Fender Frontman 65R troubleshooting, very frustrating...

[Jonah]

Hi guys!

Sorry to post a topic about a solid-state amp, but I've always found good answers here, so I thought I'd give it a go since I'm becoming desperate...

So into the shop came this Frontman 65R, with a periodical problem: at a random time, the amp loses it's power, and becomes very distorted - very much sounding like a cold solder joint. If I take measurements almost anywhere where there's voltage present - especially high AC voltage - it snaps out of it, and sounds normal again. By this I mean, when I touch somewhere with the probe.

So far I've:

-Cleaned everything, all sockets and Abiko-connectors, and the fuse sockets. Some were pretty oxidised, but none the less, cleaning didn't solve the issue. Same goes for all pots.

-Reflowed all solder joints on PCB.

-Checked filter caps and other electrolytic caps with ESR-meter. No obvious issues.

Any ideas?

I'll appreciate any input!

Thanks!

-Jonah

[xtian]

I'm guessing intermittent connection, as you suspect. When you go to take a voltage reading, you're putting a little pressure on the PCB, and flexing it fixes the problem. Confirm by using a chopstick to push. Can you repeat the experiment?

[Jonah]

Hey xtian, and thanks for the reply!

Unfortunately I can't repeat the problem. Not controllably at least, because I really can't get it to malfunction again purposely. I've tried putting pressure on the PCB, as I know from the past, that this can cause symptoms, but not on this amp. And by touching I mean TOUCHING, eg. when I merely touch one of the tabs on the power switch with the probe, the amp "wakes up"...

[xtian]

Faulty power switches are certainly not unusual. When it malfunctions again, see if wiggling the power switch connections helps, or if giving the chassis a whack has any effect.

Always check the FX loop if it has one. Put a jumper cable in the send/return to rule out that weak link.

[Jonah]

Yes, I've checked the loop, and it's not the culprit.

But strangely enough, when the problem appears, if I throw the power switch, just before the amp shuts down, the sound comes back. But only for the 1/2 second before it actually shuts down. Like there's a power surge or something when the switch is thrown. If I turn the amp back on, the problem's back, but again, goes away for that tiny moment before shutting down.

What does that sound like to you? Switch problem? Cap drainage problem?

[xtian]

Got me, brother. You can try clipping in an additional good cap in parallel with the first reservoir cap to see if its a filtering issue. But frankly still sounds like an intermittent connection. How about bypass the power switch and just use the power cord as your switch, or a switch on a power strip.

[Stevem]

Does the headphone output work normal when the amp screws up, if so the output IC pack is failing in a intermitant way and applying heat from a hair dryer to get it act up and then cooling down with some freeze spray might confirm that, but I would bet none the less that that's the issue!
You should also confirm that all chassis ground points are good and also confirm that the darn headphone jack itself is not at fault by jumping it as if the switch section in that Jack is faulty that could be the cause of your issue also, so do these checks first!

[Jonah]

Hi Steve, and thanks for your reply!

There is no headphone jack on this Frontman model, so unfortunately that's not the issue. I'm also suspecting a grounding issue, and it might be, that the PCB is grounded only through the heatsink attached to the power transistors. I haven't had a time to check that thoroughly, though. I'll go through the groundings later tonight!

[Jonah]

Hi! I've attached a soundclip of the problem. At the beginning, the sound is crapped up, but halfway through the clip, you can hear me touch one of the pins on the AC switch with my DMM probe, and the amp "wakes up", and the sound is normal again. All other distortion-like artefacts after that point, that you may be hearing are just mechanical resonation.

There's also a huge compression in the audio-file from the phone, so the sound is actually a LOT quieter, when distorted.

I've gone through the groundings, which seemed to be in order, although not very sophiticated, seeing that the PCB is indeed grounded through only the heatsink...



Thanks!

[Jonah]

I also recorded a bit of the static gargle, that's going on, while the amps going haywire. Everything's turned to zero, so it's not a preamp-based problem. Frankly, sounds to me like cap-issue, but I've yet to receive new caps, so I haven't tried to swap them out.

I can provoke this problem sometimes by for example touching the same spot on the ac switch, where I touch to get rid of it. (?) Does this make sense?

[Stevem]

If you do not have a O scope to step thru the circuit and pin down where the issue starts off signal flow wise then do not spend 4 hours guessing,order up a new output IC pack for 20 bucks and be done with it as that's my pick for whats bad in the amp!
It likely got a hit from a big static discharge from someone's clothing!

And yes they are not easy to solder in and out without a good solder sucker!, what I do is clip the leads off and then heat and pull the snipped ends out!

[fusionbear]

fyi, I am a Certified Fender Tech. The Frontman series is on the "no field service" list. In other words, its cheaper to replace than repair. They won't pay me if I work on one...

[Blacksnake]

Just wondering if the amp was fixed I have a Princeton 65 with pretty much the same problem I think it is output transistors

[Stevem]

Hopefully he replaced the output pack by now or its in a garbage can !

[Blacksnake]

Hopefully he replaced the output pack by now or its in a garbage can !

I just bought the small transistors for ten bucks Stevem hopefully it is these and not the larger ones which seem to thirty, if you send the effects send to another amp and play the other amp works fine while the fender at the same time is popping and cutting out so the power supply and the preamp work fine and I ran a multimeter over what I could in the output and all seems well. So I will take my chances I bought a studio pro and a Princeton chorus that were faulty and the output trans were the problem in them, both but this is my amp in these high powered combos I think the output trans take a beating will post if this fixes it