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Post subject: Hot Rod Deluxe Switching channels
Posted: Wed Nov 17, 2010 5:03 am
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Hello all, I am an experienced technician and am trying to repair a hot rod deluxe. The fault is that the amp is switching channels momentarily but only with the back on. I am aware of the problems with the connections on the two 470 ohm resistors and 16 v zener diodes. I have replaced the 470 ohm resistors just in case. The fault is still there if I plug the foot switch in so it's not the foot switch jack,in any case I have had this out and resprung the contacts. On the odd occasion the fault appears with the back off ( every two or three days) I have observed the +16v supply to the relay with my scope and it is constant however the other side of the relay coil does go to zero when the fault appears so the fault seems to be in the switching circuit. No amount of tapping the board will reproduce the fault. As the fault occurs so rarely with the back off I can't trace it by normal technical means.


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Posted: Wed Nov 17, 2010 5:47 am
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Check tracks that can be broken 8)

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Posted: Wed Nov 17, 2010 6:52 am
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I have looked with a magnifying glass and can find no problems


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Post subject: HRDlx channel switching
Posted: Wed Nov 17, 2010 8:28 am
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Hi fincaman,

Can you check the following TPs? TP 31, 32, 33, 34, 35, 36, the voltages are listed on the schematic, just to the left of the switching circuit where the TPs are located. Monitor each when the problem occurs, and during normal functioning, to see if that circuit is working. If you have no broken traces or bad solder joints, there could be a problem in that circuit itself, maybe a bad cap, diode, or even a bad chip. :idea:

Schematic:
http://support.fender.com/schematics/guitar_amplifiers/Hot_Rod_Deluxe_Schematic.pdf

Edit:
Check the schematic and you'll see that the 16V supply is connected directly to one side of the channel switching relay coil, so you're correct, the other side of the coil is controlled. :idea:

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Posted: Wed Nov 17, 2010 10:57 am
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Hello shimmilou, thanks for your comments, my problem is that the fault hardly appears when the back is off, in fact it went for 5 days with guitar music playing through it and didn't do it once. as soon as I put the back on it does it all the time, I was hoping that someone had had the fault before.

Regards

Kevin Mooney


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Posted: Wed Nov 17, 2010 1:38 pm
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Well, that really sounds like a heat related solder joint problem, or broken trace near a solder joint, like Tissan pointed out. Even when the joints don't look cracked, they can be cold and making a poor connection. Usually, intermittent problems almost always indicate solder joint problems. I know that it is a job, I've done it a few times, but I would just pull the circuit board again and touch up every solder joint that even looks cold (not shiny), or even touch up all joints. Also touch up all joints on the tube socket boards.

You seemed confident that the joints/traces were good, that's why I suggested the TP measurements as another possibility. The TP voltage measurements could at least confirm whether or not that part of the circuit was functioning normally, you might see something wrong, or confirm normal operation. But, the odds are that it's solder joint problems. :)

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Posted: Wed Nov 17, 2010 2:01 pm
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I give what I did in the past: I use a "hair drier" in order to warm up only a dedicated part of the circuit, by dichotomy I located the place where the problem is; Sometime it is very difficult to localize a broken track especially at the "round pad" to track border. Basically I agree with Shimmilou; 8)

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Posted: Thu Nov 18, 2010 11:24 am
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Here's a great link that addresses a lot of the HRD Series problems.
http://www.justinholton.com/hotrod/

I tend to agree with shimmilou. I'd touch up the solder joints. I'd look at the channel switching relays too. Look carefully for cold solder joints, not just cracked ones. They'll be dull gray in appearance. Be sure none of the traces lifted too.

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