Trouble finding diode in Fender "The Twin"

[rascalrick]

I replaced the LM339 as it appeared to not be acting as a comparator anymore. I have a drawer full, so what the heck, right?
Anyhow, replacing it cured part of the problem (channel switching and reverb switching not working), as reverb switching
worked again, and would switch to channel #2, but #1 still doesn't work. Also, the issue of 44vdc showing up at D started
after switching the 339 out. I have temporarily tacked a 33v zener I had in my inventory to further testing along until the
correct part arrives. Now when channel switching, the 339 output pin voltage is correct when switched to channel 2, but is
basically the 33v D voltage when in channel 1 instead of the 2.8v shown on the schematic. I have pulled all the LDR's and
they all seem to work just fine on the bench. I did find the two series diodes used for biasing the comparator inverting inputs
shorted and replaced those which resulted in the partial functioning I mentioned. The biggest mystery still is where is the 30v
zener...and if it never was there at all, how did I have 30vdc at the D supply before switching out the LM339. This is a very
convoluted switching scheme they came up with and it makes troubleshooting kind of difficult. I will get it eventually!

[J M Fahey]

Just curious why you replaced that LM339

If it was only "for kicks" then re-place it .

And/or tack solder your fresh 30V Zener under the board, from IC101 pin 3 to ground, in the proper orientation.

It "should" bring that rail voltage down to 30V

What was the symptom which led you there?

[rascalrick]

Thanks Glen. I agree that the FS switching circuitry is weird, and pretty difficult to troubleshoot. Just trying to trace the functionality of the 4 input jacks is a chore as well. The FS circuit actually uses 30vac directly through R132 to the power transformer. I haven't scoped the D supply but will do that next. Then as you say, probably start checking the straight diodes in the circuit. Man this amp has been a real bugger...with just a simple channel switching issue!

[Mars Amp Repair]

Yes, this is the part in question and it is shown in the footswitch portion of the schematic. The problem I am having as it doesn't appear to exist on my amp. A previous poster showed his Power Amp board with the D108 right next to LDR101, but my Power Amp board does not have D108 in that position, and I have traced the entire D power supply circuit and it isn't anywhere. Since the silk screen the boards it is fairly easy to see all the components and I don't see D108 anywhere. I have ordered som 1n5258B's and they should be here in a few days. I will do as previous poster suggested and tack it across C303 and see if that works. The mystery here is that prior to replacing the LM339 I measured 30vdc and D, but after replacing the 339 it measured 44v. So where did the 33v come from if there wasn't the 30v zener in the circuit? I am losing years off my life trying to figure this out. I have build/repaired/modified many tube amps in my time, and this one is driving me crazy.

AHH, OK I get it now.
I'm wondering if there is any AC on the 'D' supply that might be giving you a false reading. Have you scoped that supply line at 'D'?
It could be that Fender may have had issues with the 'D' supply drifting with line voltage & added the Zener to compensate for over-voltage.
As an aside, there is some switching that goes on for channel switching in the input jacks. Seems nutzo, but you might be certain the switching that is going on there is actually working.
This FS circuit is weird as it uses 38Vac to (as I recall) create square waves of opposite polarities to do the switching. That may actually be where you problem is. Check the straight diodes carefully with the 'Diode' check on the meter for forward drop as well as the straight 'Ohm' setting for any leakage either forward or reverse direction for the diodes.
I have had issues in this circuit that ended up being weird shit, but I can't find any notes in my Symptom/Remedy book .
Glen

[rascalrick]

Yes, this is the part in question and it is shown in the footswitch portion of the schematic. The problem I am having as it doesn't appear to exist on my amp. A previous poster showed his Power Amp board with the D108 right next to LDR101, but my Power Amp board does not have D108 in that position, and I have traced the entire D power supply circuit and it isn't anywhere. Since the silk screen the boards it is fairly easy to see all the components and I don't see D108 anywhere. I have ordered som 1n5258B's and they should be here in a few days. I will do as previous poster suggested and tack it across C303 and see if that works. The mystery here is that prior to replacing the LM339 I measured 30vdc and D, but after replacing the 339 it measured 44v. So where did the 33v come from if there wasn't the 30v zener in the circuit? I am losing years off my life trying to figure this out. I have build/repaired/modified many tube amps in my time, and this one is driving me crazy.

[Mars Amp Repair]

Hello folks,
I must be missing something here as I see D108 listed on the schemo as a 1N5256 30V zener that is readily available at Mouser/Digkey. It's part of the footswitch circuit. See attached schemo.
Glen

[rascalrick]

Wow, ok I have carefully traced all connections on all PCB's that have the "D" supply connected to them and simply don't find the zener diode anywhere front or back of boards. I have 2 revisions of the schematic, Rev M and Rev N, which the only difference is the incorrect connection from V7B plate to the treble control in Rev M. That was apparently finally fixed in Rev N. The zener diode (D108) exists in both revisions. Is it even remotely possible that they didn't include it in some version of this amp? This of course wouldn't explain the fact that I had 30v at D supply before replacing the LM339, but now have 44v after replace the LM339. I do realize that the 339 is probably toast with that 44v, and will expect to replace it once again, but need to figure out what is happening with the D supply first. I am stumped...

[rascalrick]

"Have you checked the underside of the board... just a thought."
Ha, that was one of my thoughts exactly! I did check under 3 of the 4 boards. The one I haven't checked under is going to be difficult to do with all the attached wires, but I'll do that one today.

[rascalrick]

Ok, here is the full schematic that I have. There was no schematic in the user manual provided by the customer, so I was only able to find this online. If you know where I can find the schematic for MY amp, I'd love to know that as I have not been able to do that. I did post a picture above of my board, but will repost it here rotated so it's easier to read the component screening. I have not been able to find a screening for D108 on any of the PCB's. Since there was 30v prior to replacing the 339, I am quite certain there is the zener somewhere in the amp, but since replacing the 339 the voltage is now 44v I need to find that zener (D108).

[mikeydee77]

Have you checked the underside of the board... just a thought.

[rascalrick]

I know my plans in the shop for tomorrow! Thanks for all the good ideas! BTW...love the sign "Never think up reasons not to check something"

[J M Fahey]

Please post the schematic version related to YOUR amp.

In full, not little bits here and there.

Also a PCB closeup of the relevant area.

Are there any unpopulated PCB holes?

Or maybe it smoked or cracked and an earlier Tech removed it , without placing a new one there.

Or he put a regular diode, say 1N400x, thinking it was "oh, just a diode", which of course will not "zene".

Have seen worse choices made

[g1]

The "D" supply also feeds R1 and R2 in upper left of schematic, so have a look around there as well. If nothing else, I don't think there is any problem having 2 zeners in parallel so you could just tack one across C303. If the old one came back online, it shouldn't matter.

[rascalrick]

Yes, I suspect that the 44v has probably cooked the 339. I have done some tracing, but will do further tomorrow. It is quite the mystery, as I have visually inspected all 4 pcb's on the amp and so far can't locate it! Crazy. I know it has to be there somewhere!

[g1]

Ok, seems there was a typo on some earlier schematics with regard to the tonestack, noted here: https://music-electronics-forum.com/...002#post644002

I would think the diode is in a different location, especially if you found 30V there before. Even on the schematics that show it, it is not listed on the parts list so it may have changed locations.
Could just be a bad connection to it as zeners usually fail low resistance rather than open circuit.
Locate R306 and C303 in the supply and see if it is around there. Then follow the trace from R306 to IC101/R138/R139.

* +44V may have cooked the new chip as they are rated for 36V max.