BF Bandmaster Vibrato 'motorboating'(?)

[ziggy007]

G'day all
Hope you can help in my latest problem pls.

When you plug in a vibrato lead (my shorted RCA test lead) the vibrato 'puts' at a proportional rate to the speed control and at level changed by intensity control. All other controls have no effect. Tried new 12ax7s in all positions and it has 4 new O/P 6l6s. Noise goes when vibrato lead removed. With guitar in, the channel works fine as it should (with putting underneath.) Voltages are as Schematic levels, (although I noticed Pin one goes to 430 from 280 when RCA removed)
Searching forums no luck although i just found this...
"If you can see the light flashing through the end of the heat shrink tube, then your LDR is suspect."
I can see the light easily from both ends of the heatshrink. it flashes in time with the 'put'
Why is seeing the light bad??

Thanks in advance,

Gavin

[Enzo]

"If you can see the light flashing through the end of the heat shrink tube, then your LDR is suspect."

I can see the light easily from both ends of the heatshrink. it flashes in time with the 'put'
Why is seeing the light bad??

DOn't take that advice out of context. The light shines on a photocel, a Light Dependent Resistor. If you don;t see the light flashing, then your trem oscillator is not running. If you see hte flashing light, then the trem oscillator IS running.

But if your problem is no trem effect, BUT the light is still flashing, then that leads one to suspect the LDR is bad, because the flashing light is unable to affect the sound the way it would with a functioning photocell.

All other controls have no effect.

NO effect on what? The guitar sound? The trem bumping?

What specifically is the amp doing wrong or not doing at all?

You say the amp works like it should with a guitar. OK. COmpletely? Or is there some trem issue in question? DO the other controls affect the guitar sound as they should?

or are you saying the trem doesn't tremulate the guitar, but you can tell it is doing something?

I am not sure what you are trying to solve.

[Dave Curtis, dB AudioTech]

Assuming you're just talking about the ticking sound and the circuit is otherwise working, here's the jist of Fender Service Bulletin #9:


"The ticking cuased by the Vibrato is caused by improper lead dress.


It can almost be "cured" by connecting a .01 mfd 600 volt mylar capacitor
on the 10 meg ohm resistor in the vibrato circuit. this resistor is located
on the parts panel. Remove capacitor across the 10 meg ohm resistor (old
modification) if in place."


(That is, run the cap from the junction of the 10 meg resistor and the
optoisolator to ground.)


"If this does not produce the desired results, then the leads should be
dressed as follows and excessive lengths shortened.


1. Dress the leads to the vibrato speed and intensity controls away from
the tone controls and filter leads.


2. "Bunch" the leads to the components on the parts panel which connect to
the tube socket of the 7025 (12AX7) vibrato tube."

[ziggy007]

Thanks Guys,
I will try the Cap fix tomorrow morn & play with lead 'dress'
Enzo, The amp works fine (and sounds absolutely sweet ) but when no signal is being fed in, ie guitar volume down, or not even connected for that matter, the putting is evident whenever vibrato is 'activated' by shorting plug on vibrato pedal output jack. The vibrato controls ie speed/intensity affect this in their correct sense. All other controls ie volume / tone stacks on both channels have nil effect on the 'putting'.

PS. are those Fender SB's commonly avail or do you have to be an 'authorized' repairer?
PSS Thanks Enzo, Re-reading the original text the LDR light statement was out of context. I should of known better.
Ta Again
Gavin

[ziggy007]

Morning,
Unfortunately no joy on the .01uf cap or the dressing of the leads. I am at a loss.
The intensity control does not seem to affect the 'putting/ticking' which is definitely not normal.
It is pushing out enough 'signal' that the speaker is moving a fair amount of air in proportion to the speed control.
again , any help much appreciated.
cheers
Gavin

[Enzo]

Typical bad noises from tremolo circuits are your thumping, and also ticking.

Ticking is a result of the neon bulb firing and the sudden change in current that represents. The bulletin Dave posted adresses that.

Your description is more of thumping than ticking.

If I read you right - and I have already demonstrated that I don't always do that - you are saying the intensity control will turn this thumping down. Clearly this is trem circuit related one way or another, so it is not surprising that turning the trem off stops it. If the intensity control can turn it down, then the noise is coming through the optocoupler rather than through the power supply. Or so it seems to me.

Your photocell itself could be faulty, resulting in some odd response to the trem oscillator, or the oscillator could be turning out a funny waveform. Maybe a clipped or uneven one. I would wish for a quick look at it with a scope.

And one other weird idea is a leaky coupling cap from the stages. The photocell causes a variable resistance, which is placed in parallel with some portion of the intensity pot. So pot and cell together represent a changing resistance across the signal path. There should be no DC on this portion of the signal path. But if the 0.1uf coupling cap from the Vibrato channel output, or maybe even the ,047uf from the normal channel, were to be leaky, that would put some DC on the pot and cell, and could possibly be involved. Simple enough, turn the trem off, and the intensity pot to zero, measure at the top of the intensity pot for any DC voltage. A little DC leakage might not hurt the amp otherwise because there is that 500pf cap blocking it from getting to the phase inverter.

The trem circuit has two triodes - the oscillator and the output buffer. When the trem is off, that connection to the bias supply puts like -40v on the grid of the oscillator triode. That is more than enough to cut the tube off, so when it can;t conduct, the full B+ supply voltage will appear at its plate. That is normal. WHile I'd like to scope it, I'd bet your oscillator is OK.

Are the voltages around your output buffer triode more or less close? Are the two 10k resistors and the 10 meg resistor remotely close to value? Is the 25uf cathode cap healthy?

[ziggy007]

Hi Enzo, as usual you are on the money.

I went and checked the bypass cap and lo and behold it was soldered on in reverse. It had been replaced at some stage, (probably by the same guys who decided to 'convert the amp to a 'showman', upgrade the trannies & in my opinion, butcher a classic piece of history.)
The thumping was still there but not as evident. I re soldered the .01uf in to gnd and its now all good. Its still there, but not at a level that is of concern.

Another 2 questions for you though pls (discovered during my fault finding). Is there any 'acceptable level' of DC on the op side of the coupling caps or should it be absolutely none?
and
Is a small sawtooth wave on the bias voltage acceptable (about .7V)
Thanks as usual
Gavin

[Enzo]

I really don;t expect any positive DC on a grid in your preamp, if anything a little bit of negative can be generatd by grid leak action. Once you get into the phase inverter and power tubes, then that is no longer the case. In general I expect darn little DC leakage.

.7v sounds high to me for a bias supply, after all it is going to your grids. You want .7v of hum signal? The filter is simple in the bias supply, but then the only current draw from it is the voltage divider itself. And that is maybe a couple milliamps. SO ther shouldn;t be much ripple. Schematic shows a 25uf 50v filter cap for the bias. Is that what you have in yours? Measure the voltage across that cap. Over 50 isn;t it? How old is that cap? How about a nice fresh 100v cap?

[ziggy007]

Cool,
The bias cap has been prev replaced as well, to a 100uf/100v one. (guesstimate 15yrs ago) will grab a 25uf100v today & try.

For interest, I had a 82uf/300v electro handy and tried that. It made it worse.Why would it get bigger??
ta as usual
Gavin

[Enzo]

Remember the bias is a negative supply, you ground the POSITIVE end of the cap.

[ziggy007]

yeah it was (pos gnd), but increased in amplitude to about .85v w. slightly lower frequency.
In ye experience Enzo, Why would someone put a 100uf in there , its 4 x the value.(original fender spec on bandmaster & showman 25uf) Any reason or probably just that one was handy

as a side note, geez, 25uf 100v electro caps are hard to find in Oz. I've ordered a few 27/100s.

[Enzo]

We put larger ones in there because they filter better. We increase the voltage because for some reason Fender used too low a voltage spec. Well the reason was money.

25 is not a standard value these days, 22 is. And that difference won;t be heard. I'd recommend something larger anyway - a 47 or that 100.

Increasing the size of your B+ filter caps can change the dynamics of the amp, but that is largely due to stuff like sag - the way the amp responds to changes in current demands. But the bias supply doesn;t have to provide any current to speak of, and that tiny demand doesn;t change, so the cap won;t - or shouldn;t - have much effect on other than noise.

[ziggy007]

Whilst on the subject of caps, definitely a weak spot 4 me, would a higher voltage rating necessarily affect the o/p. ie I only need a rating of say 50-100v for the bias voltage (set @ -52v),would a 22uf @ 500v affect anything adversely, and on the same boat, would putting a higher voltage cap on the bypass caps for example affect the basic job of the cap to bypass the desired frequencies?
Thanks for indulging my learning curve
Gavin

[lowell]

I have a Bandmaster AB763 on the bench with ticking and thumping. I checked all surrounding components and ended up replacing most of the resistors and all oscillator caps. I also added the .01uf from the plate of the recovery stage to ground. That cap got rid of the TICKING, but not the thumping. The circuit is mostly functional now, however when the SPEED is all the way up, there is a motor boating low frequency thumping. If I back off of 10 just a hair it goes away. So I decided to add a 270ohm between the SPEED pot and the 100k to simulate the SPEED pot being NOT UP all the way, however, perplexingly so, this made it worse. So I'm at a bit of a loss here concerning the thump thump. I even tried reducing the cathode bypass cap to 1uf. Did not help. Also tried a new tube as well. At this point I'm left with saying it's done, and telling the customer, just don't turn it up to 10. Stop at 9.

[lowell]

Ok so I started moving the wires for the oscillator tube around and definitely noticed a big effect. I twisted the grid and cathode wires together tightly. This helped, but wasn't enough. I then, after moving the wire going to the RCA Vibrato jack and noticing a big effect, pulled this wire out completely and replaced with shielded 2-conductor wire. I grounded the shield on the board next to where the hot was. The hot is on the board at the 2 1meg and 1 2.2meg junction. There is a cathode ground next to it. THIS WORKED BIG TIME. No more thumping. Hope this helps someone else.

EDIT: Also then twisted the plate wire of the oscillator with the grid wire of the recovery/amplification stage. There is really no audible thump or click anymore at all settings.