- Mackie SRM450 DC issues - 3 Updates
| "Gareth Magennis" <soundserviceleeds@outlook.com>: Jul 05 08:08PM +0100 "Phil Allison" wrote in message news:86995c69-6eb9-4eb1-8dc2-e8c390662419@googlegroups.com... Gareth Magennis wrote: > Amplifiers are not my most favourite circuit, and I don't know a lot about > how DC offsets are managed in them, but it seems this one definitely has a > problem managing the DC. ** SRM450 is the model number of a Mackie powered loudspeaker, the schem is for the amplifier module that drives the 12inch woofer. Thing to note is that such units undergo frightening amounts of vibration plus the occasional impact with the floor - which can put cracks in the PCB and or loosen components that are not extraordinarily well held down. No amount of perusing and pondering over a schem will find any of that. Nevertheless, the muting system is a common one in that the front end of the amplifier is starved of current so it can no longer drive the following class A stage and hence the output stage. Q30 does this by disabling Q29 & Q31 and hence both input diff pairs ( Qs 25 thru 28) So, one needs to engage the mute circuit ( apply +10 or more to Q30's base) and see why that is NOT happening correctly. The amplifier circuitry is symmetrical so look for a voltage that is not reflected on the opposite side in mute mode. .... Phil **************************** Belated thanks, Phil. I did get this sorted, but this amp is a bit of a pain. Q29, Q30, Q31 are commonly available MBT types - so common I already had these SMD devices in stock, so it was quick and easy to change them. The other 2SA, 2SB, 2SC diff pair SMD devices are far from common, and have very high specs. I'm very reluctant these days to purchase silicon from anyone other than Farnell or RS, as there is a risk of fake/crap parts, particularly with high spec devices. Normally I would just shotgun replace the lot. Anyway, it turned out Q7 was leaky, causing Diode string D23/24/25 to conduct, presumably via Q26 CB junction, D40, and off to the amps output. This turns on Q4 and Q5 MJE devices, causing the output to swing positive. This one single SOT device cost just short of £5 on Ebay, via the only seller I trust outside Farnell and RS. The original tests fine on a bog standard Peak transistor tester, but such leaky ones always do IME. What pissed me off originally, and hence my post, was that unmuted, the leaky transistor apparently worked OK. Cheers, Gareth. |
| "Gareth Magennis" <soundserviceleeds@outlook.com>: Jul 05 09:16PM +0100 Oops, my mistake, Q26 CB junction is reverse biased, so it must be Q7 conducting via junction CB and the 15v rail. Gareth. |
| Phil Allison <pallison49@gmail.com>: Jul 05 07:09PM -0700 Gareth Magennis wrote: > This turns on Q4 and Q5 MJE devices, causing the output to swing positive. > This one single SOT device cost just short of £5 on Ebay, via the only > seller I trust outside Farnell and RS. ** Hmmmm, Q7 has an innocent, TO92 looking number ( 2SC3645) but turns out to be a bit special: https://www.promelec.ru/pdf/2SA1415-2SC3645.pdf Finding leaky BJTs is a real PITA but you persisted and got there. .... Phil |
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