Fender Frontman 15r Blown Fuse

[Matty]

Was just given a Fender Frontman 15r that had a blown fuse. I replaced it with the spare and it also blew as soon as I tried to turn it on. Just thought I'd check and see if this might be a common problem before I begin troubleshooting it?

Thanks,

Matty

[Valvehead]

the common problem on those is the output chip . Ive replaced about 20 of those , thats the issue 90% of the time.

try mouser.com for a chip

[Matty]

I'll get one coming just in case on my next order.

Thanks

[Enzo]

And it is not just those, it is ALL the small practice amps with the power amp ICs.

[fredy2]

There is a design problem in this amplifier. It has to do with use of the headphone jack. If you try to use the jack with a cable that grounds the shell, it blows the output amp.

This has to do with the grounding of the electronics to the equipment safety ground AND the way they do feedback when using the headphone jack by floating the shell of the headphone jack to implement current feedback mode.

The result of grounding the shell with a connection to other grrounded equipment from teh headphone jack is to kill the inverse feedback and the output amp saturates and blows.

I have modified several this way: I unground the heat sink and grounding to the case by insulating washer. This results in a hum. To eliminate the hum connect a 0.1 mfd cap from the case ground to the ground of the electronics.

This method severs the DC ground loop that blows the output chip.

[Enzo]

Just which model do you have? A quick look through my schematics at several varieties of Frontman15 and FM15 show all of them with grounded headphone jack sleeve contact. Offhand I don't see any that float the phones return. The speaker returns are sampled across a resistor yes.

I have attached an example, the others were all very similar.

I would also say the headphones output was designed for phones, not conecting to other gear. COnnecting a grounded cable to it is really mis-use of the product. I am glad you found a way around the problem, but from what I see so far, it doesn;t appear to be a design flaw. Headphones are not usually grounded.

[fredy2]

The problem ONLY occurs if the headphone jack is used connecting to other equipment, rather than headphones. The ground loop via the safety equipment ground sometimes sends the 2050 into oscillation followed by destruction. This is probably due to inductance of the ground loop.

The units I have, both had the same problem are Frontman 15G units.

They have a TRS headphone jack that performs clever switching of the feedback path for the headphones.

As long as isolated devices are connected to the headphone jack there is no problem, just when a ground loop is created.

An even WORSE problem is if somebody uses the EXTERNAL speaker connection to drive GROUNDED equipment. Since that grounds out the current feedback via the safety ground that causes instant destruction of the 2050.

Given people will plug things together if the connector fits, there are a lot of these unit floating around as non-working.

My fix, breaks the Ohmic connection via the safety ground. One can still get into trouble by getting the input ground against the external speaker ground of course.

As an engineering design rule, I would never have a user connection configuration possible that could disturb a feedback path, especially one involved with amplifier DC stability.

[Enzo]

Are your units the same model as the service manual I posted? Can you explain where in the headphones circuit this feedback occurs?

[fredy2]

It is kind of hard to explain the ground loop version for parasitic as it involves inductance of wiring, but look at the speaker current sense resistors.

If one plugs in a cable into the external SPEAKER connector that gets to ground via the safety grounds, an effective short occurs across the sense resistors... this blows the amp as the DC feedback path is broken and the amp goes hard over.

Users often plug cables where they shouldn't go just because they fit...

[Enzo]

Yes, grounding the speaker jack bypasses the feedback line. But unless the phones jack is defective, it is not involved in that circuit. The sleeve contact in that jack is grounded by design already. The feedback samples the speaker return across those low value resistors, but the feedback itself is not DC. DC is blocked by C23. Only sampled signal gets to the inverting inut of the power IC.

I will agree that if ther is a way to hook up something wrong, someone will do it. Notable examples being plugging the output of some amp into the input of another and wondering why that Fender twin 100 watt power stage burnt out the input circuit of their Champ. Or the one where a guy plugs his speaker cord into the back of his head and then without thinking puts the other end of it into the input jack out front. But that is mis-use, if not outright abuse.

[fredy2]

Yes, misuse is a problem, but people do it. I try to design stuff to protect against such problems, but one cannot always do that.

The observed Parasitic oscillation occured when the input and headphone jack returns got together via the safety grounds creating a ground loop that had enough inductance to cause ground bounce in the circuit board between input jack and headphone jack ground at a high frequency.

The headphone jack problem does NOT cause the DC feedback problem mentioned when the external speaker gets grounded... these are two different issues.