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Post subject: Hot Rod Deluxe - Channel switch inoperable
Posted: Mon Jul 25, 2011 12:53 pm
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Hello,
I am beating my brain trying to trace the issue out and can not find the solution. I am not new to working on amps and have done some pretty detailed work. I have a Hot Rod Deluxe that plays great, reverb is working perfectly, all tone knobs show their sound according to the value the POT is set to. The issue is that the channel and more drive buttons seem to be inoperable. The LED is on even when the CHANNEL SELECT (CS), button in out or in the off position. When the CS is pushed in or turned on the light gets a little brighter and dims when pressed again. There is no change when the MORE DRIVE (MD), button is pushed in. I have tried running the footswitch and there is no change either. While playing through the amp and pressing in and out on both CS and MD all different possible combinations there is no change what so ever.

PLEASE HELP

Thanks
Mike


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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Mon Jul 25, 2011 1:17 pm
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Take a look at R78 and R79 -- they're a pair of co-located 5-watt cement resistors that are prone to failure due to inadequate ventilation beneath them (they're mounted nearly directly to the PCB). Damage to one or both of them usually results in a malfunction of the channel-select capability.

Arjay

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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Mon Jul 25, 2011 1:21 pm
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Yes indeed.. And they have been replaces with the same value.. I have read that 330's are replacing them. Wouk this make any difference?


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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Mon Jul 25, 2011 1:55 pm
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+1 Retroverbial

I don't agree with using the 330 ohm in place of the 470 ohm in the +/- 16 volt supplies. Tests in my simulator show that more heat is produced, not less. The shift is that more current flows in the Zeners with the 330 ohm resistors. The only real advantage of changing the resistors is that you have longer leads to allow the resistors to be mounted off of the board to keep the heat away from the board. And if the resistors are mounted off of the board, that puts more stress on the circuit trace underneath, unless some support is used for the resistors, by bending the leads down onto the board surface (top and underneath), or some other way to keep stress away from the solder pad/joint, not allowing the resistors to move with cabinet shocks or vibrations.

Which is the real usual problem anyway, broken circuit trace and/or bad solder joints near those resistors. The resistors aren't the problem, nor the charred board, but the thin circuit traces and poor solder joints on the underneath side of the board. If you tried to "reflow" the solder, or change the resistors without removing the main circuit board, you can actually cause more problems. You must remove the main circuit board to inspect the areas around those 5-watt resistors very closely. Actually, check all solder joints thoroughly while the board is out.

From you previous post, your power supply voltage was unusually low at under 400 volts, it should be closer to 430 volts. Is that still the case? There are several capacitors in the power supply, some are at a lower voltage than others. Your voltage on C31 should be about 430 volts.

Just follow the schematic from the +/- 16 volt power supply to see if you have those two voltages at the supply (top of the two resistors, R78 & R79), and then at the op amps that control the channel switching and reverb op amps, pins 4 and 8 on each op amp.

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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Mon Jul 25, 2011 2:01 pm
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shimmilou wrote:
The only real advantage of changing the resistors is that you have longer leads to allow the resistors to be mounted off of the board to keep the heat away from the board. And if the resistors are mounted off of the board, that puts more stress on the circuit trace underneath, unless some support is used for the resistors, by bending the leads down onto the board surface (top and underneath), or some other way to keep stress away from the solder pad/joint, not allowing the resistors to move with cabinet shocks or vibrations.


+1, shimmilou

Most techs I know who work on these HR series amps use a couple of dollops of high-temp clear silicon caulk to support R78 and R79 off the PCB when using resistors with longer leads. It does seem have a positive influence on the durability of the repair.

HTH

Arjay

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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Mon Jul 25, 2011 2:24 pm
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Shim- I have 399.99CDC at C31.. Also reading R78 and R79 ground lead on the chassis screw hole, an POS lead on the top of each, I get 17.57vdc at R79 and -17.39CDC on R79..

Where can I get the hi-temp fix to mount the capacitors to? I'm a mechanic in the Army and we have what's called RTV, basically liquid gasket. I'm in the El paso area as well and things normally found in the big city is hard to come by sometimes..


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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Mon Jul 25, 2011 2:33 pm
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michaelbpuccio wrote:
I'm in the El paso area as well and things normally found in the big city is hard to come by sometimes..


Check the Ft. Bliss PX first, Michael. If you don't find it I know that EP has several "Vato Zones" and Pep Boys stores in the area.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Mon Jul 25, 2011 2:38 pm
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Orelae!!! I hit up the "Vato-Zone" here in a bit.. Closer da the house..


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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Mon Jul 25, 2011 2:41 pm
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michaelbpuccio wrote:
Orelae!!! I hit up the "Vato-Zone" here in a bit.. Closer da the house..


:lol:

Don't let them pendejos chingados overcharge you, troop!

:mrgreen:

You must be a 63-Bravo.

Arjay

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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Mon Jul 25, 2011 3:12 pm
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Haha! We are now 91 series..


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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Mon Jul 25, 2011 3:13 pm
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michaelbpuccio wrote:
...399.99CDC at C31.. Also reading R78 and R79 ...I get 17.57vdc at R79 and -17.39CDC on R79....


Sounds like something wrong in the main power supply for starters, check the four diodes: CR6, CR7, CR8 and CR9. Maybe one is open, or even shorted. Also, possibly bad caps in the supply, first look for a crusty gray substance coming from the positive lead at the body of each cap, and check the values if they look good physically.

And the readings on the resistors tell me that the zeners are disconnected, as they regulate and keep the excess voltage down. Could be bad solder joints and/or broken circuit traces between the resistors and zeners, including the zener joints too. :idea:

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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Mon Jul 25, 2011 10:12 pm
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Shimmilou-
Here is what i have found. The schematic i have is # 050397 REV-B 11AUG95 along with Diagram # 050399 REV-A 07MAY96 and it is frustrating when trying to track down components.

CR6 (is actually) D6 on the morherboard with 210VDC
CR7 (is actually) D7 on the morherboard with 210VDC
CR8 (is actually) D8 on the morherboard with 210VDC
CR9 (is actually) D9 on the morherboard with 210VDC

The Power Caps have the following values and have no physical evidence of being damaged or are leaking anything like what you mentioned.
C31 = 426.5VDC
C33 = 424VDC
C35 = 387.5VDC
C36 = 343.5VDC

I am not understanding the value of R78 and R79 as they are showing voltage. How can it be an issue if there are 17VDC going through each of them? I also dont understand what else it may be that is causing the channel switch to act as it does.

Thanks again..


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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Mon Jul 25, 2011 10:49 pm
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OK, 426 volts at C31 sounds much better than 399 volts, not sure why it measured low before. The voltages that you listed this time seem good. :?

If you look at the schematic, the 5-watt resistors are connected to zener diodes (CR13 and CR14), and voltage at that point should be very nearly 16 volts, as the zeners are 16 volts regulators. Any voltage above 16 volts on the zeners should be conducted to ground. If the voltage is that much above 16 volts, the zeners are not working, either they are open, or a bad connection there. Each of the 5-watt resistors will have a "normal" voltage drop across them of about 35 volts.

These resistors and zener diodes are in the +/- 16 volt supply for the channel switching. The supply feeds op amps that control the switching relays, and the reverb op amp, and the effects loop op amp. So any problems with the channel switching, reverb, or loop, are usually caused by a problem in this supply.

There could be any number of problems, but in a case like this, the problem is usually in the power supply, most often poor solder joints and/or broken circuit trace. If you are showing more than 16 volts at the zeners, that is wrong and must be corrected before being able to continue checking anything else, and may very well be the entire problem. :idea:

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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Mon Jul 25, 2011 11:15 pm
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The following I what is read at the two points you mentioned:
D13 is 17.27VDC
D14 is 17.23VDC

Not I know you mentioned that this is not correct but what can be done to fix this issue?


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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Tue Jul 26, 2011 12:25 am
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Reviewing my previous posts, there is a recurring theme as to the likely cause, and remedy. :wink:

shimmilou wrote:
...broken circuit trace and/or bad solder joints near those resistors...Actually, check all solder joints thoroughly while the board is out...


shimmilou wrote:
...Could be bad solder joints and/or broken circuit traces between the resistors and zeners, including the zener joints too. :idea:


shimmilou wrote:
...There could be any number of problems, but in a case like this, the problem is usually in the power supply, most often poor solder joints and/or broken circuit trace....


It isn't easy, or popular, to pull the main board, but is necessary for a complete inspection of the trouble areas connections. Not a guarantee that this is the entire problem, but it is a common problem and the most likely source of the channel switching problem, especially considering that you previously had a reverb problem that could be related to this problem, and both areas of the amp have this supply in common. :idea:

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Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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