dbx 166 replacement power transformer?

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aurelialuz
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dbx 166 replacement power transformer?

Post by aurelialuz » Tue Jun 08, 2010 5:44 pm

i'm trying to fix this $15 junkshop special dbx 166. it doesn't power up and i know the transformer is not original (wrong mounting hole spacing), so i thought i would start there. only...the transformer spot of the schem available online is left blank, and if it wasn't, i don't think you'd be able to read it anyway since the copy is so bad. the back of the unit reads 117v 50-60hz, and the secondary of the transformer feeds into two LM7915 voltage regulators, so i'm thinking this hammond unit.

questions:
1- does it make more sense to go with one of these?
2 - this 166 does not have switchable power, so they can't be directly the same, but the above studio electronics link seems to indicate that the 166 and the 160x, at least at one time, had the same power needs. can i check to see if the 166 will work with the transformer of my 160x?
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Post by Nate Dort » Wed Jun 09, 2010 7:34 am

According to page 17 of the 166 service manual, the secondary of the power transformer produces 48VAC, which is rectified to +/- 24VDC, and then regulated by the 7815 and 7915 regulators to +/- 15VDC. You'll need a 48VAC secondary with a center tap to ground, or a 2x24VAC secondary, and connect the windings in series, tying the junction between them to ground.

I'd probably go with this toroidal from Antek:
http://www.antekinc.com/details.php?p=23

It's a dual 22VAC secondary, but once rectified, that will be plenty of DC for the regulators to function, and they'll probably run cooler as they won't have to drop the voltage so much.

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Post by CFB4 » Wed Jun 09, 2010 1:55 pm

I have one: a stock 110 VAC traffo from a USA dBX 166a. The deal is, I live in Ireland and bought my dBX 166a from a guy in England. It was a US model though so I used to cart around a 220-110 volt step down to run it. Finally the UK dBX people got it together and sent me a 220V traffo, I swapped them, and so I have the 110 Volt sitting on my desk - and I'm not moving back to the US :)

Anyhow, PM me if you want it ....
Last edited by CFB4 on Thu Jun 10, 2010 2:08 am, edited 1 time in total.
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Post by The Scum » Wed Jun 09, 2010 3:19 pm

It sounds like this might be solved, but a couple of notes:

The 166 is using the unregulated +24 Volts for the LEDs in the curcuit. While the rest of the thing probably works just fine with an unregulated supply more like +/-18, the LEDs mght not be as bright, or light at all, with a lower unregulated supply.

That said, transformers are rated for their loaded RMS Voltages. Peak values will be higher than the RMS, and higher still at less than max load. So if push comes to shove, an 18, 20 or 22V transformer will very likely work. As for size, the specs state 15 Watts dissipation, so look into 15 VA transformers. Amveco, Talema and Antek are the usual suspects.

Looking closer at the manual, they say the unregulates supply can be as far off as +/-3 VDC with 750 mV ripple...meaning about 20 V.

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Post by CFB4 » Thu Jun 10, 2010 2:11 am

Good point, I've never been inside a 166 so I mistakenly thought the 166a was the same. Could work, but one does want the LEDs glowing at full brightness
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Post by aurelialuz » Thu Jun 10, 2010 8:43 am

thanks a lot folks, very helpful information. so in light of the LED issue (har har), would this be a better choice than the dual 22v?:
http://www.antekinc.com/details.php?p=24
how much overage can those regulators take, anyway?
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Post by CFB4 » Thu Jun 10, 2010 9:43 am

the regs can take a substantial max voltage - problem is that they'll run hotter. Why not try dBX spares - you'll get the exact traffo with correct hole spacing & rating.
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Post by The Scum » Thu Jun 10, 2010 11:13 am

Actually, I wouldn't be surprised if the 22V one, rectified, filtered and unloaded, gave you 28VDC.

The regulators need about 18V input to run, with a max of 35V. I doubt you'd even notice the LED brightness change between different unregulated voltages

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