Analogue electronics - help ????

[sro]

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 fault. 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 a schematic.

If I dont get any replies from them, any ideas on where else to try to get schematics from ?

Steve

PS. Its a 4 string thru-neck Warwick Thumb with 18 volt 3 band pre-amp, if that helps....... PPS. Yes I did check batteries, leads etc etc

[nick nelson]

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 there.

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.

Nick

[Christian McArdle]

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.

Christian.

[The Natural Philosopher]

Can I get thus clear? Its IN the guitar itself?

Does it contian two op-amps or one?

When you say considerably reduced do you mena it goes distorted at full whack, or is simply less loud?

Simply take it out and trace it. Can't be hard.

If you are anywhere east anglia, I'll take a look.

[The Natural Philosopher]

Its almost zero probability that the chips have gone. Most likley is broken wire or dried up electrolytic.

Its almost zero probability that a pre-amp will overheat. There is no power in it, and the chips won't draw enough to self destruct unless connected backwards.

[Dave Plowman]

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.

[Niall]

Try google, I found various sites with downloadable schematics recently, or try

as a start point, an excellent resource for any electronic info.

[James Salisbury]

Try

type in the part numbers for the data sheet. Type slod006b in their search box for a very good text on opamps.

[Martin Warby]

you might want to try posting in sci.electronics.repair

Martin Warby

[The Natural Philosopher]

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.

>

[geoff]

In message , nick nelson writes

Take a step back and think about how this is transmitted down a single coax cable

My advice is to check the electrolytic capacitors if you have some means of doing so, if not, just replace them as a first finger in the wind attempt.

[Martin Angove]

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 gain/feedback.

Hwyl!

M.

[N. Thornton]

sci.electronics.design is your group for this.

Regards, NT

[Dave Plowman]

It was a common failure point on the op amp output of some early CD players.

It's all guessing, really, without a circuit.

[Niall]

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.

[Tony Williams]

Search Google for posts made by a chap called Walter Harley (sometime inhabitant of sci.electronics.design). He seems to do something around the guitar amp business and may have the information you need.

[sro]

Yes it is in the guitar.

1 TL064 and 1 TL062

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 awkward. 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.

Steve

[The Natural Philosopher]

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 that capacitor.

I used to desitn and build for a living, guitar amps using SS technology. BTW.

[sro]

Once again - thanks to all, some very useful hints on how to proceed. I will get my soldering iron and maplin magnifying glass out over Christmas.

Steve

[sro]

That sounds like a bl**dy good idea, thanks, I'll try that. A drying out electrolytic fits the symptoms because the output has dropped gradually over time.

I am just an interested amateur ( never did electronics at college ) who owns an old scope etc.

Steve