New HT5-R - No Sound!

[Jacob]

Hi,
Sorry for my bad English.
I have the exact same problem With my HT5-R .
No sound but only some crackling.
I had replaces several components in that specific circuit with no succes .
Than Is saw these posts ansd i was very happy.
But there are no carbon traces visible, also not with a magnigifer glass.
When I measure there is no resisatance , (I temp disassemble the mosfets )between the holes
I made some scratched between them , replaced them back ( after I tested them first again)
checked all other components again in that circuit but still no succes.
The dc values to the elco's (that goes to the tube )
are 50 and 320 V
That is not good I think.
Anyone any suggestions ?
I also like to have the right schematic for this model , I only could find HT5 schematics
Ref. numbers do not Always match and there are also some other differences.
Thanks in advance.
Jacob from Holland

[thephantum]

So you have no visibly burnt components, which is a key symptom of this condition...but you went ahead and pulled the mosfets and scratched between them? Why?

Have you checked the power tube? Have you tested the grid resistors? Did you measure B+ or did you measure Plate voltage?

[Jacob]

Yes I, have the same symptom.
The drain resistors are kind of burned but are still ok.
The tube is working fine, when I put a signal generator on the grid resistor I have signal on the speaker.
When I put signal on the guitar input I have signal up to the mosfet circuit but after that not anymore.
I really don't get the working of the transistor TR4.
It does'nt do anything because I measure 0,6 Volts on junction R41-R42-D21-TR4 .
I think it should be a negative voltage
Now It all goes via diode D21 to Gnd nevertheless TR4 is NOT broken ( have allready replaced that).
All other resistors in that circuit are also fine .
B+ and -15 V are also present.
I also still think the circuit is different from the schematic I use.

[thephantum]

If I remember correctly, the only way to identify the exact location of the short was under a microscope. Even then, the arc lines were very faint. It also wasn't a dead short. There was resistance measured at it...but finding where that arcing was told you exactly where a hole had to be drilled in the PCB to fix it. It was between two pads of that MOSFET, but I'm not sure which two.

Also, you are not going to find schematics for the amp. Blackstar only releases them to authorized repair shops....trust me, I've tried.

[Jacob]

okay, i measured again and now I encountered a resistance of 7k between drain and source of TR3.
I have to drill between de holes as described before instead of scratch between with a knife.
For the test I made the connetions floating above the board and its alive again !
Thanks for all your help , and all the others too

[Myles Loud]

Hi,
I'm having the same problem. My circuit board was so hot it melted the solder on R39 and the resistor fell of the board, I soldered it back before reading up on it, and it heated up again and fell off the board. . I'm not understanding the drilling the board part, ( do I just drill out the holes, or drill in between the holes , of the IRF830 MOSFET ) ? If I want to run flying leads to these and mount them on a seperate small board, do I still need to drill the circuit board? Does anybody have any step by step pictures for this repair? I can use a solder iron, but i'm not an amp tech. I contacted Korg and they said I still need to take it to an authorized center to have it looked at. The closest one to me is 2 hours away. I know I can do the repair, but I need some pictures or a video. Mine just happened last night....Feb 2nd.

[thephantum]

The problem is that there is a partial short between two pads of that MOSFET. That partial short is causing some faint arcing to occur between those two pads (in most cases, it's so faint that you need a microscope to see it). It's also causing too much current to flow to R39. Ones solution to the problem is to drill a 0.8mm hole in the exact spot where the short is, thus eliminating it.

So, ideally, you should use a microscope to find the specific spot where the arcing is occurring. That's where the hole needs to be drilled. Even then, you need to be very precise where the hole is drilled so you don't damage any other tracings. Drilling a hole in the PCB where someone else did might not be the exact location of the short on your PCB and could do a lot more harm than good..

[Myles Loud]

Hi thephantum,
What is the other solution? up the value of R39? I just want to fix, but I can't afford a bench charge from a tech at this time.

[thephantum]

Upping the value of R39 is not an option. There's a short circuit involved. All putting an increased value resistor in there is going to do is cause it to generate even more heat. You said yourself it was getting so hot that solder was melting. So unless you want the amp to actually catch fire...bad idea

The other option is build a new circuit around the short. You'd basically have to diagram out all the components in between those two pads and build a matching circuit, then make sure it's all secure enough to withstand vibrations, heat, etc.

That's why drilling the PCB is the easiest, most elegant solution...

[Myles Loud]

Fire, Bad !!!!!!!!!!!! lol
I will use a jeweler's loop to try and see the traces on the board where I need to drill. Beyond that I might have to wait and take it in after the busy season down here in Fl.
Can you tell from my serial the year of mine?
Serial #:
845644000630 0001 11051537