Overheating amplifier... Possible causes?

 

Silver Member
Username: Varney

BirminghamEngland, UK

Post Number: 423
Registered: Sep-04
Okay - I'm asking this, but armed with just the tiniest bit of knowledge and a couple of theories. I hope the answer will help me decide on a course of action. I also have a rather naughty confession to make...

Question:
If my old integrated amplifier is overheating to the point where you could definitely cook your dinner on the top - is this because:

1) It's been running with no speaker load, as a fill in for an outboard phono-stage for an amp that does not possess one (achieved by using phono-input and then out through the tape-out) ?

Or - could it be (ducks under the table):

2) That I found some white plastic round things, with screw-heads in them, on the board and decided to turn them a few times, for the sheer hell of it?

Oops! I thought where it said 'Bias' this meant the treble or something... I've just read a document I found elsewhere, which has filled me with some horror and guilt! Ulp!:/

3) Sprayed servisol all over the insides, to clean it up?

4) It's buggered up anyway. Note: This amp does not put out on the left channel, so will only serve one channel to the speakers or the headphone socket... But it WILL take a signal, in through the phono socket and put out two healthy channels, in perfectly good order, straight out of the tape/record output, to another amp's line level-only 'Disc' input.

It's just getting SO HOT, I dare not use it, without some serious advice.

Note also, I'm not worried about the old amp... I've bought a new one and I'd stuck it on a shelf, pending future repair anyway. I just need to know, so that I can relate any relevent details to the repair tech.

V
 

Gold Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 9958
Registered: May-04
.

Running a solid state amplifier without a load is not a problem. Twisting bias pots and spraying fluid willy-nilly are. It's impossible to tell what damage you've done but a good guess is you've fried the output transistors in the non-working channel. Maybe it's just a resistor or cap you've done in. Bias will need to be reset by a tech with a test bench and then the amp's other problems can be diagnosed.



Don't go turning sh!t you don't know about!



.
 

Gold Member
Username: Nuck

Post Number: 6248
Registered: Dec-04
V/A you have read too much from Andre.

Please check here before doing goofy stuff.

It takes a little longer, but helps.

Take the unit into the shop/replace.
 

Gold Member
Username: Nuck

Post Number: 6249
Registered: Dec-04
Dude, you didn't even count the turns of each pot?
 

Silver Member
Username: Varney

BirminghamEngland, UK

Post Number: 424
Registered: Sep-04
Dude - I have been a complete IDIOT about this, so no point asking sensible questions of me. Make no mistake... I am not proud of the very stupid thing I have done.

Only good thing is I have learned, once and for all is:
Don't mess with sh*t you don't understand (as said above).

The amp was fried on one channel anyway. But this is the last time I ever take a cover off. I wouldn't do this with an amp that mattered to me.

If I can get it fixed, then I will. If running in no-load condition is okay, then it will serve as a fine phono stage for a while - or perhaps the remainder of it's life.

Thanks

V
 

Silver Member
Username: Varney

BirminghamEngland, UK

Post Number: 425
Registered: Sep-04
As for the servisol, I make out I've sprayed it everywhere - but the truth is, I have only really (and carefully) directed it into the volume and tone control pots. I also squirted it leanly in and around the line-input sockets, to dispell any dust and grime. I decided it would be best not to allow it anywhere near anything that's wound with wire or onto the main board, itself.

V
 

Gold Member
Username: Nuck

Post Number: 6263
Registered: Dec-04
Well, yeah, you needed the phono anyhow.
It'll look fine in the stack.
 

Gold Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 9963
Registered: May-04
.

"As for the servisol, I make out I've sprayed it everywhere - but the truth is, I have only really (and carefully) directed it into the volume and tone control pots. I also squirted it leanly in and around the line-input sockets, to dispell any dust and grime. I decided it would be best not to allow it anywhere near anything that's wound with wire or onto the main board, itself."


Yeah, but, Varney, how do you think the signal gets from the input jacks to the output jacks? And, don't you think it has to pass through the vc at some point? Maybe not if you're just using the tape loop, but still it's all connected in there.


Where did you have the vc set when you were using the amp as just a phono stage?

.
 

Silver Member
Username: Varney

BirminghamEngland, UK

Post Number: 432
Registered: Sep-04
"And, don't you think it has to pass through the vc at some point?"

Of course, Jan... But I'm afraid I seem to be missing your point, as it's lost on me, right now.

The product is sold as a switch cleaner and adivses not to spray into live circuitry, while the unit is switched on, which I didn't. It also advises to allow to dry, before powering up, which I did.

Are you implying that switch cleaner is unsuitable for spraying into switches? Please enlighten. I'm feeling a little lost...

I played one song, after which, the unit was ready for the hot dogs. Instead of grabbing the bread rolls, I dived for the 'off' switch and pulled out the plug.

If, by VC, you mean 'volume control', then, (as in all tape loops I've come across), this control makes no difference to the output. It has been set to zero, the whole time.

My meagre knowledge begged me to ask of myself the logical question: "if an amplifier has no load to push; then where can the power, built up in the transformer go?"

If no-where, then I assumed there would be some heat build up - but not enough for a family barbecue! :-)

I have sent the unit to my friendly tech, (the same we spoke of earlier) who is, at this moment, re-setting the bias and checking for other faults...

V
 

Silver Member
Username: Varney

BirminghamEngland, UK

Post Number: 433
Registered: Sep-04
It has been over a week, since any servisol went near it.

I'm not accident prone, although I once had suspected rabies, and, more recently blew up a computer power supply, which nearly set fire to the curtains...

I'm looking forward to the removal of my first, high-current, battery-powered pacemaker, from a body... Please pray for me.

No... I'm not accident prone at all :>

V
 

Gold Member
Username: Nuck

Post Number: 6288
Registered: Dec-04
Varney, your unit is likely a Class AB.
Class A is a design which keeps a full field current available all the time, and can run quite hot as a result.
AB amps will switch modes at a low power output, and usual listeming is in B, which modifies the current according to source.
If spinning the pots left the unit in A far beyond the normal switching point, then yes, you might find evidence of overheating.
Always use unsalted butter inside the trout, when audio cooking.Do not turn the fish.
 

Silver Member
Username: Varney

BirminghamEngland, UK

Post Number: 434
Registered: Sep-04
Hahha, Nuck! :>

I know they run quite warm... NAD/solid state - but this was SO hot, like I said... I'm talking Baby Belling, get the beans on, baby. Definitely NOT normal.

No smoke. Thankfully. No really bad smells, either. That is thanks, probably to me turning it off in time. I'm constantly checking gear, to see how warm it gets. I have my rack, adjusted for maximum airflow.

V
 

Gold Member
Username: Nuck

Post Number: 6291
Registered: Dec-04
If you get smoke, then do not wrap the trout in foil, rather let the fish smoke over a few hours.
Unfortunately, the smoke does not usually last long enough for a full cook, just a glazing.
I like the Mengyue tube amp for long slow cooking.
 

Gold Member
Username: Nuck

Post Number: 6292
Registered: Dec-04
Varney, my Classe class A amp will toast bread.
It is built like a Ferrari.
 

Silver Member
Username: Varney

BirminghamEngland, UK

Post Number: 436
Registered: Sep-04
I notice this thread seems to have wetted someone else's appetite... although the dish he serves in the other thread, does not seem to have been cooked for the healthiest of reasons. :>

V
 

Gold Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 9974
Registered: May-04
.

"Are you implying that switch cleaner is unsuitable for spraying into switches? Please enlighten. I'm feeling a little lost... "


I'm saying "switch cleaner" is not approriate for all contacts and not all "switch cleaners" are equal to the task at hand. If you spray switch/tuner/contact cleaner into certain pots, you will ruin them and they will sound like frying bacon afterwards. I think most techs will tell you they see far more amplifiers being serviced after the owner has liberally used some cleaner spray than before anything is applied. Many contact cleaners also have a "conditioner" (not the lather/rinse/repeat type) which is meant to coat the contact and slow further oxidation. When this conditioner is liberally distributed across contacts, there can be a small flow of current across this product which then creates shorted circuits. I have no idea whether the product you applied has such properties, but I would take the can to the tech and ask for advice on the appropriate cleaner for your amplifier.




Bias sets the point in a push pull amplifier where, in the transistion from + to - signal flow, the signal is handed from one side of the amplifier (the + [or PNP] side) to the opposite side output (the [NPN or] - side) and back again. In a push pull design, if the amp is to run in pure class A operation, both sides (+ and - [or PNP and NPN]) of the output remain on all the time and the amp has no time to cool - if even for only a small moment - by way of thermal efficiency. In class A/B operation the outputs get to switch off momentarily as the signal goes from + to - and back again. As you adjust the bias you set the amount of time the output remains on and off - mostly in a class A/B amp you set the amount of "on" time.


By cranking up the bias you set the amp closer to class A operation, where the outputs remain on constantly, than was intended by the manufacturer. Bias is a voltage/current thing and you asked the power supply to push higher than normal levels of both through the power amplifier's circuitry. Therefore, not only the output transistors but the entire bias circuit in front of the transistors ran at higher voltage/current than they should. Heat is built up from this additional burden. In most amplifiers, and certainly in most mass market products, if the circuit is meant to run "correctly" with a 1/4 watt component or a 50VDC component, a 1/4 watt and a 50VDC component will be installed and not the more expensive 1/2 watt or 75VDC unit that would give a bit more leeway. Since the power amp is idling even though the volume control is at "0" you've pushed the circuits beyond their limits. There is no load on the amplifier since there is not a complete circuit from the +/hot side of the amplifier's speaker output to the -/ground return as would be the case when a loudspeaker is placed in the line. If you had speakers attached to the amp and you did run the volume up even slightly, you could have had more serious problems with the output bias set too high. As is, you merely cooked the amp with too much voltage/current while it was running at idle. It's rather like revving the engine to 8k R.P.M. while sitting in your driveway. The load actually acts as a bit of brake to this sort of runaway operation - albeit by shutting things down. Without the load the amp and the engine can easily be severely damaged, in the case of the amp by thermal runaway.





"if an amplifier has no load to push; then where can the power, built up in the transformer go?"



The amp technically has no "power", meaning watts, until it is fed a signal and has a load placed across its outputs. But it does have an idle voltage/current as I hope you'll understand from the above explanation. You asked the power amplifier section of your integrated amplifier to run at too high a idle bias which caused thermal damage. Whether you did any damage with the spray will be determined by the tech. You're certainly not the first person to run into problems by twiddling something they didn't understand or spraying something where they shouldn't, but it can be an expensive lesson to learn. Good luck on all counts.



.
 

Silver Member
Username: Varney

BirminghamEngland, UK

Post Number: 441
Registered: Sep-04
That's better. Thankyou. That sounds like good, useful advice. I am now re-finding my will to live on... :-)

The same tech sold me the spray. I don't, in all honestly think it is this that is the cause... but I will keep an open mind and bear in mind what you have said.

V
 

Gold Member
Username: Nuck

Post Number: 6311
Registered: Dec-04
Excellent, JV!
 

Silver Member
Username: Varney

BirminghamEngland, UK

Post Number: 444
Registered: Sep-04
So are you able to give any indication, how long it should take a qualified person, to correctly reset the bias? Given, of course, that when we say 'cooked' we don't mean permanent damage, which would preclude it's usefulness as a phono utility?

V
 

Silver Member
Username: Varney

BirminghamEngland, UK

Post Number: 445
Registered: Sep-04
I only say this, because, when it was in an overheated condition, it was still merrily doing it's job. It only stopped, when I decided to pull the plug, because I was nervous.

V
 

Gold Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 9981
Registered: May-04
.

Even if the power amp had been destroyed, the pre amp section would probably continue to operate normally, unless some catastrophic damage had occurred in the power supply. That would have been unlikely from merely increasing the bias to the outputs. Setting the bias is 1/2 hour job in most cases but the amp has to preheat and then sit and cook for several hours to ensure stability of the setting.

.
 

Silver Member
Username: Varney

BirminghamEngland, UK

Post Number: 447
Registered: Sep-04
What do you think of my idea to recycle my old amp, as an outboard phono stage, for my NAD C350 then? They say the old NAD 3020s had very fine phono stages for their day. I have no other use for it, since the output channel went... I can get a Cambridge Audio PP model for £40, but I like the idea of recycling and keeping it on, if possible.

The set-up sounded very nice, with my Rega Planar II deck.

V
 

Silver Member
Username: Varney

BirminghamEngland, UK

Post Number: 448
Registered: Sep-04
A little dark, perhaps, but I didn't expect the brightness you normally get with CDs anyway. The bass and midrange were nicely detailed, though.

V
 

Gold Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 9982
Registered: May-04
.

I should add here, if you had driven a speaker load with the amplifier's bias turned up, you might have damaged the power supply. Using just the low voltage requirements of the pre amp would not have been a problem.



You obviously know the sonic thumbprint of the 3020. If it suits your needs, then I think recycling is great. There are better phono pre amps to be had for not much money though.


.
 

Silver Member
Username: Varney

BirminghamEngland, UK

Post Number: 449
Registered: Sep-04
That;s right, Jan. It's quite nice. Actually a 3225PE, but has a very similar character to the old 3020, which is still going strong and had a lot said about it's phono-stage. They are tough little amps. No wonder everyone loves them.

Stopped putting it under speaker load a long time before this silly accident occured. I'm optimistic, although equally so, that the Cambridge Audio units are not prohibitively expensive, by any means at all. I've met people who favour them over NAD versions and use them with NAD.

V
 

Silver Member
Username: Varney

BirminghamEngland, UK

Post Number: 450
Registered: Sep-04
Cool!

V
Jump to: Home Audio Forum | Home Video Forum | Home Theater Forum | Car Audio Forum