Frontman 212 R Cliping in the negative cycle of the Power amp stage

[mbogle]

Hi im new here so if this is in the wrong section forgive me. I have a Fender Frontman 212 R that is distoring on the negative side of the power amp section.
, however when i looked at th output through the scope with the speakers disconnected i retrieved a perfect sine wave , so it only seems to be distorting when the load is attached. Any help on this would be greatly appreciated, thanks.

[The Dude]

Can you repeat that test (with speaker and without speaker) and tell us what happens with your DC power rails? A transistor's max output is directly related to its supplied DC voltage. I suspect your negative rail is dropping under load. At best a guess at this point, but possibly a supply resistor has moved towards open.

[Enzo]

I suspect the output cannot source enough current for the load, while unloaded the current demand is essentially none. I'd be checking Q19, Q21, R112, and other related parts and connections.

That doesn't mean I disagree with Dude, check his thing too. Although if he is right, I'd suspect a filter cap rather than a resistor.

I am a little foggy the last few days, so...

[The Dude]

I am a little foggy the last few days, so...

As am I! Just finished a run of 7 gigs in 9 days!

[g1]

What output wattage do you get it you back it down to just before clipping? The amp is rated at 90W into 4 ohms, it looks to me like you would be right around there before clipping.
I wouldn't necessarily expect an inexpensive amp such as this to clip all nice and symmetrical, but maybe others here could comment on that.

Edit: I was assuming the waveform was clean up to this point of clipping, but maybe the clipping is at lower power levels, can't see the scope settings on the clipped pic.

Also, is your scope ground connected to chassis, or to spkr blk?

[km6xz]

This is a current limited problem, driver is probably fine since it swings rail to rail with no load, so check the power transistor Q21(2sa1294) on that side, and emitter resistor R112 0.22. Just about anything else that could cause asymmetrical clipping would clip even without a load.

[mbogle]

Thanks for all the replys and suggestions.. I will get onto doing these suggested DC tests and check the components suggested first thing tomorrow as I am not around today and will then post the results. This just started hapening last week, the amp was working perfectly before hand, cant get any kind of volume out of it without distortion, have a loan of an amp for the week now

Yeah the signal was a clean sinewave up untill the point of clipping, the scope was connected to the speaker black, should it have been connected to the chasis instead?

Thanks guys

[g1]

You should connect your scope ground to chassis. It's possible the scope is grounding out the spkr blk connection which may create weird problems. The spkr blk connection in this amp is not grounded.
Could you reduce the signal to right before the neg. side starts clipping and post the pic, with the voltage reading?
Just want to make sure we are not chasing a ghost here, I realize you are hearing a problem, but I don't think you are only hearing it when the amp hits full power clipping. Have you verified that the speakers are good?

[mbogle]

Hi all, I performed the tests again today with the scope at the chasis gound and with speakers that are definatly good and working. Testing the DC rails with the speakers conected the negative rail was at -42.2 volts with volume all the way down and then at 40.5 with the volume all the way up, the positive rail was at 42.1 with the volume all the way down and 40.3 with the volume all the way up. I then disconnected the speakers and neither rail moved at all no matter what the volume pot was at. I checked th solder joints around Q 21 and R 112, they all seem fine.

Now to the scope tests, I just realised the pics i posted the other day were on the x10 setting with a x1 scope lead so these pictures should clear it all up. The first Image here shows the signal at the speakers just before clipping occurs,

The next shot shows the signal just after clipping has started, (at extremely low volumes, just past 1)
and i drove the signal on some more to show how the clipping develops,

[Jazz P Bass]

Have you taken any voltage readings of the output section?
At idle, no signal, Volts dc.
A few I would look at:
Base: Q10 & 11
Base: Q18 & 19
http://support.fender.com/schematics..._schematic.pdf

[Steve Conner]

Check for bad solder joints on R112.

[mbogle]

I checked the recommended transistors and the resuts were as follows



Would i be corret in saying that the voltages at Q11 are completly wrong?

I checked the solder connection on R112, it looks good and I did a continuity test between it and the surrounding parts and that checked out.

Do the voltage results above suggest that Q11 has a problem and should be replaced?

[Steve A.]

The next shot shows the signal just after clipping has started (at extremely low volumes, just past 1)

There is nothing that I can add to the suggestions already made but I was wondering if you normally find the FM212R getting way too loud way too fast which was a common complaint with the earlier Deluxe 112 and Deluxe 90 amps. I have been adding master volume controls to those amps by replacing the feedback resistor on the op amp stage immediately before the FX loop, which would be the 27k R61 in the FM212R. You could replace this resistor with a 25k linear pot wired as a variable resistor in series with a 2.2k resistor (if you want more output from the Normal channel you could goose it up with something like a 4.7k series resistor.)

I use shielded cable with two conductors. One easy way to connect it to the board is to replace R61 with a 1M or 2.2M resistor and solder the two conductors to the legs. I solder the ground to the pot case. I use shielded cable because I found that it reduces some of the miscellaneous noises I hear in these amps when close miking them (these are noises that would not be noticeable otherwise.)

Steve Ahola

P.S. I haven't checked out the Frontman 212R but I really like the Deluxe 112/Deluxe 90 amps, and the FM212R Drive channel has some tricks that the earlier amps didn't have. Do you like the reverb? I checked out the Frontman 25R which is basically the same as the Champion 110/ Champion 30 amps, but the reverb springs were total crap compared to the earlier amps, a deal breaker for me. AFAIK the Frontman amps are the last electric guitar amps that Fender makes that are 100% analog solid state- not a digital byte to be found in there at all! BTW the Normal channel in the FM212R (like the Deluxe 112/90) uses a tone stack essentially the same as a BF/SF but scaled for the impedances required for op amps rather than tubes.

I do tweak the circuitry in these amps as I am not that crazy about them stock, especially the Drive channels.