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Thread: Blackstar HT40 problem

  1. #1
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    Blackstar HT40 problem

    Hello all

    Odd job that is making my hair fall out

    Customer repaired the handle himself but let the nut on the inside drop out and rattle about
    well as you can guess the nut touched the board and now no sound at all with no channel changing LED on

    HT-Club 40 schematic.pdf

    12.6v + and 12.6v- and 3.2v all ok
    looked over the board for any burnt parts and tracks etc none found

    using the scope and signal gen i have the following

    I have signal on the V1a and V1b but nothing on V2a and V2b
    I have signal at IC2A / FET1
    I have signal at TP3
    i have signal at pin 7 ic6
    I have signal at TP17
    i have signal at TP8
    i have signal at Pin 7 IC9
    i have a bias voltage at PI -75v ( without power tube )
    Voltage on R204 correct 320v and 360v
    Heater voltages present

    UPDATE
    with tubes in and a speaker load with input jack with signal
    still the bias is at -73v


    If anyone can help or you want more info i would be truly grateful
    many thanks

    BBB

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    Last edited by blindboybenton; 11-05-2019 at 10:08 AM.

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    bias voltage -75v means the amp is in standby,to get proper bias you need to plug an input jack and a speaker or load in the output jack,then you are in play mode
    it is normal to have signal on v1 or v2 alternatively,they are on separate channels
    Bias control comes from the PIC controller,if its damaged good luck then!

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    Last edited by alexradium; 11-04-2019 at 06:38 PM.

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    Even in play mode the amp has no output I guess reading the post looks like a channel change issue sorry I can't help good luck with it

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    Senior Member vintagekiki's Avatar
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    - Do you have sound when you plug guitar in FX RET (JK3)?
    - Check fuse F1; F2 (T2A)
    - Do you have voltage at test point TP11 (+12.6V); TP12 (-12.6V); TP9 (+3.3V)
    If on the TP (9; 11; 12) some test voltage missing, check (replace) corresponding IC REG (1; 2; 3)

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    No sound from the amp whatsoever even when plugged into fx loop
    I have checked the fuses and as stated in the OP i have 12.6v+/- and 3.2v

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    What is the voltage on D15 pin 3 ( BIAS_ON sigal sheet 3 bottom right quadrant)? it should about -1.3V when in play mode with a input plugged in. Check that changes when you unplug the input. That is fed from IC16B sheet 4, that in turn from the processor (on pin 23) and that from the input jack inserted signal on pin 14.

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    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by blindboybenton View Post
    No sound from the amp whatsoever even when plugged into fx loop
    I have checked the fuses and as stated in the OP i have 12.6v+/- and 3.2v
    Voltages is OK.
    Set all volume and tone controls to "zero", all switches to "off" and put guitar cable in Guitar input jack (JK1)
    What is voltage on both sides of R1 (10k)?
    When input jack (JK1) is opened, R1 send command to IC10 (pin14) and from IC10 (pin23) via IC16B (pin7) gives command (Bias On) to bias adjustment circuit.

    Do you have voltage 50mV at TP10 (R217; D36) cathode EL34.
    Bias adjust with PR2 (10k); Bias balance adjust to output symmetrically clips with PR1 (10k)

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    Thanks nick
    Tested at junction of c163/164 i have 11.5 volts and same on pin 7 IC16 ( not changing when i remove the jack
    pin 23 on pic i have 0.5v .... pins 5-6 on ic16 i have 1.1/1.6v

    BBB

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    Last edited by blindboybenton; 11-05-2019 at 01:40 PM.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by blindboybenton View Post
    Thanks nick
    Tested at junction of c163/164 i have 11.5 volts and same on pin 7 IC16 ( not changing when i remove the jack
    pin 23 on pic i have 0.5v .... pins 5-6 on ic16 i have 1.1/1.6v

    BBB

    ..and the all important pin 14 of the processor does what with the jack in/out? Should go between 3.3v and 0v.

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    Ok further testing i believe the PIC chip has gone. With it removed i get 3.2v - 0v when the jack is inserted/removed with the pic chip in pin 14 remains at 0v
    I have already requested the part from blackstar UK who in the past have been very helpful
    I will let you know
    Many thanks for your help

    BBB

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  11. #11
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    thanks Nick. Ive just posted a reply to that about 20 mins ago. Looks like the pic chip has gone as it does go 3.2v to 0v only with the pic chip removed with it in it stays at 0v

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    Please to say that the pic chip provided by blackstar fixed the amp
    BBB

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  13. #13
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    Quote Originally Posted by blindboybenton View Post
    Please to say that the pic chip provided by blackstar fixed the amp
    BBB

    It's odd that they don't thave the same ESD protection on that signal as they do on the input. After all, both are exposed to the outside world. Maybe as simple as a 100k in series would do it.

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    would of not happened if the person who repaired the handle didnt fill the hole with filler then screwed the handle in making the grip nut fall on to the board, Funny thing is i was also given an ampeg BA210 with 4 the main boards screws loose inside and they touched the switch mode psu and blown that to hell

    oh well BBB

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  15. #15
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    Ampeg has not yet discovered lock washers.

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    or what i use is a little blob of paint

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