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Post subject: HELP - Scorch marks on PCB of Hot Rod Deluxe 3
Posted: Mon Jun 10, 2013 8:05 am
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Hi Guys,

My HRD3 has some scorch marks around two white blocks next to the bias adjuster. On the wiring diagram they are shown as TP42 & TP43. See picture below. Any ideas whats happening here and is it terminal? :(

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Post subject: Re: HELP - Scorch marks on PCB of Hot Rod Deluxe 3
Posted: Mon Jun 10, 2013 8:42 am
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The "white blocks" are resistors R78 and R79 and I believe they regulate the -16V and +16V that the diodes in the channel-switching circuit require. Left to their own devices they will eventually incinerate themselves. It's a malady common to all HR-series amps but there are some mods to help alleviate the issue.

Arjay

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Post subject: Re: HELP - Scorch marks on PCB of Hot Rod Deluxe 3
Posted: Mon Jun 10, 2013 8:49 am
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Retroverbial wrote:
The "white blocks" are resistors R78 and R79 and I believe they regulate the -16V and +16V that the diodes in the channel-switching circuit require. Left to their own devices they will eventually incinerate themselves. It's a malady common to all HR-series amps but there are some mods to help alleviate the issue.

Arjay


Thanks Arjay, what mods would alleviate the problem?


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Post subject: Re: HELP - Scorch marks on PCB of Hot Rod Deluxe 3
Posted: Mon Jun 10, 2013 11:35 am
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Here's some info.
http://www.justinholton.com/hotrod/5Wresistors.html
Here's the rest of the site too.
http://justinholton.com/hotrod/
Those resistors have always been a problem and cause lots of problems. Mine were replaced after three months. I thought they addressed that issue with the III series. Guess not. Be careful, the other thing that can happen is the traces can lift off the board.

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Post subject: Re: HELP - Scorch marks on PCB of Hot Rod Deluxe 3
Posted: Mon Jun 10, 2013 12:37 pm
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63supro wrote:
Here's some info.
http://www.justinholton.com/hotrod/5Wresistors.html
Here's the rest of the site too.
http://justinholton.com/hotrod/
Those resistors have always been a problem and cause lots of problems. Mine were replaced after three months. I thought they addressed that issue with the III series. Guess not. Be careful, the other thing that can happen is the traces can lift off the board.



Thanks 63supro, really appreciate your help.


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Post subject: Re: HELP - Scorch marks on PCB of Hot Rod Deluxe 3
Posted: Mon Jun 10, 2013 4:09 pm
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Typically, it's nothing to worry about, it's just discoloration of the circuit board from the heat of the resistors. This is very common in lots of circuit boards on all kinds of electronic circuits. The version III has thicker circuit boards that can better handle the heat from the resistors, and will probably be just fine. The previous versions with thinner circuit boards were more susceptible to damage from the heat, although most just show the discoloration with no further problems. The sky is not falling Chicken Little! :lol:

Forget about changing the resistors to 330 ohm, that is total BS, and even the reference on the JH site was nothing more than a misunderstanding, as Fender absolutely did not change the value to 330 ohms, and has no basis whatsoever in reality. The Devilles are the only amps that use the 330 ohm resistor in that circuit, and they have a different (lower) supply voltage to begin with, hence the use of lower value resistors. If the resistors in the HRDlx are changed to 330 ohms, the circuit will actually produce more heat as the current will increase, thus compounding the issue.

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Post subject: Re: HELP - Scorch marks on PCB of Hot Rod Deluxe 3
Posted: Mon Jun 10, 2013 4:29 pm
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shimmilou wrote:
Typically, it's nothing to worry about, it's just discoloration of the circuit board from the heat of the resistors. This is very common in lots of circuit boards on all kinds of electronic circuits. The version III has thicker circuit boards that can better handle the heat from the resistors, and will probably be just fine. The previous versions with thinner circuit boards were more susceptible to damage from the heat, although most just show the discoloration with no further problems. The sky is not falling Chicken Little! :lol:

Forget about changing the resistors to 330 ohm, that is total BS, and even the reference on the JH site was nothing more than a misunderstanding, as Fender absolutely did not change the value to 330 ohms, and has no basis whatsoever in reality. The Devilles are the only amps that use the 330 ohm resistor in that circuit, and they have a different (lower) supply voltage to begin with, hence the use of lower value resistors. If the resistors in the HRDlx are changed to 330 ohms, the circuit will actually produce more heat as the current will increase, thus compounding the issue.


Thanks shimmilou, that's very reassuring. I read these resistors can cause problems with channel switching and reverb, neither of which are issues in this instance, so I guess your right.

If this does become a problem, would fitting new resisters with silicon between them and the PCB help to reduce heat damage in the future?


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Post subject: Re: HELP - Scorch marks on PCB of Hot Rod Deluxe 3
Posted: Mon Jun 10, 2013 4:55 pm
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Maybe just a small dab on each end to secure the resistors, leaving space between resistors and board to allow for heat dissipation, or some other method to support the resistors so as not to put pressure on the solder joints underneath, such as ceramic tubes on the leads. If you use silicon, make sure that it is for high temp use. My HRDlx has little bumps on the bottom of the resistors that allow the resistors to be secure on the board while still allowing air flow between resistors and board.

Note, not much in the way of discoloration on mine, and it's a '98 model.

Image

I even briefly fitted some heat sinks on resistors and Zeners as an experiment, then removed them as I had no problem from the heat.

Image

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Post subject: Re: HELP - Scorch marks on PCB of Hot Rod Deluxe 3
Posted: Mon Jun 10, 2013 5:12 pm
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shimmilou wrote:
Maybe just a small dab on each end to secure the resistors, leaving space between resistors and board to allow for heat dissipation, or some other method to support the resistors so as not to put pressure on the solder joints underneath, such as ceramic tubes on the leads. If you use silicon, make sure that it is for high temp use. My HRDlx has little bumps on the bottom of the resistors that allow the resistors to be secure on the board while still allowing air flow between resistors and board.

Note, not much in the way of discoloration on mine, and it's a '98 model.

Image

I even briefly fitted some heat sinks on resistors and Zeners as an experiment, then removed them as I had no problem from the heat.

Image



Just one more question as its now 01:00hrs here, I see you have an extra capacitor top left, do you know why they have changed this for the mkIII and does it make any difference.


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Post subject: Re: HELP - Scorch marks on PCB of Hot Rod Deluxe 3
Posted: Mon Jun 10, 2013 5:28 pm
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Couldn't hurt. I still wouldn't trust them given their track record. Mine did just like Arjay stated and eventually self destructed. Doesn't mean yours will. The difference was I had one of the thin boards and it was a problem. If you gig the amp in any serious way, I'd at least have it looked at especially if it bothers you. If you gig and the sky falls, without a backup amp, you're done. Like Lou said, don't change the value. I would also leave some space between the resistor and the board for air flow. Looks like the heat discoloration is migrating to the Diode at D13.

Makes you wonder why they didn't heat sink them like Lou did with the III series.

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Post subject: Re: HELP - Scorch marks on PCB of Hot Rod Deluxe 3
Posted: Mon Jun 10, 2013 5:54 pm
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Kluson56 wrote:
...Just one more question as its now 01:00hrs here, I see you have an extra capacitor top left, do you know why they have changed this for the mkIII and does it make any difference.


I added the extra capacitor when I recapped (F&T), another 47 µF, doubling the filtering in the first stage, reducing ripple and decreasing the hum just a bit, also tightening the bass response. There is already a spot for the extra cap, just insert, solder and connect the positive to the next cap with a jumper. My original IC caps had evidence of leaking on a couple, so I recapped before any problems developed.

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Last edited by shimmilou on Mon Jun 10, 2013 5:56 pm, edited 1 time in total.

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Post subject: Re: HELP - Scorch marks on PCB of Hot Rod Deluxe 3
Posted: Mon Jun 10, 2013 5:54 pm
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63supro wrote:
Couldn't hurt. I still wouldn't trust them given their track record. Mine did just like Arjay stated and eventually self destructed. Doesn't mean yours will. The difference was I had one of the thin boards and it was a problem. If you gig the amp in any serious way, I'd at least have it looked at especially if it bothers you. If you gig and the sky falls, without a backup amp, you're done. Like Lou said, don't change the value. I would also leave some space between the resistor and the board for air flow. Looks like the heat discoloration is migrating to the Diode at D13.

Makes you wonder why they didn't heat sink them like Lou did with the III series.


A friend of mine is an electrical engineering lecturer at our local college, he's checking to see if they have the correct resistors in stock tomorrow (today), so for peace of mind I think I'll install new ones and support them clear of the board. Looking at Lou's PCB on his 98 model there's very little discolouration compared to mine .

Fender must know this is an issue so I am surprised they haven't sorted with the mkIII.


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Post subject: Re: HELP - Scorch marks on PCB of Hot Rod Deluxe 3
Posted: Mon Jun 10, 2013 5:58 pm
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shimmilou wrote:
Kluson56 wrote:
...Just one more question as its now 01:00hrs here, I see you have an extra capacitor top left, do you know why they have changed this for the mkIII and does it make any difference.


I added the extra capacitor when I recapped (F&T), another 47 µF, doubling the filtering in the first stage, reducing ripple and decreasing the hum just a bit. There is already a spot for the extra cap, just insert, solder and connect the positive to the next cap with a jumper. My original IC caps had evidence of leaking on a couple, so I recapped before any problems developed.


That's interesting, thanks. I'm logging off now as I'm totally cream crackered. Thanks for the help. I'll update once I've sorted it out.


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Post subject: Re: HELP - Scorch marks on PCB of Hot Rod Deluxe 3
Posted: Mon Jun 10, 2013 6:06 pm
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Kluson56 wrote:
...I am surprised they haven't sorted with the mkIII.


Really they have with the thicker board. The resistors themselves rarely, if ever, are the problem and they just don't fail. The problem has been the solder joints and traces on the underside of the board becoming brittle and failing due to the heat. This hasn't been nearly as much of a problem in the version III.

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Post subject: Re: HELP - Scorch marks on PCB of Hot Rod Deluxe 3
Posted: Tue Jun 11, 2013 1:21 am
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shimmilou wrote:
Kluson56 wrote:
...I am surprised they haven't sorted with the mkIII.


Really they have with the thicker board. The resistors themselves rarely, if ever, are the problem and they just don't fail. The problem has been the solder joints and traces on the underside of the board becoming brittle and failing due to the heat. This hasn't been nearly as much of a problem in the version III.



I've never seen inside the previous versions, is the board that much thicker in the mkIII.


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