Lab Series L7 (L5) low volume / intermittent distorted operation

[MatthewMuffin]

Hey Everyone-

First post.  I've been checking out different posts by Lab Series users and have been working on mine for a couple month on and off and have not been able to solve the issues.  I'm still a novice at amp repair so I'm wondering if some of you old pros could at least point me in the right direction.

I have an ooooold l7 Lab series 4x10 combo which produces very ugly distorted sounds when a note is plucked hard.  Light plucking doesn't produce a sound at all. The first two channels work at maybe 10% volume and the second two work at reduced volume as well, but are noticeably louder.  I've checked the speakers and power amp in and at this point I'm thinking it is the power supply-  So far I've replaced a dead diode, but I've found several more with black soot on the leads and am wondering if this is a sign of failure?

Also, the sound is too screwed up to give a qualitative test the compressor but when turned on, the led will be lit for about one second then quickly fade.

Anyways, thanks for your help in advance and I apologize for my lack of knowledge  :duh

[MatthewMuffin]

Almost forgot too, if I havn't used it in a day or two, there is a loud hum when powered on that lasts for about 30 - 60 seconds and then is gone. 

[DrGonz78]

Test the speaker leads for DC on output. Test it as you turn on the amp to see if there is DC voltage on the output. Lets see if there is a surge of DC as you start the amp and then see if it goes away.

[MatthewMuffin]

Thanks DrGonz-

It spikes up to 160v-180mv I think (hard to tell using a digital radioshack multimeter, but there is a definite fast swoop up and slow decrease) and then normalizes at around 26mv after 20 or 30 seconds.  Could this be related to the caps in the output section? 

[DrGonz78]

To have an amp putting out 160-180mv on the output at start up is not a huge concern really. However, the surge and steady downfall in DC on the output, with hum for about the same time, does make me question the caps too. Solder joints maybe suspects too. Do we have a schematic for this amp? Would want to test some voltage readings on the amp to see if they are healthy.   

[MatthewMuffin]

Here's a link to the service docs:

http://www.rru.com/~meo/Guitar/Amps/Lab/schem.html

I've reheated a good number of solder points on the preamp board which definitely improved the sound, but only got it from absolutely terrible to terrible.  Also, I know how to check the voltage of a live amp safely, but I don't know what to compare the readings to...  I can read a schematic, is that what the voltage markings are next to the select components?

Thanks again this is a great help to me! :tu:

[Roly]

Yes the voltages in [square brackets] are your test voltages.  It is normal that the actual value may measure 5 or 10% off the given value due to spread in component values and your meter accuracy.

Try feeding a signal in to Power Amp Input on the rear panel from your MP3 player (or similar, start with its volume at minimum).

Same problem, or clean?

[MatthewMuffin]

I've tested power amp in with a guitar, comes out clean and lovely.

Here are some voltage measurements for the power amp

Output jack positive (yellow) = 6 or 7mv vs [10mv max]
R312 measures at 31v- vs [33v]
Q301 measures at 113mv vs [<+.340v]

Power Supply:
Q205 measures 14.7 vs [14.4 nom]
R210 measures 11.5 mv vs [<80mv]
Q206 measures -16.54v vs [-14.4 nom -13min]
Q204 (right leg) measures -.693v vs [ -25v nom]
R219 measures -25.9 vs [-25v nom]

This is assuming I've labeled and measured everything correctly xP, still unsure if - means DC or a negative voltage, but either way I may have forgotten a sign or two. From the results, could q204 be fried?

I can move onto the pre-amp board if this all seems groovy gravy.

Thanks again for all your help guys.

[Roly]

I've tested power amp in with a guitar, comes out clean and lovely.

I don't have time right now to check your voltages, but that result alone looks like the power amp is okay.

Next test is to take the signal from the Preamp Out to another amp.  If this is also clean then it starts to look like the good ol' bridging contacts in the Pre Out Main in socket pair need a good clean.

Later.

[DrGonz78]

Good news on the power amp side testing good!  :tu:

Assuming you measured the correct location on Q204... That measurement really caught my eye looking it over... Everything looks good as Roly said, but still if we are looking for -25V and only measure -.693V. Well then yes Q204 could be a problem. Personally I would measure that again and make double sure. Possibly you can measure Q202 for the +25V and Q204 to show the -24V. Or measure R208 and R201 respectively, or what are the voltage readings at pins 4 and 7 on A201??

[MatthewMuffin]

Assuming you measured the correct location on Q204... That measurement really caught my eye looking it over... Everything looks good as Roly said, but still if we are looking for -25V and only measure -.693V. Well then yes Q204 could be a problem. Personally I would measure that again and make double sure. Possibly you can measure Q202 for the +25V and Q204 to show the -24V. Or measure R208 and R201 respectively, or what are the voltage readings at pins 4 and 7 on A201??

R208 and R201 measure healthy at -25v and 26v, A201 measures -15.8 at 4 and 14.5 at 7.  I think I may have recorded the measurements incorrectly before  :duh because the right lug of Q204 may be low, but at R208 and R201 the correct value is present.


All trannies center lug measures pretty well, (24 or 25 -/+) and Q206 comes in the furthest off at 23.01v.   This means the power supply is ok, neh?  :dbtu:

Next test is to take the signal from the Preamp Out to another amp.  If this is also clean then it starts to look like the good ol' bridging contacts in the Pre Out Main in socket pair need a good clean.

The pre-amp out test proved that problems are in the pre-amp.  When patched through another amp the awful sound spikes and intermittent operation persists.   I will test the pre-amp today or tomorrow for DC voltage health and post my findings for y'all.

[MatthewMuffin]

Hey Everyone-  Sorry it's taken me so long to put these values up, it's been a craazy week here in the stinky midwest...

The Pre-amp measured on the far side of sickly, here's the voltage ratings and the [suggested] ratings.  I measured each op amp to see if the -15 / +15 rails were OK and on each one below they are.

coming of Q101's S leading to C138 - Measured at 11v vs [.5-2v]  xP

Op Amp A113, lug 6 measured 13mv vs [<170 mv]  xP
Op Amp A103, lug 6 measured 1mv vs [< or =  15mv] xP
Op Amp A104, lug 7 measured 22mv vs [<350mv]  xP
           A104, lug 1 measured 13mv vs [<80mv]  xP
Op Amp A101, lug 6 fluxuates wildly, peaking around 250mv vs [<+-15mv MAX]  xP
Op Amp A102, lug 6 fluxuates between 10-100mv vs [<45mv]  8|?

Anybody have any Ideas what this could mean or how best to proceed?  Thanks!

-matt

[DrGonz78]

Well when we are looking at A101 lets measure the right legs to really see +/-15v. I mean lug 6 might have something posted on the schematic but tell us the whole story. What voltages are present at lug 7 & 4 of A101??? 

Also, Q103 find which leg is -15v so we can say something about it better... And Q104 tell us leg 4 & 8 to tell a better story on the voltages there... Really right now we are only mv on signal and not the rails on the chips that control the flow of the preamp circuit. We need all the -/+15v rails to know where a chip might be failing. Then we can pull out chips to see what is the voltage supplied. Please measure  the chips -/+15 v rails.