Lab Series L5 with 5k Hz oscillation

[Brhibler1]

A Customer brought me a Lab Series L5 with a high frequency oscillation around 5k Hz.
The noise is on the negative and positive rail voltages going to the Opamps, but I've isolated the power supply and power amp and they don't have the problem.

The noise is coming from the preamp and increases with both channel volumes and master volume. Weird side effect, the frequency slightly changes with the channel volumes. Ie, ..Normal channel frequency increases, Reverb Ch, frequency decreases when increasing volume. I've started pulling opamps and still haven't found the issue.

Any ideas?

[g1]

  Put a patch cord between preamp out and power amp in (dry), any difference?

Edit:  This post was supposed to go in here: http://www.ssguitar.com/index.php?topic=3472.msg25990#msg25990
Sorry for any confusion.

[Brhibler1]

  Put a patch cord between preamp out and power amp in (dry), any difference?

That had already been checked. I'm an experience amp tech, 

Well, I found the culprit.......
Schematic calls for LF356 in the inputs of the normal and reverb channels, there were LF351's in there. I don't know if that was the cause or if the chip had become faulty. But the LF351 in the normal ch. caused it. I removed it and the oscillation disappeared. Went ahead and ordered the correct opamps for both inputs.

Another side issue, after I corrected the oscillation, was severe distortion on the output. So I rebiased the idle current and offset Voltage to factory specs.
Plugged it back in and found a rubbing speaker, too!

This guy must have let this thing sit in storage for long time or ran it into the dirt!

It'll be sweet sounding after I get done with it! 8|

[smackoj]

Ola amigos; I have a Lab Series L7 which I am pretty sure is the same amp with 4 ten inch spkrs instead of 2 twelves. I can only get normal volume using one low gain input. The other three inputs are much lower volume than it must have come from the factory. A fellow amp dude from another forum suggested checking the input op amps. That sounds to me like there may be a weak link in those input chips because you say your L5 works good after correcting that problem. Any thoughts ??

thanks

[Brhibler1]

I would say the input Opamps or some bad solder joints.
Never know till it's put in a test setup.

[g1]

  Put a patch cord between preamp out and power amp in (dry), any difference?

That had already been checked. I'm an experience amp tech, 

lol, that post was supposed to have gone here: http://www.ssguitar.com/index.php?topic=3472.msg25990#msg25990
Sorry about that, you must have been scratching your head wondering why I would think the loop could cause that and what the "dry" was about. 
I would have thought the lf351 ok as a lf356 sub, but, as you say, maybe it was defective. 
  Or maybe someone else here more familiar with the circuit will comment on whether there is a problem using the lf351 in this spot of the circuit.

[g1]

Ola amigos; I have a Lab Series L7 which I am pretty sure is the same amp with 4 ten inch spkrs instead of 2 twelves. I can only get normal volume using one low gain input.

First, the inputs are labelled different than what we normally see, HI means for high output pickup, so there is less gain with the HI inputs.  The LO inputs will give more gain.
Each pair of inputs goes to same IC.  So for your channel that has 1 input working, the other input must have a problem with the jack or solder.  If a channel input IC is bad, neither jack for that channel will work.
  So you may have a few separate problems here.

[smackoj]

thanks g1, that helps a lot. I wondered about the 'hi' and 'low' tags? my Normal ch. has the low input that acts right. If the hi input on that ch is working correctly, then it was designed to have a MUCH higher signal because the hi input is pretty darn quiet and wimpy?

the 2nd (reverb) channel has low output from both Hi and Low input jacks. I have not pulled the chassis out for quite some time to have a look. I have just been using the Low input on the Normal ch.

thanks for the help senor.

smacko jack

[Roly]

These arrangements often use switching contacts on the sockets and I'd check these for proper switching, contact cleaner, blah.

If they are soldered on a PCB look carefully for fractured solder joints on the copper side.

HTH

[smackoj]

ok, thanks for the tips fellas. now I have to find 3 men and a dog to move this thing home so I can snoop around on it!!!

heavier than Lulu after an 'all you can eat' buffet.

8|