Hot rod deluxe RevA channel switching

[glebert]

Since the +16V is low and changing when it shouldn't then I would move back towards R78 and the components around it. This is sometimes tricky if there are compromised solder pads or traces as the act of probing can cause broken contacts to touch. R78 traces and solder pads are most likely to be flaky, so probe CR13 instead, or whatever.

[Oldmactech]

Cr13 17.11
cr14 -17.05
r78 17.3
r79 -17.25

supposed to be +/- 16v?
17 v +/- ok?

[Oldmactech]

JFets Q1-Q4 all have good continuity.
Switch LED LD1 has continuity green and red.

[nevetslab]

Thanks. I’ll check out the JFETS and the switches.
Does the fact that the yellow drive light lights even when in standby mode mean anything?

It IS normal that the Channel Switching LED's function in Standby. The Bipolar supplies are powered up as soon as AC Power is turned on. So, you can actually make all of your Channel Switching measurements in Standby. If the YELLOW LED is LIT without being selected, then that is NOT normal. It should only light up if the Channel Switch is engaged. More Drive (RED) should only light up when MORE DRIVE is engaged.

+/- 17V is ok.

[Oldmactech]

Thanks. The yellow light is always on. Normal, Drive or More Drive, only the yellow light is lit.
After the cap replacement I replaced and repositioned the resistors r78 and r79 to get them off the board a bit due to heat. When i first used it the yellow light would come on and only the clean channel functioned. Somewhere between 20-45seconds later the yellow lit would go out and if I depressed the channel switch I’d get yellow and subsequently red if I depressed more drive. At that point both Master and Drive Pots became functional. After a few short sessions it went to only yellow all the time-no drive channel. Channel switch no longer functions but there is an almost imperceptible blink when depressed.

I have replaced the filter caps, R78 and R79 along with Zeners C13 and c14 (if I remember correctly) . Also op-amp U1, U2, U3. I have examined the board numerous times for suspect solder joints and have reflowed many.

[nevetslab]

I hate it when that happens. Hard enough just getting the main PCB extracted since the xfmr wires below the board and the pot shafts constantly tell you NO YOU CAN'T. Your clues are still going to show in the Test Point chart. I know this is tedious, but, there's obviously something amiss. I'd make sure the AC Voltage that's applied to R97 is constant and correct. Your voltages at TP31 and TP32 relate to the source voltage and diode drops in the front panel switching that influence the voltage at TP32, as well as at TP33. Cursing is allowed during the procedure.

[glebert]

I am stumped how you can get such different voltages from CR13 to R91. I would measure resistance between them just to be sure.

[Oldmactech]

Results:

39v AC and firm at R97
Cr13 17.7v
r91 1.0 with negative lead Attached to ground.
Measured between CR13 to R91 in standby mode 16.5v drops to 12.8v .
Continuity between r91 and CR13 good.

[DrGonz78]

Remeasure all these test points just to make sure the previous measurements reflect where we are at now.

31 norm 1.05
Drive .83
More Drive .83.

32 norm .53
Drive 9.80
More drive 9.80

33 norm 5.06
Drive 2.14
More drive 2.14

34 norm - 12.21
Drive -12.21
More drive -12.21

35 Norm .53
Drive .53
More drive -9.86

36 Norm -15.99
Drive -15.99
More drive -15.99

[Oldmactech]

Tp31 Norm 1.1
Drive .9
More Drive .9

Tp32 .5
9.0
9.0

TP33 5.14
2,21
2.21

TP 34 -12.25
-12.25
-12.25

TP 35 -.52
-.52
-.52

TP36 -16.02
-16.02
-16.02

readings taken in standby mode only.

[nevetslab]

I'm wondering if the 3-pin RED/GRN LED LD1 is bad. The TP voltages you're getting for TP 33 and TP34 don't make sense, and the potentials at the output of U3B is off that much, it affects the results thru Q4 to correctly set the voltage at TP34 for the TP36 readings. You mentioned having issues with that LED. For test purposes, do you have a discrete RED and GRN (or other color LED) that could be substituted for LD1 (not trying to pass them thru the square hole, of course...just to see if the TP voltages straighten out.

Also confused by the supply voltage drop from 16.5V to 12.8V in Standby.

Looking at the schematic difference between the Hot Rod Deluxe and Hot Rod DeVille, there's a difference in the GND for the Bipolar supply and the Bias Supply. I usually find the secondary wires of the power xfmr on the Hot Rod DeVille having a CenterTap, while that of the Hot Rod Deluxe, there is NO CenterTap. Can this make the voltage different between Standby and Operate?

[Oldmactech]

It’s obvious by now I’m no electronics tech so....with that in mind. The supply voltage drop I referenced is testing one leg of the resistor r91 and again on the other leg.

I don’t suspect LD1 as bad. Someone pointed out green and red make yellow so I believe both are good. Last night when switching off I did see a bit of red so that tells me the LED functions?

And I don’t have a replacement.

I’m sorry, I don’t really understand a word of your final paragraph.

[glebert]

Also confused by the supply voltage drop from 16.5V to 12.8V in Standby.

I don't know that he is saying the voltage dropped that much when going to Standby. I think he was saying that in Standby there is a 3.7V (16.5V - 12.8V) drop from the ungrounded leg of CR13 to the high side of R91. At least that is how I read it, which still makes me suspect something wrong in that connection since they should be at the same voltage. Going back and rereading now I am not sure anymore

[nevetslab]

It’s obvious by now I’m no electronics tech so....with that in mind. The supply voltage drop I referenced is testing one leg of the resistor r91 and again on the other leg.

I don’t suspect LD1 as bad. Someone pointed out green and red make yellow so I believe both are good. Last night when switching off I did see a bit of red so that tells me the LED functions?

And I don’t have a replacement.

I’m sorry, I don’t really understand a word of your final paragraph.

We all can usually back down our technical jargon to make it easier to understand, as that's the beauty of this forum.....education while making the repairs to describe what happening.

The last paragraph has to do with a difference I see between the Hot Rod Deluxe and Hot Rod DeVille amps, and I was pondering the big voltage change you see between Standby and Operate. Now, unless specifically stated, all our Voltage Measurements are referenced to 0V (Circuit Ground). On many amps, there can be potential difference between Circuit Ground and Chassis ground, if the product has a ground network. Marshall Amps have such. When making differential measurements, such as across R91, then we're talking about showing difference in current flow in two different operational states. Not finding an obvious reason for the +16V supply changing between Standby and Operate, that's why I looked at the two schematics, wondering aloud for others to read, to see if I was missing something with regards to the circuit grounding.

At any rate, there's still something amiss with the potential at TP33 & TP34. I'll continue to assist you on that, both here at my shop or at home, as I have the same database to draw from. Plus, you have all the others here to assist until we figure it out. It's taken me a considerable amount of time to troubleshoot this channel switching circuit, and odd things happen when you have lots of parts that all have to work so the channel switching works corrrectly. We'll get there, if you have the patience with us. Do beware of Fender's shabby PCB quality. I've had many traces lift off the board in servicing, even with years of experience and great production tools to work with. Cursing is allowed, though it still requires repairing trace/pad damage. I've seen circuit pads separate from the connecting trace, and they're a lurking evil that can be the cause of misbehavior.

[Oldmactech]

Can’t tell you how the lack of condescension on all ya’lls part is appreciated. I am an old retired guy trying to learn a new understanding of a subject I have not much true understanding. At least I know how to discharge the caps.

I am aware that the PCBs are suspect and have Personally lifted pads on R78 and R79 but I believe I successfully made jumpers that show continuity. I have gone over the main board many times with a magnifier and have tested for continuity “all around.”

If any are gracious enough to get me through this successfully, I would be most grateful. Supposedly my o,dies band has a gig 8/20.

John

ps.done my share of cussing at it.....


also, came a cross a Deville repair utube with switching issues, related to a faulty pedal input. Don’t suppose?