Seymour Duncan Convertable 60W combo Wierdness

[Kid Squid]

Hiya Chaps,
Been busy as a busy thing, the last few weeks doing mad 16hr nights,
nice to see ya all :thumb:

Anyway,
one of my work mates, asked me to have a look at his Seymour Duncan Convertable 60W combo, after he re-capped / re-tubed, after having it given to him.
As soon as it's switched on, it hums like mad, so I looked at the power rails, and one of the rectifier diodes, was not rectifying. So, changed the diodes, and I'm still getting , what looks to be half wave rectification, coming straight off the diode junction.
Found the schematic online this afternoon, http://www.diyguitarist.com/GuitarAmps/Convert60.htm
on the bottom of the page it is.
here'sa pic of the scope, showing what i mean.

B+ after diode rectifiers, before C8,9 R55,

Her'es a pic of the Phase splitter anode volts

pics of the Anode Volts on O/P EL34's

and the 'funkiest' one

This is across the output (8 ohm 50w)resistor !! (0.7 - 0.83 VAC)

Surely it's gotta be the power supply, injecting that much ripple onto the plates ??

*Just had a little check about*

I disconnected + of C8, R61, and R55, and the B+ waveform is still the same. Surely, its the rectifier stage ??
Whats the deal with the four Diodes, on a CT secondary, and with them resistors R65,66,67,68 'shorting the diodes out?
do they pass current, until, the forward bias is exceeded on the two diodes per leg ?, 'pre-charging' C7 /C8 ??, like a soft start type of thing ?
or am i talking bollocks :wink:

edit.
* I was talking bollocks !, its a parallel RC filter - a way of getting big capacitance / votage for the buck, the two resistors are equalising voltages for the two 250V caps in series.*

checked over my mates soldering, - all ok, but the only thing different i've noticed, is that the replacement C9,10,11 are 20uF, not 22uF, but the actual measurements are ranging from about 26 - 29uF....
also, CR12, and the jumpers - jmp1,2 are not fitted, and they look like they never have been. ( something to do with the unused doubler circuit)

...........I have been working a LOT of nightshifts tho....... :wink:

Any ideas are apreciated fully !

Steve :thumb:

 

[Gus]

Did it work before being recapped?

First thing I would do is power it down and let the caps discharge.

It is an older amp I check mechanical things first. Anything that has mass and/or heat connected to the PCB etc.

Check the replaced caps are installed correctly

Measure all the diodes with a meter.

I see it has a connector J1, measure the connector resistance to make sure it is making good contact. Check for cracked or bad solder, Heat and vibration in a guitar amp can cause issues.

I will look at the schematics went I get a chance

 

[guitarbill55]

Hey Steve, I got more questions than answers for you. Like Gus asked did this amp ever work; who worked on it and are all the filter caps and diodes installed with the right polarity? This amp is set up for 120 vac operation at 60 hz. That may not mean much for now as long as its being powered from 120 vac. However some bone headed picker may have tried to power it with 220. Let's hope not, but you may be missing one leg of your high voltage right off the Power tranny or one leg off the low voltage winding. You should try checking your transformer and make sure all the AC output voltage are correct with respect to their center taps.

Another thought; make sure the output tubes have their locater pins and if not are installed correctly

Let us know what you find.

gb

 

[Kid Squid]

Hi Gus,

Did it work before being recapped?

First thing I would do is power it down and let the caps discharge.

It is an older amp I check mechanical things first. Anything that has mass and/or heat connected to the PCB etc.

Check the replaced caps are installed correctly

Measure all the diodes with a meter.

I see it has a connector J1, measure the connector resistance to make sure it is making good contact. Check for cracked or bad solder, Heat and vibration in a guitar amp can cause issues.

As far as my mate's aware - it did work previous, but he's never tried it, as he was using a different amp for years, this was put into storage.
I asked him about where the tubes were - but he said it came with none fitted..............( which leads me to think, that it might have been 'dodgy' before he got it)....it was given to him tho.... :wink:
Yep the Filter caps are discharged every time I'd work on it. - it's a bit of a bitch, getting the pcb , in, and out of the chassis, so it's essential :wink:
All Diodes checked, but even with CR6 changed, i'm still getting, a reverse reading across the diode, of about 550Kohm, it's the only diode , of the chain, to do this. ( WHY ?)
Checked all contacts/plugs, all are low resistance, also checked over new cap soldering (ok), and re-worked some joints, but nothing untoward.

Bill

Hey Steve, I got more questions than answers for you. Like Gus asked did this amp ever work; who worked on it and are all the filter caps and diodes installed with the right polarity? This amp is set up for 120 vac operation at 60 hz. That may not mean much for now as long as its being powered from 120 vac. However some bone headed picker may have tried to power it with 220. Let's hope not, but you may be missing one leg of your high voltage right off the Power tranny or one leg off the low voltage winding. You should try checking your transformer and make sure all the AC output voltage are correct with respect to their center taps.

Another thought; make sure the output tubes have their locater pins and if not are installed correctly

The amp was given to my mate, a few years ago, and he's just had it stored, untill he decided to have a go of it himself.....
there was no valves fitted, and apparently, there was some cap splurge, coming from two of the filter caps..
So, he replaced the filter caps with sprague atoms, and put in a set of Sovtek EL34's, as well as a ECC83 for the phase splitter.
I tried a ECC82 also .
All filter caps are installed correctly, as are all of the diodes.
The amp is actually set up for 240V mains Operation (50Hz), (there's a factory fitted label),
all AC voltages off the mains transformer are correct , 372-0-372 V, 3.15-0-3.15V.
O/P tubes are located correctly, but not saying that 'older tubes' may have been used incorrectly ! ( but there is no evidence of this....),

this half wave thing is gotta be the problem ?

edit
*also, on the output tubes, which is not marked on the O/P schematic, there are two diodes (1n4007), in series, connected between pins 3+8 ( anodes on pin 8, cathodes pin 3)of each O/P socket !!?!?!*

I've just got up from last nights shift, and I had a bit of a play last night, got static voltages, and a couple of pics. I'll upload 'em, when i get my head together ! - a little bit foggy at the mo :wink:

Steve :thumb:

 

[guitarbill55]

Steve, since the amp was given to your friend and had capacitor spewage
I'm taking a wild guess that this amp was in a non-working state when it was given. That means you can't take anything for granted. Some one of little skill could have mucked it up by installing parts incorrectly or of the wrong type. In short, if it was in my shop I'd be tempted to give it back!

The diodes on the OP sockets don't need to be there. That was something fender came up with in the 70's to prevent transients or some other rubbish.

The diode that reads reverse leakage should be checked out of circuit and replaced if it still reads wrong.

I would also be very suspicious that one or more of the op amps is goofed. They are so cheap I'd be tempted to replace them all to rule them out. They could cause big trouble but probably are not whats causing all the screwy waveforms.

If you test this amp section by section starting with the power supply ( I would unplug the preamp modules and not reinsert them until you are done testing each section ) sooner or later you'll get the thing going. Like I said before don't assume anything. Check everything including the bias voltage ect. I'd pull all the tubes out and check it with a good meter and the scope. Try isolating the entire power supply section and see if the
ac goes away. You might test your output tubes just to make sure they really work too. Every once in a while you can get a bad one out of the new bin.
good luck and dude get some sleep!

gb

 

[Kid Squid]

Cheers for the reply Bill,

I agree with you fully, regarding the non-working state, but I'm a sucker for a challenge, and I'd like to have a go....

What I can't get, is , I'm getting rectification through the CR4, CR5 section, but I'm not getting rectification across CR6, ( I'm picking up the half wave off the CR5,CR7 junction.)
I've changed the diodes twice, and i'm getting the same result.
I've got AC in of 375-0-375 ( 750 VAC across the windings).
I understand what the two diodes / 2 resistors are doing in each leg -
They are ganged up , for the reason of increasing the PIV of the diode string to 2000V (2x 1000V in series). The resistors, are 'equalising the voltage across the string, to compensate for the mismatching of the diode parameters.
I'll have a check of the resistors when i get a chance.
this half wave rectification is the problem..........

speak soon,

"Eat your sockeye with wild blueberries" - Copper River Red

I like the sound of that :thumb: :guinness:
Steve :thumb:

 

[Kid Squid]

Right,
Checked all tubes with avo mk.3, - all good.
De-soldered the resistors R65,R66,R67,R68 - and R65,R66 and R67 were all open circuit !?!, I replaced the four, and guess what..........


It's still there ! :shock:
I'm still getting the half wave rectification on the HT, and the output , well - is quite mad...
here's the pics.

O/P across 8 ohm resistor, no O/P

O/P across 8 ohm , low O/P

O/P across 8 ohm, higher O/P

O/P across 8 ohm, full-ish O/P

and now with B+ as bottom trace...

any ideas , as to why i cant get full wave rectification ?, I'm sure it's a straightforward explanation......

Steve :thumb:

 

[AnalogPackrat]

Just a wild guess, but looking at the schematic I see notes indicating two ways to configure the supply. You don't have CR12, JMP-1, and JMP-2 installed, do you? That would fark up the full-wave section right good...

Cheers,
A P

 

[Kid Squid]

Hi AP,

no, JMP-1,JMP-2, CR12 are not fitted, and never have, looking at the board.

thanks for the input tho :thumb:
Steve :thumb:

 

[AnalogPackrat]

I don't suppose there's any way to isolate the PSU from the rest of the circuit? Maybe theres a fault elsewhere in the amp that killed some of the replacement PSU caps (C7 & C8). Reminds of all the reasons I hate PCB tube amps.

 

[guitarbill55]

Hi Steve, do like AP and I said and isolate that supply, I know it's a bit of a challenge but it would answer your most basic question if you could clean up that power. Then go back and find the fault in the rest of the circuit.

gb

 

[guitarbill55]

One more thing to verify: make sure the center tap of the power tranny and the negatives of the hv filter caps and the positive of the bias supply cap are all making it to the ground bus.

gb

 

[AnalogPackrat]

You could also measure the resistance from center tap to plate on the output transformer primary--should be in the low hundreds of ohms or so and about equal on each side. I suppose if one side was shorted you could get the wacky stuff you're seeing. If you pull all the tubes does the B+ look better?

 

[Kid Squid]

Hiya Chaps,

got in this morning from nights, and checked the O/P transformer, all measures good, so thought i'd check the resistors around the power valve sockets,
Well, would you know, the 'bottom' EL34's 1K 5W U/L connection resistor is open circuit ! - Inbalance Right away ! :wink:

I'll try to dig some out later, but this is promising..

Steve :thumb:

Post Number 666 as well :wink: :thumb:

 

[guitarbill55]

Cool Steve; sounds like that could be at least the cause of the original failure. It reminds of the time I first tried an off brand of chinese tube about 20 years ago. A dead shorted brand new 6L6GC took out one of my vintage output tarsformers before I had time to shut it down. I'll bet a farthing that something similar happened to this amp mainly because it takes quite a bit of current to fry those 5 watters.

Like I said, you just can't take anything for granted!

gb

 

[Kid Squid]

Hi Bill,

I haven't had my replacement 5watters back yet !
I'm wondering if you could shed some light on something for me.

why does the resistor have to be 5Watt ?, what is the reason.
I thought that the U/L connection, wouldn't be carrying a ton of current, as I've seen some O/P topologies, that just connect the screen tap, straight to the U/L tap point.

I'm a little green, when it comes to theory of the O/P Stage...... :wink:
any help is apreciated.

Regards,
Steve :thumb:

 

[guitarbill55]

You want that 5 watter in there mainly for heat dissapation and long term reliabilty. Fender used those old 1 watt carbon comp (you know, the ones that fail when you're gigging ) in their 50 watt designs but most all of them will get replaced with hopefully something tougher. You're right about the current -- it's probably never gonna be that high. But just imagine that amp sitting on a hot stage in the summer with some body cranking a mess of Les Paul through it then you'll probably agree 5 watt wirewound is the nuts.

let us know

gb

 

[guitarbill55]

Hey Kid,

bump?

 

[Kid Squid]

Hiya Bill,
Just checking up on the post, I didn't have any of the post reminders tho !!,
so I missed your post from last week.

Well, I've only just got the 1K5w's, had to get them off ebay !, took bloody ages to get back, mate :shock: arrived this morning,

So, plan is, to have a go sometime today - got my daughter 'till about 4ish, so hopefully, I'll get a chance this evening.

Speak soon,
Regards,
Steve :thumb:

 

[Kid Squid]

Changed the 1k 5w's , still got the funky waveforms, on the o/p and B+.

out of curio, one ended the diodes, joined together , and i got the classic full wave , with no filtering., as soon as i connect the parallel cr filter, the waveform is DC with a 100hz ripple of 25 V.A.C., B+is 450v DC.
is this a lot of ripple from a paralell CR filter, considering, that the caps are 165uf (2x330uf series)?
this is the ripple that you can see on the B+ waveforms, on the previous page.
the two 330uf caps are new replacements also.
checked the resistors, equal volts dropped across each, so they seem o.k.
any ideas ?

Could it be those 330uf caps ?
the other voltage tap-offs , step down o.k ( (A) 2.6v ripple 440vdc, (C) 70-80mv ripple 404vdc,)
but i'm thinking, for such a big value of capacitance, in the parallel filter, there should be hardly any ripple, let alone the 25v i'm getting....

oh, and across the 8 ohm o/p, (50w resistor)i'm getting 150mv AC with no i/p.....
I've had a couple of tins, :guinness: and 450vDC is not to be dissed...

so i'm going to have a puff, then hit the sack. my daughter wore me out today !! :grin:

Nos Da,

Steve :thumb: