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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Mon Nov 14, 2011 12:06 am
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I agree! In my finding of exactly why my channel select went bad I actually used the "licht trick" I used a LED flashlight on one side of the motherboard and found a break in the circuit. It is easily noticeable when doing this. Also you can do what I call a "beep check" or a continuity check with a multimeter and make sure there is continuity from one end to the other in all circuits running off the 5w resistors. My fault was that and was easily fixed by using a razor to scrape off the section passed the area that had a break, run some solder on the newly exposed area and soldering new resistors to that area. You must ensure that the leads are NOT touching any other circuit as this will cause other problems. But if done this way it tested out to be fairly simple. Just need a little creativity in your plan.


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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Mon Nov 14, 2011 1:35 am
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Thanks, I'll try that.

The original resistors were shot, one measured in at less than 430 ohms, while the other was still at 470. There was a decent amount of heat damage to the board around them as well, which had loosened the solder pads on the other side of the board, which resulted in me running a few jumpers.

Is the .7 V over-voltage across the diodes a sure sign of a problem?
And, lastly, is there any chance this could be caused by a bad diode? The heat damage on the board extends under them as well.


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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Mon Nov 14, 2011 7:16 am
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In replacing the 5W resistors, I reccomend that you go to your nearest autozone or pepboys and pick up some high temp RTV. This is going to give you a heats ink between the board and the underside of the resistor. When you de-solder the resistors use a q-tip an some rubbing alcohol to clean the board. Then put a even and small amount of RTV on the board where the new resistors are going to go. When installing the new ones press gently, as the extra will push out from underneath the resistors. This also serves as a type of glue and will keep them from vibrating themselves off the board while playing. In addition, it will also resist the heat from the resistor and not allow the heat to burn up the board. Now as for the diodes, rule of thumb is "don't fix what's not broken" in saying this, fix one thing at a time. The result may or may not be what you hope to find. And if it turns out that the problem is not solved, move to the next issue. In my opinion using jumpers is NOT the way to go and remind me of those hack job mechanics people take their car to. I have seen them give off even more ground hum as the wire is usually close to the chassis. What I reccomend is to carefully scrape off the area passed the break in the circuit and solder the resistor lead to the uncovered portion. This makes a more solid and cleaner job. Also one that you won't have to worry about later. In my experience with this amp I have found that the diodes usually are not the problem and it is the 5W resistors.


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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Wed Nov 16, 2011 5:40 pm
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Went back in to the amp and checked every trace for resistance/continuity issues. Everything checked out, however after I powered on the amp, the channel switching worked. Probably was some kind of accidental short. At least that problem is no longer an issue.

Reverb is still inoperable though, so we're back to square one. I'm thinking it is probably the op amp, which might have overvolted when the old resistors failed. However, there are no reference voltages for said opamp on the amp schematic, you guys got any ideas on how to check it?

It's definitely not the transducers on the reverb pan, as I've checked it with the leads reversed. Some new hypotheses to test would be appreciated.


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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Wed Nov 16, 2011 6:50 pm
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On each op amp (U1, U2, U3), check pin 8 for +16 VDC, and pin 4 for -16 VDC, using the chassis for the negative meter lead.

U1 is for the effects loop

U2 is for reverb

U3 is for channel switching

All of these op amps use the +/- 16 volt supplies from the 5 watt resistor circuit. If your channel switching didn't work, but then started working, and the reverb doesn't work, this points to bad connections around those 5 watt resistors and the zeners nearby. You might need a magnifier to see any breaks, try gently wiggling the resistors and zeners while checking the solder connections and traces with a magnifier. It is most often a bad connection, but a bad relay or op amp is possible.

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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Wed Nov 16, 2011 11:11 pm
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Tey this, as i thought my readings were correct when i had a similar issue.. Open up the reverb pan. Check for resistance on both of the coils by removing the plug. One side should read aprox. 230ohms and the other side should read around 60ohms. Either way, if you get a 0ohm reading then look closely at the almost microscopic copper thred type wires on both + and - sides of both terminals at each end of the pan. I had a problem with this and found that the tiny wire was broken at the point where the small plug is. By looking very closely and in good lighting I was able to see it with my own eye. I then unwound the side that had a 0ohm reading to have something to play with, and then re-soldered it, checked again the resistance. It read about 60ohms and when I plugged it in it solved my issue with reverb for days. Another thing I found is you get a chopstick or something that is non-conductive, try sticking it into the input's of the amp. My issue was with a Deville and it was simply the jack getting stuck. So I sprayed some lube cleaner into the jack and issue was solved..


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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Thu Dec 15, 2011 10:00 am
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Pins 4 an 8 on U2 are +16.8 V and -17 V, respectively

Voltages at Zener diodes are +16.8 and -17


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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Thu Dec 15, 2011 10:13 am
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also, after running the amp for a half hour, voltages at the Zeners are now -17.2 and 17 V


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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Thu Dec 15, 2011 11:08 am
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Sergris, what exactly are you looking for? Is your reverb still not working?


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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Fri Dec 16, 2011 2:06 pm
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Oh, yeah, sorry I didn't specify that. I had to take a break from working on it to study for finals.
The Reverb still isn't working, and the channel switching locks up after the amp has been on for a while. Strangely, the channel switching was working fine for a while, but now it's gone on the fritz again.
The wandering voltage supply is probably the culprit there, but I don't know how to pinpoint the problem components. The reverb pan itself is operational, I managed to confirm that by accident while testing the supply voltages on the U2 opamp. Crossing one of the power leads to one of the other pins produced a brief signal, so the problem can't be in the reverb pan itself.
So, what components could be causing drifting supply voltages? Would it be correct to guess that this is a temperature related phenomenon?

And lastly, Would it be worthwhile just to replace the ceramic dropping resistors, zener diodes and reverb opamp? (one at a time of course)

Thanks for you help


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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Fri Dec 16, 2011 2:21 pm
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sergris wrote:
...So, what components could be causing drifting supply voltages?...


A poor/broken connection at the Zener diodes, next to the 5 watt resistors. Those Zeners will clamp the voltage to right at + and - 16 volts when connected. The supplies are common to reverb, channel switching, and effects loop. The voltage can not be over 16 with working Zeners in the circuit.

And, no, I wouldn't start replacing components that are functioning, no point in that and you can introduce other problems.

Did you actually pull the main board and double check the traces and solder joints around the 5 watt resistors and the Zeners? You can't check them unless the board is pulled. Quite possibly just a bad connection. It is the most common problem here. :idea:

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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Fri Dec 16, 2011 4:12 pm
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I've pulled the board out and checked for continuity and loose connections twice, once when I replaced the ceramic resistors (mentioned earlier in the thread) and once after that. I don't think it's a connection issue.

I suppose I will try replacing the zeners now. If that is the case, I think I see a likely chain of events. When the original ceramic resistors failed (dropped to < 300 ohms) they likely cooked the zeners. There's a bit of heat damage on the board around them that might indicate that.


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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Fri Dec 16, 2011 5:02 pm
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:?

The scenario is highly unlikely, and the zener's are likely fine. The zener's can be checked, they usually short, and I doubt if they are the problem. If you already replaced the resistors once, how is replacing them again going to help? And I thought you said that the resistance of one was 430 ohms, now it's under 300? Highly unlikely that the zener's were "cooked", you should actually check them.

Good luck randomly replacing good components. I am certain that you have a faulty connection that you've missed, or created if you tried to bridge bad connections with solder.

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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Fri Dec 16, 2011 7:08 pm
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My mistake, I meant to write 500 ohm.
Looking back on my notes, the failure value was 370 ohm, the original resistors being 450.
When I said replace, I meant return them to their original value of 450, though I doubt that would be relevant to the problem.

What is the best way to check a zener? The information I have is contradicting.


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Post subject: Re: Hot Rod Deluxe - Channel switch inoperable
Posted: Sat Dec 17, 2011 1:25 am
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Check if shorted, just like a diode, which is what they are. The original values for the resistors is 470 ohms.

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