Crate Flexwave 120H volume problem

[b-slayer]

Hi,
I have found this forum and maybe someone can help me to solve a volume problem on my Crate Flexwave 120 head :

first, how it failed : I was pushing the volume (too high of course), and suddenly, it switched itself off and blew the power fuse...
so first, I replaced the fuse and it stayed on (light on the power switch, and integrated tuner), but I only had had a big buzz on the cabinet, absolutely no guitar sound at all.
so, I decided to search for the schematics, whitch I found pretty easily, and asked an IA, (don't laugh, it's always better than no help at all), to help me decode the schematics and guide me to test and replace what was blowed by the same references, and, finally, I got rid of the buzz and now,I've got a really nice guitar sound.

BUT

the volume is way lower than before, maybe half the volume, the IA tells me that there is a Bias problem but I don't understand where to search and here I am on this forum.

at this time I have replaced :
the Transistors :
- 2 TIP147 (Q15/Q23)
- 2 TIP142 (Q5/Q11)
- 1 2N5088 (Q9)
- 4 2N5087 (Q14/Q18/Q20)
- 1 MPS-A06 (Q12)
- 1 MJE-340 (Q19)

the "power resistors"
- 4 47/5W (R23/R33/R83/R85)
- 1 (R104)
- 1 (R96)
- 1 (R95)

the "classic resistor"
- 1 (R63) that was "brownish"

the "Filter capacitors"
- (C67) and (C70)

the "Diod Bridge"
- (D35)

I tested all the Diods with one leg desoldered from the PCB and all are OK, I cleaned all solders and tested if any contact was OK and with no contact with any other components or nearby solder...



the only thing i can see is this : on the power resistors when I mesure the voltage :
- on the R23 I have 2.5mv
- on the R33 I have 2.5mv
which seems to be OK, BUT I have :
- 3.7mv on the R85
- 1.4mv on the R83
and it seems that the problem is here...

so, please tell me that someone understand this schematic and maybe help me to solve this problem

regards

David

[J M Fahey]

Sorry, your schematic is unreadable small, please post a direct link to the original schematic on the Net.
That said, I guess you replaced 4 original 0.47 (zero dot 47) 5W emitter resistors with 47 (forty seven) ohm ones, which turns this beast into a headphone amp.

[b-slayer]

Hello

sorry, effectively, it was 0.47/5w resistors and not 47/5w

and here is a link to the schematics in PDF format :

Crate Flexwave Schematic in PDF

[g1]

I have a feeling your AI assistant has read too many guitar forums, where no matter what the problem, the bias is suspected. 
But go ahead and do the bias adjustment anyway, see the note in the upper left corner of the power amp schematic.  Trim pot AP1 is the adjuster.

[J M Fahey]

Bias is important in TUBE amps, it changes amp sound and sustain or "choppyness" a lot.
In SS amps it does not, just set it high enough to beat crossover distortion and not overheat it and LEAVE IT THERE. It is NOT a "sound" parameter, period.

[Kaz Kylheku]

You've done a lot of work and replaced a substantial part of the payload silicon in the amp, front to back: differential stage input transistors, output devices as well as the transistors in the foldback current limiter.

Testing diodes with a leg desoldered from the PCB: what does it mean? If you don't actually pull that leg away out of the PCB, there is a good chance it's still making electrical contact.

I think it's time to have a conversation about your speakers. What is that head going into, a 2x12 or 4x12? Before opening and testing speakers individually, are you getting that nice sound out of all the speakers? What is the resistance of the cabinet reading? Does it pass the battery test: if you briefly connect the speaker cab to a 9V battery briefly, do all the speaker cones pop in the same direction? (Never connect speakers to DC for more than a moment.)

Solid state amps don't mind volume. What they don't like is a shorted output.

Speakers can fail in a way that they first create a short circuit (voice coil enamel degrades from heat, creating shorts between the windings). Then during the failure episode, the coil burns through, so the speaker is found open. While some amplifier parts have failed also.

If, say, a 2x12 cabinet has one speaker that is disconnected or blown, the undriven cone acts as a passive radiator. But it's not in phase: when the working speaker moves out, the nonworking one is sucked in. That causes cancellation, particularly for bass. That's in addition to (or in subtraction to? ) the one speaker being 3dB less loud than two.

[b-slayer]

Trim pot AP1 is the adjuster.

I haven't touched it because it told me that it was the pot "that calibrates the reference level from the preamp and the power amp", and it was "calibrated from factory" so I listened and Kept my hands away from it

Testing diodes with a leg desoldered from the PCB: what does it mean? If you don't actually pull that leg away out of the PCB, there is a good chance it's still making electrical contact.

I think it's time to have a conversation about your speakers. What is that head going into, a 2x12 or 4x12? Before opening and testing speakers individually, are you getting that nice sound out of all the speakers? What is the resistance of the cabinet reading? Does it pass the battery test: if you briefly connect the speaker cab to a 9V battery briefly, do all the speaker cones pop in the same direction? (Never connect speakers to DC for more than a moment.)

some explanations :
I tested all the diods with one leg desoldered AND away from the PCB, sorry, I didn't say it from the original post.

for the speakers, I burned the amp with another 4 x 12 cab than the one that originally came with the amp, (and I no longer have this cabinet), actually, it is connected to its original Crate 4 x 12 cab that I cross tested tested on another Head, and it's working perfectly, the Crate head have a similar issue on a Peavey 4 x 12 cab that i have.

PS, english is not my primary language, so please, excuse me if i'm not clear enough and don't mind asking questions if so