SVT-CL Indicator Lamp turning Orange?

[Robert Technology]

Hello!

I just got done repairing two SVT-CL amps and the customer said that last night the front panel indicator began to turn orange after playing it for about an hour. I can't seem to find any information on this happening to anyone else (or in the manual) so am curious if anyone else has ever seen this issue. Especially since I'm now curious how a Red/Green LED could even turn orange... I'm referring to the front panel LED that typically is red or green (or alternating between the two colors when in "fault" mode).

For the unit in question, the only thing that was changed was replacing the 10 ohm, 2W resistors for the fault sensing circuit with 1%, wirewound resistors that were more closely matched. The existing resistors had drifted outside of 1% of 10 ohms, causing the amp to go into "fault" mode even with a matched sextet of power tubes.

All other service bulletin updates had already been done on this amp: removing the clamping diodes on the power tubes, updating the screen resistors to 220 ohms, clamping diodes and capacitor added to fault sensing circuit.

Anyone else seen this?

*Edited to add that the amp worked fine for about a month of regular practices/shows after the repair.

[Jazz P Bass]

" I'm now curious how a Red/Green LED could even turn orange... "

Just a thought.

Our eyes perceive Red /Green as the color Yellow.

What if both LEDs came on at once (Yellow) but the Red led was more intense?

Would that not make Orange?

[Robert Technology]

AMPEG SVT_CL.pdf

I was wondering about that as well, but I actually just fixed an intermittent front panel LED on the customers OTHER SVT-CL and recall that it is a two conductor, bi-color, LED.

The LED in question is D1 on page 2 of the attached document.

I may be wrong but my understanding of this type of LED is that it will change colors based on whether it is forward, or reverse biased. The note on the schematic (page 2) showing the cathode of the LED connected to ground and the anode connected to +14 volts in "operate" mode and -14 volts in "standby" mode has me wondering if it is possible to have both colors occur simultaneously. Would this simply indicate that the LED is failing, rather than pointing to some other issue?

I know there are a handful of people here that have serviced these amps much more regularly than I have, so if no one else has seen this I'll probably just get it back on the bench and report back with what I find.

[nevetslab]

When you were seeing this Orange colored LED, was that occurring while the amp was out of Standby, and working? Or in Standby, when it's RED. I didn't see anything stating the 'Orange' light was flashing, indicating a fault condition. I've never seen this one. Now, those LED's are a rugged packaged part with small Fast-on terminals (or attached insulated wires), not the bare-leaded back-to-back RED/GRN LED meant for PCB.

I looked at the SVT-Cl schematic again this morning, just to be sure it was a 2-leaded part and not a 3-leaded part. Since it is a 2-leaded part, there's no way to get both LED cells in teh package to fire at the same time. Now, the driving circuit could oscilate, ...it's a comparator circuit that just has two states...14V and -14V on it's output. I've never seen that circuit oscilate before, but that would no doubt give you an odd color that our eyes would integrate as one color.

[Robert Technology]

Thanks for the response!

This was after the customer had used it for a set that was about 30 minutes at stage volume. He reported that the volume and tone were not affected but that the front LED had turned orange. It had not gone into fault mode, and was out of Standby and otherwise working properly. No flashing. He has since used it for a practice and the issue did not repeat itself. I haven't had a chance to get the amp back from the guy for further testing.

Thanks for the idea on the comparator circuit oscillating to create the strange color! I hadn't thought about that, although it does seem like an unlikely fault. Maybe it could occur if the amp was on the verge of FAULT mode, but hadn't yet latched in that state?

[Tone Meister]

Thanks for the response!

This was after the customer had used it for a set that was about 30 minutes at stage volume. He reported that the volume and tone were not affected but that the front LED had turned orange. It had not gone into fault mode, and was out of Standby and otherwise working properly. No flashing. He has since used it for a practice and the issue did not repeat itself. I haven't had a chance to get the amp back from the guy for further testing.

Thanks for the idea on the comparator circuit oscillating to create the strange color! I hadn't thought about that, although it does seem like an unlikely fault. Maybe it could occur if the amp was on the verge of FAULT mode, but hadn't yet latched in that state?

Just an odd, non-technical thought here. But under (presumably) intense stage lighting, could some of those stage lights have washed out the LED's own color by mixing a stronger light with either the RED or GREEN to make up the orange he perceived?

Just a thought from a stage perspective.