Clavinet C - Audio issues

[rickydemo91]

I am currently restoring a Clavinet C and have run into an issue.
Please forgive me as I am not very well versed in the field of electronics. However, I am always trying and willing to learn and would very much appreciate if anyone has any help they can provide!


I purchased and installed replacement DC jack plate from Vintage Vibe.

I am not getting any sound from the unit when I use battery or a PSU.
I noticed by accident that if I reverse the battery leads I get good tone from both pickups in all switch orientations, however it comes with a loud, constant scratching noise in the background. (I've attached a video of the sound)

If i put the leads back, no sound again.

Does anyone have any idea what would cause the unit to only work on battery with reverse battery leads? And why the accompanied noise would be so loud?

Is it likely that the preamp components have been damaged due to the reverse battery hookup?

 

[Brian Roth]

Do you have a multimeter/DVM to measure DC voltage? If so, place the red/positive meter probe on the sleeve of the output jack and the black/negative probe on either pin of the power switch (with it switched on). You should measure a positive "9-ish" volts, since that circuit is "ground is positive" polarity.

Bri

 

[rickydemo91]

Thanks for taking the time to help out Brian!

I have hooked up my DVM as you specified and I am getting - 9.63 Volts with it switched on via the Power Supply (image attached).
I get - 6.95 volts that slowly goes down with just the 9v battery hooked up.

I take it that the negative symbol on the left of the DVM is not what we were looking for, as you mentioned were looking for a positive 9ish volts.

When testing with just the battery, ofcourse the negative symbol goes away if I flip the battery around.


I've read online that Clavinet C, D7, E6 are supposed to take a Center negative9V 300mA power supply.
Am I correct in my research that the symbol on my power supply is Center Negative? (see image above)
And does that agree with the circuit telling you it is a "ground is positive" polarity?

EDIT: I had the probes backwards, the reading is positive

 

[Brian Roth]

I have no clue about the external adapter. All I can say is with the battery is what I said.

The fact with the battery reversed that it works indicates you have some sort of wiring error. The noise could be due to reverse polarity that may have degraded a transistor.

 

[rickydemo91]

I have no clue about the external adapter. All I can say is with the battery is what I said.

The fact with the battery reversed that it works indicates you have some sort of wiring error. The noise could be due to reverse polarity that may have degraded a transistor.

Sorry Brian, I just realized that I am a fool and had the probes backwards. I am now getting a positive reading of 9.65 volts with power supply and 7ish volts with the battery.

I'm thinking I will order a preamp kit to replace the components, and in the mean time I'll have to try to learn and compare the wiring with the schematic.

I take it I should not replace the components/transistors until I've remedied the wiring issue, correct?

 

[rickydemo91]

I spoke to Vintage Vibe and they were very helpful. My Clavinet now works with both the battery and power supply!

Just as Brian suspected, the wiring was incorrect. It turns out the instructions that came with the DC Jack plate were not correct for this specific model (clavinet C).

From Vintage Vibe:
"I think the issue here is that the Clavinet C preamp design uses PNP transistors and thus it is a positive ground circuit. You need to switch the polarity of the DC wiring to match this."
They provided new wiring instructions.

 

[Brian Roth]

Glad it worked out. Does it still have that crackling noise at the output?

Bri

 

[rickydemo91]

Glad it worked out. Does it still have that crackling noise at the output?

Bri

Thanks again Brian.
Yes that noise seems to have gone away!

Now I am hearing an intermittent "scratching" noise that comes and goes every few seconds. It was much louder right after fixing the wiring, and seems to have gone down in volume over time. I've attached a video of the audio.
I am wondering if that's maybe a symptom of damaged/old transistor or other component(s)?

Also, at the beginning of the audio clip, there are 2 pops heard, that is me switching the pickups. I am wondering if that is something I could likely remedy by installing a preamp rebuild kit?
Preamp kit schematic
"INCLUDED IN THE KIT":
• All Electrolytic Capacitors
• All Polyester Film Capacitors
• All Metal Film Resistors
• All Transistors

 

[Brian Roth]

You're lucky that those old Germanium transistors didn't die with the reversed power polarity!

I see that rebuild kit uses more modern "silicone" (I guess that kit would also be useful for women wanting larger breasts...lol) transistors. The added resistor at the input appears be added to adjust the bias/operating point when SILICON transistors are substituted.

Unless Hohner used really crappy "film" caps, I don't see a reason to replace those. However, ancient electrolytics are always suspect. Also, the resistors are likely carbon comp....those don't age well. New metal films MIGHT be a good idea.

If the germaniums didn't blow up, it might be a simple "rebuild" to replace the 'lytics and resistors.

How much does that rebuild kit cost?

Bri

 

[rickydemo91]

You're lucky that those old Germanium transistors didn't die with the reversed power polarity!

I see that rebuild kit uses more modern "silicone" (I guess that kit would also be useful for women wanting larger breasts...lol) transistors. The added resistor at the input appears be added to adjust the bias/operating point when SILICON transistors are substituted.

Unless Hohner used really crappy "film" caps, I don't see a reason to replace those. However, ancient electrolytics are always suspect. Also, the resistors are likely carbon comp....those don't age well. New metal films MIGHT be a good idea.

If the germaniums didn't blow up, it might be a simple "rebuild" to replace the 'lytics and resistors.

How much does that rebuild kit cost?

Bri

Haha I'll be sure to order a few sets for my wife.

I found this info

"The Clavinet C’s “Secret Ingredient” is its Germanium Transistors​

The original preamp electronics feature AC151 germanium PNP transistors that are part of the heart of the C’s signature sound. Germanium transistors have a cult-like appreciation among tone-chasing guitarists that are seeking a more “vintage” coloured tone"

Yeah, I'm very glad they didn't die from the reverse polarity!
On further inspection, it looks like someone switched out the AC151 for AC188 transistors. I wonder if there is a difference to them..

When it comes to transistors, do you know is it usually a black/white matter of either they are completely toast/blown up VS being fine?
I'd like to get rid of that scratching sound however I assume it would be ideal to keep the germanium transistors for their sought after tone.

What is your opinion on germanium vs silicone transistors?


Does that green .33 uf capacitor look like a film" cap that you speak of? (Image attached)

The rebuild kit is $20 usd + shipping to Canada.

 

[Brian Roth]

There you go again ....lol...."silicone"! Repeat after me....SILICON <g>.

No opinion here. But, maybe the germanians have some magic mojo, assuming they are still healthy.

Yes, the green one (and I'm pretty sure the white cube) are film caps.

The resistors are all carbon comps.

Bri