Ghost note on bandmaster???

[Satamax]

Hi everybody!

I have a BF bandmaster, and it has that constricted distorted note riding on every note. Could it be failing lectro caps as explained on the tube debuging page?

http://www.geofex.com/ampdbug/distort.htm

Thanks a lot for your help guys.

Bye.

Max.

[Richard]

It could be, how old are the caps? Did the problem evolve or did it suddenly happen one day?

[Satamax]

Well, caps seem brand new thought the guy said a local tech did the job, to me it looks like a hack. The caps are covered in sharpie, to identify what they are nd what they do, thought i think they're spragues (blue ones!) Seller said the amp was fine before shipping. (doubtfull!) There's few steel parts missing, chassis straps, tube shields, cap pan. There's some resistors "in the air" on the tube sockets, which have been repalced, plus some new one ohms (as if a tech would need them!) on the cathodes! (thoses are going away as soon as i can!) Both channels are affected. I'm enquiring, because someone might have heard/seen this before. I gonna take the scope to it as soon as i can.

Thanks a lot and best regards.

Max.

[Satamax]

cap testing?

Hi everybody!

I did a bit of testing. The distortion on top of the note decays faster than the note itself, dying about mid time of teh sustain, in a crackling noise, like when you crumple polyester film.

BTW, ok, it might be caps, but besides measuring the resistance of the caps, how do i check them? Do i look for exessive ripple?

[pdf64]

From your last post I supect that it is more likely to be a bad speaker - voice coil rubbing - rather than the HT electrolytics. Try plugging in a different speaker cab to test for this, if you haven't already.
If no difference then possibly parasitic oscillation, bad valves / bases, or a whole load of things. Check the speakers to rule them out.
Good luck - Peter

[Satamax]

Thanks a lot Peter!

Well, no cone cry or rub, i've tried it with another amp! Well, i've checked the caps for resistance and some have a low resistance at begining then going suddenly up, then slowly back down charging up! The caps are out of specs, there's should be 3 20µf and there's a 20 a 16 and a 40!!! Besides the two 80 which are normal! All sprague atoms. One vas marked cath follower???? No sign of a cathode follower being wired in this amp, not enough shinny solders. Seems thought that someone had soldered some of the blue caps away or back in etc! I've gotten some 25µf caps for the pre. An have two Rifa 20µ 450V for the power amp. (lower voltage value i know, but the same ones lasted 10 years in a BF bassman.)

I'll report back later.

[stokes]

"I did a bit of testing. The distortion on top of the note decays faster than the note itself, dying about mid time of teh sustain, in a crackling noise, like when you crumple polyester film."
This sounds like it could be the plate load resistors acting up.Try poking at the plate load resistors with a chopstick with the amp on and see if you can get the noise.Could also be related to dirty or loose tube sockets,try rocking the tubes in the socket to see if you get any noise.

[Satamax]

Ok thanks a lot stokes!

For the moment, i've replaced the two bad value caps in the PS for rifas (german brand may be!) of the corect value. But there's something weird, i forgot to bring the amp up on the variac , and switched 117V straight away, the amp is going click, cllick click, iregularely. And now i've pulled all the tubes out, and lowered the voltage on the variac, there's 635VAC between ground and + side of every cap??? What could that be???? Dead diodes on the power supply? Or something oscilating? I'munder the impression that one of the caps is clicking!

Thanks again and best regards.

Max.

[stokes]

If you expected more than 450v on any of the caps it is possible you shorted a cap when you fired it up.You shouldnt trust a lower voltage rated cap just because it worked before,and by not forming the cap first it is probable that you blew one or more.Try putting the old caps back in.If that 40uf cap you described as being "wrong" is on the screen that is fine and is a common upgrade to the power supply in any amp.16uf or 20uf are interchangable too.Something isnt right with that 635VAC reading,check it again,if your diodes were shorted your fuse should be blowing,as long as the amperage rating is correct.To check the diodes,disconnect the lead to the B+ rail so the only thing connected to the PT High V supply is the rectifier diodes,then check the DCV's at the rect.output.

[Satamax]

Well, that was just the everyday DMM wich was not up to the task. I ended up with 4.5V ripple on the screen cap??? (after a choke???) and i did another check, the choke cap is charging up from zero, when the standby switch is still open. How come! I switched the amp on after draining the caps, with the standby switch open, the screen cap started from zero and went up 3VDC in mater of 5 minutes! Ok, there must be something leaking some curent through, but what??? I was kind of yeah right, that's the power caps, or the cathode bypass caps acting up on me and giving me that ghost distortion, but finaly no! I have a few other things to check, but i'd say i'm stumped! .

Thanks a gain!

Bye.

Max.

[Satamax]

Just chjecked nother thing with the scope on this verry cap, and i have 170mV peak to peak on it, but it's oscilating wrt the ground, the waveform is fluctuating up and down like a snake. Weird, cos every other cap seems fine and the totempole has 14V peak to peak ripple, rather constant at 98/102 hz B node on the bandmaster is power supply for screens, and vibrato oscilator. Could it be the oscilator with messes with the power supply????

[Tom Phillips]

... with the standby switch open, the screen cap started from zero and went up 3VDC in mater of 5 minutes! Ok, there must be something leaking some curent through, but what??? ... i'm stumped!

Max,

What you are probably measuring here is the result of a property of capacitors known as dielectric absorption. A capacitor that has been charged for a long time may appear to completely discharge. However, when the load is removed the capacitor will develop a small voltage. This can be demonstrated with a capacitor completely out of the circuit.

Tom

[Satamax]

Hi Tom!

Thanks a lot for your reply! Might be this, but i realy started at 0V the amp was stopped for a whole day with stanby on , switched the stanby off before switching the amp on! Before turning on, i checked with my tekmeter what was the voltage, 0Vdc is what i got. Then, switched the amp on, and the standby switch was still off. Th evoltage starder to raise in the screen cap, milivolt by milivolt, every half a second or so.This is normal charging of caps. I stoped at three volts, i think it would have gone up more, but may be that was the meter charging the cap up??? Now i've scopped it today, here's what i got, carefull, pics and film are big. http://satamax.free.fr/v1plate.JPG http://satamax.free.fr/v2plate.JPG and the film, right click and save as! http://satamax.free.fr/v1plate.MOV .

No signal, jack plugged in the amp and in the sig generator turned off. this what i get on the plates of V1 and V2. I realy don't know how to intrepret this.

Thanks a lot for your help.

Bye.

Max

[stokes]

Pull the plug out of the input,even tho the signal generator is off the open input circuit is likely picking something up.

[Satamax]

Yes all right, i did a bit more testing! It's all over the place on the oscilator.

The only value which is good, is the 380, i have 387 there!

The rest looks like this.

http://satamax.free.fr/AB763-bandmaster.jpg On the top of the 100ohms in the PI, i have +3V when i touch the solders, in the PI on the top side, i have motorboating or fast oscilation if you preffer.

Any leads for me???

Bye.

Max.