laney probasss 150 short buzz after recap

[fireman]

hi, I decided to recap my 1980's laney pro bass head in an effort to reduce his in the limiter circuit and prevent future problems.

I changed the electrolytics one at a time being careful to make sure I got the correct polarity, position and size, after each cap was changed I turned on the amp to make sure it still worked and to see if the his had reduced, this was done on head phones, I changed what I could but realised the 470uf caps I had where only 25v and the old ones are 50v so I left the old ones in the amp till I get some more.

when the amp was run through the speakers again I noticed that when powered up there is a short (about 1 second ) buzz about 50hz triangular wave, then the amp settles down and works normally .its alost as if a cap is charging then stops the buzz when its full.

the level of the buzz goes up and down with the master volume but not the input gain or graphic eq, the gain and master are not noisey when turned.

my question is this;

could this be a bad new cap?

the old caps reacting to the new ones ?

ive buggered something up whilst changing the old caps?

I haven't changed the large 4700uf filter caps or the 470uf on the lower power supplies yet, and don't really want to change anything else until I get rid of this.

I cant find a schematic for this model, the pb400 is close but has lots of extras like footswitchable eq and stuff and also the limiter chip is used for a noise gate as well as a limiter, tha dp 150 is similar too but has some slight variations.

any help gladly received
appologies for the long winded post.

B

[Jazz P Bass]

It would be very helpful if some kind of schematic was posted.
At least of the power supply.

EDIT: well, here is the DP150.

DP100-DP150.pdf

[nickb]

Is this more like it??

http://bmamps.com/Schematics/Laney/L...assPreamp2.pdf
http://bmamps.com/Schematics/Laney/L..._power_amp.jpg

What happens if you plug your bass into the return input - do you still get the 50Hz on power on?

[fireman]

Thanks for the replies guys,

I didn't post the schematics as they are both more complex than this amp, I have been trying to find the correct drawings for the pb150 but have had no luck, I was hoping that somebody on here may have access to butter resources than me.

The buzz is the same with no input, or with a bass connected to any of the three returns; pre eq post eq on front panel and return on rear panel with the master vol up, no buzz when the vol is down.

What I have just noticed is that the limiter does have an effect, when it's out the buzz is there, when it's in the buzz is gone but a nasty loud scratchy pop replaces it.

The limiter is after the master vol, as are the rear send / return jacks.

If the amp is turned off,then straight back on before the power led has gone out then there is no buzz, if the power led is allowed to go out before turning back on, the buzz is back but only for one second still.

This amp doesn't have a relay on the speaker outs and makes a nasty pop when turned of as the led goes out regardless of volume setting, could these two issues be related?

[nickb]

Honestly, without a schematic I don't think we can be of any help. Have you tried Laney?

If it didn't do this before then it's likely, but not certain, that one of the changes you made is the cause. Carefully check you have used the correct values and that they are in the right way round.

If I'm brutally honest, just replacing a bunch of caps in the hope of fixing a problem wasn't really the best strategy. Without a schematic the chances of it ending badly were rather high. I think to fix it logically now, will require a scope and you sitting down and reverse engineering some of it. I'd figure out where the input to the power amp is and put my scope there and see what is happening. Same at the wiper of the master vol and again on the power amp output. That as least should be start in narrowing down the source and nature of the beast. I'd also check the power supplies, first of all for normal DC levels and also check how they ramp up.

Hiss, if it wasn't always there, is often caused by a bad chip rather than a capacitor.

[Jazz P Bass]

"buzz with the master vol up, no buzz when the vol is down "

That may be a hint.

If so, them the problem may very well be occurring before the MV.

[nickb]

"buzz with the master vol up, no buzz when the vol is down "

That may be a hint.

If so, them the problem may very well be occurring before the MV.

I saw that Jazz, but it is contradicted by the the use of the return jack. Also he does not hear it using the headphones which is also after MV.

[fireman]

Thanks guys, sorry but I've been really busy today, lots of answers, here goes,

I didn't try laney directly but I have tried their web site in the past, managed to get a user manual but no schematics.

I realise that trouble shooting without schematics is very difficult especially over the internet , but wondered if any one had come across this before, perhaps in another amp, and could shed some light on likely causes.

the original intention was to change the ne571 limiter chip to try and reduce the general hiss/noise level when the limiter was switched into circuit, a new chip did nothing to improve this so the old one went back in. The replacement of caps was a bit of a "while I've got it in bits kind of thing rather than a shotgun attempt".

luckily I had taken a photo of the guts some time ago which I managed to find, it showed a 47uf cap on the board which I had changed to 2.2uf by mistake after looking at the pb400 pre amp schem ( I had saved all the old caps and only had one 47uf in the bag and the photo showed its position on the board)
so much for expecting the pre amp to be the same for the series of amp, although to be fair the 400 has the other half of the 751 used as a noise gate.

as mentioned earlier in the thread, the limiter being switched into circuit changed the startup buzz to a nasty pop, this capacitor was connected to the output of the limiter/ input to poweramp.

the headphone socket is just wired from the speaker jacks with some resistors, not as in either the db 150 schem or the pb400, which shows it having its own little op amp ( I suppose the 400w output would be too high to just use a couple of resistors for the headphone feed)

once again thanks for your time and suggestions of how to try and narrow this down to a specific section/ part.
cheers

[J M Fahey]

the original intention was to change the ne571 limiter chip to try and reduce the general hiss/noise level when the limiter was switched into circuit, a new chip did nothing to improve this

Of course.

The NE571 by itself does not hiss, itīs just a very-late-in-the-path gain control and depending on designer intentions, might show more gain at idle than if not in circuit.
This increased gain will increase perceived hiss level .... which comes from much earlier in the preamp, you canīt blame NE571 for being honest.

As of "buzzing for 1 second at turn-on" and/or "turn on thump" , most amplifiers , with rare exceptions, "wake up stupid" (hey, just like me ) and for a second or two internal voltages are all over the place, until capacitors charge and everything is nice and balanced.

Many also get crazy a few seconds after turn-off, capacitors take some time to discharge and since they "slowly" travel from proper voltage to zero, at some point voltage is too low to work properly but enough to fart or squeal a little.

No big deal, at most slightly annoying, and most people get used to that, end of a (very slight) problem.

Amplifiers with speaker relays have the exact same issues, BUT hide them to the outside world by only connecting speakers when actually needed.

FWIW I sometimes have some customers which do NOT tolerate the slightest annoyance: Yoga/meditation centers, Guru Maharaj-Ji centers (no kidding), Photo-Club slide projection rooms, Art type cinemas, etc. ; to which I add a "Standby" switch: a plain switch in series with speaker "hot" lead and a small label with instructions: "turn power on, count to 5, turn Standby on ... reverse sequence when turning off"
Works like a charm , much better than relays , and everybody is happy.

As of hiss, I bet it annoys you in a home, silent room, even worse at late hours ... very much doubt you can hear it, even on purpose, in a rehearsal room and even less on stage.

Lowering original design hiss might involve re-engineering the preamp, which I guess is beyond duty.

That amplifier is very complex and looks like a serious attempt to match top line amps such as Trace Elliott, SWR and similar and they chock filled it with goodies ... you often pay that with increased hiss, each new stage adds its own.

You might replace IC1 (TL072) with NE5532 or some modern hi tech lower hiss one, just donīt get trigger happy and replace *all* in the amp, not worth it and the potential for damage is huge, tiny IC pads are easy to overheat and peel off and very difficult to repair.

And check that all caps you replaced are exact same type, value and orientation.

[fireman]

Thanks for the reply Juan

Your theory of the limiter chip amplifying noise from earlier in the preamp circuit makes sense, the data sheet shows that the device incorporates an amplifier.

The reason I thought that the ne 571 was the source of the hiss/white noise was that the amp is almost silent when the limiter out of circuit but noisy when it's in, I even tried removing the connection from the power amp, (to rule out that part of the circuit) but this didn't make a difference to the level of noise on the limiter. My next move was to look at the components around the chip, which is when I started replacing capacitors, I'm sure that new caps will not do any harm in an amp from the eighties.

another source of noise is the graphic eq section, this uses 12 opamps although I think the audio signal only passes though one of them, could this be another candidate for replacement with a lower noise part?

One thing I have noticed is that the schematic for the pb400 shows an additional opamp following the 571 chip, my amp does not have this and the phase of the output is changed when the limiter is turned on.

Thanks for sharing your knowledge with me.