I know its a bit off topic, but I know there are some electronics guys who
read this group.
I want to repair the small pre-amp in my bass guitar which has developed a
The circuit is based on a TL064 and TL062 op-amp and the fault is
considerably reduced output voltage
which leads me to suspect a resistor or capacitor going out of spec.
It is made by MEC and I've emailed them ( and the guitar maker ) asking for
If I dont get any replies from them, any ideas on where else to try to get
schematics from ?
PS. Its a 4 string thru-neck Warwick Thumb with 18 volt 3 band pre-amp, if
PPS. Yes I did check batteries, leads etc etc
TL064 and TL062 are quad and dual op amps respectively. An
internet search will throw up the pin-outs very easily and
it ought to be reasonably easy to work out the circuit from
To make a really wild guess:
Bass guitars have four strings/pickups don't they?
If I was using these devices in this application I
might well use the four op amps in the TL064 as
buffers one to each pickup, and use the TL062
as a mixer cum gain block.
If this turned out to be so and if the low output
is not confined to particular strings, then the fault
might well be in or around the TL062.
First replace the op-amps, especially if they are socketed. They are cheap
as chips. (groan). You should certainly find each chip for well under a
pound even in single quantities.
Otherwise, something this simple should be easily converted to a schematic
by examining the PCB.
Also, look out for scorch marks.
TL072 / 074 are 49p in ones from Maplin. That's if you can find a store
where the PFYs have even heard of Op Amps :-) They don't list the 062 /
064 in the catalogue.
Having said that, as TNP suggests I'd doubt that "reduced output
voltage" is down to a dead or dying op amp - you'd more likely get
severe distortion... I'd suspect something obvious like a fault in the
EQ pots first, then look at other passives forming part of the
Martin Angove: http://www.tridwr.demon.co.uk /
Two free issues: http://www.livtech.co.uk/ Living With Technology
ISTR you can use the 7 series ones instead of the 6 series, the main
difference is in the tiny cost saving for mass production.
Why are Maplin turning into Tandy? That buisness model didn't do Tandy
any good. Every catalogue has more toys and less electronics, and
their toy prices aren't even particularly competitive.
Thank goodness for an RS account.
No distortion, just very reduced output level - I think it should be at
least a couple of volts, it is barely 1/2 volt.
I will probably have to do this - actually physically removing it is
Also, some of the components ( I think they are tantalum caps ) are very
difficult to read the values on - this
is where a schematic would help.
Thanks for the suggestions guys - I think I agree that it is most likely one
of the passive components that makes
up the feedback network for the op-amp rather than the op-amps themselves.
With electrolytics, typically with -20%+100% tolerance, the exact value
is seldom critical.
The TL range of op amps are FET input, and decently low noise and highly
suitable for mag poickups wound with lots of wire...i.e. typically
guitar pickups (goes back to valve days really)
My *guess* is that there is a bypass capacitor that has hgone OC, and
that is reducing the gain.
Try putting - by hand - a tant. or other capacitor across every
electrolytic on the board. If it suddenly restores sanity, just replace
I used to desitn and build for a living, guitar amps using SS
There's usually a small value C - say about 12pF or so - that goes from
the output of the IC to the input - pin 6 to either pin 2 or 3 depending
on whether it's an inverting or non inverting circuit. That would be my
first guess, as if it goes leaky it will reduce the gain.
*If work is so terrific, how come they have to pay you to do it?
Dave Plowman firstname.lastname@example.org London SW 12
Unlikley to go leaky, Dave. Those are typically ceramics or mica types.
If it goes open circuit the thing will oscillate probably - which
doesn't affect gain but makes em noisy.
Electrlytics are the first port of call if a circuit like that dies of
old age. That ans the pots.
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