B+ variating in standby???

[Satamax]

Hi everybody!

I have the B+ variating in standby from 150 to the normal 469 on a modified sound city PA Power trany and line seem ok, i've disconected teh trany and get 354 plus or minus two volts depending on line variations. Caps are newish F&T from new sensor or TAD, changed them last winter. Diodes are the original ones, and was suspecting they were teh culprit, what's your opinion???

Thanks a lot and best regards.

Max.

[Amp Kat]

Why are you getting line variations ? You should get somewhere around 230vac but it shouldn't be jumping around as that would give you voltage swings. Are you using a Domestic tranny or an International ? At 354 volts that would put you rectified and filtered right at 500 VDC and depending on the load would take it down somemore but not from 150 to 450. Saying your line voltage is steady, the things that would cause that are leaky caps or something touching another component that's not supposed to be like a cathode resistor touching the plate resistor. A direct short would blow the fuse if it has one or possibly open up a resistor.What do you get right after the rectifier ?

[Satamax]

Thanks a lot for the reply amp kat.

Well, discovered two things in the amp while changing the recto diodes, grounding on the bottom of the totempole was badly soldered, and center lug of the normal pot was also badly soldered, freely moving into the lug That didn't adress the two probs i have tho. The sound is cuting out, and like i have some cap charging and discharging, making the sound come back and away again, and the tonestack isn't working properly, seems that it takes another path, and the normal volume is letting some sound through when i'm plugged into teh bright channel, verry weird, i gave up tonight, and i'll get back to the scope and sig gen tomorow. Thought, i'm pretty much stumped with this intermitent fault I'm working as per L100 schemo http://www.schematicheaven.com/barga...dcity_100b.pdf

Thanks a lots, and best regards.

Max.

[Amp Kat]

I would start at the standby switch, disconnecting the HT line from the Output transformer CT. and measure carefully from ground to the HT coming off the (2) 200uf caps that form a voltage divider or 100uf total. From chassis to the top of the first 220uf cap you should have a steady DC voltage of 415 volts and if it varies you have a cap or transformer or line voltage problem. Then go from there and start tracing ahead from the tubes back.

[Satamax]

Hi Amp Kat. Thanks a lot for your reply. Well, the B+ is stable now, it was the ground connection on the bottom cap of the totempole. I gonna start a new thread for the other problems.

Thanks a lot and best regards.

Max.

[Alex/Tubewonder]

Check all ground connections for continuity, if any of the preamp stages has a missing or high resistance ground connection (cold solder joint etc) its corresponding supply filter cap will charge up slowly and discharge quickly as soon as signal is present. This could cause the phenomenon you describe.

Just a speculation but I have seen this before.

[Satamax]

Thanks a lot Alex.

That's what i did yesterday, i had one falty connex on the totem pole bottom can. Now, i still have the prob of teh B+ variating again! basicaly, when i hit the chassis on the side, it's dropping down from 465~ down to 170 or thereabouts, cuts of the sound, and then the voltage when it has reached that low level, raises again quicker than it decreases. I suspect it could be something like oscilation loading down the power supply. I have Between 5vdc, deacreasing down to about three VDC on the tone controls. Seems almost normal.

Well, thanks a lot for teh replies guys.

Bye.

Max.

[Alex/Tubewonder]

DC Voltage on tone controls is way wrong, there shouldn't be any DC on them. Most probably a leaky capacitor in the tone circuit.

If you get B+ to change so much as you say when you hit the chassie on the side it means you have a bad joint somwhere causing intermittent open circuit. Or a piece of wire or perhaps a glob of solder lying somewhere shorting part of the circuit.

[stokes]

The leaky cap or caps in the tone circuit are likely causing oscillations.Oscillations will cause the B+ to behave as you describe.Replace the leaky caps and you should be okay.Check the entire amp for leaky caps as well.The B+ being affected when you bang it could be a loose connection as Alex pointed out but could also be related to the leaky cap,the cap is probably very close to shorting out internally and when you bang the cabinet the vibration is causing it to "leak more".I am sure this would also be the answer to the problem you describe in your other post.