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**Johnny Creeper** · 02-12-2009, 11:57 PM

Mate of mine has a pair of Fender twins, which I've done a fair bit of work.

One of them had a very similar problem to yours, in that mid gig it would randomly change channels.

After a fair bit of poking around, turned out it wasn't any of the usual suspects (foot switch, cables etc).

There are two cement resistors, labelled as Dale 7W, soldered side by side. They're about 10mm x 35mm greyish white in colour (may be slightly longer depending on the model). They do get very hot when in use, which is normal as far as I know, but it turned out each one had become detached from the board at the solder joint. Difficult to tell by sight, but easy enough to confirm by moving them. A quick clean up and resolder, and it's been fine ever since (best part of a year now).

Would sorta tie in with the fact that it seems to happen with yours once the amp is nicely heated up.

On the current Fender Twin I have here in bits, they're marked on the PCB as R106 and R107 and are on the left hand side of the layout if looking from the front of the amp.

Heres hoping,

Johnny Creeper

**bnwitt** · 02-13-2009, 12:01 AM

Wow, JC. What a great first post. Welcome to the board.

**scustom** · 02-13-2009, 12:59 AM

I agree. I've had similar problems with both the Hot Rod/Blues Devilles and Deluxes.

In the Hot Rod Series of amps, those resistors are part of the +16V and -16V power supply for the switching circuitry. The original values were 470ohm, and Fender had those changed to 330ohm. I generally look for that to be the problem with any intermittent switching problem, I'm sure that the circuits in the Twin are similar, and you may want to check with Fender to see if they would recommend changing the component value from what you have in there.

Even with the new values, there are still problems due to lack of clearance between the board and the component. If you wind up having to replace them (which is a good idea), when you solder the new ones in, use a toothpick or something similar to hold the resistor just off the board as you solder it in. When removed, this will give the resistor the room it needs for thermal expansion and eliminate the stress on the board, the component and the solder joint. A dab of silicone between the board and component prevent any mechanical vibrations while allowing for expansion and contraction.

Hope this helps.

**Enzo** · 02-13-2009, 01:11 AM

Wow, three first posts in one thread.

Welcome to you all.

I'd agree, look for that pair of resistors. Push then with a wooden chopstick or something and see if it triggers any symptom. It shouldn;t unless they are loose.

Alternatively check the FS jack cutout contacts and their solder.

**Don Moose** · 02-13-2009, 01:40 AM

So does Fender just not know how to build a low-voltage supply?
They keep using the same proven-broken design over and over again.

**bnwitt** · 02-13-2009, 02:49 AM

Originally posted by Don Moose View Post

So does Fender just not know how to build a low-voltage supply?
They keep using the same proven-broken design over and over again.

Gee Don, this one made it 15 years before breaking.

Granted, not a Leo tube amp reliability record, but better than some.

**Robert M. Martinelli** · 02-13-2009, 06:02 AM

Originally posted by bnwitt View Post

Gee Don, this one made it 15 years before breaking.

Granted, not a Leo tube amp reliability record, but better than some.

Well....
Actually I think this "15 years before breaking" to be the exception.....FMIC continues to use the same crappy and cost-saving design on a number of amplifier models, and usually these amps fail much earlier than that.

Serious manufacturers recognize their mistakes and recall their products for a free upgrade.....FMIC goes straight on the road of poor design and equally poor reliability, some amps fail within the warranty coverage, many amps fail shortly after the warranty's expired, only a few of them last longer than that.

I wonder what Leo would have thought about this if he still was with us.....knowing his attitude he probably would have ( rightfully ) fired a number of design engineers at FMIC.

Unfortunately for us all, those were the times when "contents" were important; nowadays, it's all about "appearance and advertising". For an amp, it's enough to look like a Fender or a Vox, then it's up to the advertisers to convince the people they "indeed" are Fenders or Voxes.

Best regards

Bob

**Enzo** · 02-13-2009, 08:57 AM

I don't know, to me Leo was the master penny pincher, the king of just good enough. Leo would make a nice amp design with a choke in teh B+ supply, then the exact same model would come out the next year with a resistor in place of the choke. He could have used 5% or even 10% resistors, but he didn't, 20% resistors were cheaper.

There are so darn many of those current Fender amps we like to gripe about. Even a small percentage of failure results in a large number of units. It is easy to point out ones that failed, they make themselves known. But we cannot ignore all the thousands of them that work and never did fail. I think it is important to recognize the difference between THIS is GOING TO fail, and IF IT FAILS, This will be what is wrong.

**Robert M. Martinelli** · 02-13-2009, 09:55 AM

Originally posted by Enzo View Post

I don't know, to me Leo was the master penny pincher, the king of just good enough. Leo would make a nice amp design with a choke in teh B+ supply, then the exact same model would come out the next year with a resistor in place of the choke. He could have used 5% or even 10% resistors, but he didn't, 20% resistors were cheaper.

There are so darn many of those current Fender amps we like to gripe about. Even a small percentage of failure results in a large number of units. It is easy to point out ones that failed, they make themselves known. But we cannot ignore all the thousands of them that work and never did fail. I think it is important to recognize the difference between THIS is GOING TO fail, and IF IT FAILS, This will be what is wrong.

Maybe that came from the fact that Leo was an accountant ( electronics was only a hobby to him in the first place )

.

About modern Fender amps, well, it's true there are many around still working, but OTOH FMIC could have been be more "open" to complaints and suggestions ( at least to the good and right ones ). They have used the "zeners plus droppers" arrangement on a number of models for some 15 years now, and I bet they have received a lot of complaints over the years, ( not to mention the ones that can be found here ) but never did something about it. And bear in mind we're not talking about rocket scientists' stuff, it's just about Ohm's law and power dissipation ratings.....

Maybe I'm narrow minded and biased due to my tendency to always consider the "worst case" in my designs ( after all that's what I do for a living ) but I wonder who would be happy and comfortable to know that, inside his amp, there are resistors getting hot enough to melt solder joints.....

Cheers

Bob

**rene2k** · 02-13-2009, 09:59 AM

Thank you guys for your effort and answers.
Í´m going to find those resistors, removing them and resolder a pair of new one. But first I gotta tell you, that I´ve found probably a piece of "metal" on that board which is mounted on backside of the amp. It is very small, looks like solder, don´t know, and was just on footswitch jack circuit, I removed it with my finger (it wasn´t soldered) , played a while (2hrs) and nothing really happend. Tonight I gotta gig that amp again but with a sub on stage (FenderVibrolux) this time. If it failures again I will go for that resistors and check. A friend told me that it´s probably the footswitch jack,which is not in use, cause I´m just using the clean channel of the amp and got a 2 ch stomp preamp on the floor. Through the years he told me it becomes dirty even if nothing plugged in, but I doubt this opinion.
regards
rene

**Robert M. Martinelli** · 02-13-2009, 10:09 AM

Hi Rene,
If you don't want to perform "heavier" mods ( like putting in voltage regulators ), my advice would be to put in a couple of resistors with a 2x power rating ( if the originals were 5W, put in at least 10W ones ). This won't heal the cheap design, but at least will lower the resistors' temperature. As already said, some lower the resistors' value as well, ( e.g. from 470 to 390 Ohms ) to reduce the voltage drop ( and the dissipated power ).

Hope this helps

Best regards

Bob

**bnwitt** · 02-13-2009, 02:31 PM

Originally posted by voxrules! View Post

I wonder what Leo would have thought about this if he still was with us.....

I imagine Leo would have opened up Fender warranty repair centers across the country to make money off of the situation.

**rene2k** · 02-14-2009, 02:40 AM

Hey guys, thank you so much for your help. My amp worked fine tonight.

It seems that it really was that small piece of metal that shorts the footswitch circuit. Glad to found a friendly community here.

Keep on rocking

regards
rene

**Robert M. Martinelli** · 02-16-2009, 06:31 AM

Originally posted by bnwitt View Post

I imagine Leo would have opened up Fender warranty repair centers across the country to make money off of the situation.

I must say I'm surprised to read such comments....or maybe I should blame all the bios I've read, in all of them there' s a sort of "halo" or mystique about Fender's ( company ) early days, so probably my perception of Leo' s attitude was "softened" by the known tendency of writers to "romanticize"........Enzo's and bnwitt comments about Leo tend to depict the image of a terribly greedy man in place of the image of the inquisitive, curious, perfectionist genius I had in my mind....

It's true, there's always something new to be learned....

BTW, rene, glad you've managed to fix your amp!

Regards

Bob

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