Aloha All from Hawaii Island! Coreblade problem

Again, Aloha All from Hawaii Island!
So I was brought a H & K Coreblabe to attempt repairing. I have never seen one before. A tank for sure!

So, the problem(s):
#1) No preamp functions, but a low volume sound is produced by turning up master with something plugged into input. All pre controls have no effect on that output volume. Also no effects.
#2) The Clean/Dirty/Ultra I/Ultra II is stuck with 1st 3 lights lit. (all but Ultra II) No effect by moving switch position
#3) The boost switch? is "on"? (lit) Pressing button has no effect.

I am guessing a microprocessor failure of some kind, but no clue where to begin to troubleshoot. Any ideas?

Mahalo,
Steve

Hi Steve.  'What fettle bonny lad' from the North East UK.

I would say first try a factory reset if you haven't already done it.  The manual says this:

Coreblade Service Manual wrote:7.3 Factory Settings and How to Restore Them (Factory Reset) - A factory reset is a seldom needed feature. Even so, be sure to read the explanation carefully to ensure you don’t accidentally delete your presets.
7.3.1 Triggering a Factory Reset - Pressing STORE and FX SERIAL simultaneously while powering the amp up resets all settings, including the 128 MIDI-switchable presets and the basic MIDI configuration.
CAUTION: This procedure is a last-resort option! It irrevocably wipes out all stored settings. Before you trigger a factory reset, be sure to first create a backup by copying your amp’s internal memory to a stick.

Now I'm sure we are all aware that a reset will wipe out all of the current settings on the amp.  You don't have the option of exporting those settings so I would see this as a necessary starting point to rule out simple software corruption in the memory. From this point on I'm assuming you have experience of working inside high voltage gear. It can KILL! I'm not sure of the layout of the Coreblade but I would really stay away from anything near the valve stage area if you aren't really confident and have the right knowledge of how to work on them. Be realistic there!!!

Ok, that said... If that doesn't change things then I would see how far the signal is getting through the chain. I would put a signal in at the front end and check it through the preamp circuitry to see where the signal path goes quiet.  Do you have the Coreblade Service Manual with the schematics in it?  If not try sites like Elektrotanya to see if it is available there.  The Coreblade has a similar philosophy to the TM and GM series.  The input stages are solid state opamp circuits and they should be easy to check out if you have a scope to simply look at their outputs with a signal in the amp input.  You are looking at ICs U11 pin 1 and U12 pins 1 and 7 there.  The signal is then fed to the valve preamp stages through RL2.  These relays do fail occasionally sop that could be sticking but it won't account for all of the other symptoms you have reported. If there is a problem in the genuine digital microprocessing area rather than any analogue connecting or driver circuitry you are extremely unlikely to be able to fix it.  The only viable answer without specialist digital troubleshooting gear is replacement of the board.  As you are no doubt aware, that won't be cheap!

How far have you got with it at the moment?

Looking a little more carefully I can see that the channel selector switch which you described sits on a small PCB on the back of the switch.  There is an 8 pin socket JP10 receiving and feeding signals to and from that PCB via JP7.  Is it connected correctly?  Try breaking and making both ends again.  Connectors like that can go dirty or even lose their wire connections.

It looks to me as though that switch is simply sending the microprocessor a 0, 1, 2, or 3 digital signal (00, 01, 10, 11) on two of those lines to indicate the channel selected.  The socket pins 1, 3, 5, and 7 down one side are carrying in +5V on pins 5 and 7 (linked) and ground connections on 1 and 3.  The switch then simply grounds 1 and 3 to 5 and 7 when turned to produce the digital 0s.  It will work like this:  (for lines 5-7)  Clean 5V-5V,  Drive 0V-5V,  Ultra1 5V-0V, Ultra2 0V-0V.  If you have constant Ultra1 selected with the first 3 LEDs lit then it is possible that the 5V line is down.  Have you checked out the various power supply lines?

Look for a fuse FU2, a T1A.  It looks as though it is on the end of the main board carrying the valves at the end away from the sockets.  This feeds the regulators for the 5V and 15V and the unregulated 22V lines.  If that fuse has blown then you will not be getting any LT supplies which would do the sort of thing you are experiencing.  If that fuse is ok then check out the 22V output and the outputs of the two regulators.

I'm holding my breath on that last point!

OK, so I have attempted the reset. No change. The 3 leds and boost light come on as soon as power is switched on while still holding down the store/serial buttons. Also FU2 is good.
I did look into the JP10->JP7 connection for that rotary switch. Actually it's an 8 pin header soldered directly into the pre amp board and the switch mini board. No chance it's a dodgy connector. So, next will be to see if the 22V is present. I did download the service manual, and I appreciate your warning about high voltages inside of tube, or as you folks east of the Atlantic say "valve" amplifiers, but I do have many years of experience in this area. Unfortunately none at all with this particular amp. More later. Mahalo!

BTW I like your Humble Pie "30 Days" quote. I saw them live a handful of times, 1st being in Dec 1971. They were a fav!

Good to hear you have experience with HT Steve. It's always a worry advising on what things could be wrong if you feel the owner is going to probe into their amp without any knowledge of the risks and how to mitigate them.

I am getting a bit tied to the idea of something around the selector switch being at fault. That is certainly the case but I mustn't forget you have also lost signal. If the selector switch alone was at fault I would have thought you would simply be stuck on the Ultra2 channel. There must be a problem with the signal circuitry too to lose sound as you have done. Again, I first suspect the +15V supply to the solid state stuff. You checked that fuse but did you check the actual supply lines themselves, maybe at the regulators near the fuse and even at the chips themselves.

I'd better mention here, don't trust yourself to judge that fuse by eye. I've been caught out many times by fuses which have looked absolutely fine even with a magnifying glass but have blown inside the end caps where it can't be seen. I have once or twice found fuses which are great at switch on but which lose contact after they heat up, (which they all do in use), but there is little you can do about that unless you spot it after it happens. Did you take it out and check it for continuity with a meter because that is the only way to be pretty sure. Personally, to be absolutely certain barring a faulty new part, I check them all now by swapping with a new one. No difference and I can swap back again.

Really the best way is to just measure the +5V, +15V, +22V lines after the fuse. We need to guarantee they are working before looking into anything else. The opamps run on a single +15V supply with a generated half supply on their own board to centre them. You would of course get nothing if that line was down. As you will know, the 8 pin connector just shows on the schematic as a bland rectangular block in the schematic. I assumed it was a plug/socket, your explanation removes that as a potential source of problem.

The workings of the channel selector switch which I went into in a bit of detail could be very significant here and give us clues. It looks very like the two binary selection lines are stuck on 0V-0V. And the fact that the LEDs are lit up means they are being fed voltage by the processors which suggests the +5V line is ok. However, you are seeing exactly what you should see with the LEDs if the switch has no 5V feed to it. Or maybe the switch itself is at fault. Are you able to rule this out by checking the state of pins 5 and 7 on that connector as you change the channel selection? They should both show +5V for Clean and 0V for Ultra2 (or maybe the other way around) with one at 0V and the other at +5V for the other two channels.

Aloha, and yes, I did pull the fuse and checked it with my meter. t was good. I have had similar experiences with fuses that looked good and then turned out bad when checked for continuity. I have a gig tonight and won't be working on it today. I will get further into it tomorrow or Sunday. I will be checking those DC voltages & see what I find.
Many Thanks, Steve