Op amp choice

[Steve Blackdog]

Hi folks

i am looking to replace some op amps in the KMD GV60 circuit. There are 7 Motorola MC34002p JFET input op amps. Data sheet attached.

what do people think about replacing them with TL072s? Is there a better choice? The only MC34002s I can find are exorbitantly priced.

thanks in advance.

steve 33-MOTOROLA-MC34004P.pdf

[Jon Snell]

TL072s are absolutely fine and dirt cheap.

[mozz]

Are they bad? I see them listed for $1.60 If you are just trying to upgrade, i doubt very much there will be any sonic differences.

[J M Fahey]

If you NEED to replace some because they are confirmed broken, TL072 should be fine,since pinout seems to match.

If you just want to "improve" amp or are throwing parts at it in hope to magically repair it without troubleshooting, please don´t.

If available, post KMD GV60 schematic to see what they actually do.
Is there some specific problem you are trying to solve?

[Steve Blackdog]

If you NEED to replace some because they are confirmed broken, TL072 should be fine,since pinout seems to match.

If you just want to "improve" amp or are throwing parts at it in hope to magically repair it without troubleshooting, please don´t.

If available, post KMD GV60 schematic to see what they actually do.
Is there some specific problem you are trying to solve?

You are quite right. I am not looking to improve, more as you say to throw parts at it. I am struggling on the troubleshooting.

Will upload schematic when system allows!

s

[Steve Blackdog]

Here’s a copy

https://music-electronics-forum.com/...d-hi-lo-switch

[J M Fahey]

Ok, I found a copy displayable inside the post, any actual problem you are trying to solve?

[Steve Blackdog]

Yup, here are the key points:

channel 2 not working. Footswitch not switching channel. Don’t think prob is f/s.

channel 1 sound distorted and erratic at low volume.

I have swapped the electrolytics for new.

I will check for cold solder joints etc, but visually looks good.

cheers

steve

[g1]

How are the low voltage supplies, +/-16V, +/-8V ?

[Mick Bailey]

If your voltage supplies check out, then take a look at your opamp outputs. A failed or failing audio opamp (as distinct from one configured as a comparator) will often have significant DC on the output and it's a very quick check to measure the voltage on each one.

[loudthud]

I went looking for a TL072 data sheet. As I remember, the rail to rail Voltage rating of the old TL072C was only 30V. Now there is a new and improved version, TL072H. It's rail to rail Voltage rating is 42V ! The new data sheet is 79 pages. Not sure I will ever get through it. Can't even find the TL072C.

Link: https://www.ti.com/lit/ds/symlink/tl...duct%252FTL072

[Steve Blackdog]

How are the low voltage supplies, +/-16V, +/-8V ?

they appear to be correct.

[Steve Blackdog]

If your voltage supplies check out, then take a look at your opamp outputs. A failed or failing audio opamp (as distinct from one configured as a comparator) will often have significant DC on the output and it's a very quick check to measure the voltage on each one.

On close inspection I have found that 3 of the op amps are MC1458cp, presumably replacements mc1458.pdf

Of those one (ic6) has significantly higher output voltages, 14v on pin one and 16.5v on pine seven. All the others are less than one volt. But if I swap ic6 and ic7, the voltages on ic 6 are still around 14v and 16v. That seems to be telling me that this is not necessarily a fault?

[Enzo]

Depends on what the IC is used for. In the audio path, that would likely be a fault. But the IC could be driving an LED or relay or something, or even just channel switching, in which case it looks normal. In your case it looks like IC6 is directly connected to IC5, which could be outputting DC and so the IC6 has no choice but to follow along.

Remove the IC from position IC6 and see if DC voltages are still present on signal pins.

[Steve Blackdog]

Thanks, Enzo, will do.

I think this may be the culprit (or something around here) for my problem with switching on channel 2. I am having a look under the board.