Tannoy Reveal 6D

[Khron]

Re: Tannoy Reveal 6D

And you're positive those issues stem from the digital board?

The speaker connectors in (at least) one of my Precision 6D's had gone wonky, and upon closer inspection, one of the "tongues" inside the connector going to (i believe) the woofer just fell out - i ended up just stripping & soldering the wires to the bottom of the board.

No, swapping connections shouldn't be an issue. If a voice coil goes open-circuit, you'll "just" not hear sound out of that particular driver.
In fact, that's a basic troubleshooting step

[Valden]

Re: Tannoy Reveal 6D

Sorry, I should have been clearer. The loud hum/buzz noise a couple of seconds after switch on happened at first with the faulty digital board uninstalled, then again with that board installed. So it appears to be independent of the digital board.

I will go ahead and swap the woofer over to the good monitor tomorrow and test it. Hopefully it's OK, though the 9.2 ohm reading has me wondering. I guess the fact it was producing the buzzing noise is a positive sign.

Thanks for your help so far.

[Khron]

Re: Tannoy Reveal 6D

That's kinda what i figured - the only real way the digital board could cause noise is if it was selected with the switch on the back.

Other than that, perhaps if the dropper resistor on the positive rail crapped out (ie. went high-resistance or open-circuit) - although that might "kill" (ie. turn off) the front indicator led as well...

Well, in that case, my prime suspect would be the speaker connectors

[Valden]

Re: Tannoy Reveal 6D

Ah ha! The front LED has indeed crapped out, I noticed last night. I will go looking for that dropper resistor. I realise we're talking about an 8D here, but from the schematic for the 6D (which I have from this thread - at post 28), can you name it or pinpoint it please? Thanks.

[Khron]

Re: Tannoy Reveal 6D

It's one of the two big white 5W ceramic resistors, on the bottom of the "power board".

As you might've read in this thread, it's really common for those to fail open.
On the previous page (page 5) you can see i up-rated them to 10W ones, "just in case". I kept them at the same values (120ohm for the positive rail and 220ohm for the negative one).

[Valden]

Re: Tannoy Reveal 6D

Ah, the white ceramic resistors. I know those ones well. In one of the pics in a post of mine above you'll see I've floated the 120 ohm one away from the board. As experienced by TJC on page three, it was cooking the board.

In the 8D, it's a 5W 100 ohm ceramic resistor rather than 120 ohm. I've measured it in situ and I'm getting 100 ohms. So it looks OK. I'll continue checking other components. Any suggestions welcomed.

In trying to identify what's causing this loud hum, a fresh summary of symptoms is:

- digital board is not working
- shortly after switch on, a loud hum/grinding buzz noise comes from the speakers, irrespective of digital/analog input selection
- front light is not working but mute function is working. It stops the loud hum.
- transformer seems to be OK, delivering 27.1V each side, which is the same as the transformer in my good 8D.
- the rectifier is measuring as 54.4V AC at the two inside AC pins and 73.8V DC on the two outer DC pins.
- the two white ceramic 5W resistors seem OK, measuring as labelled; 100 ohm and 220 ohm.

I'll report further findings. Suggestions welcomed.

- OK, I just plugged the woofer from the broken 8D into the power board of the good 8D. The woofer works perfectly. I do wonder why I was getting 9.2 ohm measured through the woofer's connector, but won't be dwelling on that.

- power supply to the upper filter board looks Ok. I'm getting 15.4V between pins 1 & 2 of connector 1 on the power board, which links up to the filter board.

- thermal fuse adjacent to amp chips is OK

[Valden]

Re: Tannoy Reveal 6D

Tannoy Reveal 8D symptoms continued:

- using an audio probe reveals that the audio signal is not getting off the filter board. I'm picking the audio up on the board near the input jack and then at some components nearby, but not further across the board towards the connector that takes it to the power board. Will check the digital/analog input switch.

[Valden]

Re: Tannoy Reveal 6D

Tannoy Reveal 8D symptoms continued:

- correction to the measurement of the voltage being passed from the power board to the filter board. I measured the wrong pins. TBA.

[Khron]

Re: Tannoy Reveal 6D

See if you can use a crocodile / alligator clip to hook the black multimeter probe to a ground point (solder a piece of wire to one of the ground legs of the two big filter caps, if needed), and then probe around for voltages "down the line", with the red probe.

I guess you can follow the 6D schematic - apart from maybe a 2nd amp chip for the woofer, i doubt the circuitry's much different.

Right on the rectifier bridge (and, i assume, on the main filter caps) the voltages look normal. Check the voltages on each leg of the two 5w resistors, then on the input & output pins of the 7815 & 7915 regulators (datasheets are easy to find).

Then move on to the connector going to the filter board. Measuring on the underside of each end (on the amp board & filter board) would be the most certain, and remember to try to pierce the oxide layer.

Also, check the voltages on the connector going to the front panel - there should be a +15v and a-15v going to that as well.

I assume they "went to town" with the crap brown glue in your speakers as well? If that's the case, copper trace corrosion is (also) very likely.

Re: the 9.2 ohm measurement - oxidation and other crap on the outside of the solder joints or connectors can easily account for that That's what we have those sharp tips on probes for

[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.

[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

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

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

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.

[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

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.

[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

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

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

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.