Tannoy Reveal 6D

[Khron]

Re: Tannoy Reveal 6D

That doesn't sound like a short-circuit.

With the 7915 removed, there won't be much of a voltage reading to read

Am i mistaken, or had you at some point mentioned already replacing that 7915? Or were you just thinking about ordering a new one?

Given this fresh information, that would indeed point toward that 7915 being the actual problem. As mentioned before, 33v at its input would point to little or no current being drawn through the 220ohm 5w resistor "upstream" of it.

[Valden]

Re: Tannoy Reveal 6D

Ok, I've got the 7915 off. Not easy. The heat sink mounting pin and the three legs of the 7915 are tricky to de-solder and move. The track that runs from the 7915's outlet hole to pin 2 of connector 1 was already damaged/lifted. To re-establish the connectivity that would have run through that outlet hole, to connect pin 2 of connector 1 to the negative terminal of what is cap 12 on the 6D schematic, I've soldered in a jumper wire between those locations. Are you happy with this? I believe they should be connected for the intended tests.

The diode test performed with the circuit inactive, with the black probe on the ground under the big caps and the red probe on pin 2 of connector 1 gives a reading of 656. I'll post again in a minute with voltage tests.

[Valden]

Re: Tannoy Reveal 6D

Thanks Khron. Great explanation. I'll get that 7915 off so we can see what's up. Stand by.

[Khron]

Re: Tannoy Reveal 6D

Either the 7915's faulty, or there's something shorted between the -15v rail and ground. My bet's on the latter

But that's why a measurement on the board, with the 7915 removed, would make things clearer.

With the 7915 removed, we could measure ONLY the resistance (or reverse voltage drop, with the multimeter in diode test mode) of the circuitry that's connected to the output of the 7915. Basically removing the 7915 as a suspect, or at least eliminating a variable / an unknown. You can then also test the 7915 out of circuit, to make sure whether it's good or shorted.

The "new" piece of the puzzle is nothing new - it just re-confirms that the problem is in the electronics of the "bad" monitor

With the diode-test-the-negative-rail thing, we would be determining whether there's something shorting the negative rail to ground or not. The lack of -15v on the filter board pushes its output towards the +15v rail, which in turn, causes the amplifiers to put out DC into the speakers.

Think of these dual-supply (positive & negative) circuits as keeping the speaker cone in balance, at idle. Each supply rail is pulling on the cone in opposite directions. If one of the rails is missing, the other rail has no opposing force countering it

[Valden]

Re: Tannoy Reveal 6D

Hello Unique.

Thanks for your encouragement. You might not have read in post 116 that I've already replaced the 7915 negative voltage regulator. It made no difference.

I'm hunting for causes of voltage drop in the negative half loop, possibly on the filter board. Insightful suggestions will be warmly welcomed.

Another piece of the puzzle:

- when I plug the speaker and front panel cables of the faulty monitor into the electrics of the good monitor, both the front panel light and switch work (as well as the speakers). The light does not work with the faulty monitor's electrics.

[Th3_uN1Qu3]

Re: Tannoy Reveal 6D

Remove the 7915 then, it is almost guaranteed to be faulty.

[Valden]

Re: Tannoy Reveal 6D

The component numbering with the 8D is indeed different from the 6D. The 6D schematic will be useful as a guide only.

I removed the capacitor I suspected was shorted, but testing off-board shows it's not. This is the capacitor that has its negative terminal connected directly to the outlet of the 7915. It's in the role of cap 12 in the 6D schematic. It certainly appears shorted when tested on the board. The same capacitor in my good monitor does not do this; it counts up like a good cap should. Something's fishy.

Khron, can you please explain how removing the 7915 and testing as described in post 117 will help? Your guidance was 'With the whole thing powered off, set your DMM to diode test ( -|>- ), put the black probe on a ground point (remember to pierce through the oxide layer on whatever solder joint you choose), and then with the red one, probe the output of where the 7915 was (or alternatively, if it's easier, on pin 11 of any of the 14-pin opamps)'. I can do this, but what are we looking for here? Are we looking for a problem on the filter board that is putting DC into the audio circuit, in part causing the woofer cone's extension?

Thanks.

[Valden]

Re: Tannoy Reveal 6D

Yes, I'll have to remove any suspect caps from the board for testing.

While the schematic for the 6D is helpful I need to remember that I'm working on an 8D now. There are some differences, including with cap numbering. That schematic for the 8D would be really handy - which I will pursue, with Tannoy UK and the repair guys at my local audio store.

[Khron]

Re: Tannoy Reveal 6D

Caps are best tested out-of-circuit, ie. removed from the board.

That C22 appears shorted due to the reason i've stated above

Re: finding caps - they're all pretty clearly marked on the top of the board.

C15 & 16 = the 8mm diameter ones, closest to the connector on the right: https://www.badcaps.net/forum/attach...1&d=1467844995
C22 & 23 = the 5mm(?) diameter ones, next to C15/16: https://www.badcaps.net/forum/attach...8&d=1467845455
C11 & 12 = the blue 5mm diameter ones, closest to the small black electrolytic, near the middle of the board: https://www.badcaps.net/forum/attach...1&d=1467844995

[Valden]

Re: Tannoy Reveal 6D

Khron - thanks again for your help with this. To catch up on a few points from your last post:

- the speakers have been detached except for a few quick tests. I switched off very quickly when I saw the woofer extension. No more than a second.

- I've got the regulator pins identified correctly. Your labelling of my pic confirms this. Thanks.

- Yep, 33V at the input pin of 7915. On the good monitor it's 17V. There's something screwy with this negative voltage regulator circuitry. That shorted cap 22 is getting my attention.

- I'm checking all the caps in the regulation circuit, including the ones you've listed. Some are hard to find. Will resume in the morning.

[Khron]

Re: Tannoy Reveal 6D

C22 is in parallel with C19, the 10.000uF cap. Keep the probes connected for about 10 seconds, to allow the big cap to charge, and then you'll see the resistance reading rise (assuming all is well).

I guess you've yet to get around to removing the new 7915?

[Valden]

Re: Tannoy Reveal 6D

Capacitor C22 looks like it's shorted. Resistance test at 20 k setting dives straight down to 0.01. The same cap on the good monitor counts up like a good cap should. Would that short in C22 be causing all this?

[Khron]

Re: Tannoy Reveal 6D

"Inlet pin of the regulator" = the middle leg of the 7915, correct?

You really don't wanna keep the woofer "extended" like that - that's a clear sign of DC on the output, and that's really bad, it toasts voice-coils pretty quick.

In fact, i'd rather keep the drivers disconnected at all times during testing, at the very least until you fix the missing -15v. It won't hurt the amps, they work fine without a load as well.

Since there's no coupling cap between the filters and the amplifiers, and given the missing -15v rail, that DC's definitely coming from the filter board.

I'd remove that new 7915 you installed in the bad monitor. With the whole thing powered off, set your DMM to diode test ( -|>- ), put the black probe on a ground point (remember to pierce through the oxide layer on whatever solder joint you choose), and then with the red one, probe the output of where the 7915 was (or alternatively, if it's easier, on pin 11 of any of the 14-pin opamps).

You might also want to consider replacing the small capacitors near the regulators (C11, 12, 15, 16, 22, 23). I've seen caps that have failed shorted a few times.

Hmmm... -33v at the input of the 7915 is suspiciously high. What can you measure on the input of the 7915 in the good monitor? I'm gonna guess it's gonna be a lot less, possibly less than -20v.
Something's really fishy there...

Just to make double-sure, i've marked what each leg of each regulator's supposed to be, on the bottom of the board, using one of the photos you posted - see attached picture.

[Valden]

Re: Tannoy Reveal 6D

I bought a new L7915CV negative voltage regulator this afternoon and installed it. Testing results in the same measurements as before, i.e. only 15V between the output pins of the two voltage regulators and only -0.5V between the output and ground pins of the 7915 itself. That is, no change. Hmmm.

Possibly worse, instead of the loud hum/buzz sound, now the woofer cone extends all the way out with a pop and stays there. Not a good look. The front panel LED continues to not work. I will check if the front panel mute switch works. ........ Yes, it works, even without the LED.

Khron - I'll go back and read your suggestions and see if there's anything I haven't looked at yet.

I should add, the speakers in the bad monitor still work fine with the electronics from the good monitor.

[Valden]

Re: Tannoy Reveal 6D

With my faulty Tannoy Reveal 8D, I've just run another check of the white ceramic 5W 220 ohm resistor in circuit with the 7915 negative voltage regulator. Firstly, the resistor measures 231 ohm. Then, with the circuit active and measuring from the rectifier negative DC pin with the positive DMM probe, there's -33.1V at pin 1 of the resistor and at the inlet pin of the regulator. It looks to me like the resistor is OK.

So the thing to try now is to move the good 7915 voltage regulator from my good monitor into the faulty monitor and see if it works. Not exactly trivial but worth trying.

[Valden]

Re: Tannoy Reveal 6D

Tannoy Reveal 8D repair. Diagnosis continues.

Results from probing the voltage regulators on the power board are:

- with my working 8D monitor, the L7815CV positive voltage regulator measures 15V from output pin to ground pin. The L7915CV negative voltage regulator measures -15V from output pin to ground pin.

- with my faulty 8D monitor, the L7815CV positive voltage regulator measures 15V from output pin to ground pin. The L7915CV negative voltage regulator measures -0.5V from output pin to ground pin. Hmmm. This 7915 is not looking good.

[Khron]

Re: Tannoy Reveal 6D

Flukes might be a bit overkill imho (unless you'll be using them in some professional / industrial settings). I've actually had my eye on some Brymen meters, which offer considerably better bang per buck

I'm suspecting either the 7915 itself, or some corrosion of the trace between the 220ohm resistor and the 7915. Although, since you DID get different voltage readings on the two ends of that resistor, that should mean there's SOME current going through it...

[Valden]

Re: Tannoy Reveal 6D

Yes, my initial findings this afternoon led me to be concerned about the 7915. It is looking rather guilty. I'll compare results against my good 8D tomorrow to confirm.

My multimeter is a pretty shitty generic POC, dressed in Dick Smith Electronics labelling. It does measure negative voltages, though I sometimes forget to make note of them. I'm sizing up a Fluke 179 or 3000 FC, to make life more enjoyable.

[Khron]

Re: Tannoy Reveal 6D

What voltmeter / multimeter do you have?

Most digital multimeters (DMM's) WILL put a "-" sign in front of the reading, if the black probe is, say, connected to the ground of the circuit, and the red one to a negative voltage

Also, the pinouts of the 7815 and 7915 are NOT mirrored.


versus


Probing between the output of the 7815 and the 7915 should've produced a reading around 30v, actually: 15v - (-15v) = 15 + 15 = 30v (difference between the two points).

So i guess you have your culprit right there - the 7915 regulator's... not really regulating much anymore
Either that, or there's something fishy between the 220ohm 5w resistor, and the 7915's input pin.

A reading of 17.6v between the 7815 output (which should be +15v relative to ground) and the 7915's input would mean that the latter is around -2.6v relative to ground, when normally that should be in the -18v area.

[Valden]

Re: Tannoy Reveal 6D

Thanks for these suggested checks. I will pursue them in the morning.

Given my concern that power supply to the filter board might not be good I've been testing the 15V regulators. My findings so far are:

- the L7815CV positive voltage regulator is delivering 15V, measured from outlet to ground. Looks good. This one was easy to measure.

- the L7915CV, being a negative voltage regulator, was tricky to measure. In the end (and after consulting the schematic for the 6D and comparing the 8D's board truth - similar in this respect) I probed from the outlet of 7815 to the 'outlet' of 7915, then to the 'inlet' of 7915. The results were 15V and 17.6V respectively. I think this means that the 7915 is working, and is chewing up a couple of volts in doing that work.

- Not being an expert, I was curious about results from probing the 7915 directly. Sure, the inlet and ground pins are reversed relative to the 7815. Apart from that, I found that measuring from outlet or inlet to ground would not produce 15V, but only 0 or about 2.5V. My son and I think this is correct and is due to this regulator being situated in the negative half of the 15V loop.

Thoughts?

More testing tomorrow.