1937 Admiral 488 Radio Dial Belt Repair

From Rain City Audio:

Most tube radio repairs at Rain City Audio are full overhauls, but occasionally there’s something that’s just in for minor maintenance, like this Admiral AM 488 from 1937. This radio was fully restored in exacting detail both electrically and cosmetically by a gentleman several years ago who has since retired. This radio served faithfully in its new home in Spokane, WA for many years but a few problems recently came up which prompted its trip across the pass and into Seattle for some work.

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This radio came with two problems: first was that the volume was very loud and the control no longer responded; second was that the tuning belt had snapped and could no longer adjust the station.

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The volume control issues was very easy to sort out: the pot’s lock nut was loose, allowing the control itself to rotate; this motion ultimately caused the low side of the volume control to break free, causing it to no longer work as a voltage divider and making the volume grow much louder. Simply re-soldering that connection and tightening it down was all it took to fix. Then, it was on to a more challenging problem, the belt.

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Dial strings are more common than belts and are, in my opinion, a bit more challenging to replace. Not physically speaking, but because they weren’t really made with the understanding that belts break in mind, and thus you’re left to figure out how to wrangle the mechanism to get a closed belt around all the necessary pulleys. A string, in contrast, is usually threaded and joined as the last step. In this case, the first task was to remove the retaining ring and celluloid dial scale, exposing the bulbs (one of which was burnt out, and was replaced) and the gearing. This radio especially was challenging because it uses a two-speed, two-hand reduction gear system for the dial scale.

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Removing one screw let the top pulley loose, allowing you to remove the gear and thread the belt in from the top.

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Next was the lower half. The belt wraps around two guides, then to the lower string pulley. By loosening the lock-nuts and pushing the shaft inward, a gap opened up between the front plate allowing the belt to be slipped over the lower pulley.

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Now, just to re-install the scale and pointers, then re-align the dial with the tuning capacitor.

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This radio had developed some hum, too. I replaced the electrolytic capacitors in the power supply for good measure, which significantly improved the hum and improved the richness of the tone as well.

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That’s it for this radio! After a trip back across the mountains to Spokane, its owner reported it was as good as new.

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Posted in Radios and Tubes, Vintage | Tagged , | 1 Comment

Bose® 901 Series IV Active Equalizer #509052

From Rain City Audio:

This Bose® 901 Series IV Active Equalizer, #509052, was just purchased and sent to Rain City Audio for an overhaul.

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This unit was originally a 220V model but had already been converted to 110V on a previous repair. The op-amps had been replaced as had a couple of electrolytic capacitors.

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Unfortunately, the pads and traces on the bottom of the unit were badly damaged around the op-amps. Ultimately, it ended up being too risky to replace them, but since they had already been replaced with reliable models this wasn’t a major loss.

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New electrolytic capacitors, and silver solder later, and this one works and sounds good as new.

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Mercury Switch Find

Digging through a seldom-used parts bin, I made a cool discovery: a set of four mercury switches, still intact:

These are used as tilt and vibration sensors in mechanical systems. They’re heavy from the mercury inside, and it sloshes around with a very satisfying motion. I’m not sure I’ll ever need these for repairs, but they’re probably worth keeping in a drawer!

Posted in DIY, Gadgets | Tagged | 3 Comments

Phase Linear 2000 Upgrade and Overhaul

From the Rain City Audio Repair Blog:

I recently got to work on another Phase Linear 2000 pre-amp. These well regarded components are a fantastic part of any stereo system with their high output rating, multitude of inputs and outputs, and low distortion – especially if they’re paired with a Phase Linear power amp. This particular one came from it’s original owner where he purchased it in the ’70s and variously used it as a stereo amplifier, a guitar pre-amp, and more.

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The owner reported it was sounding uneven between the channels and generally not working as well as it had. No wonder, it’s on all original parts!

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First up was a re-cap, and that did improve performance, it was still very uneven especially on the phono inputs and had an incorrect response curve. Phase Linear released a service bulletin a few years after these came out warning about the longevity of some of the early op-amp chips they used. While the chips in this particular model weren’t in the date range called out, they are known to have a reputation for failing, so it was time to purchase some new chips.

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The RC4136 op-amp is manufactured today, but the RC4739s have been out of production for many years, as they didn’t really fill a distinct niche and also had a non-standard pin-out. Fortunately, there are drop-in replacements available which use a very much improved NE5532A op-amp chip instead.

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In addition, I hand matched and replaced the passive components surrounding the phono pre-amp which delivered perfect channel balance and a correct RIAA equalizer curve.

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Both RC4739 chips got the same treatment. Time for testing:

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Testing showed that distortion was within spec, no doubt helped by even better op-amps than original, and the channels were balanced within 0.5 dB of each other through the full frequency range.

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Now it’s good as new!

Posted in Audio, Electronics, Hi-Fi, Stereo, Vintage | Tagged , , , , | 1 Comment

AUX in Volvo Stock Head Unit Radios

Dangerous Prototypes shares an article detailing how Karl Hagström hacked an aux input to his 2007 Volvo V70 stock stereo.

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Why, why, why isn’t there an AUX input on my car stereo from 2007?
Yes 2007 was before the big era of smartphones, but everyone owned a couple of dirt cheap mp3 players and iPod was a big thing.
The HU that my car is fitted with has two super retro 8-pin DIN-connections on the back. One of which is for connecting a CD-changer “CD-CHGR” that you could have installed in the boot of the car – but who uses CDs these days?
It didn’t take allot of research to find out there is already a product out there that lets you add an AUX to you HU-xxxx. The only drawback is that it sets you back $80 and most of all: It doesn’t come with the awesome feeling that you get when you have hacked the stereo yourself.

Even being heavily involved in electronics, I’m pretty sure my time is worth more than the $80 a manufactured adapter would cost. Looks like a neat job, though!

Dangerous Prototypes

Posted in Commentary, DIY, Electronics, Gadgets | 1 Comment

DBX SFX-10

I saw this interesting set of speakers on Craigslist, but they sold quickly before I could investigate. They’re the DBX SFX-10s. There’s a very interesting article about their design in the May 1990 issue of The BAS Speaker, the publication of the Boston Audio Society which goes into quite a bit of detail, but the long and short of it is that they’re a pair of speakers designed to provide a fully omnidirectional sound field – and in the paper, there are polar plots to show they did a great job of it.

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These are another uncommon design of an active-equalized speaker, similar to the Bose 901, EV Interface, and an early McIntosh line. Has anyone experienced these before? If you own a set, I’d be interested in studying the Soundfield Imaging Controller in my shop; I’d pay shipping to borrow it for a week or two and hook it up to my Audio Precision System One to see what it does.

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Bose® 901 Series II Active Equalizer #66380 Repair

From Rain City Audio:

This Bose® 901 Series II Active Equalizer #66380 came through for an overhaul just recently. The owner in California reported that he got these from his father, and the equalizer was sending only the right channel signal.

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After all these repairs, and a new set of matched transistors and upgraded film capacitors, it’s as good as new and sounds fantastic!

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