Repair facility recommendation - London England - Dolby Multichannel amplifier

[Tristan Summers]

Hello
One of our Dolby Multichannel amps died and Dolby UK and dealers have yet to suggest a good technician to assist.
This seemed like a useful place to ask for repair recommendations around London.

Tristan

[Steve Guttag]

Oddly, that amplifier is designed for field repair. One would think that there would be someone in the UK that fixes cinemas that would take on that unit.

[Marco Giustini]

Who installed that DMA? Plenty of integrators in the UK to choose from, surely you are familiar with their names if you run a cinema? Maybe you're not happy with your current one and you're looking for an alternative?

[Leo Enticknap]

I left the UK in 2012, and so am by no means up to date on who does and doesn't offer cinema equipment service in the London area. However, three companies that were around when I was still appear to be in business and offering service and support: Sound Associates, out of Molesey, Surrey (near Hampton Court, about 15 miles south of Central London), Omnex, based in West London, and Bell Theatre Services in Hertfordshire.

[Tristan Summers]

Thank You Leo. I tried Sound Associates first as they were very helpful with Trinnov parts before. I will try the others
Marco, I am neither the original owner or a commercial cinema. Just trying to use decommissioned equipment. Luckily Dolby don't have time bombs built into ALL their equipment, just some of the media blocks.
Also the official channels are not always the best places for out of contract repairs and this is a good place to find the people who care enough to repair stuff

[Dave Macaulay]

These amps are designed for a form of serviceability, but Dolby made it a pain. Modules are accessed from the top and side, so the chassis has to be disconnected and taken out of a rack to get at them. The (many) speaker cables better be labelled!

The basic design has a common power supply for all configurations, and several 4 channel amplifier modules sharing the power available. This is great for something like Atmos where a lot of amp channels are needed. There is a limit to total output power from the unit though, not usually an issue for a cinema but it has to be considered: having screen and subwoofer channels all in one chassis can reach the PSU power limit where all channels get reduced gain automatically.

There are several modules for control and power supply that are accessed from the top.
Then there are multiple amplifier modules, a few types for different output power ratings. These go in from the side.

The Dolby DMA service manual is good at describing the testing of the control/power modules and there are fuses on the amp modules that can be checked.
The warnings about hazardous voltages inside are no joke, be careful.

Beyond the diagnostic steps, only replacement of failed modules is possible from Dolby's point of view.
I don't think actual schematics are available for any parts.

Some electronic technicians can probably diagnose/repair at the board level - but Dolby doesn't make that easy.

What symptoms do you have when you try to turn it on?

[Marco Giustini]

Tristan

Ah, I see. I believe Sound Associates should be able to help you somehow. They might want the DMA in their warehouse though. The issue with buying parts is "what if it doesn't fix the item" or, even worse, "what if the part gets damaged by another part when I turn it on"?

One thing with the DMA, for your own safety: earlier versions didn't have bleeding resistors on the main bulk capacitors. That means that once you apply mains, you'll have 390V there for many hours after they've been unplugged. So be careful.

[Leo Enticknap]

Many years ago I did the Atmos training class, the first day of which was basically a guided tour of what was then Dolby's current product offering. I remember that when the instructor covered the DMA, he warned us to be super careful when swapping out an individual module, due to the risk of receiving the Ted Bundy experience (those weren't his exact words, but you get the idea); but he didn't go into that level of detail. As a result of that warning, I put a pair of thick electrician's rubber gauntlets in my tool cart, but have only ever used them since to handle Barco SMPS cards, after once having been bitten by one. I have literally done two Dolby DMA installs in total (the vast majority that we do use QSC or LEA amps), and thankfully have never had to open one up.

[Marco Giustini]

I believe the official process asks for an 8-hours wait before the unit is dismantled because of those caps. Later units have bleeding resistors (tiny resistors "burning" energy from the capacitors: in normal operations the amount of energy they dissipate is negligible but then you pull the plug, they discharge the capacitors in minutes.) so you don't have to worry too much.

If you're comfortable with electronics, there are ways to discharge those caps manually. Which is NOT by using a screwdriver ?

[Tristan Summers]

Thank You everyone for help. Omnex have sent out a quote and I am seeing if I can get the Service Manual from Dolby so I can work out which board needs replacing in advance (safely), though it may have to be relagated to spares for now.

[Leo Enticknap]

Is the web UI giving you any clue as to where the trouble is? If it thinks that a channel(s) is bad, you should see a yellow warning button and details as so:

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If some or all of the channels on one card are showing as NFG, that's the card that needs replacing.

[Marco Giustini]

Slightly OT, I miss simple amplifiers. Those with an ON/OFF switch, two trimpots, NO coloured display and a fan.

[Leo Enticknap]

The valves (vacuum tubes on this side of the pond) had a nice friendly glow to them, too.

[Tristan Summers]

The unit's main fuse blows the moment it's plugged in, so no access to interface or ability to reset. Dolby think it's had a power supply issue before and I think I'm going to take it in to BTS.

[Marco Giustini]

if the fuse blows, I'd expect a short on the primary section - the power supply basically. It could be "simple" to find.