Clavinet E7

[tommypiper]

Hi guys. 

The thing is very noisy, buzzy.  The buzz goes away somewhat when the two pickups are played out of phase with each other.  When switching between pickups sometimes it dies and goes silent, then slowly revives.  Bizarre.

I've checked pickups and wiring. 

I've replaced all the caps except the 8.2 pF.

What might cause all the noise and the odd behavior?

What kind of diode is that on the lower right in the schematic?

Anyone know off hand what modern substitutes for the transistors?  I'll look them up.

 

[livingnote]

This was one of the ones I installed in Zürich, he had dug up the old one from Blue Note Records...and it behaved just like that, only minus the dying part. Me too, no idea why except "standard guitar misbehaviour"...

 

[tommypiper]

Well, I had this one working a few years ago without buzz.  So something has broken.  It sounds like something is not grounded, a real loud buzz.  

Then the signal dies after awhile, as if the battery suddenly gave out.  But then it fades back up again after a few seconds.  (And it's not the battery.)  This makes me suspect a bad component.  I'll eventually replace everything if I don't get a better clue.  I don't know what kind of diode to get for that one on the lower right... What's ZPD mean?  Zener --- Diode?  What's the number mean?

 

[geoff004]

I've got an E7 on my basement that has the same symptoms as described above.
I haven't had a chance to get in to it.
The only thing I have noticed is that the buzz/hum will change depending on it's orientation to my speaker - just like if you move a guitar around in front of the speakers until the buzz goes away.
But the one I've got has a bad transistor too, and I think I'd like to update them all.
Is that the entire schematic?

 

[tommypiper]

Yes, that's the entire schematic.  I've more or less verified all the parts visually and they correspond, so it's correct.  (I didn't check resistor values yet.)  I think the one tant cap had a slightly different value, but all the others were the same as the schem.  Yes, I'd like to upgrade the transistors...

Yes, they are single coil pickups, so they are prone to noise and interference.  When I had it working though it was pretty quiet.

Does yours also fade out at times and go dead for a few seconds?

How do you know you have a bad transistor?

 

[RuudNL]

Could it be that an external field (from a transformer) is picked up?
Try to connect the blue and yellow wire from the transformer together. (Secondary) Is the buzz still there?
If not, have a look at the screening of the wires connected to the pickups.

 

[bpucekov]

What kind of diode is that on the lower right in the schematic?

zener diode 8,2 V

 

[mad.ax]

Make sure that the harp is properly grounded.

The main difference between D6 and E7 models is  the voltage regulator on the left of the schematic (BC238, 8.2V Zener, and 1.8K). This was added because most people wanted to use an external power adapter rather than batteries.
Put a voltmeter across the 100µF and see what happens when the signal dies...

BC550C are quite good transistors. Imho, no need to 'upgrade' them...

Axel

 

[tommypiper]

Could it be that an external field (from a transformer) is picked up?
Try to connect the blue and yellow wire from the transformer together. (Secondary) Is the buzz still there?
If not, have a look at the screening of the wires connected to the pickups.

When I connect the blue and yellow, to short the transformer secondary, it goes silent.

Also when I disconnect the pickups the unit goes silent.

Does this mean I have a pickup grounding problem, or could it still be a fault in the preamp?  I've seen stranger things...

 

[tommypiper]

Make sure that the harp is properly grounded.

Grounding the harp chassis directly to the preamp PCB, lowers buzz/hum by ~25% using the neck pickup, and lowers it ~50% using the bridge pickup.  Still not a fix, but does indicate something weird with grounding and pickups maybe...?

Put a voltmeter across the 100µF and see what happens when the signal dies...

The voltage is very steady at 7.73v across the 100uF.  (Would that be expected normal voltage?)  When I switch between the pickups back and forth for a minute or two eventually something gets triggered/shorted, there's a thump in the audio and the voltage drops to 0 almost immediately, then comes back up to 7.73.  This is just from toggling the pickup switches...  At one point I even got a momentary negative voltage of -3 or -4v during one of these outages.

 

[mad.ax]

Quote: “lowers buzz/hum by ~25% using the neck pickup, and lowers it ~50% using the bridge pickup”
I would say normal behavior. That's one of the flaws of the Clavinet switching scheme. If look at the schematic, you will see that when the left switch is in the upper position, the left pick-up is not grounded, but is still connected to the 'rot' side of the input transformer. In this configuration, it acts like a big antenna and picks up all the noise it can!
So keep the harp grounded as it should.

“The buzz goes away somewhat when the two pickups are played out of phase with each other”
Normal behavior too. Hum-bucking effect.

Toggling the pick-up switches should NOT affect the power supply! This points out either a bad grounding scheme, either a mechanical issue (bad battery connector, bad ext PSU connector, bad power switch, cold solder joint...)

You could try to connect a good 9V DC source directly on the PCB, across the 100µF, and see if it solves the issue...

Axel

 

[RuudNL]

The voltage is very steady at 7.73v across the 100uF.  (Would that be expected normal voltage?)

Yes. The voltage is 8.2 v (zener) minus the B-E voltage of the transistor (~ 0.6 V).

 

[tommypiper]

Toggling the pick-up switches should NOT affect the power supply! This points out either a bad grounding scheme, either a mechanical issue (bad battery connector, bad ext PSU connector, bad power switch, cold solder joint...)

Axel

I wonder if I have a bad transistor which occaisionally shorts to ground and that would draw down all the voltage from the 100 uF and turn off the amp.

What I don't understand is why toggling the pickup switches would trigger it, or whatever it's triggering.  Grounds are stock and this instrument worked fine previously.

 

[QUEEF BAG]

most of the ones i have had to dick around with have been D6 models

with them it's mostly lack of shielding.  if the pickups are not shielded it's like
a single coil fender in a bad rehearsal space.

i have seen people make shields for the pickups and they help a lot!

if they are stiff metal instead of foil, they should be shock-mounted or
they may increase microphonic noise from the chassis.

 

[tommypiper]

Update:

1) I found the neck pickup cable shield was touching the metal shielding below the harp and causing a ground loop of sorts.  Insulating that has helped.  The PCB is wired to the harp chassis ground at only the bridge pickup normally, so a second connection of harp chassis -> PCB at the second pickup may have been causing extra noise.

2) Something is very funky with the voltage regulator. Voltage is now around 4v across the 100 uF and drops to 3.xx whenever I toggle the pickup switches.  The 3.xx voltage is so low that the amp sags and dies.  Voltage remains in the 3's and after some seconds returns to 4.xx and then revives.  So, instead, putting a 9v battery straight across the 100 uF I get a steady amp and no dropouts.  Toggling the pickup switches has no effect anymore on the power or amp dying out.  

Maybe it's super sensitive to lower battery voltage?  My battery was running ~8.8v under load.  Does it take about that much to pass through the voltage regulator (8.2 zener + .6 transistor)?  How would that give me 3 or 4 volts though?...

Q: Why would toggling the pickup switches cause the voltage to drop?  It's not a mechanical or switch issue, nothing's loose, I've tested that.  Is it a load issue?  

I thought maybe I had a bad BC 550 transistor or cap, but it runs fine with the voltage regulator bypassed, and I've replaced all caps except the 8.2 pF.  (See below on the 8.2 pF.)

3) The PCB requires the metal switch bracket to be securely screwed to the PCB in order to complete a ground circuit.  Testing the PCB without the switch bracket screwed on results in a dysfunctional amp with no signal.  (When I press a finger on, or in the air near, the first transistor or the 8.2 pF in this state I get a squealing sound!  What causes that?!!)

So I may experiment with a jumper to bypass this ground connection to make a more reliable connection and use insulating washers on the switch bracket to prevent a loop...

With these changes (bypassing the voltage reg and a jumper instead of switch bracket and fixed ground shield in neck pickup) I'm getting more reasonable levels of noise.  When I hold the metal cover over the pickups noise drops further to still-annoying-but-probably-normal and usable levels...

TBC...

EDIT: If I decide to wire a battery harness that bypasses the voltage regulator, how can I put in a protection diode to prevent an accidental reverse polarity from frying the circuit...? (Maybe I can find something similar in guitar pedals, I'll research..)

 

[tommypiper]

most of the ones i have had to dick around with have been D6 models

with them it's mostly lack of shielding.  if the pickups are not shielded it's like
a single coil fender in a bad rehearsal space.

i have seen people make shields for the pickups and they help a lot!

if they are stiff metal instead of foil, they should be shock-mounted or
they may increase microphonic noise from the chassis.

Funny.  The D6s that I've seen have less noise than my super shielded E7 which has never been noise free...

 

[mad.ax]

(1) That's a progress. This ground loop issue was certainly not 'stock'

(2) Now you know that the circuit works fine with a proper power supply. From there you have two options: If you plan to use it on batteries only, just add a diode like in gtr pedals and be done with it. The regulator was mainly aimed at filtering external PSUs anyways.

Or fix the regulator. 4V is definitely not enough. Strange that you now measure  4.xx, and 7.73V 3 days ago, though... Is your battery discharged?

Keep in mind that the Clavinet IS a batteries eater... That's why they added the regulator!

Quote: “The D6s that I've seen have less noise than my super shielded E7 which has never been noise free...”

The Clavinet always been noisy. Therefore many of them have been moded or shielded in order to reduce the noise... Perhaps the D6 you have seen where 'improved' ones?

Axel

 

[geoff004]

Has anyone tried these Jim Williams clavinet mods:

http://www.gti.net/junebug/clavinet/d6upgrade.htm

Are the amps the same in the D6 and the E7?
I finally cracked my E7 open today and it doesn't look anything like the pictures.  Of course that doesn't necessarily mean that it's all too different, but I don't know.

 

[tommypiper]

Geoff, the D6 is different.  I'll attach schem.  I don't care for all the JW mods though the basics are good to do.  Probably replacing the transistor is good (D6 only) as it's higher gain.  I think replacing many of the caps is a good idea but his obsession with bypass caps is over the top IMHO.  There's no need to add "air" to this circuit IMO, there's plenty of top end already and in fact it's the bottom end and a richer tone that I want.  Your E7 is doubtless the same as mine, they do not look like the D6 at all.  I replaced all the caps, and despite the noise problems in mine I can already tell the tone and clarity is a bit better.

 

[tommypiper]

(1) That's a progress. This ground loop issue was certainly not 'stock'

(2) Now you know that the circuit works fine with a proper power supply. From there you have two options: If you plan to use it on batteries only, just add a diode like in gtr pedals and be done with it. The regulator was mainly aimed at filtering external PSUs anyways.

Or fix the regulator. 4V is definitely not enough. Strange that you now measure  4.xx, and 7.73V 3 days ago, though... Is your battery discharged?

Keep in mind that the Clavinet IS a batteries eater... That's why they added the regulator!

Axel

Thanks Axel.  I'm done with this round of fixes and the Clav is sounding GREAT!   ;D

1) Yes, correcting the second shield contact helped, and reduced noise.

2) I wired a direct input for the battery, bypassing the voltage regulator.

I considered a diode protection circuit, but decided I'd like to fuse it as well, didn't have all the parts, so I'm going direct for now.

I don't know why the battery voltage was sagging with the regulator.  I believe it was the load when switching from one to two pickups while toggling the switches (while the battery was below 9v) but who knows?   It works fine with a wall wart supply into the regulator.  I will eventually replace the regulator transistor and diode just in case...

3) I bypassed the ground connection made by the switch housing with a wire jumper.  Doubt if it helped noise, but it allowed me to run and test the preamp as a naked PCB without the switch housing.

4) I found a lot of noise was entering the pickups from the side of the Clav, where there is no metal shielding -- the one place where there is no shielding.  It was a desk fan I use to blow solder smoke away.  Even though it wasn't running, it has a circuit with a temp control etc which was radiating noise, 18 inches away!  

Now my clav is quiet and sounds great!