Lab Series compressor

[lowell]

got a L9 on the bench... compressor is down. when testing op-amp (4558) pins for -+15v the voltage is there but quickly over about 30 seconds goes down to 0v when I connect the meter. i checked diodes CR101 and CR102 for the -+15v and same result. i don't know why this is happening or what it means... the amp sounds great aside from the compressor not working.

here's link to schem:

http://www.netads.com/~meo/Guitar/Am...L11_scheme.pdf

[Don Moose]

Is the op-amp socketed? Do the rails collapse if you measure the socket pins as opposed to the IC pins?

If this is the only op-amp who's rails collapse under measurement, I have to think there's either a solder issue or a broken trace or two.

[Enzo]

Before you go any further, do you realize how the compressor control works on the Lab Series?

The control is NOT the amount of compression, there is only one level of compression. The control is a threshold control. The higher you turn the control, the louder the signal must be to trigger the compression. CLick the compressor on but leave the control all the way down to the lowest setting. That is the most sensitive setting and the most compresion will occur there.

Every single time I have had a Lab Series come in with "compressor doesn't work" as the complaint it has been this misunderstanding. The compressor itself works as designed.

Not saying yours can't be broken, but before you troubleshoot the whole circuit through itself and out the other end, check that first.

[lowell]

well i just tried the compressor on 0 and was getting no sound... turned out the speaker wire had broken of the speaker lug... when i reconnected and turned on the amp there is a horrible noise. i tested the output with a 100ohm resistive load and there is .011vdc on the output... is this bad? Did I blow something cause the speaker was disconnected? Also, this amp says min load 8ohm. do I need a load on this amp to troubleshoot? Can the load be larger than 8ohms?

[Don Moose]

It says MIN load 8 ohms - Solid state amps are perfectly happy with open outputs and higher-impedance-than-usual loads (too low a load is their BadThing(tm)). It's the tube amps that need something speaker-like as a load.

11mVdc on the output isn't great, but probably not a disaster unless it has been much higher.

You can try the speaker with a known-good amp to rule that out (you might want to use a low-powered SS amp at first, set to a low volume before turning on).

[lowell]

Don,
I swapped speaker and same horrible noise. I again measured the output for DC and w/ my meter on 200mv setting it reads OL but on the 2v setting it reads .011...?? This does not makes sense to me. what do you make of it?

[Don Moose]

Well, the speaker is toast, and was most likely fried by a toasted power amp.

The meter reading -- I could see an AC waveform/noise riding the top of the DC in a way that takes the peaks over 200mV and pissing the meter off at the lower range.

At this point, I would probably try unhooking the power amp board altogether and seeing what the power supplies are up to.

If that's all happy, check out the Preamp output and all the controls. That should give you a pretty solid inventory of what's broken.

Solid state power amps are not my thing (heck, I'm barely competent with tube power stages). The overall debug process is similar, but takes a more (w)holistic approach because a toasted driver or feedback component can keep killing replacement power transistors.

I think Enzo is your man for the rest of this.

[lowell]

Don,
I don't think the speaker is toast. I took the power amp out and disconnected it but now there's no supply voltage. I see that the thermal switch is in the primary ac line so that's probably why. can I short those 2 clips together w/ the power amp disconnected w/o any problems for troubleshooting purposes?

[lowell]

ok i realized the thermal switch gets hot from the output's heatsink... w/o that connected it's a closed switch. I shorted the thermal switch wires and measured the power supply for output and it looks great... right at the specified +-63vdc per the schematic. I will now check the output transistors and report back. If there's something in particular you suggest let me know.

[lowell]

ok I tested just about everything on the output board... servo, output transistors, ballast resistors... I can't find anything wrong on the board. I also cannot locate Q305 or Q306. They must be in a different model or something. I then thought to test the rectifier diodes for the output board. In forward bias diode test they all test as shorted. Could this be? Could the caps across each one possibly be bad? Should I replace them or is reading them in circuit inaccurate?

[Enzo]

REmove them from the board and retest them, it is doubtful all the caps are shorted. Far more likelt they themselves are shorted.

[lowell]

I disconnected the power supply board from the "screw in" caps on the chassis and the diodes all tested perfect... then I realized that ONLY when the center tap of the secondary on power transformer is connected to board AND when the filter caps w/ screw in terminals are connected - do the rectifiers read shorted. I then narrowed it down to the negative terminal on one filter cap... all other cap terminals can be connected and rectifiers are fine, only if I touch the negative of one filter cap to the main board do the rectifiers read shorted. The cap and negative terminal causing this is the one directly wired to the - main supply rail. I tested both filter caps for DC and test showed them as good. All rectifiers are fine too. I don't know what to test next.

[lowell]

Ok I the diodes in rectifier are fine. However only when the power tran is hooked up do the diodes test as shorted. I disconnected the secondary on the power tran and it measures 1ohm from each end to CT. Is this a typical secondary PT winding for a ss amp?

Also, the output section's transistors are testing short from emitter to base. I guess I'll pull 'em and confirm.

[lowell]

Ok so both power rails are good w/ amp all together... output board and all. The -+33v zener test points are good. The preamp out sounds fine. There is now a negligent 5mvDC on output. However, there is a 8.6vAC signal on the output w/ no signal applied. Aside from a bad filter cap could this be a bad transistor causing oscillation? What could I check for AC on output w/ no signal applied to input? Finally the AC signal is only on the output side of the 22k feedback resistor.

[Don Moose]

Finally the AC signal is only on the output side of the 22k feedback resistor.

To my simple mind, this sounds important. If I'm looking at the right spot in the schematic (R314), at the other side of R314 there's a 750 ohm resistor to GND (R315), you should get something like 8.6*(750/(22,000 + 750)) = 0.283Vac at that node.

Sort of like there's some noise being injected and amplified that should have been corrected out by the feedback.

I give this suggestion about a 20% chance of being on-target, though.