Tannoy Reveal 6D

[Valden]

Re: Tannoy Reveal 6D

Khron asked 'Resistance between where the 7915's output should be, and pin 11 (of 14) / pin 8 (correction - pin 4 is ground) (of 8)? Answer - 976 ohm for all them. The 8 pin IC is labelled JR448AC LM833N. Another opamp? Google might know.

Edit - these measurement were taken with the 7915 back in circuit and apparently working.

The current challenge is, what's making the woofer cone extend outwards? This could be because the negative voltage rail is not reaching one of the amplifier chips. I'll try the stunt-double speaker with the HF output.

[Valden]

Re: Tannoy Reveal 6D

The HF output is extending the test speaker too. The problem is affecting all the amplifiers.

The amplifiers run off the +/- 36V rails, not the regulated +/- 15V rails. Will look for a problem in the 36V rails.

- I'm getting 36.7V and -36.7V from the rectifier.

- looking at the LM3886 amplifier ICs, all three have 36.3V at supply pins 1 and 5 and 16V at audio output pin 3, except for the amp chip closest to speaker connectors, which has 0V at pin 3. A shorted amp chip?

[Valden]

Re: Tannoy Reveal 6D

I proposed 'A shorted amp chip?' Probably not actually.

The amp chips' output is on pin 3. Pin 3 of the middle chip goes to one of the woofer connector pins. Pin 3 of the left chip goes to the other woofer connector pin. This suggests that the middle and left amp chips work in reverse phase. One pushes the cone while the other pulls.

Yep, from following the tracks, that is how the left amp chip works. While the audio input goes to the positive audio input pin 10 of the middle amp chip, it branches off and goes to the negative audio input pin 9 of the left amp chip.

Remember, this is for a Reveal 8D, which has three amp chips. The 6D has two.

[Valden]

Re: Tannoy Reveal 6D

I've scraped goop off the even numbered pins of the three amp chips. I'm getting -37.1V at each pin 4, being the negative rail connection.

[Valden]

Re: Tannoy Reveal 6D

OK, this is interesting. I've checked voltage at the speaker connector pins. Red probe to positive speaker connector pin.

- with the good monitor, it's 0 for woofer and tweeter, with filter board connected.

- with the faulty monitor, its 14.9V for the woofer with the filter board connected. With filter board disconnected it's 0V.
- For the tweeter connector, it's -15V with filter board connected and -22.2V with the filter board disconnected.

Explain me that! Wait - I've found a problem with my temporary 7915 wiring. Because the power board was upside down when I was soldering I put a wire from the 7915 output to the high frequency audio connector pin. I've corrected this. The pic below shows the red wire has moved from pin 5 to pin 2, comparing with the pic in post 139. I'll now recheck those speaker connector voltages. Hopefully nothing's blown up.

New measurements are: tweeter pins are 0V with filter board connected and disconnected; woofer pins are 14V with filter board connected and 0V disconnected.

I'll go out on a limb and suggest it's a shorted low frequency opamp on the filter board, putting 15V into the low frequency audio track. I spoke with the tech at my audio gear supplier and he talked about the opamps being skittish in these monitors. He suggested I try to feel if one is a lot hotter than the others. BZZZZDT. Or look for smoke, of which I've seen none.

[Valden]

Re: Tannoy Reveal 6D

A quick test with that sacrificial stunt-double speaker shows that my voltmeter readings from my previous post play out as expected. The speaker cone sits still when hooked up to the tweeter pins. It pops out fully extended when connected to the woofer pins.

[Khron]

Re: Tannoy Reveal 6D

Yes, sounds like the 8D has two of the LM3886 bridged, driving the woofer.

So... With the new 7915 in circuit (and wired up correctly? ), you ARE getting -15v at its output?

Well, if the finger-thermal-testing doesn't reveal anything relevant, the only other step i can think of is quite tedious, but should pretty much totally clarify things.

Black probe clipped / "permanently" connected to ground, and probe every leg of every opamp on the filter board. Note down the readings and post them here.

The outputs of the LM837 (quad opamp) are on pins 1, 7, 8 and 14, and the outputs of the LM833 (dual opamp) are pins 1 and 7.

https://cdn.badcaps-static.com/pdfs/...a12f80b5d7.pdf
https://cdn.badcaps-static.com/pdfs/...2cc9f50a3d.pdf

If both the woofer and tweeter are affected, and the -15v rail works as it should, i'd expect the issue to be "upstream" of the crossover stages, ie. before the high/low-pass filters for the tweeter/woofer.
Not sure if the filter board is the same between the 6D and the 8D, but on the posted schematic, that would be anywhere between the inputs and IC2B - you can see the green signal line splitting off to the right (entering a subsonic-high-pass-filter section) and upwards (entering the tweeter high-pass)

[Valden]

Re: Tannoy Reveal 6D

So... With the new 7915 in circuit (and wired up correctly? ), you ARE getting -15v at its output?

Yes, the relocated and custom wired 7915 is producing -15V at the output. Having that working correctly makes diagnosing the other fault, up on the filter board by the looks of it, easier!

[Khron]

Re: Tannoy Reveal 6D

Well, as the old saying goes - even a kick in the a*s is a step forward

But yeah, with that 7915 issue out of the way, that indeed narrows down the possibilities considerably.

[Valden]

Re: Tannoy Reveal 6D

Bugger. The Eagles killed the 7915 regulator!

I played my iPhone into the analog audio input on the filter board and the 7915 is now reading -0.5V at the output, just as it was before. Now it might not have been the audio that caused this, but that's the only thing I did. I'll buy a few more 7915s tomorrow and try to identify what's wrong on the filter board before playing audio into it. Sheesh!

[Khron]

Re: Tannoy Reveal 6D

Yeah, can't really imagine why you'd wanna do that

I mean, at least in theory, first you'd wanna get the whole thing to work ok in static conditions...

Now that i'm looking more at the filter schematic, it seems like the whole thing is DC-coupled, apart from the high-pass sections.
So at least in theory, if some sort of too-large signal(?) somehow messed up (some of) the input opamp stages, that might've blown/shorted something inside IC1.

But until you re-replace the 7915 and measure all the pins of all the opamps, i'm only speculating

[Valden]

Re: Tannoy Reveal 6D

I played audio so I could trace the signal through the opamps, to find if any are not working. Seems a reasonable approach. Luckily, the 7915s are only $1.85. But, the electronics store up the road has only 13 of them. Might not be enough.

[Khron]

Re: Tannoy Reveal 6D

A noble intention, but perhaps a bit premature / hasty

As i said - first i'd want to check / establish how things look from a DC perspective (voltages, operating points at idle etc). Once that's cleared up & settled, THEN it's time to move to check the AC behaviour.

Perhaps a bit late, but here's a "testing checklist" that might come in handy:
http://sound.whsites.net/troubleshooting.htm#test2

Although it must be said that some of the details there refer to an amplifier made up of discrete components, but the same principles apply
Also, see lower (section 11.0) for tips regarding testing the filter board.

[Valden]

Re: Tannoy Reveal 6D 8D repair

Status update for repair of Tannoy Reveal 8D studio monitor.

I have confirmed that the power board is working, by using a dummy load to simulate the filter board being connected. In this configuration the two voltage regulators are supplying +15 and -15V, as they should. However, once the filter board is connected, voltage at the output of the negative voltage regulator drops from what should be -15V to -0.1V, relative to ground measured under the big caps on the power board. Measurements taken at the connector of the dismounted filter board show that there is a short circuit between pins 2 and 4, being the -15V rail and ground. I'm presently hunting for that short circuit.

Using the filter board schematic for the 6D as a guide, the first suspect was capacitor 49, one of the two electrolytics on the board. With it removed from the board the short circuit persists. Thoughts then turn to the five LM837 quad opamps and the one LM833 dual opamp. The negative rail connects at pin 11 for the 837s and at pin 4 for the 833. However, resistance testing reveals no short circuit between pin 11 (or 4 for the 833) and any other pin on these ICs. Even if there was, none of these IC pins connect directly to ground. At least one other component would need to be shorted for a short in an IC to be part of the overall short circuit between the negative rail and ground. The suspect components are surface mount capacitors and resistors. Are these prone to failing and going open? If it's more obvious than this, I haven't seen it yet

So, I'm continuing to look for the cause of the short circuit between the negative rail and ground on the filter board. I'll have to start lifting the surface mount components one by one …

Don't call the priest for this board just yet.

[Khron]

Re: Tannoy Reveal 6D

There needn't be a "hard" or permanent short from the negative rail to ground inside one of the opamps - it might just be a transistor inside that is made to conduct fully when the opamp in question is powered.

A simpler test - get a nice sharp box-cutter knife / stanley-knife / whatever it's called "down under"
And then just (carefully but firmly) slice through the trace going to the negative supply pin of each opamp, one by one, and power-test the board between each operation.

Once you've detemined which one's the culprit, you can then just scrape off the solder-resist from both sides of the unnecessary cuts and bridge them with a solder blob (or a piece of resistor leg / wire, if needed).

I'd start from the opposite end of where the connector is, and work your way towards it. Once the semiconductors are cleared, then we can start suspecting the passives.

That being said though, you miiiiight wanna check the negative rail bypass caps (10nF between ground and -15v), if they're surface-mount ceramics - they've been known to fail shorted (i've revived several laptops that way )

[Valden]

Re: Tannoy Reveal 6D

Thanks for the suggestion re cutting the tracks. I'll have to start calling it the Frankenfilter board.

The short/s I'm hunting for is/are present when the board is unpowered, as well as powered. Bewdy!

*************

I've been checking out the Brymen BM869S. There's a very interesting test video on YouTube, by Joe Smith.

https://youtu.be/K-4L2JarVxA

[Khron]

Re: Tannoy Reveal 6D

That's the only realistic way to hunt down short-circuits - separating the potential culprits.

Fingers crossed

[Valden]

Re: Tannoy Reveal 6D

I decided to focus on the surface mount components that were single points of failure in terms of creating a short between the negative rail and ground. I lifted one end of C18. Nope. The short's still there. What about C21? Holy crap! The short's gone. Connect it again with a molten strand of solder. The short is back. Break the strand? The short's gone again.

With a bit of luck this could be the fix. Now I'm on the scrounge for a 10nF SM ceramic capacitor.

[Khron]

Re: Tannoy Reveal 6D

There ya go then! Glad to hear you nailed it that quickly

You'll want the replacement to be rated for at least 25v though.

[Valden]

Re: Tannoy Reveal 6D

I rang Jaycar and to my surprise they said they had just the thing. But when I got there, just before closing, and showed the girl the surface mount cap I needed, she said. 'Oh, I don't think we have any of those!' I settled for the next best thing - a ceramic cap of 10nF and 50V rating.

The monitor is working! But, the high frequencies are significantly attenuated. Could it be the ring-in cap I'm using? I'll check the schematic to see if opamp 2 (IC2) has anything to do with the highs. The tweeter itself is fine. The highs problem moves with the repaired electrics; the tweeter works with the good monitor's electrics.