LED not working

[fritterandwaste]

Hi

As perhaps betrayed by my previous thread(s), I am a novice to electronics but hope to change this. I thought a simple project might be to replace an LED that is not working in a guitar multi-effects pedal (Boss BE5) which includes chorus, delay, distortion and compressor pedals and controls. The compressor function appears to be working OK but its LED is not lighting when the pedal is toggled to 'on'. I opened up the box and used the diode test function on my multimeter and the diode lit fine. This suggests to me that the actual LED is fine and need not be replaced. I have attached a word document showing the area of the circuit diagram that I believe is relevant.

Towards the far right you will see 'CS LED' which I think must be the component. I have tested the 'nearby' resistors R25 and R26 and these are showing values that match the diagram. Any suggestions as to what to do next would be greatly appreciated thank you.

[fzabkar]

What is the voltage at CSON (base of Q008)?

[fritterandwaste]

Hi, thanks very much for the reply.

I have to show my ignorance here as I know Q008 has 3 terminals and I'm not sure which 2 I should be applying the multimeter contacts to. If I test the two outer terminals I get 8.75v. Other combinations yield 9v or what appears to be a rapidly fluctuating mv figure (around 40mv?).

Are you able to tell me what CSON actually is? I see it on the diagram but does it represent a component of some sort?

Thanks again.

[tooki]

Please upload or link the full service manual so we can see the entire schematic. CSON is a signal, the symbol means “I connect to the same signal somewhere else”, often used to avoid crazy long traces on schematics.

[fritterandwaste]

Thank you and understood re CSON also thanks.

The manual is too large to upload but you should be able to see it here (do let me know if you have problems)

https://www.synthxl.com/wp-content/uploads/2018/04/Boss-be-5-service-notes.pdf

[BrokenYugo]

I have to show my ignorance here as I know Q008 has 3 terminals and I'm not sure which 2 I should be applying the multimeter contacts to. If I test the two outer terminals I get 8.75v. Other combinations yield 9v or what appears to be a rapidly fluctuating mv figure (around 40mv?).


Thanks again.

When it is asked what the voltage at a single point is, the implication is between that point and circuit ground. If you're probing CSON at Q008 you'd go between the base (the middle pin coming out the left in the diagram) and emitter (the lower pin on the diagram), since we can see that the emitter is connected directly to ground, any point in the circuit at ground potential will do. Note that the pinout of transistors can vary, check the datasheet or follow the circuit to confirm which pin is which.

Datasheet for RN1211: https://pdf1.alldatasheet.com/datasheet-pdf/view/94692/TOSHIBA/RN1211.html So in this particular case it would be the outer two pins.

[fritterandwaste]

Thank you for your explanation, persistence and patience - they are greatly appreciated. I have uploaded a photo of the underside of the board. You will see Q8 near the centre. I'll refer to the pins as left, centre & right working left to right. On the circuit diagram I will refer to the pins as 1, 2 & 3 working down vertically.

I believe the following "analysis" confirms what you've said...

If my eyes are working OK (not always the case!) then:

  Left pin (board) corresponds to pin 1 (diagram) and visibly connects to R27 on the board
  Centre pin (board) corresponds to pin 2 (diagram) and visibly connects to R25/R26 on the board
  Right pin (board) corresponds to pin 3 (diagram)  (ultimately leads to C28 where I believe ground symbol is shown on the board).

The 'base' therefore is 'left' and 'emitter' is 'right'. If I understand correctly you are therefore asking for voltage measurement between 1 & 3 (diagram) or left and right (board). This is 7.3v. For completeness the other measurements are:

1 & 2 (diagram) centre & left (board) fluctuates a lot, last reading 'settled' at about 600mv
2 & 3 (diagram) centre & right (board) 7.67v

I confess I confuesed myself a lot getting this far! I am using a different mains adapter from yesterday which might explain the reduction in the higher readings. I hope this helps!

[fritterandwaste]

Thanks to all contributors so far. Any further thoughts on this would be appreciated as I haven't a clue what to do next nor how to interpret these voltage figures!

[sausage]

Hi fritterandwaste, going off your original schematic diagram, could you please tell me what voltage is measured coming into D039 (the LED) and R025? Measure where it appears to say +9V.

Might have to double check the guitar pedal is active in case you forget, so that you get a reading.

[fzabkar]

As far as I can tell, the base of Q008 is being directly driven by an output pin of IC024 which is a TC9130P. According to the datasheet, the output circuit [of the TC9130P] is a complementary construction of bipolar NPN transistor and Nch FET".

https://datasheet.datasheetarchive.com/originals/scans/Scans-007/Scans-00153593.pdf

Therefore it sems to me that there should be a series resistor in the base of Q008. This is absent from the circuit diagram.

BTW, if the OP's voltage measurements are correct, then the base-emitter junction of Q008 is open.


Edit:

Q008 has a built-in base resistor. Sorry.

http://www.elektronikjk.pl/elementy_czynne/tranzystory/RN1210.pdf

The OP's measurements confirm that Q008 is being turned on by IC024 (7.3V into the base resistor). However, the collector of Q008 is not dropping to 0V (it is sitting at 7.6V), so this would suggest that Q008 is faulty.

[BrokenYugo]

Yeah, bad transistor if I'm understanding the somewhat cryptic measurements correctly, again, you should generally reference everything from ground, and call out transistor pins by name, e.g. "collector is X volts (above ground)", also helps to print out the schematic and write down the measurements at the relevant nodes. Then you can look at it and see what the transistor is or is not doing.

I think one could use just about any TO-92 NPN here (minding the pinout), 2N3904, 2N2222, S9014, BC547/8/9, etc. provided an external 10k base resistor is added in. I'm not sure you can even get through hole biased transistors anymore, a quick look on Mouser comes up with nothing.

[fritterandwaste]

Thank you once again, all. Really appreciated. You will have gathered the word "newbie" probably doesn't do my incompetence justice given that I'm not really sure even how to take a reading. Please excuse my ignorance.

Sausage - assuming I'm taking readings correctly (!) then there is no voltage coming into D039 or R025 (pedal is definitely active). That kind of makes sense to me with D039 at least given that the LED illuminates when tested with DMM but not when pedal active.

fzabkar, BrokenYugo - if this boils down to a defective transistor (Q008) does it make sense that all the functions of the pedal still APPEAR to work OK and only the LED is faulty or would you expect me to be able to find fault elsewhere in the compressor feature of the unit? There are 3 controls within the function: sustain, attack & tone.
 

[fzabkar]

Q008 controls the LED and the uppermost MOSFET, so there will be other faults. Basically Q008 determines whether Q004 gets its input from IC1a or IC3a. IC3a would be the "compressed" signal. IC1a would be the unmodified, flat signal, without tone, level, sustain or attack controls.

You can observe the difference by carefully shorting the collector and emitter pins of Q008 with tweezers. Don't make a mistake with this, otherwise you could damage the TC9130P chip.

[fritterandwaste]

OK thank you very much - I will try and get hold of a suitable replacement for Q008.