Hot rod deluxe RevA channel switching

[Enzo]

The two large resistors that get too hot are there to make the 15v supplies. If you have +15v and -15v, or something close to them, then those resistors are working.

The switching circuit is complex and does a number of functions, so I am sure there are a variety of repair videos and other material out there. But the likelihood that your amp will have the exact same flaw is tiny. A good video will lead you through a step by step systematic troubleshooting approach, which is what we are trying to do here.

[Oldmactech]

Thanks. I’m getting +/- 17v and a bit more at R78 and R79 resistors. Would that be within tolerance or a symptom?

[DrGonz78]

Probably way off here but did you try cleaning the foot switch jack? Not sure it would make a difference but it only takes seconds to try.

[Oldmactech]

Not yet, should it be removed or can I put deoxit or something in it?

[DrGonz78]

Not yet, should it be removed or can I put deoxit or something in it?

Oh yeah just some deoxit and then insert a plug in & out a few times. First you could check continuity from R97 to CR 21. Check it for zero ohms and not just continuity setting (The beep) on the meter.

[g1]

The talk about the differences between the Deluxe and Deville was kind of about you taking your voltage readings in standby mode.
There is no major amp fault to worry about, so might as well be doing those checks in 'play' mode as that is how they are specified on the schematic.

[nevetslab]

One place I've made a mistake is misreading NJM4580 Dual Op Amps for NJM4560. The circuit requires the higher current drive capability of the NJM4560 to work correctly. Other's have mixed up U2 with U3, where U2 is a TL072. It also will NOT work for U3 in the Channel Switching circuit. Just a thought.

Regarding the Footswitch Jack, I assume you've already checked that jack for solder joint fractures. All of the jacks on that Panel, as well as the Pot solder terminals get fractured solder joints. If the switching contact on the Footswitch jack is intermittent, you could jumper the junction of R97/CR21 anode/CR25 Cathode/CR23 anode, which is what the switching contact is doing. That would, at least, answer that question about the jack being involved in the problem.

[Oldmactech]

Thanks. All the op amps U1-3 are new, almost did what you suspect but u1 and u2 are TL072 and U3 is NJM 4560. But I will look again with a magnifier for sure. I have touched up all of the pots and jacks. I swapped an audio taper pot for the Norm volume at that time. Normal still works as advertised, and maybe I’m just going to live with it. I will perform the check others above have suggested and clean the pedal jack. When my friend gets back from the beach I’ll borrow his pedal to further test.

thanks all.

[Oldmactech]

I took some measurement on the tube board this morning and it seems very odd.

tp 23 expected 744mv Read -.06mv
Tp 24 expected 11.5 v Read 257v
Tp25 expected 11.5v Read 241v
tp 26 expected 37v Read 35.1

Does this mean anything?

[glebert]

I took some measurement on the tube board this morning and it seems very odd.

tp 23 expected 744mv Read -.06mv
Tp 24 expected 11.5 v Read 257v
Tp25 expected 11.5v Read 241v
tp 26 expected 37v Read 35.1

Does this mean anything?

I think it means something is wrong with your amp, or your meter. What condition is this in? Standby or Play? Normal, Drive, More Drive?

In looking closer, I'm guessing you are confusing AC volts (RMS) and DC volts. In the schematic AC volts are in ovals, DC volts are in rectangles. The AC voltages are if you have a test signal input that is spelled out in the schematic and controls set accordingly.

[nevetslab]

Also, the DC Voltages published are 'nominal', read at the factory at 120VAC/60Hz AC mains @ idle (no signal applied). The voltage difference you see at TP26 is in the ballpark, and may be due to the line voltage at your location, perhaps being lower than 120VAC, which would produce lower voltage at that test point. It's only off by 5%. Many of us have metered Variacs for setting Line Voltage as needed, including slow-starting an amplifier having known defects, and being able to shut down immediately if hi current is being drawn.

[Oldmactech]

I’m not near the schematic but I’d bet your AC vs DC reading mis-read is likely what has happened.
oops.

[Oldmactech]

Advice reading schematic?

The op-amps U1-U3 and the Switches K1,K2 have numbers beside various points next to them on the schematic. Are those PIN numbers or voltages or something else?

[g1]

If it says TP it is a test point. If the numbers are in a rectangular box they are DC volts. Numbers in an oval are AC volts with signal as specified in the notes.

[Enzo]

Yes, the pin numbers are printed next to the pins on the schematic.