Altec 801-8A dividing network repair.

[alancohen]

I bought a pair of 801-8As for my A5s before I decided to build the Hiragas. One didn't pass HF so I shelved them for the time being. Now I'm selling them as is, but getting lots of questions.

So I decided to open them up and see if I could diagnose the issue. All connections seemed OK and all caps looked and measured good, so I thought I'd put my new signal generator/scope to work. I set the SG to a 2000Hz sine wave and hooked it up to the XO inputs. I started tracing the HF signal and it was fine entering the L-Pad but that where things went bad. I soaked the L-pad with Dioxot and finally got a good output signal with the knob wide open, but as soon as I started to attenuate, the signal got squirrely.

Prognosis: It will be fine with a new L-pad.

So does anyone know off hand which Parts Express L-pad will fit the bill?


As a side note, while I had the scope hooked up I ran the signal generator through the range of frequencies from 100Hz to 15K. It was fun to see the LF voltage output drop as the Hz rose. I could have plotted the slopes if I felt like it, which come to think of it, might be a an interesting way of profiling what a crossover is doing independent of its reaction with the drivers. If I'm not careful, I might actually learn something.

[bowtie427ss]

You're giving up too easily. If it's salvageable that stock pad is made with as good or better materials than most any replacement you'll find. Leftover NOS Violet L-pads are getting hard to come by these days, and they're the only thing i'd offer as "better".

Those old pads Altec used will come apart with a little persistence and care. A light buffing of the wiper and resistive NiChrome winding with some deoxit soaked extra fine steel wool and follow up with de-oxit preservative should buy you another 20 years out of that old pad.

Unless it's literally burned up, there's not much to go wrong in there, it just gets tarnished and cruddy over time. If you have really bad luck like me, some slag of a backyard technician will have sprayed WD-40 in the pots which will clean up with hot soapy water and a lot of love for the hobby.

[alancohen]

You're giving up too easily. If it's salvageable that stock pad is made with as good or better materials than most any replacement you'll find. Leftover NOS Violet L-pads are getting hard to come by these days, and they're the only thing i'd offer as "better".

Those old pads Altec used will come apart with a little persistence and care. A light buffing of the wiper and resistive NiChrome winding with some deoxit soaked extra fine steel wool and follow up with de-oxit preservative should buy you another 20 years out of that old pad.

Unless it's literally burned up, there's not much to go wrong in there, it just gets tarnished and cruddy over time. If you have really bad luck like me, some slag of a backyard technician will have sprayed WD-40 in the pots which will clean up with hot soapy water and a lot of love for the hobby.

Great...another project Thanks, I'll get it outta there.

[bowtie427ss]

Great...another project Thanks, I'll get it outta there.

The satisfaction of fixing them will make you feel good, and you can put the money you'd have spent on replacements into something for one of "your" projects.

[alancohen]

The satisfaction of fixing them will make you feel good, and you can put the money you'd have spent on replacements into something for one of "your" projects.

Even better than that, I was going to sell the for $100 as is. Now I get double plus saving the money on new ones. If this works, I just got my Hiraga's for free. Hoo-ah.

[alancohen]

Well...not exactly a straight up clean job. I got the L-pad out and open, no biggie. But there is a break in the wire coil right at the open end. This is why it was passing HF when turned up, but stopped as soon as the knob was turned down a bit.

The wire is .018" in diameter and I happen to have some .018 guitar strings around. I'll try soldering in a replacement.



- - - Updated - - -

So I tried making an extension for the coil, but I made the mistake of washing off the dioxit in soap and water first. There is a paper insulation under the coil and over the aluminum core and it started disintegrating.



So instead of messing with it too much, I slid the insulation back on and just extended the coil wire across the top of the mandrel and made it smooth to maintain contact and reassembled everything.



The good news is it passes HF signal throughout the range now and attenuates smoothly.



Now on to making those subwoofers...

[bowtie427ss]

I'd still call it a "success".

Thanks for sharing your adventure and posting pics.

I hate seeing those L-pads get tossed as a matter of course. It's very difficult if not impossible to get replacements that are made with the same quality. For the fella with the inclination i think it's worthwhile venture to try to save em where you can.

Ideally, if you like the CAIG products, follow up a good de-oxit cleaning with their Gold conditioner(used to be Pro Gold), it's great stuff for that application and will help keep pads attenuating smoothly for a good while.

[alancohen]

I'd still call it a "success".

No doubt. Anytime I fix something, just not making it worse is a success.

[Alien_Shore]

Alan, I'm impressed yet again with your inventiveness, creativity, electrical know-how, and persistence. You get the AUB DIY award!!

[GM]

It's very difficult if not impossible to get replacements that are made with the same quality.

Worse, these are ~20 dB attenuators designed to work in conjunction with compression horn loading, so is quite a bit different electrically from the 'textbook' L-pads PE or similar sells or what a typical on-line fixed resistor designer will calculate.

GM