Laney LC50 MK2 Low Normal channel

[blindboybenton]

Hi Folks

been asked to have a look at a Laney LC50 MK2 with a low normal channel

The Dirty channel is ok very loud but the normal channel wherever you are in the low or high inputs is low. Possible about 1/4 of what it should be
If you have it up full its got a overdriven sound
Its been though 2 other repairers in the area and customer gave up on it. til now.

One of the preamp tubes has been repalced V2.
The output valves are 6L6GC ( 38-45ma biased ) not matched up. using a matched set of EL34 ( 30ma ) at present

I dont have the schematic for this amp and the only one i can find is for the LC50 not mk version. Im alittle lost without one!!

Im a bit of a newbie to valve work just all day computer mainboard repair yawn yawn

so without the schematic this is what i got

V1 anode 195v and 189v Cathode 1.5v and 1.5v
V2 anode 277v and 222v Cathode 2.8v and 1.8v
V3 anode 330v and 185v Cathode and 1.1v ( is the 189v the problem?)
heaters are 6.8v ac
Check the pcb for djs and all valve bases resoldered and all pre amp valve been swapped for known good ones all are ECC83

any ideas

BBB

[Jazz P Bass]

Have you tried contacting Laney?
They should be able to supply a schematic.

[blindboybenton]

Thanks ive sent an email and i aint had a reply yet. Ive heard though another forum. that laney take the view of You aint a tech. you aint having one. The email i used was my business one.

[mozwell]

try this

[blindboybenton]

Many thanks.. Now i got to try and fix it.. have you have experience on theses amps Mozwell? I spent some time on it last night without the schematic but not seem to be getting anywhere

Blindboybenton

[mozwell]

Hi Blind Boy
No, i havent had experience. I am sure Enzo, Jazz & others here will be able to help you out.
The rule of fault finding is to start at one end & work through, measuring as you go. When you get a signal you dont expect, then thats where the problem is.
Can you inject a signal ? say a 1kHz sine wave. The schematic is nice, as it gives AC and DC expected voltages throughout.
Pull the power tubes, so we can turn the preamp gain way up without sound coming from the speakers

So with an input signal, do we get 220mVAC on hte anode of V1a with an input signal of 22mV into the LO input. I woudl expect this stage is ok, as it is shared between the two channels.
Turn all clean tone controls & clean volume up full.
Do we get 3VAC on the anode of V1b with the same input signal ?
Measure the SEND output, you should get a reduced signal there, approx 1.2VAC

Do the same thing for channel 2. the drive channel, in the middle the schemtic tells you where to set the controls for the testing.

Hopefully the other guys here can help you further over the weekend.....

[blindboybenton]

Hi Moswell

once i had a quick read of the schematic this morning, i did think that its time to dust off the signal gen and scope..
thanks for the info..

Blindboy benton

[g1]

Only V1 is used for the clean channel, your voltages seem ok. All voltages seem high but that is probably due to high line voltage (240 to 250VAC ?), as the heater voltage is up at 6.8V.
Not much to the clean channel, V1, treble pot, relay RL1-B, volume pot, relay RL2. Or a bad connection in that path somewhere.

[blindboybenton]

Hi folks

Thank you all for the help. It turned out to be V4. I thought i had swapped it out at some point.. But i didnt ;-( . Used the signal gen and scope and found clean channel to be ok. I knew the dirty channel to be good. so i turned my attention to V4. Wasnt 100% sure that what i was looking at was correct just didnt look right.. as im a newbie to the world of tubes.. I swapped out V4 and up it came. Just pluged in the guitar and as loud as the dirty channel... Case closed

Blindboybenton

[g1]

Glad you got it working, but a bad V4 should have also affected the dirty channel. Has the dirty channel volume level or distortion character changed? Maybe it was weaker but less noticeable than the clean channel weakness?

[blindboybenton]

Hi

Well what happened was my old Gould signal gen started to play up and i was getting alittle bored at this point. So i unpluged everything a put it back to guitar amp and cab and refitted the output tubes.
when i turned it on the clean channel was loud and proud. I had changed v4 for a test one during the testing. so i swapped it back and normal channel was low again.Ive since found out that its possibly a tube base or solder joint on the base. As it did it again when swapping out tubes... I only play 4 strings and at present dont have a 6 string. Im going to run it for abit. I got to learn some songs for a bit of bass depping tonight. It will save my own valves :-) tahnks again for the help. I do find this site very helpful and spend my lunchbreak at work reading the posts, before going back to replacing fets/ic's/caps and BGA chips on computer motherboards yawn yawn

Blind boy benton otherwise known a John

[Jazz P Bass]

I would have to recommend reflowing the solder on the tube sockets.

[blindboybenton]

well i checked them .. they are all fine resoldered them anyway. it turns out to be a dirty v4 base... cleaned it all up and all seems fine.. Ive got a fender usa Jazz bass with S1 switching and a fender P bass MiM. Would love to have a deluxe.. Jazz neck with Jazz and P pickups.. I love my p bass use it for 70% of the gigs

BBB

[Mr_Void]

try this

Hi all, sorry to bring up an old-ish thread but this attachment doesn't open for me... what am I doing wrong!??

[DrGonz78]

I think this is the right one that he had posted before...? The site had a glitch that is affecting the ability to download many files.

LC50.pdf - LANEY - amplifiers head combo drawings | GuitarElectric.eu

Also, would have just uploaded but having trouble attaching files today. Yup just tried again to upload and not working still.