No output on SR 2 band preamp.......need some troubleshooting help.

[rcl]

Hey guys and gals, I need some troubleshooting help with my 1987 SR 4 2 band preamp.

Long story short....... years ago the pot on the bass control stopped stopping. It would spin all the way around. Rather than solve the issue I opted to "up grade" to the SD basslines 3 band in 3 holes. Never really liked it............fast forward 20 years. After having the bass passive for a number of years I decided to go original.

I fixed the pot. Re installed the original preamp........ no out put. I have 8.26V+ on pin 7 and oddly enough around 7.6 V+ on pin 6 ( output). Any one have any trouble shooting tips here. I feel comfortable replacing anything on this preamp. I am in rual Brasil but can get parts for this as they are common.

Thanks

 

[tbonesullivan]

I'd definitely get in touch with customer service, along with the serial number and such so they can tell what year it was, and then look up the preamp design for that year.

 

[Rod Trussbroken]

As TBS said, get in touch with Customer Service:

Here

You can also get an other preamp board from them at a very reasonable cost (as long as you give them your problem preamp).

 

[rcl]

Thanks guys, The electronics are March 3, 1987. I am trying to avoid shipping. If the package makes it out and if the package makes it back in.....I will have bought all the parts 10 times due to tarriff costs. Even if there is no money exchanged.

I figure Customer service peeps in here and maybe this conversation helps others.

 

[Basstech]

I recently repaired a classic 77/78 stingray preamp. It had lots of little problems and was potted in epoxy, but I got it going again.

You should be getting the bias voltage (1/2 the battery voltage) at the output pin of the opamp (6) and also at the two opamp input pins (2 and 3). You should also have no DC voltage at the input and output wires of the preamp, because of the coupling caps.

If you are reading 7.6 on pin 6, then I'd say the biasing resistors and/or caps might be faulty, but also the opamp could be damaged.

The original preamps don't have a protection diode, so are vulnerable to reverse polarity damage (which will harm the polarised caps and the opamp). They also don't have the 1k output resistor that all the modern preamps have. This is because that particular opamp can be damaged if its output is shorted.

Here's a video I made about fixing the 2-band preamp. I hope it is some help. FWIW I also tested the pickup.

 

[rcl]

Thanks Basstech. I actually saw that video before I saw your post. Excellent Video! I am going to do some more troubleshooting based on your video and report back here. You answered My biggest question already. And that was about the voltage divider. The voltage I see on the output is pre cap. there is actually none post cap. So it seams my voltage divider isnt functioning or chip is bad. Ill let you know.

 

[Basstech]

Thanks mate. I'm glad you liked my video.

You sound like you have some experience, so you've probably already done this, but also check over all the solder joints with a magnifying glass. It may be just a dry joint. It is a 30 year old PBC after all...

Oh, and if they had shifted to electrolytic caps by '87, I'd think about replacing them.

Good luck with the trouble shooting! And keep us up to date...