Mesa Boogie Lone Star Special 4xEL84 Low voltage supply unstable, causes relay dropout

[BiBi]

Hello,

I am wondering if anyone has had a similar problem with these amps-- namely the relays activating randomly. Unfortunately, there is no known schematic for this amp on the webs,
but the low voltage supply is similar to the one on the bigger brother seen below.
So far, I have checked all the diodes; played around with the capacitor values before and after the 7812; tried an LM317; tried a different 7812 with input .3 and output .1 caps as well as a diode from out to in, all to no avail. The voltage starts at around 14 volts after the rectifier before dropping wobbling all over the place dropping the output voltage to 11 Volts before the relay eventually trips and the process starts again.

Could a relay be drawing current in this situation? I know the low voltage supply provides the heater voltage for the first two tubes but this should remain stable. And yes, I have replaced both tubes just in case... Thanks for any help!

[drewl]

Yeah, I had this problem, as soon as you plugged in the fan it would drag the supply down.

I think I upped the 150 ohm resistors.

Check the big ass filter cap and four diode bridge first.
They sometimes start failing but still measure okay, especially in the Dual recitifers.

[Dave H]

The voltage starts at around 14 volts after the rectifier before dropping wobbling all over the place dropping the output voltage to 11 Volts before the relay eventually trips and the process starts again.

14V isn't really enough input voltage. A 7812 could have a drop out voltage of 2.5V. It will be struggling to regulate. I'd try a low dropout regulator, something with a drop out of 0.5V.

[BiBi]

Thanks for the replies!

As there is a switch to turn off the fan, I did switch it off so it wasn't the source of the problem. Although, it is a useful way to hear the current draw as the fan slows down as the voltage drops! I also disconnected the pilot light but that didn't help either. The "big ass" filter caps were exchanged first-- my thought was that with the marginal input voltage a high value cap would bring the whole thing under. I tried a 4700uF in the first position (In the Special it is a 6800uF) and then an LM317 with a smaller capacitance on the output but it still wobbles, which leads me to believe that the input voltage or current from the transformer is marginal. I also used an external bench supply for the 12V and everything worked just fine.
I haven't replaced the diodes yet (perhaps Schottky diodes would be a good alternative?) but that would be my next guess, and/or a low dropout regulator. Alternatively, I could change the primary voltage input. Currently, it sits at 230VAC but where I am that is rather high-- we have almost always 220VAC and there is a tap for that.
What I don't understand is how the current is varying however. Could that be a result of the mains voltage going up and down? Or is it because something on the 12V circuit is drawing more current intermittently?

[Pedro Vecino]

Maybe this can help:

pow
swt

[g1]

I could change the primary voltage input. Currently, it sits at 230VAC but where I am that is rather high-- we have almost always 220VAC and there is a tap for that.
What I don't understand is how the current is varying however. Could that be a result of the mains voltage going up and down? Or is it because something on the 12V circuit is drawing more current intermittently?

If the mains voltage is varying, I would expect the current would also vary.
So is the votage switch set for 230V? And your voltage is normally less than 230V? Then yes, change the switch to 220V.
Check the heater voltage at the power tubes. It should be around 6.3VAC. See what it reads with the switch set at 230V, then see what it reads with the switch set at 220V.

[BiBi]

Maybe this can help:

pow
swt

Hey Pedro,

thanks a lot for those schematics. If you have the complete set, would you mind putting them up here or sending them per PM?
Or, could you verify for me that the output tubes are put in fixed bias when switched into 30W mode? It doesn't seem to matter to the
problem at hand as the relays activate also in 15W mode but I am curious nonetheless. When in 30W mode, the HT is also sitting very low
around 320 versus 360 on the schematic while in 15W mode the HT is spot on.

Otherwise, I measured the input voltage here and it is indeed very low. There is a bit of variation of a volt +/- but mainly it sits around
118VAC. I will adjust the power transformer input to compensate and see how it goes from there. What I find particularly strange however
is that the relay switching problem is something that popped up recently-- I believe the amp is around 5 years old at least.

[BiBi]

If the mains voltage is varying, I would expect the current would also vary.
So is the votage switch set for 230V? And your voltage is normally less than 230V? Then yes, change the switch to 220V.
Check the heater voltage at the power tubes. It should be around 6.3VAC. See what it reads with the switch set at 230V, then see what it reads with the switch set at 220V.

Hmm... I have now switched to the 220VAC tap on the power transformer. In the good news, voltages appear to be more or less normal for the amp, i.e. I am getting 6,3 volts on the heaters versus 5,8 previously. The HT in 30W mode is also closer to the published value at 355 (although now the 15W mode has an HT of 290VDC). As well, the low voltage section is now regulated quite well with a low drop out regulator at 12VDC.

However, the variance in the low voltage supply is certainly very noticeable. The HT for instance doesn't have near this amount of variance. The low voltage supply goes from a high of around 15VDC at startup down to a low of around 13VDC. It does seem to stabilize after a period of twenty minutes or so at around 14.3VDC but I still get the occasional hiccup evidenced by the lamps blinking for a quick second.

As the amp appears to stabilize after some time, I do imagine there might be something involved with the output tubes. The cathode resistor for the EL84s switches out in 30W mode, then a 12VDC is introduced onto the cathodes. I imagine the 12VDC is produced somewhere along this low voltage supply which could explain the dropping out and instability.

[Pedro Vecino]

blk

opw

pr1

pr2

That is all.

[Gregg]

14.3VDC is too low for a regular 7812. My advice is to replace the 4007 with Schottky diodes (like 1N5819 for example) to minimize the voltage drop. Even better would be also to replace the 7812 with a 12V LDO like LM2940.

[Jazz P Bass]

"14.3VDC is too low for a regular 7812"

Not too sure that this statement is all that correct.

The datasheet specifies a minimum 2 volt input differential compared to the output.

LM7812.pdf


With that in mind I would monitor the input voltage.
Is it relatively ripple free (Vac)?

Is the output ripple free?

[Gregg]

The input voltage must remain typically 2.0V above the output voltage even during the low point on the input ripple voltage.

Key word is "typically".

[Jazz P Bass]

The very next consideration on the LM7812 is the current being drawn from it.

[TimmyP1955]

Double check there the chassis is smooth where the 7812 mounts, that the bottom of the 7812 is smooth, and that there's no foreign matter on the insulator. Maybe even try another 7812 for grins.

[diydidi]

Bridge rectifier diodes have been known to go bad in these amps. Replace them all and stand them off of the board.