Preamp noise - repairable?

[jordan86]

So its not a TUBE amp, but this is a preamp (also a mixer/DI) for acoustic guitar.  I got this unit 15 years ago. About 4 years ago the power supply crapped out on me. Uncommon 18V ac wall wart. And shortly thereafter it started making terrible noises. White noise, sort of like a jet engine. It's been discontinued for a long time so there is no support or repair from the manufacturer. They didn't even truly make it to begin with. Outsourced it. So they can't help at all.

So it's sat boxed, as I didn't want to throw it away. Didn't want to dump $100 into a diagnostic either to just learn its shot. But I've learned a ton about amp building and circuits since then, and thinking maybe I could repair it? Just wondering if the brilliant minds here might be able to offer any insights.

Here's a video of what it is doing. Also attached some pics for reference. Just sent an email to the company wondering if they could shoot me a schematic.
https://vimeo.com/189042447/f35ebffc2e
***notice the top cover plate is off in this video. I am adjusting the master volume.

It's a high dollar unit so it would be rewarding to get it working again. It was $500 new. I wouldn't normally spend that kind of dough, but my guitar was stolen and insurance covered enough for me to "upgrade" the whole pickup system with the new guitar.

[shooter]

with a meter, scope, and schematic, you can fix anything electronic 
without a schematic, experience in "foil trace following" is real handy
being able to identify the basic blocks on the PCB comes in handy.
so;
while waiting on schematic, trace out and draw up the basic power supply section  (usually takes 3-5 passes to get real close)
determine the 1/2 split point in the signal path (pre/PA).  Figure out if there's a good break point to test each 1/2  (maybe MV)
by the time the schematic shows up you'll have a good working knowledge of the layout and ready for the fun stuff

[glass54]

Jordan

Have a look at P20 in manual for Power Supply requirements, this may help as well. (If the PS is gone, you could re-engineer a new section)

Kind regard's
Mirek
https://www.manualslib.com/download/814708/Highlander-Pro-Acoustic-Mix-Di.html

[shooter]

I went to the link twice, both times Microsoft threw up a RED page, said someone is trying to steal my bank data, call.  browser locked
just an FWI

[PRR]

> Microsoft threw up a RED page

Wasn't Microsoft. What did you click?

Ah, that page has several "bait" links.

[jordan86]

Here is the manual. I tried to attached it earlier but it is 1.2Mb. Just barely too big. Found a way to compress it more and deleted the two pages that I already attached above.

FWIW...its does run on two 9V batteries, so the charger issue is not the end of the world. But I wouldn't mind having a new charger. Its weird though bc it is 18v AC.

[jordan86]

That is very strange about the 9-12v dc power requirement? I never noticed that before. The old PS clearly says 18v ac. I opened it up and it looks like a little choke. Just a step down transformer. It also uses a 1/4" trs connector. Just odd all around.

[glass54]

Sorry about the link, worked correctly in Aussie 
Your power Supply requirement is +/-9V to +/-12 @ 4.2mA.
Obviously that was generated from your 18VAC Wallwart. You only need 12VAC to generate raw +/-DC rails, then post regulate.
If you can prove what components you have, ie + and - Rectifiers and capacitors for raw DC and regulators, then you could check
with a 12VAC transformer or anything between 10VAC and 15AVC. (Small transformer).
If it was a TRS feed from the old Wallwart, you need to check if it utilized a 3 wire supply eg 9V-0V-9V.
You have a little circuit analysis to do  . Can you recognize PS diodes and Electrolytics? Maybe a photo?
Kind regards
Mirek

[glass54]

....Just had a listen to your video. Assuming your power supply is OK (now) it sounds like you have a few problems. Obviously Master Vol sound like it has intermittent wiper contact and it also sounds like you have an open/fracture in board (?) earlier in the circuit as I don't hear a broad spectrum lower level noise of instrument plugged into unit as you wind the volume up and down.
Is it possible for you to carefully measure the DC volts on Surface mount Op Amp on second PCB below MV Control? (VERY Carefully, fine probe). DC Volts, relative to ground, Possibly pins 4 and 8 depending on the IC used?
This would determine the status of your Power Supply to some degree.
Kind Regards
Mirek

[Latole]

I see and listen the video. Slide pot you are slide look it is very dirty / scratchy. Slide pot makes a terrible noise
Did you try to clean it ?

[jordan86]

Slide pot makes a terrible noise
Did you try to clean it ?

I cleaned all the jacks and wipers with deoxit d5 numerous times.

[jordan86]

SYour power Supply requirement is +/-9V to +/-12 @ 4.2mA.
Obviously that was generated from your 18VAC Wallwart. You only need 12VAC to generate raw +/-DC rails, then post regulate.
If you can prove what components you have, ie + and - Rectifiers and capacitors for raw DC and regulators, then you could check
with a 12VAC transformer or anything between 10VAC and 15AVC.

I have the old supply somewhere boxed I opened it up and try to resolder connections. Will post a picture. I recall there only being two wires. So it might have been a TS connector. I do recall it being AC though since there was no mention of polarity. I know some of the old Line 6 equipment was this way. Had a bridge rectifier and took either AC or DC input. I’ll check for diodes and electrolytics.