Laney vc30 -112 sound level problem

[bunzonoise]

Hello everybody, I'm pretty new here.
I've a problem with my Laney VC30 - 112, and can't find in any troubleshooting guide the symptoms I'm getting from it.
Hope somebody can help: Power is on, all 7 tubes are normally orangy, but I can get only a faint sound(like headphones) from the amp, and only if I max the volume knobs. This happens in both clean and drive channels.

What I tried already: plugging the send/return jacks - swapping tubes - checking the power supply voltage - looking for evidence(by sight) of something burnt on the board, or broken joints on the valve tube grids.

Another thing I notice is that when you turn the standby switch on, it's not doing the low frequency sound that usually does(something like 60hz hum) when is working properly.

I didn't change any tubes recently and the amp has always been well kept and didn't see much stress.


Hope somebody can spread some light.

[J M Fahey]

1) the trivial test: check your speaker, unplug its terminals from the amp and briefly touch them with a known good 9V battery. Any clicks/pops?
2) I guess you have the schematic. Post it here (or link to it) so we all talk the same.
On the Power tubes, check the voltage on all plate pins, then on all screen pins, then touch briefly the grid pins with the red probe; you should hear pop/click/buzz on the speaker.
Post results.

[acorkos]

and if the speaker checks out ok, make sure you're plugged into the internal speaker jack and not the extension speaker jack

[bunzonoise]

hi thanks for your help so far.
Speaker responds to 9v battery.

I checked the pinouts for EL84, and if I have read that right:
pin 7 (plate): 306V on all 4 output tubes
pin 9 (screen): 304V on all output tubes
pin 2(grid): no sound touching it and 0.02-0.03 voltages

Here I found the schematics: http://www.bymcomunicacion.com/pveci.../laneyvc30.pdf


Hope to hear from you guys soon.
Cheers

[Jazz P Bass]

Please check Pin #3 of the output tubes.
The schematic indicates 9Vdc.

[bunzonoise]

it is in fact 9v in the four of them. Can you tell me why you think the wrong is in the output tubes and not in the pre amp ones? you see I'm a total noob and trying to understand a bit! thanks

[g1]

I think JPB may be focusing on the power tubes because there was no popping sound when you put your probe to the power tube grids. Also, the problem is in both channels, the only preamp tube common to both channels is the first half of V1.
Can you check the speaker cable for continuity (unless you did the battery test through the speaker cable).

[J M Fahey]

agree and add: there is no connection between those power tube plates and those speaker terminals.
Why?
2 tests:
1) with amp off , unplug 1 lead from the speaker and measure resistance from this free terminal to the other. Value?
2) with amp off and unplugged for a couple hours, first check thay you don't have any +b voltage at all, only then apply one end of a 9V battery V6 or V7 plates, the other end to V4 or V5 plates. Any click from the speaker?

[bunzonoise]

Hi guys, thanks again for your support.
J M, I carried the two tests:
- the resistance between the ends of each speaker lead was 0.6 ohms
- there were clicks connecting the battery, in every combination: V4 vs V6, V4 vs V7, V5 vs V6, V5 vs V7
g-one: I did the battery-speaker test through the speaker cable and the speaker produces very distinct pops.

What now?

Cheers

[J M Fahey]

OK.
Test 1) tells me that the transformer secondary to speaker path is not open; those 0.6 ohms (really around 0.2 ohms, discounting the multimeter cables own resistance) are a reasonable DC value for the secondary.
I was mistrusting the impedance switch or any connector in between, but it does not seem to be so. Good
Test 2) thells me that the transformer does its thing (coupling plates to speaker) so it's strange touching the tube grids does not produce clicks (even if faint) at the speaker.
Test 3): check that the phase inverter, V3a and b has all voltages as shown, and then touch with the red probe the union of R28 and C10, you *must* hear clicks at the speaker.

[bunzonoise]

Hi J M,
If I get it right, I'm measuring at p31 and p32 and i get
V3a :225V
V3b: 217V
instead of 195V as indicated in the schematics.
I see the C10, but I can't locate R28, can that be in the hidden side of the board? Anyway, touching one leg of C10 doesn't do anything, the other one produces a click and a voltage of 20V.

[J M Fahey]

Yes, P31/32 look fine, voltages can vary +/- 20%.
Your click (and 20V DC) on the union of C10/R27/P27 tells me your power amp looks fine.
Follow the track on the other end of C10 and you *must find R28,
A track is a track, doesn't matter if it leads you one mile away (ok ok, a few inches )
the other end of R28 comes from U3b, the effects mixer.
Inject some music, either an MP3 or just your guitar into the FX return jack, you should hear it on the speaker.
Your volume control will be the FX return level pot.
Good luck.

[bunzonoise]

Same thing with the FX return, the sound is over distorted and weak as in the normal inputs.
Should that tell me something?

[g1]

That tells you the problem is after the effects return, or power supply related. Most of the power supply voltages have been verified, but check the + an - 15V lines to the IC's. Also check that the diodes D3 and D4 at U3b ic are not shorted.

[bunzonoise]

g-one: I'll try this as I'll be back from work.
J M: I finally found R28 and I got not sounds touching between that and C10.