SOUND ON SOUND

[Lionel Red]

Hello Everyone,

Long time lurker first time poster.
I have a TR606 with a non functioning bass drum and was hoping you might be able to help.
I swapped out IC 14 (2904) thinking it would solve the problem but no joy. Have on order 4558 IC 15 as thought that would be the next logical place to go...
I now have a clear schematic after acquiring it from a previous discussion from 2010.
Thank you! the copy I had was so blurry I thought my eyes were shot - Sooo much better to read those tiny values.
It has been pointed out to me kindly by Folderol that IC5 is responsible for the bass drum, my bad. I had tested it and thought it was working but am new to this so likely a mistake my end
Since then I have been going through the signal path checking resistors and capacitors.
Any help with this would be much appreciated

One thing I did want to check was should I be measuring in AC or DC on the multi meter?

hope everyone is safe out there

Best

[Folderol]

A quick check (DC) to start, would be the voltages on pins 1 and 7 of IC 5. At rest, they should both be around +8V relative to ground. If not, C18 is a likely suspect, or the one directly below in the drawing - looks like a 2 and a smudge so probably either 22u or 2u2.

Both sections of IC 5 are bridged T filters, so not a lot to go wrong with these. They don't actually oscillate, just shape the incoming pulse.

The next thing to check would be that there actually is a pulse coming in. Q14 and Q15 turn the incoming 5V trigger into a solid 15 V one. At rest Q14 collector should be at 15V and and Q15 collector at 0V. With your meter on DC you might get a quick flick of the value when the drum is triggered - it will depend on how fast your meter responds. Also it is quite safe to briefly short Q14 collector to ground to artificially create a pulse. This just mimics the switch action of Q14 if it sees an incoming one.

[Lionel Red]

Many thanks, I will check this when I get home.
Just to add I was checking R 58 and the meter was jumping all over the place - not sure if this was down to me in not being able to fit the probes in properly as it pretty tight in places.

[Lionel Red]

Just Checked and got 0 out IC 5 switched in rested state. Should I change this chip or would that sugest an O/C before this?

[Folderol]

Well R58 probably would ((where it meets the two capacitors) if the circuit was being triggered, but otherwise it should be at zero.

Having zero on the chip output pins rather suggests that C18 is S/C. That should be higher maybe 12V. Also check the resistors around it, and that the 'top' one is actually getting a supply.

[Lionel Red]

Just had a quick test and Getting 13v on C18 and 0V on R56 when running

[Folderol]

Very strange.
Going on the chip pins themselves, what are the voltages on pins 1, 2 & 3?

For some reason most meter manufacturers seem to thing you want to use barge poles for probes, wheres for delicate work like this you want something with a fine, sharp point. If nothing else, try jamming a fine sewing needed down the end of your probe. You'll not only be able to get accurately on the pins (and poke through any corrosion or varnish, you'll be able to see them too :)

[Lionel Red]

Q14 0.6 v Q15 0v
Is that normal as is opposimg the other right?

[Folderol]

I need to know the voltages on the actual pins of IC 5, like this:
Pin 1 {some value}
Pin 2 {another value}
Pin 3 {a different value}

The figures you gave me on those transistors aren't much help without knowing which legs you were on.

[Lionel Red]

Ok will check again.
Sorry for the silly question, which pin should the negative probe be on when doing this?

thanks

[Folderol]

Ground. Any part of the chassis should do, or the 'cold' leg of the output socket.

[Lionel Red]

Great! here are the readings for IC 5

pin 1 8.93v
pin 2 8.96v
pin 3 8.95v

[Folderol]

That's much better. I'm pretty confident that the chip itself is OK.
So now, always using the same point for the negative meter probe, what is the voltage on Q15 collector - i.e. where it meets R63, and does it blip if you trigger the bass drum?

[Lionel Red]

Perfect! Right okay, when playing it doesn't appear to be getting above 0.2 v on outer pins although is jumping all around on the MM.
Middle is Ov which is the base right?

[Folderol]

That suggests it isn't seeing the trigger at all. Is it only the bass drum that's not working. If so it might be damaged trackwork.

If you look at the drawing, you'll see the emitter (the leg with the arrow) of Q15 is also connected to the emitter of Q18 and Q29.

With the unit powered OFF and your meter on ohms (or continuity/diode check if it has it) put the probes between these points. You should read a short circuit.