Fender Frontman 212R low guitar sound with popping and crackling

[Crane]

Hi Folks,

This is my first post after reading a few Frontman 212R repair threads here. My Frontman does send some sound out to the speakers, the guitar sound is very low and there is a lot of crackling going on. Sometimes I hear very loud pops. After a few minutes, I sometimes get no sound at all.

I visually inspected the board and see no signs of heat or water damage. I have resoldered all the major components and the only thing I have found is R119 (a metal film resistor) with a pin hole in the epoxy coating. There is no sign of heat damage, just a tiny hole. I will upload pictures asap.

If I had a replacement on hand (1.2k 2 watt) I would replace it without question, but I don't have one and the shipping on a $1 part is $8.
So I would like to know if the pinhole could be causing my issue so I can better decide my next step.

TIA

[The Dude]

Does the resistor check good? I doubt it's the cause of your problem, although anything is possible. The problem I've seen multiple times on this amp is defective transistors Q12 and Q13. I would test them first although they will often check fine out of circuit, but fail in circuit.

[Crane]

Forgot to mention that, but yes the resistor (one leg up) measures fine.
Will look at transistors next.

[J M Fahey]

Visual inspection is sadly a very poor tool, unless damage is GROSS.
And even so, it´s not enough.

You must inject 100mV 1kHz at the input and follow that signal along the path, from input jack to speaker out , and find where it stops/distorts/garbles.

Best is a scope, of course, but if not available, at least build a signal tracer and listen to it with headphones or a small speaker.

Otherwise you are just guessing.

[oc disorder]

My 2 cents worth is The Fender Frontman does have a pre-amp out jack and a power amp in jack (there was a sticky somewhere here on these troublesome loop jacks).
If the switch contacts in the power amp in jack are dirty this could cause similar faults like you describe.
Quick fix to check that is to plug in a lead from the pre-amp out into the pwr amp in bypassing the possibly faulty internal switch.
If it is not causing the fault/s it can be a test point to determine if the fault lies in the pre-amp or the power amp.
If you have or can borrow another amp run a lead from the pre-amp out into another amp bearing in mind the signal will be far greater than just the guitar plugged in
to the second amp so proceed with caution - and you will need to plug in a single jack plug (not connected to anything - a new spare will be fine) into the pwr amp in
jack to open the switch so the power amp in the 212R will be silent.
See if there is any noise or crackle coming out the second test amp and that the pre-amp works ok in the 212R.
If that works fine (apart from being extremely loud) and doesn't cut out etc then the fault must be in the power amp and could possibly be what The Dude suggested.
Plug your guitar directly into the power amp in jack. It should work although it will be quiet at that input as it is expecting a line level signal not a weak signal
un-amplified from a guitar pick up..
If you have a graphic eq or something similar placing that between the guitar and the "power amp in jack" may produce a more "full bodied" signal for testing the power amp.

If the fault seems to be in the pre-amp building a signal tracer as J.M. Fahey suggests (if an oscilloscope is not available) is a good idea. Mounting an axial capacitor
inside an old biro /pen case as a test probe say .1uF 100v for a solid state tester and running it to say an old pair of cheap plastic computer speakers with built in
power amp that uses a 3.5mm mini jack plug.
Join the tip and sleeve together converting them to mono and connect the ground wire from the shield to a flexible wire with an alligator or crocodile clip on it
(depending what they have in the pet shop) to clip on the the chassis under test. The joined tip & sleeve wire obviously go to the one end of the capacitor in
the biro/pen casing and the other end pokes out as your probe.
Hope that makes sense. You can probably download a test tone to a phone or computer .. in a phone you could lay it over the pickup in your guitar and with
a bit of luck and a fair breeze with your guitar plugged into the input you can hear it in all its faulty distorted glory coming out of the amp !!
Perhaps now you could use that single jack plug to plug into the power amp in to mute it.
Using your probe now as J.F. Fahey suggested work your way along the circuit using the points shown on the diagram as AC volts and listen for a
radical change in the signal.

We forgot to ask if it happens on both channels .. if it does it has to be something common to both channels in the pre-amp or... will be the power amp.!
This method wont be much help in the power amp apart from determining if U6 the TLO72 is ok or not .
Of course you could take it to your local repairman who is probably desperate for work but that wouldn't be as much fun !!

[Crane]

Thank you everyone for all the info. The noise happened on both channels.
I tapped on 2 capacitors near an input jack and like magic, the amp sounds great.
I tried wiggling he capacitors to try to get it to make the crackle noise again, but could not. So I flipped the board over, resoldered the area with the capacitors and put the amp back together. I have used the amp for over an hour today with no issues. I hope it stays o.k.

[Chisililltan]

That crackling and popping sound can be frustrating, but it's awesome that you've already resoldered components. The pinhole in R119 is curious, but it might not necessarily be the main culprit. It's worth considering but exploring other possibilities is also good.Sometimes, issues like these can stem from loose connections, worn-out tubes, or even minor internal wiring problems. If you're up for it, you could try wiggling connectors gently while the amp is on to see if the sound changes to rule out loose connections.If you're not getting much luck, replacing that resistor might be worth it in the long run, especially if it's a relatively cheap fix. Also, you can always check out How many strings are on a guitar. They have some good points.

[Petr]

Hi there,

I had on my bench a Frontman 65R with very similar symptoms which occurred once per 2hours (very annoying). The issue was definitely in power amp, distorted sound and low power which persisted when signal was injected directly to PWR IN jack. Well, the tricky part of this issue was the tracing. I could easily trace the signal up to U7 (I'm referring to FM_65R_schematics.pdf) where it became distorted. The problem was tracing it further, because when I touched junction of D18 and D20 or junction of D25 and D26 the signal jumped back to normal and I had to wait for another hour or two to reappear. The root cause were faulty Q10 and Q11 (I changed them both) as suggested here by The Dude (he's referring to Q12 and Q13 in 212R's schematics). Strange thing, both transistors were fine on tester, but one has indicated extremely low hFE = 47 only. On the picture below you can see distorted signal on speaker output (yellow) and on pin 1 of U7 (blue). The amp is now fine and I'd like to express many thanks to the contributors here. Great forum! Greetings from the Czech Republic!

[The Dude]

FWIW: I have found that, when those transistors are the cause of the issue, using diode check on a DVM will not reveal the issue. You have to measure transistor voltages in the circuit when the problem presents itself. You will see that the B-E junction of the defective transistor will read either much more or much less than the typical .7ish volts.