Tannoy Precision 8D active studio speaker dead...

[LektroiD]

Tannoy have ceased production of this model and will no longer service them. Neither will they release a service manual for this model. So I'm stuck with a pair of expensive studio monitors that serve little more purpose than a door wedge.

So far I have replaced the regulators, the rectifier, the LM3886 power amp chips, but can't seem to bring it alive. The driver is fully working, as I tested it with the other amplifier from the other speaker. I just don't know what to replace next.

Any ideas where else I could be looking or where to test next? I'm at a dead end here.

[wraper]

Measuring voltages? What is on mute pin?

[wraper]

testing != blindly replacing the parts.

[nitro2k01]

First rule of troubleshooting, thou shalt check supply voltages!

[LektroiD]

testing != blindly replacing the parts.

It's the only option, since there's no schematics or service manuals.

First rule of troubleshooting, thou shalt check supply voltages!

I have no idea what the supply voltages should be. The correct voltage is coming from the regulators, but I have no idea how it is distributed after that, as I have no info...

So really the only way I can go about this is by blindly changing parts that may be damaged. Just wondered if there was a common problem with active speakers, or if anyone knows about this particular speaker?

I have no idea where to go next, which is why I'm asking here...

[krivx]

I have no idea what the supply voltages should be. The correct voltage is coming from the regulators, but I have no idea how it is distributed after that, as I have no info...

You can figure out likely supply voltages from the IC datasheets and ratings on the filter capacitors. You don't need to know exact values, it's unlikely a voltage rail is a little off. It's fairly likely a supply has been pulled low or disconnected somewhere. What's are the voltages at the power amp IC supply pins?

[wraper]

testing != blindly replacing the parts.

It's the only option, since there's no schematics or service manuals.

wtf LM3886 datasheet is for? I have a schematic for less than a 5% of devices I repair, still able to measure somehow.

[wraper]

And yeah, replacing diode bridge??? That can be checked for sure. Just by checking AC voltage before and DC after, checking internal diodes with multimeter. Is there some voltage after diode bridge on filter capacitors at all? With such an approach you may end up reworking all pcb with just a blown fuse at fault.

[Yago]

Compare voltages/waveforms etc to the working one?

[Lightages]

A friend of mine had a bad set of Tannoys. After looking at his pictures of the circuit board I suggest resoldering every joint. They were very poorly built. This fixed his problem.

[simingx]

And yeah, replacing diode bridge??? That can be checked for sure. Just by checking AC voltage before and DC after, checking internal diodes with multimeter. Is there some voltage after diode bridge on filter capacitors at all? With such an approach you may end up reworking all pcb with just a blown fuse at fault.

... or a faulty power switch, blown line fuse, defective power cord etc...   

[Riotpack]

a chip amp like this should be simple to repair.
Start at the power supply, is there both positive and negative voltages on the amp IC?
Is there any protection replays after the IC going to the speaker? (download the datasheet and find the pinouts) Can you input a signal straight into the chip amplifier input pin? (lift factory coupling cap and feed signal in to test)

Usually after confirming power supply you simply start at one end and work through the amplifier stages until the signal ceases.

[Riotpack]

Here are the schematics for the 6D which uses the same chip amp - I am unsure of the differences but it may help.

[LektroiD]

Here are the schematics for the 6D which uses the same chip amp - I am unsure of the differences but it may help.

Thanks for the file, not sure how you managed to find it, but I wish it was as simple as the 6D, which only seems to have 2 power chips, there's 3 boards which all interconnect on the 8D, one is the power, then what appears to be a filter board, and a digital board. Looks like a pretty complex circuit on the 8D

[LektroiD]

I've done some tests, the 7915 is taking -36.0VDC in and throwing out -26.66VDC. This is a part I have already replaced, so I'm wondering if there's something else in line with it that might be causing the spurious reading?

I can't do live tests for long as the 3x LM3886 chips are roasting without heat syncs. The rear panel provides heat syncing, and I can't access anything when it's bolted to it, as you can see in the pic above...

[wraper]

I've done some tests, the 7915 is taking -36.0VDC in and throwing out -26.66VDC. This is a part I have already replaced, so I'm wondering if there's something else in line with it that might be causing the spurious reading?

I can't do live tests for long as the 3x LM3886 chips are roasting without heat syncs. The rear panel provides heat syncing, and I can't access anything when it's bolted to it, as you can see in the pic above...

Either it is dead or you measure it wrong. Common (GND) terminal of the 79xx is not the same as on 78xx regulators and is not connected to the heatsink tab.
First of all check if LM3886 is using single supply or dual supply, if don't know how, measure if pin 4 is connected to GND. Single supply will have output electrolytic capacitors of 1000s uF too. Then measure (GND referenced) if there is a power on the pins 1,5 V+, and negative supply on pin 4 if it isn't connected to GND (using dual supply) . Also measure voltage on the pin 8 (mute). I'm asking this because if the controller mutes the power amp, you wont get any sound even if amplifier is completely ok.

[LektroiD]

Seems both the 5W resistors are short, and sit before the regulators. Now I need to source a couple of these in the UK... Anyone know where I may buy a couple?

Would using a higher wattage mess things up?

[wraper]

They do not become short, only open. They might be short only if there is something dead in parallel. unsolder and check if they are 100 and 220 ohm and if there still is a short without them.

[LektroiD]

After a few tests and comparing to the other speaker, there should be 36V/-36 going in and 20V/-20V coming out of the 5W ceramic resistors. On the faulty board, it has the same voltage into the ceramic resistors as what's going out (36VDC) - would this not suggest a short? Whatever it is, something is not right there... I'll desolder them now and see what results I get...

[wraper]

After a few tests and comparing to the other speaker, there should be 36V/-36 going in and 20V/-20V coming out of the 5W ceramic resistors. On the faulty board, it has the same voltage into the ceramic resistors as what's going out (36VDC) - would this not suggest a short? Whatever it is, something is not right there... I'll desolder them now and see what results I get...

That suggests there is no substantial load after those resistors, not that they are faulty. You shall check resistors by measuring resistance  . This only indicates that something is not ok after those resistors.

[wraper]

BTW, the problem might be artificially introduced as well when you blindly changed parts there. Compare with working one if you didn't solder wrong parts or in wrong direction by accident.

[LektroiD]

Aha! Resistor checked out ok, but the 100µF electro behind it had an ESR of 0!!!

Unfortunately I only have 100µF in 35V, which wouldn't last long with 36V going through them. I'll grab some on my next shopping spree...

[LektroiD]

*update*

I put the 100µF/35V in as a temperary measure just to test, now I have 24V going in, and 9.6V coming out, so looks like I'll have to replace the negative reg again (I have plenty of those in stock).

Stay tuned though, chances are, with that reg throwing out more than double the required voltage, I would imagine there may be further damage...

I'll order some new 50V or even 63V electro's (just to make sure) and go from there...

[wraper]

Aha! Resistor checked out ok, but the 100µF electro behind it had an ESR of 0!!!

Unfortunately I only have 100µF in 35V, which wouldn't last long with 36V going through them. I'll grab some on my next shopping spree...

What, 0? the less the better, don't know how precise your esr meter is thou, usually it should read about 0.1-0.5 OHm. However capacitor shouldn't affect anything at all in this situation unless it is shorted. However it definitely is not.

[wraper]

now I have 24V going in, and 9.6V coming out, so looks like I'll have to replace the negative reg again (I have plenty of those in stock).

It might be overloaded as well. The best would be to desolder it and check using some power supply to power it. Thou voltage before it should drop lower than in working amplifier. So likely it is blown.