Trouble finding diode in Fender "The Twin"

[mikeydee77]

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

[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).

[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]

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...

[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]

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]

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]

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!

[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]

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!

[rascalrick]

I am having trouble understanding how channel switching is supposed to work when plugged into only
channel one. I can't see the path to ever turn off LDR2 & 3 which I presume is needed to kill the signal
going through channel 2. I can see how pulling the volume switch on channel 2 or the foot pedal causing
pins 1 & 2 on the LM339 to connect to ground would turn on LDR1 effectively killing the signal passing
through channel 1. I also don't see the path to kill the channel 1 LED either, when switching to channel 2.
I also don't understand the purpose of D109 connecting to pins 9 & 11 of the LM339. Can anyone enlighten
me?

[g1]

It's probably the most convoluted switching scheme Fender has ever come up with.
If it's not your amp, I would suggest first going through the owners manual to understand the normal/parallel setup of the channels. From there you will have to check out all the switch segments of the jacks not in use, as they are part of the switching scheme and those 9pin Fender jacks are common sources of switching issues.

[rascalrick]

I totally agree about how convoluted their switching is in this model. I have done some measuring of the input jacks, and may
actually end up pulling them from the board and testing them out of the amp. This amp is definitely testing me!

[J M Fahey]

Thisis acually a BRILLIANT design.

Any 4 button pedal needs *five* wires at least: ground + 4 "control" ones so a special multi wire cable, plus a 5 contact connector, that´s why 5 pin DIN sockets are popular.

OR you need a sophisticated microprocessor based system at each end, to code>send>receive>decode control commands.

GENIUS Fender achieves that using single conductor plain Guitar cable, available anywhere, what´s not to like?

Of course some complexity is needed, but it´s nothing compared to other options.

To boot, shielded Guitar cables are used, simple "because they are there" but I am quite certain plain unshoelded twisted wore or even zip cord will work just as well.

NO audio signal there, and voltage level is low impedance *30VAC*

Pure GENIUS.

PS: PLEASE don´t disassemble the whole amp just to test all parts "outside" one by one, this only leads to amp destruction (PCB destruction counts as such).

That is not a valid repair system, problem may very well NOT be a "bad part" at all and it´s an admission of "I have no clue" or "I have nothing to do here".

Send it to a proper Tech.

Whatever he charges will be way less than the replacement cost of a "The Twin" amp, even if second hand.

[rascalrick]

Sending the amp to a proper tech isn't my call, the owner is trying to get it done on a budget. I am not unskilled and have been
working with electronics my entire life, so please don't assume I don't know what I am doing. I have repaired amps for many
respected musicians in my area, and built several as well. This amp just is giving me more trouble than usual.