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

Quote from: DrGonz78 on December 19, 2013, 05:50:17 AM
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.

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

Quote from: J M Fahey on December 18, 2013, 05:57:44 PM
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.

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

Quote from: Roly on December 18, 2013, 04:39:11 PM
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.

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

Many thanks Roly - that was very educational sir!
Cheers,
Tom

Quote from: Roly on May 02, 2012, 10:13:29 AM
Hi @noddyspuncture; Rod's amp doesn't use a SMPS, and the particular par starts;

Thanks for the link Phil - but I am now confused. The 2nd paragraph says to "use a bench supply and increase voltage slowly"... but didn't we say that using a Variac on SMPS was a no-no...?

Cheers,
Tom

Quote from: phatt on May 01, 2012, 10:43:01 AM

Re safety on supply rails;
http://sound.westhost.com/project3a.htm

2/3rds down read *Powering up* 22 ohm in  place of fuse but this might need to be a different value for other voltage/current requirements.

I'm sure other here will know more. :tu:
Phil.

Thanks for the replies guys... OK just thought of something else. Would like to run it by you. Obviously there are amps out there with SMPS... I already came across a faulty Sub-Bass unit, but one where the SMPS was the problem, and managed to fix it. It was dead. I found some (rather thin) traces open circuit - maybe 'fused'..? I jumpered them and the SMPS sprang into life... sparks everywhere, coming from under the large 16 legged transformer on there. I ended up removing the T/X and found the arcing path... blackened and conducting. I cleaned everything up, replaced the T/X (added a few more jumpers) and everything was working fine. (I have attached a photo....) anyways, this leads me to my question/scenario...

If that SMPS had been OK and the actual amplifier needed attention, obviously I could not use the bulb-limiter on the mains input side.... BUT, I noticed that the output(s) from the SMPS were two really thick wires carrying DC (cannot remember the value) and also a small muti connector. Now then, would it be feasible to say that you could connect your 'bulb'... in line with the main HT out from the SMPS going into the amp section? It was all very obvious and modular, using spade connectors and this could have been easily done.

Just a thought for future reference....
Cheers,
Tom

Quote from: J M Fahey on April 30, 2012, 11:30:31 AM
It depends on the PS design.
Most of them do not like anything blocking current.

Hi again...

I just had another thought on the bulb/variac debate.
It is concerning "switch mode power supplies", and of course any gear using them... it is well documented that using a variac is not a good idea with them!

But what about the bulb-limiter? My initial thoughts were that it would be OK and work in the same way... but I thought I'd just ask and see if there were any other opinions on it?

Cheers all,
Tom

Quote from: joecool85 on April 20, 2012, 01:16:30 PM

Quote from: noddyspuncture on April 20, 2012, 07:09:37 AM
Which one first though - I was earlier thinking Varic first - but now, thinking about it the Bulb first would seem to make more sense... or does it on matter?

It may not matter, but I would put the Variac first, then plug the Light Bulb Limiter into that, and the project amplifier into the limiter.

Thanks guys... glad I joined this forum, you're a great bunch of friendly and knowledgeable chaps. Who'd have thought this little problem amp of mine (well, my mates...) could have provided this much debate...;c)

Looking forward to my next problem/project...haha!

Stay good!
Tom

Quote from: phatt on April 25, 2012, 10:09:53 AM

Quote from: J M Fahey on April 24, 2012, 11:54:22 PM
1) The voltage on Q1's base will be a few mV DC, because its base current flows through R2 to ground.
The voltage on Q2's base *should* be the same, because it also flows through a 10K resistor to ground : R7 .
And where is that ground connection?: through the speaker.
I didn't even mention its value because 4 or 8 ohms are *nothing* compared to 10000.
In fact it won't be *exactly* the same because Q1 and Q2 do not have the exact same current gain, but anyway they will be only a few mV from each other.
So , in a nutshell, Q2's base will also be a few mV (less than 100mV ) from ground.
Exactly what Phatt said from his considerable experience, only I added the *why*.
2) those 220 ohms shown are necessary so the amp has gain, otherwise you would need to hit it with over 20V RMS.
The AC (signal) gain is= (R7/220)+1=(10000/220)+1=46.5 ... call it 50.
Which in practical use means , since it can put around 20V (20000 mV) into that speaker, it will need to be driven with = 20000mV/50=400mV .
Easy for any preamp, even a 9V battery powered one.
The 47uF capacitor in series with the 220 ohm resistor blocks DC, so the amp has AC gain around 50, but DC gain of 1 , which is good because otherwise any DC at the input would produce a speaker burning disastrously high DC voltage at the output.

This amp uses a classic design, time tested and used by the Millions , basically for one reason: it works ... and very well.
Long live SS !!!!!!  :dbtu: :dbtu: :dbtu: :dbtu: :dbtu: <3)

Arrh,, Mr Fahey you are very kind about my experience :lmao:
Thanks for the maths as it does help explain all those transistors that sacrificed there guts to help an Amateur learn.
Re discrete Pwr Amps,, I have one of my first ever attempts at hand drawn PCB somewhere in the shed and the schemo from a Electronics mag, So I'll try and find it,, might be an interesting exercise to analyze the merits or lack off.
Oh Yes the first one did actually work but the second one blew instantly.
Oh Well 50 % success for a beginner was not bad ay?
Cheers, Phil

Hi Phil,

Thanks for the reply and the comments. My redrawn cct does indeed have Q5's emitter amended! Also I'm pretty sure that the resistor is 220ohm... and as for the voltage on the base of Q2... well the amp is now all reassembled and I am loathed to open her up, in case I disturb something again. I was fixing this amp for a friend and have told them it's all done! Success...etc... Hahaha!

I wish you'd have asked these questions a little earlier, I could have checked it all for you. It has been quite a learning curve for me... and I have now also built a Variac/Light Bulb station which I shall be using more in future, I get the feeling...;c)

Can anyone else comment... about the 220ohm... or even what volts there 'should' be on Q2's base...?

Cheers
Tom

Quote from: phatt on April 24, 2012, 09:52:39 AM
Hi Noddy,
Well done you worked that out well  :tu:
just need to flip Q5,, which was my booboo.

Others here will know more than myself ,, but I'd be a bit worried about that *220R* off the base on Q2.

In my real world testing Destruction of perfectly working transistors. :duh
I did realize that if that is set around 1k it seems to stay safe but depends on circuit/voltages and much other stuff which is mostly beyond my ability to understand let alone explain clearly.
One quick Q?
what is the DC voltage at the base of Q2?
Circuits like this are a very good learning tool for understanding DC coupled stages as they are very basic and in my experience can often sound better for guitar work.

From what I've understood (while blowing things up) the idea is to keep the base of Q1, Q2 and the speaker out should be all close to Zero Volts.
Phil.

Well I think my Torque amp is now fixed... just on soak for a few days... and no issues yet, touch wood. I am attaching my revised circuit diagram - I slightly edited the one posted by (phatt) Phil, making it a circuit of the one I have here. A couple of changes... removed the protection circuitry, added a few components and moved one 'trace'... please let me know if it "looks about right", it's quite confusing as you probably know, trying to follow the path!

It might be useful to someone with this same model combo...!?

Thanks for all the help and advice.
Cheers
Tom

Quote from: J M Fahey on April 22, 2012, 10:28:45 AM
Yes, all analog meters had a single resistance setting for everything. Oh well.
Believe it or not, I sometimes still use my old analog (needle) meter , both to keep trained with it, and because some things are best shown by a wiggling needle than by rapidly changing numbers.

Yes, of course... my meter doesn't have a dedicated transistor tester, as some do. And I never really got out of the habit I developed on my old analogue meter which only had an ohms range... anyway here's an update - as of this afternoon the amp is fixed!

But I think there is more to this than meets (or met) the eye!

All I did was to remove those two transistors... then replace them, possibly swapping them around!? I also lifted all three zeners and checked those again...remember this is all treble checking as I have gone over and over this amp already. So I ran up the variac again - and there were zero volts on the output..! I monitored both rails and it all behaved beatifully. I next attached the speaker... no hum!  So it seems to have 'fixed itself'.

Now remember, this is an amp I already fixed once and after a couple of days use it blew again. I am expecting it to do the same as I haven't actually found anything definitely wrong. Maybe I 'disturbed' something - I might never know....

My next step will be to reattach the preamp section ribbon cable and feed some audio through it... but this begs another question... what would the recommended procedure be here?

Soak test it all via the bulb just in case the problem rears it's head again? Tap around (lightly) and see if I can 'force' the fault to reappear? I am going to have to put all my faith into this bulb to protect it should this be an intermittent problem...hehe!

Cheers,
Tom

Quote from: J M Fahey on April 20, 2012, 09:55:54 PM
Transistors are always measured in the diode setting, that's what it's for.
Google how to measure transistors with a multimeter, as not to repeat the same time and again.
Good luck.

Hi again. I have an update... I *was* searching for shorts on this board... but I found two transistors (would be Q1 & Q2  on this schematic although they are numbered differently on mine...) that, when removed measure "open" all round with my meter on the ohms setting... however using the diode/capacitor setting they read 'open' (3volts) one way and 0.7volts the other between the three legs. Now, which should I go with? Are they suspect, would you say?

Quote from: phatt on April 18, 2012, 08:20:36 AM
Yeah I found it  :cheesy:
Don't take it as gospel but it will likely be close to what is in front of you.

From memory give or take a few brain cells the rails where around 35 VDC.
Not a rock concert level of power but a big 12 inch driver makes it OK for a fold back Amp.

My guess at why it blew??  Likely someone added a dodgy external speaker jack and did not bother to check the internal speaker was already 4 Ohm and adding another driver would definitely blow it up.

The owner who knows me well has had no problems since we fixed it  and removed the ext socket.
(over a year ago now)
Phil.

Which one first though - I was earlier thinking Varic first - but now, thinking about it the Bulb first would seem to make more sense... or does it on matter?

Quote from: DrewV on April 19, 2012, 12:58:17 PM
Tom, Yeah that's how I've always done it. You'll know right away by the brightness of the bulb if excessive current is flowing.

Cheers guys!

Quote from: J
@ noddy: it would be nice to have some gut shot of your Torque, never seen one inside.
/quote]