High Voltage Regulator - Page 1
Forum section: electronics / repair

author: Birstall
author_rank: Contributor
timestamp: July 13, 2020, 08:33:46 pm
content: I have a Linn Sizmik 12.45 subwoofer. It is powered by a Linn Class V 500 watt amp.After rectification 250 - 300 v DC is spit and delivered to two 500uF 200v capacitors and a VB 408 voltage regulator. The Vout on the schematic for the regulator is 12v.The voltage regulator has blown and no longer available, Mauser have no alternatives. They are still available on Ebay but repeating this failed design seems foolhardyThis is a common problem with this particular amp and usually solved by installing a modern plate amp at a cost of $500.I am a novice but I am curious to know if there is a workaround to solve this issue. Obviously 500 watt SM amps are still around so how are they designed with lower voltagesYour comments and advice will be most welcome

author: Pulsepowerguy
author_rank: Regular Contributor
timestamp: July 13, 2020, 10:06:55 pm
content: I can think of at least one approach to making a discrete version of the VB 408. The part looks like an integration of some fairly standard items. It would take quite a bit of effort - maybe a fun time for a EE hobbyist. All it really does though is take the rectified line voltage and drop it down (with tons of loss).A pragmatist would take small 15v 'wall wart' supply and use a linear regulator to take it down to 12V. The wall wart should be a chunky, 60hz type (not a switcher) in order to satisfy the audiophile. This would be highly reliable.

author: TheMG
author_rank: Frequent Contributor
timestamp: July 14, 2020, 12:49:04 am
content: 7812 with a suitable resistor in series with the input to bring the voltage down to something it can handle.If the load is non-constant add a zener diode across the input to clamp the voltage down if the load get very light on the regulator. I would put the zener there regardless as a precaution anyways.Seen this sort of thing done in plenty of designs where a linear 3-terminal regulator needs to run from a supply voltage greater than it can normally handle. This avoids the use of oddball rare parts like the VB408.

author: Pulsepowerguy
author_rank: Regular Contributor
timestamp: July 14, 2020, 01:52:52 am
content: Quote from: TheMG on July 14, 2020, 12:49:04 amA resistor in series with a 7812? With 250-300V upstream? For a current on the order of 40mA at 300 volts input the power dissipation of a 7k resistor is on the order of 11 Watts. The big problem though is what happens if the input is 250V. For that case the resistor is too large and the regulator is starved. A 50 Volt variation in input voltage makes this approach fail, to say nothing of line surges or dips. At any rate, I would not go that route.Birstall: Do you have a schematic of the power supply?

author: coromonadalix
author_rank: Super Contributor
timestamp: July 14, 2020, 07:47:28 am
content: I had tl783 regulator in mind, but they are i think obsolete too, would an ac-dc adaptor be good ?? an 120-240 vac to 12vdc

author: shakalnokturn
author_rank: Super Contributor
timestamp: July 14, 2020, 08:34:34 am
content: If you want to follow the low efficiency route you could use a capacitive divider rather than resistive one at least that avoids the heat to dissipate.Still a dedicated transformer sounds a better approach.

author: magic
author_rank: Super Contributor
timestamp: July 14, 2020, 08:40:53 am
content: Capacitive divider doesn't work for DC.I would prefer the solution with additional 15V transformer and postregulator. It's trivial, it's efficient, it's reliable.If you want to stay true to the original (i.e. inefficient and expensive), cascode a 7812 with some TO-220 depletion mode MOSFET, SiC JFET or similar species, bootstrapped to the regulator's output.If you want cheaper and cruder, use a standard enhancement MOSFET instead and tie its gate to a low power shunt regulator at 20V or so.

author: shakalnokturn
author_rank: Super Contributor
timestamp: July 14, 2020, 11:16:53 am
content: I'm aware that a capacitive divider will not work (or only for a very short time) on DC, neither will a transformer AFAIK, intended use was before the rectification of course.Totally agree that a small transformer would be the most suited if there is enough room available.

author: David Hess
author_rank: Super Contributor
timestamp: July 14, 2020, 01:11:53 pm
content: Quote from: magic on July 14, 2020, 08:40:53 amThat is what I would do but there is no need to use a depletion mode FET although that is the simplest way; a zener diode can set the bias voltage on a conventional bipolar transistor or power MOSFET and I have done this many times.One thing to watch out for is that during fault conditions, the regulator's full current will be applied to the high voltage transistor so it is advisable to either include this factor into the power transistor's derating or include a separate current limit. Vbe current limiting is usually sufficient but old designs tend to use foldback current limiting to limit stress on the high voltage transistor.There is a trick which can sometimes be used with integrated voltage regulators. If the bias circuit for the high voltage series transistor sets the voltage at the input of the regulator high enough, then the regulator's built in safe-operating-area protection will implement foldback current limiting helping to protect the high voltage transistor. This assumes the the regulator's current limit is low enough however.

author: CaptDon
author_rank: Super Contributor
timestamp: July 14, 2020, 01:41:56 pm
content: You do see that the vb408 is a 'nightlight' or 'standby' supplydevice which does nothing during the actual 'run' mode. The14.5vdc supply will overrun the vb408 when the amp is upand running. During the redesign, take care to observe whatyour kludge circuit will do when the 14.5vdc comes up.

author: magic
author_rank: Super Contributor
timestamp: July 14, 2020, 08:03:22 pm
content: You know, DIYing a high voltage downregulator could be moderately tricky, but adding a transformer-based auxiliary 12V PSU is pretty straightforward.It seems you would need about that much:- a 12V transformer or AC wall-wart- rectifying bridge like BR402 (perhaps smaller) or four diodes to make one- 35V 330µF or larger capacitor- one 7812 chip- one more diode to protect the 7812 from backfeeding when the LM2594 converter comes upOr alternatively, since your amplifier is already rendered impure by the presence of the LM2594:- a single 12V switching DC wall-wart with a two-prong mains plug (three prongs will be instant smoke)- a Schottky diode, I doubt that the circuitry downstream would care about seeing 11.6V instead

author: Birstall
author_rank: Contributor
timestamp: July 15, 2020, 12:22:30 pm
content: Thank you Magic, I appreciate your response. I wonder if you could draw that out for me if and when you have a moment.

author: magic
author_rank: Super Contributor
timestamp: July 15, 2020, 12:45:50 pm
content: Hmm, there is actually another question.If the original regulator was blown, it may have applied full input voltage to the 12V circuits.What actually is powered by that 12V rail? Do you have a schematic of that? Did you look for signs of damage to those parts?

author: Birstall
author_rank: Contributor
timestamp: July 15, 2020, 02:32:27 pm
content: On the second drawing in my original post, it shows that rail going to VCCThe only visible sign of damage was a burn mark below the vb408 and a blown electrolytic capacitor, which I replaced. Whether there is additional component damage, I couldn’t say at this time. I was hoping to solve one issue at a time.If you are implying that I shouldn’t be wasting time on the voltage regulation when there are other issues is probably correct. However, if I understand you correctly, using a transformer may not be so difficult. in which case I would be interested in understanding exactly how to do that but without a drawing I can’t imagine how it all goes together.

author: magic
author_rank: Super Contributor
timestamp: July 19, 2020, 06:38:08 pm
content: Forgot about this thread.It looks like there is a whole bunch of circuitry there and it may be busted if the 12V regulator blew. Was that exploded capacitor on the 12V rail too?Did you verify with a DMM that there is no short from the 12V rail to ground?

author: TheMG
author_rank: Frequent Contributor
timestamp: July 19, 2020, 07:55:11 pm
content: Can you post the complete schematic of the power supply, or a link to it?

author: magic
author_rank: Super Contributor
timestamp: July 19, 2020, 08:04:11 pm
content: Two images in the first post, if you want just the power supply.

author: TheMG
author_rank: Frequent Contributor
timestamp: July 20, 2020, 12:09:12 am
content: Quote from: magic on July 19, 2020, 08:04:11 pmThose only show small portions of the primary side of the power supply circuit.

author: winniethepooh_icu
author_rank: Contributor
timestamp: July 20, 2020, 03:23:05 am
content: Do not give up on this VB408. If 25C ambient and 300V input you can not exceed 6mA through this device before risk of thermal shutdown. Consider that failure may simply be to lack of heat sink. You are not looking at a burn mark from failure in the picture. You are looking at the effects of long term heat. In the picture the only heat sink is a small copper area on the board. Improve heat sinking ability with a aluminum heatsink and it should not fail. Do not short the heat sink to anything, it will be at the same voltage as Vin, maybe put a hazardous voltage label near it as well.

author: magic
author_rank: Super Contributor
timestamp: July 20, 2020, 06:30:11 am
content: Quote from: TheMG on July 20, 2020, 12:09:12 amInteresting idea. I assumed that here is no secondary and this is it, the amp simply runs on rectified mains.

author: drvtech
author_rank: Regular Contributor
timestamp: July 20, 2020, 07:48:53 am
content: You could use an MP174 non isolated switching regulator. I've found them excellent except when they have absolutely zero load. This is easily solved by putting a small (500mW) zener across the output. But if you always have at least some load (less than a mA) they do the job.

author: Birstall
author_rank: Contributor
timestamp: July 20, 2020, 08:35:19 pm
content: I was thinking along the same lines it did last for over 12 years.Unfortunately I was not able to find a similar VR from a reputable US supplier. The alternative was buy from China a VB408

author: bdunham7
author_rank: Super Contributor
timestamp: July 20, 2020, 08:45:37 pm
content: Quote from: Birstall on July 20, 2020, 08:40:40 pmI'm pretty sure posting here for the purpose of discussion would be fair use, if US law even applied to an Australian forum.

author: Birstall
author_rank: Contributor
timestamp: July 20, 2020, 09:04:40 pm
content: Here's what I have

author: TheMG
author_rank: Frequent Contributor
timestamp: July 21, 2020, 01:30:42 am
content: That's a rather interesting power supply design...It almost looks like the regulator's sole purpose is to provide power to the primary side switching control circuits until the power supply starts switching, then it gets power from the auxiliary winding of the main transformer, labelled as "BIAS1" and "BIAS2" (strange naming, as it has pretty much nothing to do with biasing of anything), this is rectified and regulated down to 14.5V by a switching buck regulator. Since the 14.5V is greater than the 12V that the VB405 regulator is set to, it basically supplies no current at all once the SMPS is up and running normally.Since the linear regulator is only supposed to supply power for a brief amount of time during the initial power-up, that would explain the lack of heatsinking.This also brings about another question: is the buck regulator working as it should and supplying the 14.5V it's supposed to during normal operation of the power supply? If this circuit is not functioning, it would explain why the linear regulator ran so hot and eventually failed, as it would be powering the SMPS control circuits full-time instead of only at startup.What I would do first, is to apply about 20VDC to the "VBIAS" test point with the unitdisconnected from mains, using a bench power supply. Then check for 14.5V VCC. If that's good, check for a switching waveform at the gates of switching FETs. That way you can assess is there is other damage or issues with the primary side of the power supply, in a safe way without ever applying mains voltage to it.

author: Birstall
author_rank: Contributor
timestamp: July 23, 2020, 01:11:29 am
content: Quote from: TheMG on July 21, 2020, 01:30:42 amI tried that, sort of, but my PSU (TTi PL303) dropped from 20v to 2.4v whenever I switched on the output. However, when i checked the voltage across the FET's i got a reading albeit small.

author: TheMG
author_rank: Frequent Contributor
timestamp: July 23, 2020, 04:17:10 am
content: What did you have the current limit set to on the bench supply? The primary control circuit shouldn't draw much power anyways, even 100mA should be more than enough. This may be a sign that there is other problems. Perhaps a fault in the control circuit drawing excess current and not allowing the SMPS to start is what caused the high voltage regulator to cook itself in the first place.I would lift one side of D414 and try the experiment again. If the current drops down to something low and you measure 14.5V on the anode (positive) side of D414, then you know the buck regulator is good and the fault is further downstream. If not, then you know you've got an issue with the buck regulator circuit.

author: Birstall
author_rank: Contributor
timestamp: July 23, 2020, 01:40:07 pm
content: Quote from: TheMG on July 23, 2020, 04:17:10 amI had it set to 5oomA.I couldn't find the test point, so I ran it from the output of the VB408 to the negative side of the power input as it was the only thing I could grip with a crocodile clip. There's supposed to be veropin on the negative rail but couldn't find it.

author: Birstall
author_rank: Contributor
timestamp: July 23, 2020, 05:08:07 pm
content: I tried again this morning and in the same configuration I set the current to 100mA at 12v and it dropped to 1.7v when I pressed outputI found two nodes marked PWR+ and PWR-I ran my DMM from the VB408 out to the both of these nodes and both showed OL the same fro D414I ran 15v 100mA from the North terminal on the D414 to PWR - with no effect and then to the mains in negative prong and the voltage dropped to a volt or so. The D414 is not a pin through diode, I don't think my soldering skills will stretch lifting a leg.There is connectivity between PWR+ and PWR- but non from PWR + to Mains + or PWR- to Mains -

author: Birstall
author_rank: Contributor
timestamp: July 23, 2020, 09:17:36 pm
content: Well it looks like the end of the road for this one. I found a lifted trace on the back.Thanks to everyone who tried to help me,

author: Pulsepowerguy
author_rank: Regular Contributor
timestamp: July 23, 2020, 10:29:59 pm
content: I burned trace can be mended, but one must know the cause. The 'bulging' areas just look like extra solder to me - nothing to worry about if I am correct. If you follow the damaged trace to the components it was connected to, then you can get an idea of what went wrong. It's all part of the process.

author: Birstall
author_rank: Contributor
timestamp: July 24, 2020, 10:24:22 pm
content: Quote from: Pulsepowerguy on July 23, 2020, 10:29:59 pmThanks for the encouragement. I did mange to repair it after watching a few YouTube videos. Quite pleased with the result.I had one accident insofar as I scraped away too much and exposed a parallel trace but I managed to seal it again with Bondic UV liquid plastic. Once insulated I was able to complete the repair.

author: Pulsepowerguy
author_rank: Regular Contributor
timestamp: July 25, 2020, 10:59:48 pm
content: Quote from: Birstall on July 24, 2020, 10:24:22 pmBTW: that coating that you scraped off (solder mask) does not qualify as insulation so its OK to leave it exposed. It is more professional looking with the sealant though.

author: W1ngs
author_rank: Contributor
timestamp: May 31, 2023, 05:37:40 am
content: I just repaired another one of these tonight; I service a reasonably large quantity of Linn stuff. In any of the 90s Linn gear, keep your eye on the electrolytic capacitors, especially the brown Elna "Long Life" capacitors, and especially especially the 100µF 35V ones... Every one of those I encounter lately is leaking electrolyte (sometimes a great deal of it) and causing corrosion and other damage.In the Linn Sizmik I dealt with tonight, the VB408 was thermal cycling because it was being forced to keep trying to support the supply's initial power needs indefinitely because the supply itself was unable to start. The root cause of all this was the pair of 47µF 200V capacitors which form the coupling for the primary of the main transformer (I'm not fond of electrolytic-capacitor coupling of inverters, especially when any appreciable current is involved). The Linn power supplies in a lot of their amplifier products like to bake these coupling capacitors to death.On inspection the supply inverter was stuck at max duty cycle (50%) and the inverter transistors were getting relatively hot, yet the output at the transformer secondary was just brief pulses rather than a square wave, indicating that the majority of the input waveform was being dropped across those 47µF coupling capacitors, which were getting very hot!On replacement of the two aforementioned coupling capacitors (right up next to the power transformer) and all three of those 100µF 35V Elna capacitors (which were all leaking), the amplifier returned to normal operation and VB408 runs cold as it is only doing startup duty, as intended.

author: lagas
author_rank: Newbie
timestamp: January 11, 2024, 06:36:21 pm
content: hi to every one.i am trying to repair Linn Majik 126 active subwoofer.Looks like have issues with standby power supply. But this PCB/boards are different design.Maybe some one have schematic for this one in the pictures?Looks like bad CAPS but i checked all them and all pass all tests.If connect power and mess around, its starts ticking noise, then disconnect ac, caps discharges and then as soon you connect ac back, then power up and works. If you leave like this it can work for days. As soon you disconnect and leave longer, then again to start it takes play...Ticking noise coming from transformer near small cap 100uf 50V. And standby chip are TOP223GN. Looks like its faulty. I tried other caps no go, same situation.re-soldered and cleaned this area because all over heated and solder cracks every there.