Electro Voice Speakers - ZX1A - Amplifier repairs

[James Bushell]

Does anyone have any experience with the Electro Voice ZX1A active speakers. I have a pair and both the amplifier modules have died. Anyone got any information on how to repair them. I cant find any circuit diagrams or much other information on the Electro Voice Speakers - They don't seem to make anything available !!

Many thanks.

James

Based in Devon, UK

 

[Khron]

Are the amplifier modules themselves shot (lower left in your photo), or the power supplies, or both?

 

[James Bushell]

From the testing I have done to date one definitely has a dead power supply and so far found that the bridge rectifier on the input is short circuit - Looks as if someone may have replaced it previously.

The other powers up so assume power supply is OK but nothing coming from the speaker - not even the normal background hiss !?

I actually have a third module (Power Supply and Amplifier) I purchased as a replacement but this just makes a buzzing noise through the speaker when powered up.

I am hoping to make at least one good module from the three I have.

These speakers have a major design fault as the knobs on the amplifier board stick out the back of the cabinet and very prone to being knocked. If they get damaged the board is basically dead and replacing the pots is very very fiddly as there is no access.

 

[Khron]

the bridge rectifier on the input is short circuit

It's a lot more likely that the (half-bridge?) switching MOSFETs of the power supply are shorted through. That, and/or you might not have allowed your meter to charge those 1000uF / 200v caps up to your meter's test voltage (those can seem like a short-circuit, because it takes current to flow into them, to charge them up).

The other powers up so assume power supply is OK but nothing coming from the speaker - not even the normal background hiss !?

If you're brave (and ideally also have a "series bulb tester"), you could swap the amp module from the shot-PSU one to the other one, and see what happens. Preferably with a dummy-load first, not a speaker driver.

Safe to assume you've otherwise tested each woofer & tweeter that they're not toast / open-circuit?

 

[James Bushell]

I have four of these speakers. Two fully working currently configured as passive speakers. I have one speaker with a open circuit tweeter and good Woofer (Amplifier Working although one pot - Input Gain Control - damaged). I have a forth speaker I am using as a source of spares - This is the amplifier that just makes a buzzing noise when connected to speakers. The Tweeter is open circuit and I purchased it without the Woofer present.

The Power Supply board I have been working on appears to have had the Bridge Rectifier and the FETs connected to the 250v input side replaced as the solder is fresh. Rest of the board appears original. This just blows the 4A fuse when plugged in to 240V.

I have some electronics ability (worked for Texas Instruments for many years) but I have no circuit diagram or any other information on these boards. I have both a Soldering Station and Hot Air Rework station. I have built my own pre-amps and Valve amps in the past but this Digital stuff is quite new to me (both Amplifiers and SMTP Power supplies).

My master plan was to try and find a working Power Supply Board and pair it with a working Amplifier Board !! If this fails then I will first need a good Power Supply just to test the Amplifiers !!

I am assuming I can power up the Power Supplies with no load and not do any damage !!

Electro Voice really didn't make these easy to work on - Never seen so many screws just to separate the boards !!

I still need find a Woofer and repair the two tweeters (new diaphram) or get replacements so I have a set of two Passive and two Active Speakers. Well at least that was the goal !!