Wurlitzer 720A (B era) restoration

[wordsandsigns]

I'm restoring my 720A (B era) - see the doc wurly page here: https://docwurly.com/700-series-consoles-keep-em-straight-720-720a-720b-726-b/
It's the solid state version with a 140B amp.

The main issue I'm having is hum in the speaker.  Also when I first switch it on, there's a fuzzy sound that comes through the speaker, and intermittently there's some fuzzy sounds and pops that come through the speaker.

So far, I've replaced all of the polarized electrolytic caps, as well as some of the resistors.  I tied in and soldered some of these components into the leads of the original components, and there's a slight burning small when the amp is switched on which I bet is caused by these tie ins.
I also removed the cable that runs to the phono input, which reduced some noise and increased the clarity of the amp.  I also replaced the built in power cable with an IEC port and removed the "death cap" that runs from the 120V power source to the fuse.

I'm still planning to:
-solder the replacement resistors straight to the circuit board to try to get rid of the burning smell
-replace the can caps with electrolytics mounted inside the chassis (I have some ideas of where to put the replacements, but suggestions are appreciated)
-replace more caps and resistors (I have ordered this kit from Tropical Fish Vintage and haven't installed all of the components yet: https://www.tropicalfishvintage.com/keyboard-parts/wurlitzer-140b-amplifier-rebuild-components )
-I also want to get into the volume and tone knobs on the cheek block and apply contact cleaner.  Does anyone know how to access these potentiometers?

Do you have any other suggestions for removing the hum from the amp?  Am I on the right track?

I also will need to make adjustments to the action of the keyboard, but I want to take care of the electronics first.  Some keys require more force to hit than others.  There are also some keys that are not muting properly.

I can update with some photos later if that would be helpful to see what's going on in and around the amp.

Many thanks!

[Alan Lenhoff]

Regarding the hum, I'm not familiar with your console model, but on my 140B, the piano will hum badly when the lid is removed. The inside of the lid is painted with electrostatic paint, which provides shielding. The lid also is grounded by being tightened so it makes good contact with a braided cable under the lid.

Does the lid on your piano perform a similar shielding/grounding function?  Have you been testing your amp with the lid removed?

Beyond that, the 140B amp is an early solid state design. Re-building the amp will improve it, but some hum and hiss is to be expected. If you need yours to be dead silent for recording, you might want to explore a modern replacement amp from Retrolinear.com (their Warneck Research amps) or other suppliers.

Alan

[wordsandsigns]

I had been testing it with the lid removed, and I'd also wondered if the hum would go away if I put the lid back on.

But then yesterday I turned it on, same as I had before with the lid off, and it was dead silent.  The piano sounded great and clear through the amp, and there was no hum.  Amazing!  I'm not sure what changed, but it sounds great.

Still a small burning smell, so I just soldered the high watt resistors straight to the PCB, and it's still very quiet.  There's still a slight burning smell, but I'm thinking that's just the smell of the resistors getting hot, and that I didn't notice it before because the lid was always on?  Either way, I feel better knowing that those components are making a solid connection with the rest of the circuit.  And in the future I may replace them with higher watt resistors and see if that helps.

I'm going to pause replacing electrical components for the time being because I'm very happy with the sounds, but I have a couple more questions:

-The little cap that goes over the coupled LED/photoresistor came off.  Right now I have it taped onto the board, is there a better way to attach it?  The glue/adhesive between the LED and photoresistor has also come undone, is there a best way to reattach these?    These are ocmponents 20 and 21 in the attached schematic
-You mentioned a shield cable that's attached to the lid screws.  I'm attaching an image of a shield cable at the back of the piano that appears to have been cut.  Should I reattach it?

My next step is to adjust the action on the keys.  As I mentioned, some keys need much more force to play than others.  Also some keys are not damping properly.  Is there a guide to performing this kind of maintenance?

Thanks for all your help!

[wordsandsigns]

Here's a picture of the high watt resistors.  The green ones get the hottest when the amp is on, and they look a little sickly and twisted.  That's my best guess as to where the burning smell is coming from.

[Alan Lenhoff]

>>There's still a slight burning smell, but I'm thinking that's just the smell of the resistors getting hot, and that I didn't notice it before because the lid was always on?>>

Those resistors get very hot.  I've generally seen them mounted a bit above the board, presumably so they don't burn the board.

>>-The little cap that goes over the coupled LED/photoresistor came off.  Right now I have it taped onto the board, is there a better way to attach it?  >>

The original is an incandescent lamp. Maybe yours has been modified. I know Vintage Vibe was selling an LED replacement mod.  Maybe check with them? 

>>I'm attaching an image of a shield cable at the back of the piano that appears to have been cut.  Should I reattach it?>>

If the shielding on the 720A is accomplished like the 140B, that lid definitely needs a connection to ground. If you look closely at the extreme left of this photo (which is of a different Wurli model), you can see a braided ground cable that will make contact with the left edge of the lid when the lid is replaced. https://paleophone.net/?attachment_id=2035

>>As I mentioned, some keys need much more force to play than others.  Also some keys are not damping properly.  Is there a guide to performing this kind of maintenance?>>

Yes. The official service manual goes through this in great detail. I'd never regulated a piano before, but following the manual's instructions, my 140B now plays like a dream. Lubing the action centers is a great place to start before you start making let-off or lost motion adjustments. I tried to attach a copy, but it is larger than this forum will accept.  If you need one, send me a private message with your email, and I'll send it to you.

Alan

[wordsandsigns]

Thanks for all your input.

I lifted the resistors a little bit above the board and that helped a lot with the burning smell.  Makes sense that they shouldn't be in contact with anything when they're that hot!

And I'd love a copy of the service manual.  I'll pm you now.

Thanks again, this is super helpful!

[Alan Lenhoff]

I lifted the resistors a little bit above the board and that helped a lot with the burning smell.  Makes sense that they shouldn't be in contact with anything when they're that hot!

And I'd love a copy of the service manual.  I'll pm you now.

Thanks again, this is super helpful!

Glad to help. I've sent the manual.

Alan

[pianotuner steveo]

The hum in this model is sometimes caused by a worn out volume control.

[pianotuner steveo]

Keys that have to be hit harder are usually letting off way too early. The Letoff tool is available on eBay. This is the single most common action adjustment needed in Wurlitzers.

[wordsandsigns]

Keys that have to be hit harder are usually letting off way too early. The Letoff tool is available on eBay. This is the single most common action adjustment needed in Wurlitzers.

Thank you, I see the letoff tool on ebay for the 720s.  Messaged them to make sure it'll work for mine, then I'll order it.

[wordsandsigns]

Thanks for the tips!  The hum is gone, but there's still some crackling and fuzzy sounds most times I turn it on and play.  I have some time off work in a couple weeks so I'm going to replace more electrical components then.

I want to replace the can capacitors.  I ordered electrolytics with the same capacitance, but do you have any tips on where and how to mount the replacement capacitors inside the chassis?

[pianotuner steveo]

Crackling noises in Wurlitzers can be caused by dust,dirt, pet hair or condensation on the reeds. Condensation happens when moving a Wurlitzer in cold weather, then turning it on too soon when brought indoors. Don't keep wasting money on parts until you are sure that parts are actually bad. Also, condensation can happen if you clean the reeds with compressed air.

If hammers are letting off too early, use the tool to adjust the capstan that trips the jacks. You will be turning the capstan clockwise (it's upside down, so left to right). Don't go too far or hammers may block against the reeds or even break them when hit hard. You may need to use long, needle nose pliers on the E and F n the middle of the keyboard where the metal braces are.

[DocWurly]

If it has a solid state amp, it's a 720B. Unless someone replaced the tube amp in a 720A. The badge (outside, under the keys, front left) should confirm this.

[wordsandsigns]

Maybe someone replaced the tube amp in a 720A?  Here's a picture of the badge

[wordsandsigns]

Thanks so much to everyone for your help.  I'm a teacher, so I've finally been able to get back to working on my wurly on my summer break.

I adjusted the let off on some keys and that really helped the action.  Forgot to lube the action centers first, so I'll do that soon and then readjust the letoff on keys as needed.

I mounted the power resistors above the board, and that's working great.  No smells, no noise.

I'd eventually like to replace the can caps, but since the piano's working well now this isn't a priority.  But does anyone have advice for mounting/wiring replacement caps when the time comes?

The vibrato isn't very deep.  I'm thinking that there might be some light leaking under the enclosure for the lamp/photoresistor coupling.  Any tips on how to get that as close to stock as possible?

Thank so much for everyone's help!

[wordsandsigns]

One more thing!  I'd like to modify the headphone jack to be dual mono instead of only coming out of the left channel.  I'm doing some research on this now, but tips are appreciated.  Thanks!

[cinnanon]

If you convert the jack from Mono to Stereo, BE CAREFUL. If someone unknowingly plugs in a Mono jack, the Amp signal will be shorted to ground and you'll likely blow the amp. The safer option is to make an external "Mono-to-Stereo" converter using an aluminum Hammond box or something similar.

Most amplifiers these days have dedicated Headphone circuits that can be shorted to ground without issue. The Wurlitzer Headphone signal is tapped directly off the amplifier's power output.

[pianotuner steveo]

RE the headphone Jack: you need to replace that Jack with a stereo Jack and just connect the 2 (+) terminals together, then the sound will come through both sides. Be sure to use a Jack that cuts out the speaker.