carvin legacy 2 phase inverter problem

[shane]

Hi Folks
I got an issue with the LTP phase inverter on a Carvin legacy 2.
when coming off standby the anode voltages are taking ages to settle.
The inverting half anode voltage starts at supply voltage but takes about 30 seconds to get to get to its operating voltage.
The other anode starts at about half the supply and rises upto its operating voltage.
both voltages are about 20V higher than stated in the partial schematic I have but then again so is the supply voltage.

I tried a new valve, same result, all the components appear to measure correct ie resistance values etc but I have only measured them in situ on the board.

The guy whos amp it is has complained it has low output on start up but come to life after a while
also says the boost channel dont bosst but thats another matter

anyone got any ideas?

Cheers
Shane

[Stevem]

Does the amp have EL34s in it and the bias switch is set for 6L6 / 5881?

[shane]

The amp does have EL34's in but the bias switch is set to EL34.

[TUBEDUDE]

Is the PI stable afterward, no intermittent gain changes? When the triode is not fully conducting, is the bias voltage on the grid high? Solid connections at the cathode resistor? I'd reflow the socket pins on the board, and the Rk and grid components leads.
Did you mean all the HV voltages are high?

[Stevem]

And what is the negative voltage you read on pin 5 of the output tubes?

[shane]

The voltages do stabilise, will try reflowing pins etc. Negative voltage on output valves is -44v.
Saw something in an old post on another forum where guy had a similar issue on an older legacy and it turned out to be the 10uf feedback cap, which on that model was non polarised but on the legacy 2 is a normal electrolytic. May look at that too.
Going on holiday today for a week so will put it on hold. I'll report back when I get back on it.

Cheers
Shane

[Anger]

Hi!
Did you solve it?
regards,
Joao

[shane]

I did but I'm damned if I can remember what I did exactly. I definitely changed that cap so thet could have been the issue. Sorry I can't be more help.

[Mars_Amp_Repair]

Hello folks,
I'm relatively new to this forum having been a senior member on Music Electronics Forum until they went dead.
I just had this issue with the Legacy PI. Out of standby, the output sinewave formwould either be clipped on the top for about 20 seconds as it slowly filled out or the amp would have no output for about that period of time, slowly fading in.
It did turn out to be the neg feedback cap C-69 being the incorrect value.
The schematic indicate '10' with no designation to 10 what? The cap in there was an unbelievable 220uf @ 160V! No NFB cap needs to be that high. So, since there's about 80Vdc on the PI side of the cap, it is necessary to have a blocking cap to keep the FB network from carrying & affecting that PI bias. Every time the amp was taken out of STB, the cap would take forever to charge up. I'm sure this is a factory faux pas. I also tried a 10uf there and it still took some time to charge and acted a bit wonky.
I finally settled on a 2200pf (0.0022uf) as it didn't affect the frequency response & behaved just like the 220uf (when charged). It could have been a 0.1uf or really just about anything under 1uf to provide adequate NFB w/o affecting this charging issue.
Glen Whatley - formerly of Mars Amp Repair now at 12th Fret Music in Boise, ID.
I'll be posting stuff like this as I run across it for the benefit of other techs.
I sure miss our MEF.