Blackstar series One 200, OD1, OD2 & Crunch NOT passing Signal

[nevetslab]

To my "amusement", a Blackstar Series One 200 Bass Amp was sent over from inventory, label stating No Output. And here I thought all those Blackstar amps we have (had) were just for stage props. I love their design. One handle in the middle, with all of the transformers on one end, tubes at the other for good balance.

The AC mains Fuse holder was missing the fuse cap. Of course I didn't have one like it, so I first tried swapping it out with one of the stage prop chassis here in the bone yard. I gave up on that quickly, as all of those fuse holders deserved to be in the trash, so had to settle for one that uses 3AG style instead of GMC style fuses.

Seeing the KT88 tubes, I looked to see if there were discrete cathode resistors. Nope. Just one common buss that tied to a common 1 ohm 2W metal oxide resistor to ground. So, no simple way of assessing plate current balance. I powered it up, while in S/B, looked to see what was at the grids of the power tubes. -98V. This pot looks like it might be the bias pot (only one other to choose from), and diddled it to see if the bias level changed. Yup....-90V thru -105VDC. Put it back to -98V. Switched out of S/B, and no change in AC mains current, still sitting at 1A/112W @ 120VAC.

I only have schematics for the HT Stage 60/100 that I got off of this website, and looked thru it for some clues. While looking further for anything related to Blackstar 200, I came across Randall's thread 'Damn You Blackstar!', where he found after spinnning his wheels a while, it takes inserting a plug into the Input jack for the Bias to turn on. Cute. I plugged in my 100k termination plug, and sure enough, I had 88mA of current flowing thru the common 1 ohm cathode resistor, that settled down to 80mA. I did find Enzo's contribution to Blackstar documents 'Bias Guide General' and downloaded that. Many thanks, Enzo!

I finally plugged in my test signal, only to find the Clean Channel passed signal. The OD1, OD2 & Crunch channels all just yielded hum with the faintest bit of signal in the background. I swapped out the other three 12AX7's, one by one, leaving the 12AU7 in place, guessing it was the driver tube for the output stage. Swapping two of the tubes with each other, I did get the hum to go down, but, no passing signal thru those three channels.

I'm guessing it's on the Preamp front panel PCB....J175 JFETS, maybe one or more op amps, or who knows.

Anyone have the schematics for the Series One Model 200?

[drewl]

Sorry, no but from what I've seen the preamps and topology are very similitude.

Got an HT20 and 60 in this week.

HT20 sorted out, trying to get the 60 hot enough to find the intermittent loop fault.

[nevetslab]

Sorry, no but from what I've seen the preamps and topology are very similitude.

Got an HT20 and 60 in this week.

HT20 sorted out, trying to get the 60 hot enough to find the intermittent loop fault.

After pulling all the knobs & spacers off the pot shafts, removing all the cable assemblies from the top edge of the front panel PCB, I went thru carefully and pried off the PCB from the circular metal fingered friction PCB spacer/holders and worked it free. Then, you gotta figure out how to shift the board to extract it from the chassis, as the middle PCB's in the chassis all say no. I did finally get the board free....also unplugging the input PCB assy's plug.

The board is full of TL072's and J174 P-Ch Switching FET's, and steering diodes (1N4148's). I went thru and just did Semiconductor junction tests on all the JFET's, and when the board was still in the chassis, verified all the op amps were sitting with nominal DC offset (all less than 5mV). All the steering diodes tested good, as did all the JFET's.

Comparing the PCB with the schematics of the JT60/100, it doesn't look much like that, as far as the gain stages go. So, I was faced with some choices. What seemed the easiest was to replace the TL072's associated with the gain stage area of OD1, OD2 & Crunch channels, as well as their associated switching FET's, then re-install the board and see if I have all four channels working.

The first J175 I tried to remove, I was greeted with LEAD-FREE solder, which has always defied the use of my well-serviced PACE SX70 Desoldering Iron and clean fresh/tinned desoldering tip for .030" ID pads. I was greeted with deformed blobs of LEAD-FREE solder as a result, so I applied some 60/40 Kester solder, 0.031" solder, which in the past, has been the solution to then melt and allow the rest of the LEAD-FREE solder to melt with it and get sucked up by the PACE Iron. Not on Blackstar's PCB's! I was immediately greeted with solder pads being sucked right off the PCB, had to go to the top side of the board and use solder-wick to get the rest of the solder free, which upon finally removing the JFET, along came the plate-thru holes still clinging to the component leads. Carefully inserting the new JFET caused one of the solder pads to catch the tip of the lead, it already haven broken free of the foil adhesive, and peeled the trace/pad away from the board to please me all the more. WHAT A PIECE OF DUNG!!!! I though Fender's Hot Rod series PCB's were bad. Those are WONDERFUL in comparison to the crap BLACKSTAR has bought from the Orient!!

I made one more attempt...that was trying to remove the first IC that appeared to be associated with OD1. Got what Lead-free solder I could from the holes, which also gave me two unattached solder pads for my trouble, and applied lead-free solder in attempt to get the solder out of that IC solder pad. Nope. Bonded too well to the top side of the PCB, and sandwiched in too tightly by adjacent box caps to get any common soldering iron in to deal with that. I had to go to my micro-surgery sized tip to re-solder the two pads that disappeared on the bottom side of the board, re-soldered the IC back into place and wanted to take this board outside and sail it as far as I could to get it out of my sight!

This amp doesn't deserve to work again, at this point. My only other possibility is to extract the front panel boards from the two 200 amps from the bone yard, and see if those even work. If so, and this recovers, it will be the LAST BLACKSTAR amp that will be admitted into this shop! If it doesn't work, that's what it deserves.

[drewl]

Did you try tracing a signal through the bad stages first before shotgunning it and ripping out opamps?

[g1]

From your description, I would guess that the factory or repair depot do not repair these boards at the component level. Probably board swaps.

[nosaj]

From your description, I would guess that the factory or repair depot do not repair these boards at the component level. Probably board swaps.

Ok I had a similar issue today on a Marshall 250dfx Had to replace outputs to get it back up and the MV pot. What I found was it would play clean in both channels no grit.
At one point I bumped the input jack pretty hard on accident and got some dirty marshall tones. Pulled the PCB out pulled the input jack out continuity on the jack good with the jiggle test
What I did find on re-installation was the circular pads were not quite connected to the tracks. Scratched some green coating back and reattached. Now I have good sound on both channels. I started at the first opamp and worked backwards till I got to the input jack.

So maybe it's something simple.

An again thank you guys for giving me the knowledge to work some things out on my ow. I am eternally grateful for all the nuggets you guys through.

And Nevetslab your posts are friggin awesome the way you lay things out.

nosaj

[nevetslab]

Ok I had a similar issue today on a Marshall 250dfx Had to replace outputs to get it back up and the MV pot. What I found was it would play clean in both channels no grit.
At one point I bumped the input jack pretty hard on accident and got some dirty marshall tones. Pulled the PCB out pulled the input jack out continuity on the jack good with the jiggle test
What I did find on re-installation was the circular pads were not quite connected to the tracks. Scratched some green coating back and reattached. Now I have good sound on both channels. I started at the first opamp and worked backwards till I got to the input jack.

So maybe it's something simple.

An again thank you guys for giving me the knowledge to work some things out on my ow. I am eternally grateful for all the nuggets you guys through.

And Nevetslab your posts are friggin awesome the way you lay things out.

nosaj

To answer drewl's question, I didn't scope it out, before pulling the preamp board. There's so many cables in the way to do that, and, the documents for the HT 60/100 aren't all that much help on this amp. After removing the front panel PCB, which is part of the preamp, while the rest of it, as far as the tubes are concerned, they're on the first middle board. I only got as far as checking DC levels on all of the op amps, not finding anything obvious.

But, I was going to try powering up the front panel board with an external supply and try and trace the signals thru the four channels', as far as that went, though still no idea if the signal leaves the board before coming back to it. The solder pad/traces issues ended that pursuit.

But, having two boneyard Series One 200 amps, I extracted the first one on the most empty chassis, not having noticed two of the right-angle multipin headers were absent. I set it aside, pulled the second chassis out of the cabinet and set it on the bench upside down to see if everything looked intact on it. It did, so I pulled the harness connectors off of it, pulled all the pot knobs/bushings off of it, then carefully pried that board out of the chassis snap/standoffs, fiddled with the position until I was able to extract it from the chassis.

Then, sat down with this board and checked all the FET's and steering diodes, found all were ok. I moved it into the 'working chassis', got it re-installed, connected it all back up, put all the pot bushings and knobs back on. I plugged in my current probe amp into the Tek mainframe, set the current probe up with the scope, and clipped it onto one of the four plate voltage wires....there being sufficient cable length for measuring each of the four KT88 power tubes' plate current. And, had the common 1 ohm cathode resistor connected to a DMM channel for total current.

Powered up the amp and all came up ok, no surprises. I saw I had 100mA total current, which is what Blackstar calls out for. I did, though see significant differences between all four power tube plate currents, and recorded those. I'll be able to swap two of the tubes to even out the plate current balance, if/when I got there.

I patched in my test signal, speaker connected, and switched it back out of S/B. I still had the Clean Channel, and this time, with a different front panel PCB assy, I had all four channels. So, it WAS the front panel PCB, though WHAT specifically.....I dunno.

g1 has it.......these PCB's are not designed for service.....they're designed for board swapping. Unless you're a gluten for punishment. What a shame. With all the complexity they designed into these things, higher component count usually boils down to more frequent service issues. Anyone want a front panel board for free?

So, it looks like there will be a working amp as a result of this exercise. Not one of my better service adventures, but, I can live with that.

[drewl]

Yeah, I'm not reading all that right now, maybe later.

Did you have signal up to any of the tubes?
The front end is SS.
Signal splits after two opamps to ch2 and 3, which are driven by their own tube.

It seems like you did some unnecessary things before troubleshooting.

I say this to help, not to criticize.

[nevetslab]

Yeah, I'm not reading all that right now, maybe later.

Did you have signal up to any of the tubes?
The front end is SS.
Signal splits after two opamps to ch2 and 3, which are driven by their own tube.

It seems like you did some unnecessary things before troubleshooting.

I say this to help, not to criticize.

Initially, after I had restored the AC mains fuse holder circuit, I did have signal thru the tube stages....though only the Clean Channel was getting thru properly. The other three, trying to get thru OD1, OD2 and Crunch, those were loaded with hum and very faint signal. Swapping the second and third tubes improved on the hum aspects, but didn't change the faint signal level. I never swapped out the 12AU7, assuming it was the driver tube, though from the only documentation I had, being that of a HT 60/100 amp, and seeing there was MosFET's involved in the LTPI circuit, and easy access to the signal flow on the back side of the front panel preamp board was a bit impaired, I jumped past that step, while agreeing with you on what appeared unnecessary.



It made more sense at that time to extract that front panel PCB assembly, and, not getting changes from tube swapping, made the assumption (the mother of all screwups, of course) that swapping out the two op amps in the grouping around OD2, OD3 and Crunch channels with new op amps might speed things up. The PCB quality and failure of solder pads, together with trouble dealing with the lead-free solder on this board drove me to extract another front panel assembly from a boneyard amp, on the outside chance that board MIGHT work. I flat out lucked out on that, as that board DID work. My feelings about the serviceability of their PCB(s) remains unchanged at this point.

Now that I had an operational Series One 200 amp on the bench, I did find a couple features I like. Their sensing circuits that prevents the power tube biasing to be turned on UNLESS there is an INPUT and a Speaker Cable plugged in IS, I think, a good feature. However, their Output Jack sensing circuit does NOT detect if there is a load attached to the speaker cable, as well as whether there is a guitar or signal source plugged into the input.

I had to resort to using a current probe to check the plate current balance, since the only measurement point they discuss in their Biasing document is across the common 1 ohm cathode resistor.



I ended up swapping two of the power tubes to balance out the plate current on the two halves. They also provide bias balance, and that function worked ok.

I also found their variable output power circuit works well.



I don't know if the MIDI functions on this amp have the ability stop it from working. I'm not set up for doing any MIDI interface, and never wandered down that path. There is, of course, a lot of circuitry and cabling associated with all that.

Maybe it's just me having poor luck with this amps' inferior PCB quality and not being able to get Lead-Free solder removed with my PACE SX-70 desoldering iron. I've still been able to deal with Lead Free solder without the breakdown of PCB foil adhesives allowing solder pads to disappear or peel up on other mfgr's boards. Blackstar's board quality chased me away from any further attempts to get near it with a soldering or desoldering iron.

[minim]

Whilst I admire the designers for coming up with these amps, there's no way I'd feel confident having one sitting behind me atop a couple of four-by-twelves, vibrating away like a pimped-out CLITMASTER3000. They just seem so WEAK.

[drewl]

I'd bet the design is pretty close to the HT60/100.

Either a shared opamps or switching chip.

Oh well, most importantly you were able to get it working.

[nevetslab]

I'd bet the design is pretty close to the HT60/100.

Either a shared opamps or switching chip.

Oh well, most importantly you were able to get it working.

Could be, though this 200 amp has two more 12AX7's than the HT60/HT100, uses different steering diodes for the JFET's. It also has a separate input PCB with a dual op amp on it besides those in the four channels. I didn't take the time to really follow the signal path.

[J M Fahey]

What I did find on re-installation was the circular pads were not quite connected to the tracks. Scratched some green coating back and reattached. Now I have good sound on both channels. I started at the first opamp and worked backwards till I got to the input jack.

A widely found problem is that PCB software writers compete with each other and one way o do so is to brag about "how many tracks can I pass between 2 pads" which also eases autorouting so typically starting presets are tiny pads (62 mil diameter) which when discounting component lead holes end out becoming 10 mil rings and narrow 10 or 12 mil tracks.

Fine in Motherboards, PC boards and similar, which stay still in some place, but ludicrous on Musical Instrument who vibrate hard sitting on speaker cabinets and get tossed all over the place.

I suspect many PCB makers use software as is, or just upgrade to next larger size ... not enough.

And round pads meet tracks at sharp 90 degree angles, which tend to concentrate tensions, so when pad solder cools down it is already prestressing the pad to track joint.

[nosaj]

A widely found problem is that PCB software writers compete with each other and one way o do so is to brag about "how many tracks can I pass between 2 pads" which also eases autorouting so typically starting presets are tiny pads (62 mil diameter) which when discounting component lead holes end out becoming 10 mil rings and narrow 10 or 12 mil tracks.

Fine in Motherboards, PC boards and similar, which stay still in some place, but ludicrous on Musical Instrument who vibrate hard sitting on speaker cabinets and get tossed all over the place.

I suspect many PCB makers use software as is, or just upgrade to next larger size ... not enough.

And round pads meet tracks at sharp 90 degree angles, which tend to concentrate tensions, so when pad solder cools down it is already prestressing the pad to track joint.

Well if nothing broke, we'd have nothing to do.

nosaj

[nevetslab]

A widely found problem is that PCB software writers compete with each other and one way o do so is to brag about "how many tracks can I pass between 2 pads" which also eases autorouting so typically starting presets are tiny pads (62 mil diameter) which when discounting component lead holes end out becoming 10 mil rings and narrow 10 or 12 mil tracks.

Fine in Motherboards, PC boards and similar, which stay still in some place, but ludicrous on Musical Instrument who vibrate hard sitting on speaker cabinets and get tossed all over the place.

I suspect many PCB makers use software as is, or just upgrade to next larger size ... not enough.

And round pads meet tracks at sharp 90 degree angles, which tend to concentrate tensions, so when pad solder cools down it is already prestressing the pad to track joint.

What often happens in manufacturing is Engineering hires new graduates with a BA in Electrical Engineering, so many having no experience in building or repairing equipment down to component level and assign them to laying out PCB's using their layout software. They don't know any better, and use the defaults, having no clue about creating proper pad stacks, have no clue about pad sizes and what the importance of using as MUCH copper as possible so there's adequate anchorage. How that gets past design reviews bewilders me! Of course, the bean counters select the least expensive board house, unless there's enough intelligence in quality control to circumvent engineering stupidity. So, we end up with boards like these, or what you get at GK, with tiny little annular rings for solder masks, even though they're placed on adequate sized solder pads. At least GK DOES use decent PCB material....I seldom loose traces or pads on their boards....they just don't have sufficient exposed copper, until you scrape away the solder mask.