Switching problem in Fender Twin red knobs (the Twin)

[Pedro Vecino]

I have a problem in the switching circuit of a Fender The twin:

With the IC that brought (Motorola LM339N, I dīont know if it works well or not):
- manual channel selection works OK. Status led alternates in each channel and both channels sounds OK.
- Reverb does not work.

With original footswitch connected (ledīs light up when activated):
- Channel selection does not work through it.
- Reverb does not work.

I tried another new LM339N. National Semiconductor brand (unknown origin. I know that also distributes Aliexpress) with these results:
- manual channel selection does not works OK.
Selecting Channel 1, led lights OK, but at the same time led of channel 2 ligths dimmed. Channel 1 sounds with very little volume.
Selecting channel 2, led lights full and sounds OK
- Reverb works OK

With the original footswitch connected (Ledīs light up when activated):
- Channel selection does not work through it.
- Reverb works OK and also on/off through it.

The circuit is revised (resistors values, diodes, etc.) and also the footswitch. The IC has a good socket.
Only one component of this circuit was not original and the surface of the pcb under him has blackened. R306. Value is correct (1K2 / 4 watts). Is the first resistor after rectifier diode (next to the transformer).
I can not find explanation for all this and some ideas or guidance is appreciated. Thanks in advance,

Here a picture of ICs:


Here the amp schematic:
www.pedrovecino.com/thetwin.pdf

[Enzo]

Don't try new parts, try finding what is bad first.

LM339 is just a quad voltage comparator, similar to op amp.

Look at your schematic, there are voltage readings on it at the output pins of the sections, how do yours compare? Look lower right just under the power transformer, see the box marked D-readings? That refers to test point D just above the IC.

If the voltages happen but still no reverb, then LDR101 might be bad or not be triggered for some reason. Find the reverb control and then from its wiper R113. At the end of R113 away from the control, measure resistance to ground with the amp running. Does it measure some low figure? And does the reverb footswitch make that resistance change?

[Pedro Vecino]

Thank you, Enzo. Reverb optocoupler works with the new IC. The other optocouplers (channel change), also work manually with the original IC.
The starting point, puzzling, is the different effect of each IC, which leads me to believe that the original (Motorola) may be operating incomplete (does not activate never the reverb, but it changes channels manually) and the second is probably not original or another type (Itīs difficult to understand that the first active channels perfectly, at least manually, and the second not allow the first channel sound with full volume).

EDIT: Second IC is LM339N / LM339NOPB . Apparently he has different characteristics.
I have commissioned new Texas Instruments standard ICīs to test with them and to have an accurate reference.
To be continued..

[Enzo]

Look at the schematic, the channel change switch is after the IC, so unless the output pins are stuck in one condition, the pull switch ought to work.

LM339 is LM339, the brand should make no difference, this is not some high strung circuit, it is just a basic comparator.

"OPB" on the end means zero lead, as in RoHS compliant. The inner circuits are the same. If two ICs work different, it is likely one of them is bad.

[Pedro Vecino]

Thanks again, Enzo. I finally received others LM339N and operation is identical to the new LM339NOPB. I read some different characteristics in Farnell information but probably for other applications, not in this. The original that comes with the amp is definitely defective.
Thus, the switching of the reverb works well, but not the channel switching. Either manually or through the footswitch.
Leds associated to each channel have a strange behavior. With manual selecting, as follows:



Ch1+ Ch2 mode works good, with boths channels sounding fine.

Voltages at D point are as follows. Iīm not able to deduct any of it. The amp is selected to 240 VAC and network has 230 VAC.



These are the other voltages, and I think that is all.



I checked the jacks (internal contacts) following a new one as reference and work well. Diodes marked on the diode test 0.550 in one direction and infinite in the other, so I assume they are good.

[Pedro Vecino]

After leave it three days and return with him I found a short circuit between pins 4 and 5 produced by something like a strand of desoldering mesh or something similar between IC socket pins. Barely visible by shining a flashlight on the rear side of the pcb.
The advantage of this is that I already know the ideal formula to sabotage integrated circuits and that goes unnoticed.
Thanks to all.

[jalexquijano]

After leave it three days and return with him I found a short circuit between pins 4 and 5 produced by something like a strand of desoldering mesh or something similar between IC socket pins. Barely visible by shining a flashlight on the rear side of the pcb.
The advantage of this is that I already know the ideal formula to sabotage integrated circuits and that goes unnoticed.
Thanks to all.

Could a damage LM339N (Quad Voltage Comparator) cause signal bleed at volume 0 on either of both channels?