Had the same with an Orange OR200 amp. Worked fine up to half power then major distortion. Replaced output tranformer at a huge cost but it is a 200w amp and no problems.
Pretty peculiar little amp, because if I have understood correctly, the triodes of that 12BH7 work as a push pull output stage. Those around 450V plate voltages seem quite high but those tubes have been widely used as oscilloscope deflection tubes at those voltages, and often been tortured at max plate dissipation as well. Possible no load situation could have killed that OT. Very nice that you could fix it. 👍 The output power of that high voltage beast could be even 3-4 watts, I guess. 😂 That kind of output stage could make quite a nice reverb driver when using 8 ohm reverb tank.
Yes I was surprised at that HT. Yes it's a standard push-pull arrangement but they drive the cathodes via those FETS for some reason. I think this died due to being thrashed solid for 2 hours on max volume. Crazy. Not a gigging amp at all!
@@stuartukguitarampguy5830 Thanks, because I was already wondering the purpose of those FETs. Whether no load or output stage driven to clipping, voltage spikes in both cases, and OT does not like either option at all. And no wonder if those fets are prone to failing as well. 😁
For that price, it should have been a Heyboer or Mercury Magnetics, instead of some cheap Chinese thing! I suggested to Stewart in another comment that he probably should have measured the turns ratio of the replacement transformer and included that info in the video so that other technicians would know what to use as a replacement in future. The blazingly high 463 V at the plates of the 12BH7 output tube might have been a clue that the tube was drawing virtually no current due to a wacky transformer load. I bet the B+ dropped by at least 20 to 30 volts when a new transformer was installed.
@@gingercat777 I would have gotten a similarly speced OT from eBay before I would spend that much on a Blackstar brand component. It's not like Blackstar makes those OTs anyway- they get them from some supplier in China, probably the same one selling them on eBay for $20.
Excellent diagnosis and repair Stuart.. About five years ago I had the same thing happen to my FBJ .. All the readings on the output TX looked okay.. But I had another TX kicking around so I thought there's nothing to loose and I connected it up. Hey presto it worked perfectly. I still have the faulty TX but can't figure out to this day what's actually wrong with it, but there's something going on for sure. I'm pleased to say I'm one of your 'million' as I've been with you from the start of your YT channel. Congratulations and well deserved.
@@pda49184 test it with a megger and the truth will be revealed. Many times you check something with a VOM and it's fine, but the component fails under load. The VOM only puts out a few milliamps, not enough to find some problems.
To remove the FET's without damaging the pads, just clip the leads off and then de-solder each lead and pull it out. It'll save you de-laminating the through hole pads and possibly damaging the barrel. By the way, I learn much from your channel, thanks!
I wish as I had read this comment before I fixed my Bugera G5, which is probably based on this amp. Yeah, mistakes were made, things were learned. Now the MOSFETs are wired and mounted on heatsinks at the upper side (where the tubes are). The board is a bit messed up, I didn't want to risk messing that area again
@@DennisCameronMusic Sometimes it helps if you instead of sucking solder away just add solder first to every pad, then it can be possible to enough heat them all at the same time and pull the component out. Sometimes those holes really are very small and tight and the leads can get very much stuck in them.
I actually have one but yes, they don;t supply them. Can;t have other engineers stealing those top secret ideas like using op amps and FETs 'n' stuff, can we???
@@brivington2011 I worked for a manufacturer that scrupulously guarded their designs like it was a covert CIA operation. I guess they didn't want the competition to know that the machine hydraulics were straight out of the Bosch -Rexroth handbook available to the public.
Consumer electronics mentality. They knew when they made these that they were destined for the landfill in 10 years or so but hey, you can't sell as many new amps if your old ones last too long and are readily available on the used market. People love to bash Mesa for lack of repairability, but smt components are the mark of a true sociopath imo.
At 11:25, the whopping 462 volts B+ measured at the 12BH7 was probably a clue, because it appears the tube is idling at virtually no current, which indicates an incorrect load, due in this case to the bad output transformer. It would have been instructive to measure the B+ after the transformer was replaced; I would bet that it dropped by at least 20 or 30 volts. As for the blown output transformer, your megger might have shown internal leakage or capacitive coupling. PS, 75 pounds for a puny transformer like this is highway robbery! You could probably have looked at a tube manual and figured out an appropriate impedance match so that you could replace the original transformer with a Hammond or something else for half that price, or measure the turns ratio of the brand new transformer before installation so that you can find a more reasonably priced substitute if you ever need to replace one again. You'd be doing a service for other Blackstar owners and their long-suffering technicians!
Yes I thought that BUT those power resistors in the FET path were getting warm and drawing current, so I assumed thart must be going through the primary? Yes outrageous! It's super rare to replace one though, so I probably won;t seek out an alternative.
Poke holes in the solderwick with an awl or tiny jeweler's screwdriver and then you can place the wick directly over the stub of each component lead. Adding the special low-temperature solder known as ChipQuick is also helpful; it stays liquid for almost 10 seconds without further heat being applied (ChipQuick is also very helpful for removing SMD parts with multiple pins). PS, I'd recommend adding heatsinks to those Mosfets; the discoloration of the board indicates that both the mosfets and the resistors are generating heat. Unfortunately you can't heatsink the resistors, nor easily fit larger ones, but replacements could be mounted a little above the board for better air flow.....
Good tips thanks. Particularly you have reminded me to get some low melt solder. There's an excellent RU-vid site caleled Northridge Fix if you haven;t seen it. Amazing work on smd boards.
Brad's guitar garage has been working on a replacement DSL board for those ones where the board itself goes conductive but I think (a) it's the 100 watt version and (b) he's maybe just at the prototype stage and hasn't fully populated one and installed into an amp yet but check him out
Funnily enough I diagnosed the exact same problem in my HT5R Combo last week. I'm not paying £75 for their crappy OT though! The Hammond 125B is a direct replacement if I'm not mistaken.
Every SMT component amp chassis is a foundation for a point to point all tube circuit. Install a power amp resistor on the output jack to prevent a no load scenario. Great work sir!! You made this amp alive again!! :)
Stuart, It's always exciting to see new content from you. I think the fact that you show the process you use and explain the issues simply is why so many people watch you. I know I've learned a lot watching your channel. This was interesting, but I don't know if I would have clipped a new transformer like you did. However, I'm a novice and voltages scare me. Always informative and entertaining. Thanks, Mike
Hi Mike Yes super dangerous what I did, but of course I was very careful and checked all connections wer ok before switching on. Not for the faint hearted!
@@stuartukguitarampguy5830 Using two POWER FETS as a phase splitter/phase inverter is very unusual for guitar amplifiers. I think maybe Music Man amplifiers from the late 70's used also a Solid Stage Phase Inverter/Phase Splitter also but I'm not sure. Its Rare to see amplifiers using a Solid State Phase Inverter plus only using ONE output tube in a push pull mode which is also very unusual.
Good tip about marking the bad part. Even though I've never once ever took a bad part off then put it back on thinking it was the new part causing me hours more diagnostic time, I mark the old part with a paint marker pen- red for bad and yellow for maybe still good.
@@stuartukguitarampguy5830 I also put a yellow dot on valves that are weak but still useful for testing or something. Otherwise I would have to throw it away which is wasteful, or mistakenly think that it's good a year or two later when I take it from the spares box. The chips you replace on this amp I would either chuck them in the bin because they're not rare or expensive, or I would mark them with a white dot and put them in the spares. White or green I know that it's a good used part (I still would test it before installing)
