Fender Frontman 212R (help)

Hello to everyone new in this forum, i have a question and I thought someone might have a pointer to start, so i got this amp from a friend, i have fix and mod some tube amps in the past, nothing fancy but i do feel more confortable working with them than transistors, so the thing is i open this amp, first i noticed is that someone did a horrible “job” to it , specially to replacing the 5Watter resistors, i read a lot before writing here, and found out that this resistors are mostly the common issue in this model, so the problem im having is:

The amp turn pilot light on, but theres no sound, looks to me that the whole preamp is not working im not getting any leds in the front panel just the pilot hence no sound, the value of the 330ohm 5W ( R144&R145 in schematic) were wrong and i think this might took something that caused this, but honestly i do not know where to check, i visually inspected and double checked carefully and i do not see anything burnt.

Already replaced
R109
R112
D38
D39
R101
R84

To spec in schematic, because this components were cut and poorly soldered, so i just replaced em

Any pointers would be great

Thanks
 
Hello,
R 145 side to diode D58 17.6 vdc, upper side -1
R145 side to diode D57 17.0 vdc, upper side 1

Now has the two “Stock” resistors that came with the original parts that the guy gave me, the ones that were put wrong were .33ohm resistors, also this two resistors get waaay to hot if that helps in some way knowing, thanks ill be waiting for new instructions
 

Tarzan

Member
2004-05-23 6:54 pm
Genk
So you have your +/- 16V.

You say that no led is lit when you push the switches?
I would say that the solderjoints on the diodes D57 and 58 are bad.
They become really hot during service.

Check if you have +/- 16V on the IC's pin 8 and 4.

You also have no sound.
Try to plug a sound source, guitar or so, in the "Poweramp In" jack, J4.

Maybe the speaker is gone. Check the ohmage or us a 9V battery to see if it "pops".

Let us know.
 
Ok, thanks for helping me I’ll clarify some stuff to get out of the way,

No LED never, its supposed to be green when is clean, yellow when push and so on i got no lights on panel just pilot light,

lets take out the speakers off the equation, they’re ok, as i tested with other amp, actually sounds pretty neat.

When you ask for the IC can you point me which one?

Also as I understand amp worked fine but the clean channel distorted, so someone tried to change R144 and R145, problem was they used the wrong value, amp blowed the fuse, and then they handed it to me, but I don’t know if it took any components nearby or in another area of the amp ,since i only have looked tube amps, i dont see anything burned which to me its more suspect. also i have noticed that this two components (r145&144) get really hot and i mean hot as burning hot, i dont want to start changing components before knowing where to really look at, for information i just put the stock value resistors that i understand the amp had before they changed them, i did put new 5W resistors in R114, R115, R109 and R112 because it came also with wrong values there but i did have some spares with me and also the fuse,

Thanks again pal I’ll be waiting for any ideas
 

Tarzan

Member
2004-05-23 6:54 pm
Genk
The parts you changed in post #1, let's forget about it for a moment. Why you changed them is unclear to me.
1.
Check if you have a dc voltage on the speaker connector.

2.
Did you try to plug a sound source, guitar or so, in the "Poweramp In" jack, J4?
What's happening?

3.
If R144 and 145 have been changed, what whas the value?
If it's been to small, the parts after these resistors probably are fried.
They, and the following zeners, provide the preamps with the required 16V.

4.
Where you have to check on wich IC? Any one.
If any of these chips have a voltage near the supply (16V) on pin 1, 2, 3, 5, 6, 7, then it's most likely dead.

Step by step...
 

Tarzan

Member
2004-05-23 6:54 pm
Genk
point #3. That's been the problem.
I believe that the voltage has been to high for the opamps and fried them.
Both the zeners may have gone to because the voltages of -19 and +18V is to high.

Now comes the tricky part.
You have to remove all opamps. Be carefull, the tracks are very thin.
Do not rusch and use desolderbraid to clean the holes.

Replace both zeners.

Measure if the voltages are +/- 16V.
If so, Do not try to use one of the opamps you just removed.
Use new ones, they are not so expensive.

Good luck
 
Ok, thanks just to be sure because im not familiar with these terms or names,
Opamps = the ICs like the one i measured?
Zeners = the diodes? If so which ones?

Sorry about the question is just i never heard of it, and also when i googled thats what i got, if is not anproblem please confirm or clarify, thanks
 

Tarzan

Member
2004-05-23 6:54 pm
Genk
OK,
Opamps are the ic's, small black multipeed thingies, 8 pins. Labeled U1 to U8.
Zenerdiodes, D57 and D58.

1. Remove the opamps first.
2. Change the zeners.
3. Check the +/- 16V. Should be there.

If all is wel, insert the opamps, one by one. Check each time if you have the +/- 16V.
Do not continue if the 16V drops with,let's say, 0.5V or more.
Then, maybe an electrolitic cap coul be bad. But that's a bit to far to think about.

May the electrons be with you
 
Hello, sorry for reviving this so late but after things went rare this year , finally i could get my hands at the parts i needed, so the status is this:
Everything works normal, but indo have the same first problem, the channels sound distorted, even the clean, i read a lot and i did connect from the pre out to another amp, the sound is great so that tells me the problem is in the output stage, so where can i look now?