NAD - Harmony L3500? - weird time period / anybody know this thing?

[billyjack669]

I recently acquired a 1979 Harmony Solid State Piggyback amp with 2x10 (Oxford 10j) cabinet.  Speakers, pots, etc., match the 1979 date.

It looks like online photos of an L-3500 but as you can see from pictures it only has the rear sticker model number 7096-90PB and it was built "after Harmony stopped making amps" sometime in the mid 70s.  No "L-3500 label" to be seen. 

I took photos of the inside and made a little composite photo to make it easier for me to view / trace.

Anybody recognize its guts?  The reverb "works" after I made it float again, but it's pretty weak. 

The tremolo works well, and as a bonus I figured out that you can turn the trem off if you keep turning depth CCW until it clicks (haha). 

Funnily enough (to me at least), they chose Bass to double as the power switch instead of Volume.

I read some stuff the other day about a company called Sound Design Products and a few little Chicago-based outfits that possibly assembled these amps and perhaps had extra stock to build / sell after Harmony closed up shop, but I'm starting to wonder if ChatGPT hadn't just gotten drunk on electrons and started hallucinating a lot of that "information".


Thanks!  (btw, it sounds really cool with the Revv G3 clone pedal from Mosky in front of the amp.)

[J M Fahey]

Very interesting, specially that weird tiny reverb tank.

Maybe you can take a gut or sideways picture to show how springs are driven?

It might very well be Piezo at one or both ends, like Silvertone.

As of the rest of the circuit, many of those amps were made for different resellers, often department stores, but by the same OEM supplier, so you might find a very similar one under other brand, with minimal changes.

But as is it looks very well preserved.

Those hand drawn PCBs using flexible black paper tape and stick-on donut pads certainly bring back memories

Back in the day, before widespread personal computers, I used to draw PCBs using Bishop Graphics tapes and pads.

[dmeek]

Can you post a photo of the component side by itself? The composite photo is hard to sort out.

I've designed hundreds of PCBs over the years with donut pads and tape. Prefer it to computer design because you can lay the actual components on the layout to see how much room there is. I would still like to do it that way but all my pads and tapes dried out years ago. No stickum.

[billyjack669]

I just added another version of the composite without the overlay transparency in the original post (the overlay version was hindering me last night and i kept referring to the other one anyway.)

[billyjack669]

Very interesting, specially that weird tiny reverb tank.

Maybe you can take a gut or sideways picture to show how springs are driven?


Those hand drawn PCBs using flexible black paper tape and stick-on donut pads certainly bring back memories

Back in the day, before widespread personal computers, I used to draw PCBs using Bishop Graphics tapes and pads.

When I slide the chassis out again I'll disassemble the suspension and take photos.  Where can I find new compatible "suspension" springs?  2 of the 4 are pretty played out.

Your experience with the Bishop Graphics tape is the kind of thing I love to learn.

I got into computers in Junior High (early 90s) and missed out on a lot of the earlier "pioneering" that would be lost to time if we didn't share our information and knowledge. 

Thank you!

[dmeek]

Here's a messy first draft. There may be mistakes. *added some minor corrections

You cannot view this attachment.

[billyjack669]

Wow.  Nice work! Now it's documented online for the ages
! 
When I pull it back apart and photograph the reverb tank, I'll take closeup shots of whatever component labels are visible and report back with those too. 

[J M Fahey]

Can you post a photo of the component side by itself? The composite photo is hard to sort out.

I've designed hundreds of PCBs over the years with donut pads and tape. Prefer it to computer design because you can lay the actual components on the layout to see how much room there is. I would still like to do it that way but all my pads and tapes dried out years ago. No stickum.

You can get a way around that problem, with some limitations:

the hardest part is the black crepe tape.

Still available down to 1/16" width which roughly means 62 mil track width, an old PCB standard.

https://macphersonart.com/product/144311/Crepe-Graphic-Tape.html

Not sure about a fresh donut source but you can "revive" old ones by pulling one from carrying sheet and lightly wiping back on some paste adhesive, maybe stick adhesive holds?

I used them up to early 90s when I bought my first desktop PC specifically to run Tango PCB and Protel Autotrax for boards and Corel Draw 2    for front panels and Magazine ads.

The good old days. 

PS: you can draw round pads with a transparent center hole using Architect´s China Ink pens (0.8 o1 or 1.2 mm tip) and a "circles" draftsman ruler.





Saved my bacon many times.

[dmeek]

The most important property of the glue on the pads and tape is that it's
re-stickable since often things need to be re-posistioned.

It's like the glue on masking tape. I don't know where to get that or how you would
prevent it from peeling off the pad. Maybe there's a primer

[J M Fahey]

With my half cooked workaround, you can do that (literally "rip and reroute") with tape, which is the most complex partb of thejob.

You would be stuck with drawn or permanently glued pads though.

There is a so called Tolex Glue which is water based.

It remains tacky when dry, for a long time (many days at least), it works as a permanent contact cement only if you apply it to both surfaces, even so not *that* permanent, British amps are cabinets (Marshall, Laney) are (in)famous because cabinet wrapped in back edges often get loose, and have to be reglued.

IF used on a single surface you can stick and pull it from a non porous surface, such as drawing Mylar, then reattach elsewhere.

I have used it to attach acrylic or thin aluminum front panels (think Marshall Plexi or golden aluminum sheet) to punched amp chassis.

Not saying you buy a bottle for testing but IF you re Tolex a cabinet using it, you might try some leftover to check it.

[dmeek]

I researched a little and found this type of glue is called Pressure Sensitive Adhesive (PSA) It comes in spray cans too. 3M, Gorilla etc. Not hard to find. I'll try it out.
I could stick the donut pads upside down to a post-it note sticky area and then spray.

[Tassieviking]

I used to use carbon paper to transfer the tracks onto the PCB's and then "borrow" some nail polish from mum to paint the tracks onto the copper.
It felt like watching paint dry when you waited for the acid to etch the boards, then a little "borrowed" acetone to clean the nail polish off with.
Some of my early PCB's were a bit rough but I don't remember any not working.

[J M Fahey]

Exact same here 

[dmeek]

I tried many methods early on: wax crayons, stick tapes directly to the copper foil,
but quickly found that the photo sensitive resist method was the best. I would make my
layout on Mylar, then contact print it to photographic film to make a negative,
which freed up the donut pads for future layouts, me being that frugal. Then I would
make contact prints of the negative to return to positives. Now I use a scanner and
laser printer for that step.

Of course it makes more sense to use a PCB program, generate Gerber files and send them
off to Korea or wherever for better boards than I could ever make, but I find the computer
method a big ordeal and annoying. And I have a good supply of photoresist in stock.

[J M Fahey]

I also went through the methods above.
I guess we old timers went through the methods popular at each "era".