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Post subject: Hot Rod Deluxe wont switch channels-no reverb
Posted: Sat Apr 30, 2011 3:05 pm
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Hello this is my first trip to the Fender Lounge- I was giging my Deluxe when 2nd to last song she died. Got her home and found that I only had the clean channel (although the yellow drive light is on) and I have no Reverb. They say these things are notorious for these problems in the R78 and R79 +/- 48 volt circuit. So, I opened up the amp and found the top leg of R78 and the diod to the right of it d13 had came unsoldered from the board.

I went ahead and resoldered the diod back in place as it seemed to check out ok. I went ahead and replaced the resistors with 330 ohm values instead of the stock 470's as I have read a lot on that fixing channel switching problems with these amps. No luck, the amp is the exact same.

I have my +48 and -48 volt dc on the bottom side of these resistors but according to the schematic I should have + and - 16 volts on the top side. R78 is reading +9 R79 is reading -8. I own 2 of the amps so I pulled the back off, fired it up and sure enough I have the +/- 16 on this amp.

The only thing I found in this part of the circuit that may affect these these voltages were Caps 41 and 48 so I replaced them as well as put the stock value 470's back in place. Still the same problem! The 2 5 watt diod's are in that circuit but seem to be working correctly. What am I missing????? There are several test points to troublshoot this circuit in the schematic but without the 16 volts that seems pointless.

Any help or suggestions on who to talk with about this problem would be awsome. Thanks a ton! My other HR Deluxe is a limited edition maple case so I wont gig, it's for recording so I am left playing a backup Marshall in a country band. Please Help!!!!!!!! lol.


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Post subject: Re: Hot Rod Deluxe wont switch channels-no reverb
Posted: Sat Apr 30, 2011 3:28 pm
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If the +/-16V is lower than expected then there are components that sink more current than foreseen.
Did you check CR13 and CR14 ?
Did you check all opamp (4560 + TL072)

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Post subject: Re: Hot Rod Deluxe wont switch channels-no reverb
Posted: Sat Apr 30, 2011 6:59 pm
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+1 Tis-san

Hi Hot Rod591,

First check the two zener diodes as Tis-san suggested for short or open. If you have the +48VDC and -48VDC on the bottom of each resistor (means that the DC supply for that circuit is likely OK), and less than +16VDC and -16VDC on the top, it sounds like a resistor is open, or you may have a cracked/broken circuit trace at one or more of the solder joints on the resistors or even the zener diodes, it may take a magnifier to see but it is usually right where the solder pad connects to the trace. Also, check the resistance of the 470 ohm resistors that you put back in to make sure that one or both aren't open. I would use the 470s myself (if they are good), I don't agree with the change to 330 ohms. If you do find a broken trace, carefully scrape away a small amount of the protective coating on the trace with a small flat blade screwdriver so that solder will stick to it (scrape towards the pad), then using the end of the resistor lead bent across the break, or a small piece of wire, bridge the gap and solder. :idea:

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Post subject: Re: Hot Rod Deluxe wont switch channels-no reverb
Posted: Sun May 01, 2011 12:07 pm
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Thanks a ton for the input guys. Im almost positive the diods are ok, they dont read open and with power on I have to ground my +9 on one leg and 0 on the other. The other diod I have my -8 on one leg and 0 on the other. I have ohm'd both sets of resistors 330 and 470 multiple times and their value's are correct. I have not done the magnifying glass thing but I did do continuity check's from each leg of the resistors to it's next connection and everything read fine, but thats cold. Going to try the magnifying glass ;)

I do have my suspicions about the opamp chip, some of the test point values are'nt even close but I was'nt sure without a full 16 volts. If your saying that a possible bad component upstream can effect the 16 then that's my next move. Once again thanks!! You've been a huge help!!


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Post subject: Re: Hot Rod Deluxe wont switch channels-no reverb
Posted: Sun May 01, 2011 2:53 pm
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Do the +/- 16 volt supply readings at the resistors (+9, -8) change at all when the channel select switch is switched?

You may be right, it could be a chip problem. I agree that it's not much help to check the test points at the chips without the full +/- 16 volt supplies. Maybe you even have one of the relays sticking. While checking your traces, check the contacts on the relays to be sure that they are open for the NO and closed for the NC. :idea:

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Post subject: Re: Hot Rod Deluxe wont switch channels-no reverb
Posted: Sun May 01, 2011 6:56 pm
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No the voltage does not change when switching the channel switch. I have already checked both the K1 and K2 relay's and they seem to be fine as well. I have 1.4K across both coils and both the NO and NC are reading what they should.

After going through all the components up stream I could not find any bad resistors or diods, all values where what they were supposed to be. Im even more convinced it may be the 4560 chip. Im not an amp tech by no means, Im a maintenace tech with some general knowledge so im not sure if my methods are right but this is what I did.

I set my meter to the ->+ setting and with black lead to chassy ground I checked the values on each pin. I did the same thing on my good amp and compared. Pin 8 seems to be a problem. This looks to be the +16 input to the chip, when I follow the trace on the board back it goes to the top of R78 wich is the leg that came unsoldered from the board. Just speculative thinking.

The value on pin 8 on the good amp is a gradual climb up to 1.950 or so. The value on the bad amp rises up to .800 then stops. I used the same method on the TL072 chip in the Reverb section and both amp's were spot on.

I think im gonna try and get a new chip, if there are any other way's of thinking about this I'll be glad to listen!! Thanks shimmilou, you've been a huge help!!


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Post subject: Re: Hot Rod Deluxe wont switch channels-no reverb
Posted: Sun May 01, 2011 7:52 pm
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Dude! Seems like you are on top of it, good methodical approach....just like a real amp tech. :wink: Whether an amp, or a PLC, or a pump control, you seem to have a sound analytical process for troubleshooting. You might consider using a chip socket with the replacement chip, since you are already going to change the chip anyway, it might make things much easier in the future and it's just as easy to solder the socket as it is to solder the chip, and soldering the socket will keep the chip from getting too hot while soldering. Keep us informed as to your progress. :)

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Post subject: Re: Hot Rod Deluxe wont switch channels-no reverb
Posted: Sun May 01, 2011 9:14 pm
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That my friend is a brilliant idea :D Got the chip ordered, I am definatly going to hunt a few of those down. Probably take a week or so but I will definatly keep you posted! Thanks a ton you guys are great! Later


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Post subject: Re: Hot Rod Deluxe wont switch channels-no reverb
Posted: Mon May 02, 2011 4:25 am
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What you can do is unsolder the VDD pin (+) of the IC (one after one) and test if the 16V is back. When it is back just use you probe to short the VDD pin to the PCB in order to verify the supply down to 8-9V.

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Post subject: Re: Hot Rod Deluxe wont switch channels-no reverb
Posted: Mon May 02, 2011 10:53 am
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Thank You Tis-san!
I started by unsoldering pin 8 since that was the pin in question. When I got the solder removed I noticed that the pin was very very loose feeling. I wiggled back and forth a few times then the pin fell through the other side, this tells me the pin must have been broke where it bends to go into the chip. I went ahead and fired the amp, my voltages are still the same but I believe I have found a huge problem here! Yaaaaa :D

Im gonna stop right here and wait for the new chip. No garentee's this is the entire problem but it's the first thing I have actually found wrong since the finding of components pulled away from the board! Stay tuned :wink: Thanks again!


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Post subject: Re: Hot Rod Deluxe wont switch channels-no reverb
Posted: Mon May 02, 2011 2:01 pm
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There are three chips, all tied to the same supply. Which chip are you looking at with the broken pin? U1 is for the effects loop, U2 is for the reverb driver and recovery, and U3 is for the channel switching. :idea:

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Post subject: Re: Hot Rod Deluxe wont switch channels-no reverb
Posted: Mon Nov 05, 2012 7:36 pm
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Hi Guys

Im experiencing a similar problem with my HRD.

I installed a mod kit, after I noticed the Caps needed changing (decided to do the whole thing) I had problems for years with the amps channel switching not working.

I replaced R78 and r79 with 2x 5W 330 ohm cement resistors. (the solder side of the circuit board was pretty scorched by the old resistors)

Anyways, the amp wont switch channels, the yellow light stays on, and the amp stays in clean mode. When i first fired up the amp after completing the mod, both channels worked and switched, but then stopped working later that evening.

I read through the information above, and my readings don't match.

on R78 and 79, bottom I get + and - 45v. At the top of r78 I get +9.6. and at the top of R79 I get -9.0v.

Can anything be deduced from the lower than spec voltage. (48, and 16) could this be because of the change from 470 to 330?

Let me know If there is something I should be looking at.

Thanks


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Post subject: Re: Hot Rod Deluxe wont switch channels-no reverb
Posted: Mon Nov 05, 2012 9:04 pm
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You almost certainly have broken solder joint(s) or trace near the 5 watt resistors, likely at one or both of the resistors themselves. See the link below for excellent instructions on how to repair these broken traces and/or reinforce the solder joints.

Now, as for the myth of using the 330 ohm resistors in the HRDlx and BDlx, it's just plain wrong. Using 330 ohm resistors in place of the 470 ohm resistors allows more current to flow in this circuit, so you get the same amount of heat from the new resistors, with added heat from the zeners dumping the extra current/voltage to ground. There is no free lunch, if you reduce the resistance, the current has to increase, no way around that fact. Sure, the zeners can handle the extra current, but the heat produced in the circuit is the problem to begin with, and the 330 ohm resistors actually allow more heat in the circuit, making the problem worse. I had correspondence with Frommel, and he just doesn't get the problem with using the 330 ohm resistors in the HRDlx, oh well. :(

The advantage to using new resistors is that the new ones have longer leads and allow the resistors to be mounted up off of the board so that the board doesn't get as much heat on it. Of course the resistors then have to be supported in some way to relieve the stress on the solder joints and traces, which can be fragile. The resistors used in the Dlxs should definitely be 470 ohms and NOT 330 ohms.

My belief is that someone noticed that the BDvl and HRDvl (very similar amps) use the 330 ohm resistors and those amps don't exhibit the heat related issues that some of the HRDlx and BDlx do. What they failed to notice though, is the fact that the Dvls have a lower supply voltage before the resistors, therefore the DVls don't need the larger 470 ohm resistors to begin with. The 470 ohm resistors are necessary in the Dlxs because of the higher supply voltage feeding the resistors in those amps.

http://billmaudio.com/wp/?page_id=204

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Post subject: Re: Hot Rod Deluxe wont switch channels-no reverb
Posted: Tue Nov 06, 2012 12:23 am
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Hey Shimmilou

Thanks for the info.

Just to double check, would I still be getting the 45 Volt readings, and 11 volt readings on those resistors with bad traces/continuity?

I thought that with a bad solder joint I would get zero or something. Its close, but not correct...

Thanks


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Post subject: Re: Hot Rod Deluxe wont switch channels-no reverb
Posted: Tue Nov 06, 2012 7:43 pm
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It really depends on where the circuit is broken (if that is the problem), the voltages can be all over the place. The +/-45 VDC will be there regardless of an open circuit after the diodes CR11 and CR12. But, the voltage dropped by the resistors R78 and R79 depend on the amount of current flowing through them, and the amount of current depends on the circuit connections after the resistors. Any open circuit connections, at or after the resistors, will affect the voltage readings. If the connections to the zeners are good, you should expect to see the +/- 16 volts at the top of R78 and R79. If the one or both of the zeners are not connected, there would not be as much current flowing through the resistors, so the voltages could be low, or even high, again, depending on which part of the circuit is disconnected. Time to pull the board again and closely inspect all joints and traces. You might even wiggle the components while looking at the joints to be able to see a break in the joints or traces.

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