More fun refurbishing audio stuff
Posted: Sun Sep 09, 2018 9:10 pm
I'm not crazy about the modern "electronics are disposable" mind-set. I understand why it exists... modern electronic devices are so complex, and use such small components that the labor cost of repairing them is usually prohibitive... but I hate to see stuff thrown away. There''s still a lot of very usable gear of a certain age that's going off to landfill (or, if we're lucky, to some sort of authentic recycling) simply for lack of some TLC and gentle servicing.
On the other hand, that means there's lots of stuff for hobbyist nerds like me to pick up for-cheap. I've been lucky lately at my local electronics-oriented swap-meet/flea-market.
A couple of years ago I picked up a thoroughly filthy Proton D940 receiver - I think it had been stored in a barn - with a broken power switch. I paid around $30 for it. After a thorough blowing-out, some contact cleaner on the switches, a power switch, and a tuner alignment, it was ready to make music. This is a really sweet little receiver - 40 watts/channel continuous, a huge ability to handle musical peaks without clipping, and a sensitive tuner with the Schotz noise reduction circuit. I stuck it on a shelf until I needed it.
That turned out to be May, when I stumbled across a pair of Minimus 77 mini-speakers... one of the long line of metal-cabinet bookshelf speaker systems Radio Shack marketed... they're surprisingly good for the price, and there's a big cult-enthusiast community updating and modifying them. The ones I found had rotted-out surrounds (inevitable at this age). I got the pair for under $10, replaced the surrounds (another $10), built an upgraded crossover (the "Zilch" design), and put them up in one of our bedrooms with the Proton receiver driving them. Very nice-sounding although they're too small to have much "low end". They really need to be used with a subwoofer, so I've had my eyes open for one (or for drivers that I could use to make one from scratch).
A subwoofer was this Saturday's big find. One seller had a Definitive Technology ProSub 800 (8" cone, 8" passive radiator, and a MOSFET power amp and active crossover built in). Quite good reviews on-line. It looked to be in very decent shape, so I asked about it... he said "it works, sounds good, but the control knob on the back is broken off". Sure enough, the low-pass crossover control knob was gone - the shaft was broken off inside the plate amplifier. For $45 I figured it was worth a try - the cones were pristine - so I bought it.
The control was well and truly busted - I think the owner must have slammed the knob against a wall somehow and jammed the shaft into the amplifier. I took out the amplifier and found that the potentiometer on the amp board was completely shattered - the body was broken and the leads were bent. No way it would work, in that condition - I rather suspect that the seller thought he was scamming me by selling me a dead subwoofer. A look at a schematic, and a few minutes with a soldering iron improvised a solution - I took out the pot, and jumpered around it with a couple of 7k-ohm resistors (fixing the crossover frequency at about 90 Hz) until I can get a suitable "dual 20k" replacement pot. Closed it up, hooked it up, and tried it, and it works... it can rattle the room if I turn it up too far. At a modest setting it works very nicely with the Minimus 77s - it fills in the low octave-and-change that the little bookshelf systems can't manage.
A few minutes after I bought the subwoofer, I saw another guy selling a sadly-dirty-looking pair of Minimus 7 speakers (the original, smaller variety). He wanted $5! I paid. Got 'em home, found that both were filthy behind the grills (easily cleaned) and that one had a badly-busted tweeter. Best as I can tell, somebody dropped that cabinet onto its back from a height, and the weight of the shielded tweeter assembly actually broke the tweeter loose from its mounting plate (the screw attachment points were shattered). Disassembled the tweeter, carefully epoxied the plate back together, sanded it down flat when the epoxy cured, re-mounted and re-sealed the tweeter. Put everything back together today, and after a short break-in period both speakers sound good. The repair seems to have been successful and there's apparently nothing else wrong with them other than dirt and some paint chips. I did the "quick" version of a crossover upgrade for them (replaced the electrolytic cap with a film cap for better fidelity) and put them in the garage to wait for a new home. These will probably end up being hooked to our TV, as A/V speakers.
On the other hand, that means there's lots of stuff for hobbyist nerds like me to pick up for-cheap. I've been lucky lately at my local electronics-oriented swap-meet/flea-market.
A couple of years ago I picked up a thoroughly filthy Proton D940 receiver - I think it had been stored in a barn - with a broken power switch. I paid around $30 for it. After a thorough blowing-out, some contact cleaner on the switches, a power switch, and a tuner alignment, it was ready to make music. This is a really sweet little receiver - 40 watts/channel continuous, a huge ability to handle musical peaks without clipping, and a sensitive tuner with the Schotz noise reduction circuit. I stuck it on a shelf until I needed it.
That turned out to be May, when I stumbled across a pair of Minimus 77 mini-speakers... one of the long line of metal-cabinet bookshelf speaker systems Radio Shack marketed... they're surprisingly good for the price, and there's a big cult-enthusiast community updating and modifying them. The ones I found had rotted-out surrounds (inevitable at this age). I got the pair for under $10, replaced the surrounds (another $10), built an upgraded crossover (the "Zilch" design), and put them up in one of our bedrooms with the Proton receiver driving them. Very nice-sounding although they're too small to have much "low end". They really need to be used with a subwoofer, so I've had my eyes open for one (or for drivers that I could use to make one from scratch).
A subwoofer was this Saturday's big find. One seller had a Definitive Technology ProSub 800 (8" cone, 8" passive radiator, and a MOSFET power amp and active crossover built in). Quite good reviews on-line. It looked to be in very decent shape, so I asked about it... he said "it works, sounds good, but the control knob on the back is broken off". Sure enough, the low-pass crossover control knob was gone - the shaft was broken off inside the plate amplifier. For $45 I figured it was worth a try - the cones were pristine - so I bought it.
The control was well and truly busted - I think the owner must have slammed the knob against a wall somehow and jammed the shaft into the amplifier. I took out the amplifier and found that the potentiometer on the amp board was completely shattered - the body was broken and the leads were bent. No way it would work, in that condition - I rather suspect that the seller thought he was scamming me by selling me a dead subwoofer. A look at a schematic, and a few minutes with a soldering iron improvised a solution - I took out the pot, and jumpered around it with a couple of 7k-ohm resistors (fixing the crossover frequency at about 90 Hz) until I can get a suitable "dual 20k" replacement pot. Closed it up, hooked it up, and tried it, and it works... it can rattle the room if I turn it up too far. At a modest setting it works very nicely with the Minimus 77s - it fills in the low octave-and-change that the little bookshelf systems can't manage.
A few minutes after I bought the subwoofer, I saw another guy selling a sadly-dirty-looking pair of Minimus 7 speakers (the original, smaller variety). He wanted $5! I paid. Got 'em home, found that both were filthy behind the grills (easily cleaned) and that one had a badly-busted tweeter. Best as I can tell, somebody dropped that cabinet onto its back from a height, and the weight of the shielded tweeter assembly actually broke the tweeter loose from its mounting plate (the screw attachment points were shattered). Disassembled the tweeter, carefully epoxied the plate back together, sanded it down flat when the epoxy cured, re-mounted and re-sealed the tweeter. Put everything back together today, and after a short break-in period both speakers sound good. The repair seems to have been successful and there's apparently nothing else wrong with them other than dirt and some paint chips. I did the "quick" version of a crossover upgrade for them (replaced the electrolytic cap with a film cap for better fidelity) and put them in the garage to wait for a new home. These will probably end up being hooked to our TV, as A/V speakers.