Fault symptom - only with speaker connected...!

[noddyspuncture]

Hi Folks,

I've perused the site but couldn't find any pointers to my strange symptoms...

I am working on an old Simms-Watts transistor amplifier. It was blowing HT fuses. I found two shorted transistors which I have replaced. No other components are showing as 'faulty'...

I powered up via a Variac and a light bulb. The bulb lights and gets brighter as I increase the voltage... so I back it off and chicken out...! You would assume there is still a fault.

I do measure DC on the output... BUT only with a speaker connected...!
Disconnect the speaker... bulb don't light and there is no DC on the output.

Now I would have thought that a *fault*  would give me DC on the output all the time, speaker or no speaker...!?

Also, with such a fault which lights the bulb, I would have expected to read a dead short to ground from say the HT lines or maybe some where around the output transistors...!?

Does anyone have any ideas please...?

Many thanks,
Tom

[Roly]

Do we have a circuit, or at least some indication of what sort of output stage we are looking at here?  Failing that, how about some pix?

There are some output stage designs that depend on a DC path through the speaker to correctly bias the pre-drivers.

[J M Fahey]

Turn amp on, variac on minimum, no speaker.
Raise voltage slowly. checking for no DC on output.
If all looks normal, briefly connect speaker. Any DC/spark/smoke/thump ?
If none, apply signal to input.
Does it work?
Some amps "wake up stupid"if loaded and with a restricted supply, so what I suggest is to wake it up first and only then connect speaker.
If everything fine, turn amp off and then on again, with full line voltage and speaker attached.
*Shoulde*  work.
Post results.

[noddyspuncture]

Thanks Roly,
No schematics that I could find. It's a Simms-Watts TriSound MkII.
I could take photos - you'd need the print side as well... would they help..?

CHeers,
Tom

Do we have a circuit, or at least some indication of what sort of output stage we are looking at here?  Failing that, how about some pix?

There are some output stage designs that depend on a DC path through the speaker to correctly bias the pre-drivers.

[noddyspuncture]

Thanks for that..!
So, as I basically I don't want to blow the new transistors I am assuming that because I have the bulb connected - even if there is a fault (spark, thump, smoke even) they should still be OK...?

I am somewhat of a chicken with this you see...

Cheers,
Tom

Turn amp on, variac on minimum, no speaker.
Raise voltage slowly. checking for no DC on output.
If all looks normal, briefly connect speaker. Any DC/spark/smoke/thump ?
If none, apply signal to input.
Does it work?
Some amps "wake up stupid"if loaded and with a restricted supply, so what I suggest is to wake it up first and only then connect speaker.
If everything fine, turn amp off and then on again, with full line voltage and speaker attached.
*Shoulde*  work.
Post results.

[DrGonz78]

One quick question... Is this a 100 watt amp? Oh one more question... What wattage is the light bulb you are using?

There are some amps that I have fixed the output section and they were 100 watt amps. I then put in a 40 watt bulb, then 60, and then when I put in the 100 watt bulb it was shining bright. It never went dim with 100 watt but went dim with the other bulbs. I was perplexed just as you are and did not want to plug the amp into line level voltage. Everything I tested said the amp was good to go and I went to plug it in, it worked just fine. Not saying that is what is happening here, but need to know answers to those questions.

[noddyspuncture]

Thanks for the info!

Here's a link to what this amp looks like - this isn't my photo by the way - it's just a quick one I found on the net. The one I have here has an orange face.

http://simmswatts.homeunix.net/twiki/bin/view/SimmsWatts/AllPurpose100TriSound

Looks like it's a 100watt amp. And yes it is a 100watt bulb I am using.

Cheers,
Tom

One quick question... Is this a 100 watt amp? Oh one more question... What wattage is the light bulb you are using?

There are some amps that I have fixed the output section and they were 100 watt amps. I then put in a 40 watt bulb, then 60, and then when I put in the 100 watt bulb it was shining bright. It never went dim with 100 watt but went dim with the other bulbs. I was perplexed just as you are and did not want to plug the amp into line level voltage. Everything I tested said the amp was good to go and I went to plug it in, it worked just fine. Not saying that is what is happening here, but need to know answers to those questions.

[DrGonz78]

Cool amp! Also I remember another time I plugged an amp in, via light bulb limiter, with the speaker connected and it made the light shine bright. Without the speaker it was fine, but yeah I went through the whole amp to make sure there was nothing shorted. Turned out the bulb only shined bright when the speaker was attached. I guess it is how the amp is designed and I have read about this before. Mostly I rely on the dim bulb test without a speaker attached, but still a good idea to check both ways.

[noddyspuncture]

Thanks for the info DrGonz.

OK, so could/should bite the bullet and wind up the volts... after all, the bulb (even at full brightness) *should* protect my new transistors... right?

Hehe...
Tom

Cool amp! Also I remember another time I plugged an amp in, via light bulb limiter, with the speaker connected and it made the light shine bright. Without the speaker it was fine, but yeah I went through the whole amp to make sure there was nothing shorted. Turned out the bulb only shined bright when the speaker was attached. I guess it is how the amp is designed and I have read about this before. Mostly I rely on the dim bulb test without a speaker attached, but still a good idea to check both ways.

[DrGonz78]

Did you follow all of Juan's steps? Ooops I just re-read your first post again... I guess my question is how much DC voltage are you seeing with the speaker connected Volts/MV?

[Enzo]

Understand what the bulb does.  The point of a bulb is you can turn the circuit on and IF there is a problem, instead of blowing fuses, the bulb lights.  It offers some protection to the unit under test.   But the amp often will not work normally with a bulb.   Once you can power it up with the bulb remaining dark, then get rid of the bulb.

Another problem is that amps are not always stable at low voltage.  An amp designed to work with 50v power rails will not necessarily function with 5v on each rail.  If you can turn it on and find no DC on the output with the bulb and without a load, then eliminate the bulb and turn the amp on, still no load.   And I mean just flip it on, no variac-ing.   Does it come up with no DC?   

Just reading between the lines, are you bringing it up on the variac each time, load or no load?   With no load, very little current flows, so the amp is able to keep hold of itself.  But when there is a load present, it then has to control the currents as well as the voltages.  ANd when loaded, if you bring it up slow on a variac, you will go through those stages where there are only a few volts present, and the amp will not be stable there.  It will swing over to one rail , and likely stay there, which causes large currents in your load.

So once you find - unloaded - no DC on the outs WITH a bulb, then try WITHOUT a bulb.  If that works with no DC on the outs, then put the bulb back on it, connect a load, and then flip it on.  No variac.  Does teh bulb come on bright?   or does it blink maybe but otherwise stay dim?   If it stays dim, then lose the bulb, and flip it on loaded.


The bulb is a great thing, but it only does what it does.   A whick is great for making whipped cream, but lousy at carving a turkey.  Always get the right tool for the job.

[noddyspuncture]

Many thanks for the detailed explanation Enzo!
Today I sat down and was going to go through and perform all the checks as you listed... but then there was yet another spanner in the works, so I've delayed..!

It was mentioned before about what wattage bulb I was using... I used a 100watt bulb, which I've since changed for a 60watt bulb. Now, whereas the 100w bulb didn't light with no load... this other 60w bulb DOES light - albeit dimly with the variac wound up to full volts. I treble checked and put the 100w bulb back in and it still does not light - at all!

So, a 100w bulb doesn't light - but a 60w bulb does light, but dimly.
On both counts there is no DC on the output.

So I am now confused... again....

Cheers,
Tom

Understand what the bulb does.  The point of a bulb is you can turn the circuit on and IF there is a problem, instead of blowing fuses, the bulb lights.  It offers some protection to the unit under test.   But the amp often will not work normally with a bulb.   Once you can power it up with the bulb remaining dark, then get rid of the bulb.

Another problem is that amps are not always stable at low voltage.  An amp designed to work with 50v power rails will not necessarily function with 5v on each rail.  If you can turn it on and find no DC on the output with the bulb and without a load, then eliminate the bulb and turn the amp on, still no load.   And I mean just flip it on, no variac-ing.   Does it come up with no DC?   

Just reading between the lines, are you bringing it up on the variac each time, load or no load?   With no load, very little current flows, so the amp is able to keep hold of itself.  But when there is a load present, it then has to control the currents as well as the voltages.  ANd when loaded, if you bring it up slow on a variac, you will go through those stages where there are only a few volts present, and the amp will not be stable there.  It will swing over to one rail , and likely stay there, which causes large currents in your load.

So once you find - unloaded - no DC on the outs WITH a bulb, then try WITHOUT a bulb.  If that works with no DC on the outs, then put the bulb back on it, connect a load, and then flip it on.  No variac.  Does teh bulb come on bright?   or does it blink maybe but otherwise stay dim?   If it stays dim, then lose the bulb, and flip it on loaded.


The bulb is a great thing, but it only does what it does.   A whick is great for making whipped cream, but lousy at carving a turkey.  Always get the right tool for the job.

[DrGonz78]

It is also known in many corners as the "Dim Bulb Tester"... So idea is that it might grow bright at first as you flip the power switch, but will go dim. So if the light is dimly lit then it would not be holding back high amounts of current from your amp. If it was brightly lit then it is holding back current from your amp. Dimly lit suggests your unit is not drawing an overage of current. So it appears good. Ditch the variac and try it with the light bulb limiter (Dim Bulb Tester) with out a load. Light bulb will go bright the first time turned on and then go dim. That is a healthy test.

[Roly]

Just a little point about light bulb limiters.

Something that is not obvious about globes is that they change their resistance quite significantly between cold and hot.  When cold/warm they have a very low resistance, but as they heat up their resistance rises by a large factor.

This means that when used as a limiter they tend to have two states, cool/low resistance, and hot/high resistance, and they kinda flip between the two states, so they act a bit like a self-resetting fuse.

HTH

[noddyspuncture]

Many thanks to everyone for their help and advice....  all sorted now. I was just a overly cautious - probably not a bad thing - and thus ended up asking lots of questions just to make sure. Everything was fine and it's working a treat....

I've learned a lot on this one.

Thanks again,
Tom