Hi guys!

Having found this electronic model of speaker impedance, I set about doing some simulation.

Here is a screenshot comparing two current feedback situations. Both achieve almost exactly the same voltage transfer curve against identical speaker models. But one is some 7 decibels louder, due to using a smaller current sensing resistor. Using a smaller resistor in the CF line and larger DC blocking cap makes the curves nearly the same.

Indisputably, less is not more: more is more! Except maybe in current sensing resistors.

LTSpice .asc file is incluced; strip the .txt suffix. (Maybe the BBS should allow .asc files).

The speaker impedance page I linked to is guitar oriented: the first set of parameters given are for a "typical guitar speaker", so I used exactly those values.

Keeping things bone simple, I didn't model amp details like the output inductor, and Zobel network.

The CF topology and choices of feedback resistors, as well as the way input is is patterned after a power amp I use; adjust to taste.

I have never seen this amp in person so I was going by the schematic and photos I have seen on the net, all the photos I have seen has 2 sockets next to the switch on the right front of the amp. The outlet nearest the switch is an external speaker jack and the second one on the right side is the headphone jack, I cant go any further unless I see some photos of the front and inside the amp of the PCB near the headphone jack.
The good new is that if the headphone outlet works then the amp should be ok but you most likely have a dirty contact or a bad solder joint somewhere near the headphone jack, just my opinion of course but that is where I would start. It could be the speaker being faulty as well I guess so if you have another speaker you can try that would be good.
I'm just a retired electrician who loves to build and repair amps and pedals and stuff so hopefully one of the many great technicians who are on this forum will be more familiar with this amp and will jump in with some more experienced answers.
Cheers
Mick

Quote from: Den. on April 28, 2026, 07:05:02 PMThanks for the service manual, this will be very helpful.  Den

And thank you, Den, for the detailed photos and testing information - this makes it a WHOLE lot easier to known what you are looking for.

Good luck in the repair!

I plugged a pair of headphones and can hear my guitar. It doesn't have an external speaker jack.
I sprayed some contact cleaner and pushed the plug in & out to clean them but still no sound coming out of the speaker.

This might help explain The VA relation in AC circuits
https://www.youtube.com/watch?v=15dskimsfyM.

A-V phase in the electrical grid can cause problems,, otherwise they waste power.
Phil.

Try it with some headphones, if headphones work then it could be dirty contacts in the headphone jack or the external speaker jack, when inserting a plug in the headphone jack or the external speaker jack the main speaker gets disconnected.
Try running a plug in /out of those jacks to see if the main speaker comes back to life, if it does it is dirt in the other jacks.

You cannot view this attachment.

Hello there, new to the forum and thanks for the add.

I've got Crate GX-20M that has power but no output sound. I was playing on it and the sound was becoming intermittent, initially I thought it was my wireless, plugged in a guitar cable and same thing.

Threw it on a table for inspection and noticed the positive lead going to the speaker was dangling. Plugged it back it thinking it was the fix but unfortunately not. And now there's absolutely no sound at all.

I disassembled and inspected for any cold solder I can see but nothing, looked for any bulge or swollen or burnt capacitors again nothing. I checked the speaker with a 9v and it works.

I don't know what else to check. Any advice would be greatly appreciated. TIA.

Quote from: Tassieviking on April 28, 2026, 07:29:01 AMU115 Volt Regulator -15V L7915CD2T  D2PAK  ST
U116 Volt Regulator +15V L7815ACD2T D2PAK  ST

Service Manual:
https://studylib.net/doc/27942375/l1-model-ii-service-manual?p=32

Thanks for the service manual, this will be very helpful.  Den

U115 Volt Regulator -15V L7915CD2T  D2PAK  ST
U116 Volt Regulator +15V L7815ACD2T D2PAK  ST

Service Manual:
https://studylib.net/doc/27942375/l1-model-ii-service-manual?p=32

I've got a G120H with no output, and have been searching for a schematic.  I was about to give up when I stumbled on to your post.  Can't thank you enough for sharing it!   RZ