Tann-44]

that's the feedback resistor, and lowering the value would lower the gain...... there's no need to mess with the gain of the power amp IC.... 15 watts is 15 watts, and you can only get so much sound out of it...

Yes, that is the feed back resistor...there is so many different chip type amps out there using TDA2030 40...50.. form 10w to 32w they just change this resistor from say, 150k 100k to 22k.. psu etc...

Regarding the post about this amp, by now he may of got it to work or replaced it with a better amp.

 

[fossil]

I'm coming in quite late with this request but, I've traced my problem to the TDA2050 on a Fender 15R. I've taken it apart, found the IC and ordered another that's on its way. In the meantime I want to know if there is a way to bypass the existing IC and see if the amp turns on long enough to not blow a fuse?

I'm not an electronic wizard but I can get this kind of thing sorted out by reading you guys on these forums ..so here's a question, what does this IC do? Most people have had this problem by plugging in an AUX. and I can't remember now if that's how mine shorted but if I were to take out the old IC and didn't want to bother with an AUX., would the amp still work?

Thanks.

 

[Tann-44]

Remove the faulty ic the will still power up so the preamp can be checked..

 

[Tann-44]

You can check the preamp circuit via line out to a second amplifier..or wait till new ic as turned up and refit it, make sure the plastic spacer is ok.

 

[fossil]

The plastic spacer ? I don't remember seeing that... can I stick a piece of paper instead?

I read on another thread someplace a person recommending thermal grease be used when replacing the chip... any thoughts?

 

[Nigel Goodwin]

The plastic spacer ? I don't remember seeing that... can I stick a piece of paper instead?

No, assuming it needs an insulator you MUST use the correct mica or silicon type.

I read on another thread someplace a person recommending thermal grease be used when replacing the chip... any thoughts?

Only if the insulator requires it, mica does - silicon normally doesn't.

 

[dr pepper]

The tda 2050 is a power amp on a chip, its sometimes called a power op amp as it has similar inputs to an op amp.
It takes line level signals from the preout of the tone control/amp section and amplifies them up to around 50v or so and drives the speaker, producing plenty of heat in the process hence the heatsink.

The metal tab with the hole in connects to one of the pins, not neccesarily the ground pin, if your amp chassis is grounded then you'll have a short and blow the chip or the amps power supply, an insulator is a must, I buy silicon insulator as a sheet and cut out what I need, dont forget you'll also need the plastic bush around the screw as well, an old pc power supply is a good source of salvaged washers and insulators.

 

[Tann-44]

Fossil, these small guitar amplifiers fail due the common things like broken front controls / blown speaker and the main one is the power amp ic... Your notice how the tad... Chip is mounted on small heat-sink via fitting a better one or hard wiring the power ic to the chassis using correct parts to give better thermal life for the tad.. Amp.

 

[fossil]

I've got the chip all set but every time I power up the amp. I get a hum through the speakers but no guitar sound. I checked for continuity as well but nothing by hum...