Anyone here ever worked on an Epiphone Firefly 30 DSP ?
I have one in for repair. The problem is hum.
It's caused by an imbalance of the idle current at the output transistors.
Q34 base to emitter -0.38 volts
Q36 base to emitter +0.52 volts
I have checked every component and replaced all the transistors to no avail.
I noted the schematic shows the voltage rails as +42V and -42V whereas the rails on this read +30V and -30V. The transformer is 22V-0-22V.
I see a schematic for an Epiphone Triggerman 100 DSP on this forum.
It is the exact same schematic as the Firefly 30 DSP.
I can only assume Gibson used the same board and used different transformers for different output power.
I powered up this board with a variable transformer and brought the rails up to +42V and -42V. The outputs became more balanced and the hum disappeared.
Also I can balance the outputs and get rid of the hum if I change R323 from 330R to 470R. I was wondering why I would have to do that and if I leave that change would it be stressing any of the components.
I would appreciate any thoughts or ideas on this.
I don't know how to upload the schematic here but it is exactly the same as the Triggerman 100 DSP here on forum except the name.
Quote from: DrLev on May 26, 2026, 05:38:14 AMAlso I can balance the outputs and get rid of the hum if I change R323 from 330R to 470R. I was wondering why I would have to do that and if I leave that change would it be stressing any of the components.
You tell us where you came up with the idea to change that resistor. :)
Likely what is happening is that you are compensating for a damaged or out-of-spec component, which is not how you want to do the repair.
2 things happen when you adjust R323, you alter the balance of the differential pair (Q312,Q313), and you change the bias of Q39, which affects the conduction of the output devices.
You did not mention whether there was any DC on the output, or the voltages of the drivers (Q310,Q315).
Differential circuits can be very picky about out of spec parts. In some modern circuits, replacing the differential pair with new parts from the same batch will cure issues like this.
But it may also be a fault somewhere else in the circuit.
In any case, modifying component values is not a good fix, if it was a poor design, all amps of this model would have complaints about the issue.
Thank you for replying. I will do some voltage readings tomorrow.
I did some voltage checks today.
20 millivolts at speaker output with or without speaker connected.
Q315 voltages referenced to ground
With speaker connected
Base -0.915 volts
Collector -30.4 volts
Emitter -0.377 volts
With speaker disconnected
Base -0.974 volts
Collector -30.4 volts
Emitter -0.435
Q310 voltages referenced to ground
With speaker connected
Base +1.07 volts
Collector +30.4 volts
Emitter +0.535 volts
With speaker disconnected
Base +1.00 volts
Collector +30.4 volts
Emitter +0.467 volts
I becomes more balanced when the speaker is disconnected.
Would that indicate the problem is the output transistors are out of balance even though I did change them?
I would appreciate any help.
Thanks.
Measure voltage across R320, with and without speaker. It will be millivolts DC, so use whatever range is most accurate.
Then do same for R321.
Thank you for getting back to me.
R320
With speaker
1.5 millivolts
Without speaker
0.0 millivolts
R321
With speaker
0.0 millivolts
Without speaker
0.0 millivolts
Try removing or disconnecting one end of R35 (10K), then repeat those last measurements.
Thanks again.
R35 disconnected.
R320
With speaker 1.2 millivolts
Without speaker 0.0 millivolts
R321
With speaker 0.0 millivolts
Without speaker 0.0 millivolts
It seems like Q34 is not conducting at all, and Q36 just barely.
edit: I think I went the wrong way with R35, to increase idle current of the outputs, you need to reduce R35.
If you have a 10K pot you could wire it up as a rheostat and start with a 10K setting, then slowly reduce til you see a few millivolts across each of R320 and R321. From there once you have a good setting, you can measure the pot resistance and install a resistor of that value for R35.
If you don't have a pot to use, you can do trial and error with values less than 10K for R35, or add a parallel resistor if that is easier than replacing it.
The way it is set up, R35 and R318 in parallel are making about 900 ohms. You want to slowly decrease that to get the outputs conducting.
Thanks again for your reply.
Here are the results.
Combined resistance of R35 and R318 decreased to 790ohms
Q34 base to emitter -0.572 volts
3.0 millivolts across R321
Q36 base to emitter +0.566 volts
4.6 millivolts across R320
The hum is still there.
Might be a little louder.
If I reduce the resistance lower than 790 ohms the hum gets a lot louder.
As I sweep the resistance,using a pot,from 900 ohms down to 790 ohms the hum does not reduce at any point.
I had another SA1695 so I changed it.
All the voltages stayed the same.
I don't have another SC4468.
I had already replaced both of these before.
I'm just wondering if they are all unbalanced.
If the hum is increasing as the outputs turn on, I suspect it is ripple on the power supplies rather than DC imbalance.
With speaker connected, measure both AC and DC volts at speaker with stock bias setting, then with bias setting that gives loud hum.
Also measure AC volts at + and - 42V supply rails under low and high hum conditions.
Thank you for your help.
Yes, you are right.
The more turned on the outputs are,the more ac is at the speaker.
The dc stays much the same.
There is 120 millivolts ac on the +42 rail. No ac on the -42 rail.
When I first looked at this amp I suspected the smoothing capacitors.
They were reading a little on the low side (4400uf) so thought they should be ok. I checked the bridge rectifier and found no problems.
Then I moved on to looking at the imbalance at the outputs.
Now I have replaced the capacitors and the bridge rectifier but still have 120 millivolts ac on the +42 rail.
I even doubled the capacitance of C320. Although it lowered the hum ever so slightly I still have 120 millivolts ac on the +42 rail.
I discovered today that my meter is giving me false readings on the power rails. If I reverse the probes I'm getting 0 volts ac on the +42 volt rail and 120 millivolts ac on the -42 volt rail.I assume the meter is being affected by the dc voltage.
So now I'm wondering where the hum is coming from.I have tried it with the preamp connected, disconnected and disconnected with the power amp input grounded.
There will always be some amount of ripple on the supply rails. Yours are measuring less than 1% of the rail voltage which is quite acceptable. You did not post measurements for the amount of AC hum across the speaker. That would be helpful to know the amount of hum we are talking about. Both with the minimum amount, and with the increased amount when bias is adjusted.
If the hum was still there with the power amp input grounded, that indicates it is coming from within the power amp somewhere. That is assuming the grounding of the input was a short run to a good ground. Sometimes our test jumpers to ground the input can pick up hum themselves.
Thanks again.
Here are the results of my tests.
Q34 B-E R321 Q36 B-E R320 Speaker voltage
-437MV 0MV +543MV 1.3MV 21MV DC 9MV AC
-592MV 3MV +573MV 4.5MV 23MV DC 12MV AC
-612MV 11MV +588MV 12.3MV 22MV DC 20MV AC
-625MV 22MV +600MV 23.1MV 25MV DC 30MV AC
I hope this makes sense to you.
Ps I have just discovered this does not display properly on my phone. It was ok on the laptop, so I have rewritten it. Please see my next post.
I will have to rewrite my last post as it doesn't display properly on a phone.
It was perfect on a laptop.
Q34 B-E -437MV
R321 0MV
Q36 B-E +543MV
R320 1.3MV
Speaker 21MV DC 9MV AC
Q34 B-E -592MV
R321 3MV
Q36 B-E +573MV
R320 4.5MV
Speaker 23MV DC 12MV AC
Q34 B-E -612MV
R321 11MV
Q36 B-E +588MV
R320 12.3MV
Speaker 22MV DC 20MV AC
Q34 B-E -625MV
R321 22MV
Q36 B-E +600MV
R320 23.1MV
Speaker 25MV DC 30MV AC
Those readings across the speaker are within what is considered the normal range for hum in a guitar amp. Are you using it in a particularly quiet environment? Do you have other amps you have compared it to?
Thank you for your reply.
This amp belongs to someone else who insists the hum came into it recently.
To me it sounds a little noisy.I have heard quieter ones.I don't know anyone with the same model to compare.
I guess I will just have to give up on this one.
Thank you for all your help.
Sorry I don't have anything more for you to try. These are the worst kind of repairs, qualitative judgements that are just borderline 'maybe' worse than normal.
I have been in your shoes before and spent way too much time only to find there is no fix within economical means. Sometimes it turns out the owner's memory is 'rose-colored', or they have changed something else in their rig that they are overlooking. Or they have something in their environment that makes it worse at their location and what you are hearing is not even what they are complaining about.
It's ready to go back to the owner now.
Thank you very much for taking the time to look into this.