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studio45 
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Joined: 16 October 2007 Location: United Kingdom Status: Offline Points: 3881 |
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 Topic: Markbass ampsPosted: 24 October 2023 at 2:59pm |
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Hi guys,
I'm in the middle of a difficult repair and I have a question about the design of the power modules in Markbass 500 watt heads. Not strictly PA I know, my apologies  So I have worked out - sort of - how they get their awesome power-to-weight ratios. The design seems to rely on very closely matched MOSFETs, thus allowing them to run at a high rail voltage (+/-80v) and squeeze every last watt out of the devices. So they have only six TO247 devices in the module, but true 500 watt output into 4r. Yeah....not so clever when it comes to repair. I have managed to construct a test and matching jig, that allows me to measure Vgs (gate-source voltage) at a constant current, source-drain voltage and temperature. The surviving original devices match Vgs to within 0.001v, very close indeed. However, the stock of ten new parts I bought to select replacements from are ALL OVER THE PLACE, relatively speaking. None within even 10mV of each other let alone the originals. So it looks like I will need to buy a very large number of new parts to find a set of 3 that match. So...I was considering another possible course of action. Because this is always going to be a massive problem with repairing these things, and they turn up broken fairly often. Could I slightly redesign the amp so it doesn't need such closely matched devices? The two things I've thought of so far are: - reduce the rail voltage. There are three taps on the primary of the power supply transformer, if I move the jumper to the lowest one it should reduce the rail voltage somewhat. Not sure how much though. Unloaded, the power supply shoots up to +/-120v....very rudimentary design  - Increase the source resistor value to force more current sharing at the expense of output power and damping factor. EG I could change the 0.22r resistors that are installed, for 0.33r. -Use different, more highly rated MOSFETs that will just survive the abuse. Current devices are IRFP240/IRFP9240, not particularly modern. Are any of my ideas likely to work, or am I entirely wasting my time? Thanks in advance for any help
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Studio45 - Repairs & Building Commotion Soundsystem -Mobile PA
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kedwardsleisure
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Joined: 20 January 2009 Location: Staffordshire Status: Offline Points: 5157 |
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Posted: 24 October 2023 at 3:18pm |
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Well cloud used these in their CV1000 amps and specified Vgs within 25mV and 0.33R resistors. And they're pretty solid. You can always stick those in and do a quick check of the idle currents across each resistor and under load.
But therein lies the problem with using fets not specifically designed for use in the linear mode, and I remember Ecler's PAM amps boasting how their 'switching fet' output stages were super clever (without actually denying they weren't a class-D switching design). When IR say their fets are 'easy to parallel' I think they meant when they were in switch mode, probably the RDS-on being the tighter tolerance in question. |
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Kevin
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ceharden
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Posted: 24 October 2023 at 11:25pm |
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Those devices were also used in the rail tracking circuitry of the Crest 9001, not exactly a light duty application. Gut feeling is that changing to a difference device might just cause more issues.
I think I have a tube of those devices somewhere (can't remember how many) and might be able to assist with putting together a reasonably matched set. They're at my 'other' office, not sure when I'm due there next. As Kevin suggests, maybe go for the closest matched set you can, increase the source resistors and see how it goes. Increasing the resistance shouldn't have a huge impact on power and the damping factor shouldn't be affected because they're inside the feedback loop. The main issue could be increased power dissipation in the resistors themselves. |
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studio45
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Posted: 25 October 2023 at 2:27pm |
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Thanks guys that's very helpful.
On checking, there are actually already 0.33r source resistors in there, so I guess the next step would be 0.47r. There's still no schematic for the CV1000 online, that I can find. Do you happen to know the rail voltages in that one, or how many devices per channel? I have found several forum posts from people who have tried fixing these amp with non-matched parts. Usually what happens is, it works on the bench - but then once back with the customer, fails shortly afterwards. No good. Given that it was taking me about 5-10 minutes per transistor to obtain a stable reading, it's a long job to match them, unless you have a jig with proper temperature control. Very nice of you to offer but probably not practical! I also emailed Markbass to see if they would sell me a set of matched parts. No reply yet...senza trattenere il respiro.
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Studio45 - Repairs & Building Commotion Soundsystem -Mobile PA
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kedwardsleisure
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Posted: 25 October 2023 at 3:07pm |
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the CV1000 is +/- 80V
It was originally designed for hitachi 2SJ83 / 2SK227 then went to 2SJ119 / 2SK414 but eventually to the IR mosfets. THe VTX is very similar. Originally 5 pairs but then reduced to 4 pairs in the mk2 as was the VTX1200 (VTX1500 retained 5 pairs). |
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Kevin
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Earplug
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Posted: 26 October 2023 at 9:08am |
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I've mentioned these before:- I've used these many times to replace burned out (Class D  ) modules and they are great. They use IRFP240/IRFP9240 and don't seem to have any issues with rail voltages up to +/- 90V. |
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Earplugs Are For Wimps!
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ceharden
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Posted: 26 October 2023 at 1:15pm |
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I bet those are just clones of the old BK Electronics modules which in turn are just based on the old Hitachi Application Notes. As are basically all the Matrix amps!
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Earplug
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Posted: 26 October 2023 at 3:13pm |
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Ok, yes. I'm not familiar with the Matrix stuff, but they are very similar to the Hitachi modules sold by Maplins, etc. What's pretty good is that they've managed to "tame" the cheaper MOSFETs. Makes them affordable!  |
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Andylaser
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Posted: 26 October 2023 at 8:57pm |
Those old BK modules (and amps) were rather good. Built like the proverbial brick shithouse, took untold abuse and still kept running.
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"music so loud, that if we move in next door to you; your lawn will die" - Lemmy
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studio45
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Posted: 27 October 2023 at 3:11pm |
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Hmmm. Very interesting! So it looks like Markbass aren't really doing anything that unusual with these rail voltages and this many devices. I wonder how they could have departed from the design of these other modules, such that theirs is so much more finicky? There's a bunch of over-temperature, DC and output device SOA protection circuits in theirs, which frankly seem to work as well as a chocolate teapot.
Unfortunately the power supply and amp are on the same PCB, so fitting a replacement module would involve sawing it in half, which is obviously a rather extreme course of action! |
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Studio45 - Repairs & Building Commotion Soundsystem -Mobile PA
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kedwardsleisure
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Posted: 27 October 2023 at 3:28pm |
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THe IR mosfets aren't lateral fets so they dont have the same thermal stability and SOA that the hitachi fets do. A bit more thought has to go into the design
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Kevin
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Earplug
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Posted: 27 October 2023 at 3:48pm |
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Yup. You do have to keep them cool - extra heat sink & fans, but they cost a fraction of something like the Exicon laterals:- |
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ceharden wrote: