Shiva Posted October 21, 2004 Share Posted October 21, 2004 I have a Forte model 1A 50 watt class A amp. It has played beautifully for the few years that I have had it and still does. the problem is that as with most class A amps it has big cooling fins on each side that used to get nice and toasty to the touch. Now both sides while playing stay nice and cool and they never heat up. What has happened to the excess heat. I would like to get it cooking again to help heat my house during this winter. Does anyone know what is happening to this amp and how to fix. Thanks Quote Link to comment Share on other sites More sharing options...
tpg Posted October 21, 2004 Share Posted October 21, 2004 That is a very strange problem... 50W class A should be quite warm. Is there any air blowing over the amp? Maybe an air conditioner vent or something? Perhaps the folks over at diyAudio would know a cause for this... Quote Link to comment Share on other sites More sharing options...
mikebse2a3 Posted October 21, 2004 Share Posted October 21, 2004 Hi shiva I'm not familiar with the design of the Amp you've asked about so maybe someone will respond that has actually some experience with it but to try to help you out some I've got a couple of questions. 1st question: Did both channels start running cooler at the same time? 2nd question: You are saying that as far as sound everything sounds OK to you? I would run test on the power supply voltages if both channels went cool at the same time. (this assumes the pwr supply is common to both channels) If the amp still sounds ok but runs cooler than normal then I would think the output transistors bias points have changed and for both channels to be like this at the same time would make you look for things that are common to both channels like the pwr supply if the channels share a common pwr supply. If one channel started running cool first then the other one sometime later then I would look at the Bias circuity of each channel for problems. Some designs use a bias control that can become dirty or bad after years and cause a shift in the bias settings as well as other componets in the bias circuity could have failed. To really start to check this out properly you really need a service diagram and specifications to know if the bias is really off and what value to set the bias when the problem is located. Again without a diagram of the amp its hard to help more and what I've said is some general thoughts based on some common ways amps have been designed. Again hopefully someone will respond that is more familiar with this amps design and can offer more specific help. mike Quote Link to comment Share on other sites More sharing options...
Shiva Posted October 22, 2004 Author Share Posted October 22, 2004 Hey Mike, that was a great and informative post by you. To answer your question, first one side cooled off then the other. While the insides of this amp are not that complicated, discerning the bias circuitry from the rest of the bits and pieces in there, looks to be a bit over my head. UNder the hood of this amp, there are two big capacitors per side, a very large torroidel transformer I believe it is called and quite a few litle soldered doodads. you wouldn't happen to live in S. California by chance would you. Quote Link to comment Share on other sites More sharing options...
mikebse2a3 Posted October 22, 2004 Share Posted October 22, 2004 Thanks shiva, Sorry but I live in East Tennessee. From your description it sounds like this could be a dual mono design on one chassis in other words two completely seperate amps in one box.If thats the case and they did cool down at different times then it looks even more likely there might have been a change in a componet in the bias circuit. Since both have the problem the same way it would be possible that there is a componet that fails more commonly in this design. Sometimes a manufacturer specs a componet's tolerance (like say a resistor) to closely and over time they can fail.When componets fail in a bias circuit it would be possible for the amp to run cooler or to over heat( which if the amp has some fail safes built in will cause the amp to shut down to protect itself (like a Hafler DH200/DH220 design. I believe Nelson Pass designed some of the forte amps- if my memory is right. Do you have any literature on the amp at all? Are they still in business? I haven't seen any in a long time. I was just trying to think how we could maybe see/get a service diagram to maybe help track your problem down. If I can think of anything else I'll try to get back with you or if you get more information let me know. mike Quote Link to comment Share on other sites More sharing options...
Shiva Posted October 22, 2004 Author Share Posted October 22, 2004 Hey Mike, Thanks again for taking the time to answer my questions. This amp is said to be a nelson pass design.I have no schematics for this amp which came out around 1989. I was hoping to see what this amp would do with my recently acquired Klipsch epic cf3's. But I would not want to hurt the speakers in any way by hooking up a defective amp to them. Is there a simple visual way to find out which circuits control the bias settings. Do they have a particular shape,size or color or perhaps have a certain value printed on them that would allow me to find them from amongst the other circuits on the two circuit boards in there. It may be going up on audiogon pretty soon as a fixer upper. Take Care Quote Link to comment Share on other sites More sharing options...
lynnm Posted October 22, 2004 Share Posted October 22, 2004 Errr...Ummmmm The amp sounds as good as ever and isn't getting hot! And your problem is ?? It's somewhat like owning an elephant......You sometimes gotta ask yourself "Is constipation necessarily a bad thing " ? AKA Are you running the same speakers you used when it was helping reduce your heating costs? Is the amp in the same rack and on the same shelf as before ? Have you made equipment changes ? Have you changed the rack's location within the room or house ? Are you in the same house ? Why Is There Air....Bill Cosby has to know! Quote Link to comment Share on other sites More sharing options...
Shiva Posted October 22, 2004 Author Share Posted October 22, 2004 Your observations are without merit. A class A amp by its very nature runs hot. Because of that they are built with big cooling fins to dissipate the heat generated. If you put a fairly big class A amp outside on a winters day it would still get hot. They are inefficient by their very nature and generate a lot of heat no matter what they are driving or where they are put. It does not take a very big leap to discern thas it is not running properly as it was intended. If your car does not work properly do you ignore it just because it still gets you down the street or do you find out whats wrong and attempt a fix. Maybe its in your nature just to ignore problems and let things run down and die but as for me I would rather have things run as they were intended. YMMV. Quote Link to comment Share on other sites More sharing options...
lynnm Posted October 22, 2004 Share Posted October 22, 2004 . Quote Link to comment Share on other sites More sharing options...
mikebse2a3 Posted October 23, 2004 Share Posted October 23, 2004 hey shiva one thought that did come to me although if both cannels changed at different times it couldn't be the problem but check if you haven't yet. Some componets have a switch to reconfigure the pwr supply transformer to except different AC voltages so lets make sure (if yours has one) that its not gotten accidently changed and is in the correct position.look around where th AC cord comes into the amp on the back and if your inside follow the wiring to the transformer.Most likely I would guess you want have a switch but I wouldn't want to overlook such a simple possibility. AS far as locating the bias circuits by site that would be difficult because most of the componets will look the same as others on the boards. If it does have a control to set bias you might recognize that on the board but it might not even use a control depending on how the circuit is designed. If you have a digital camera and want to try to take some pictures of the boards and amp so we could see more what you see but its probably going to be difficult to solve any problem without a schematic/specs. If one channel was working correctly a tech could compare the channels and maybe find some problem but with both the same thats probably not possible. Sorry wish I could be of more help but without information avaible to you it would be very involved to try to trouble shoot this problem and you still would need to know what levels to set the bias if its adjustable by you. mike Quote Link to comment Share on other sites More sharing options...
sunnysal Posted October 23, 2004 Share Posted October 23, 2004 Somethin has certainly gone wrong with your amp, one question that was not asked is if the amp has a class a, class a/b switch? I assume that there is none and if there is it is not set on class a/b...two locations could be of asistance; Nelson Pass was the president of Forte audio and designed this amplifier, he is a nice guy and I am sure he would lead yo uin the right direction if you sent him an e-mail. nelson@passlabs.com Jon soderbegt specializes in the repair of threshold and forte amps, he could give you some specific advice about how to troubleshoot this amplifier, http://www.rsvlonline.net/vintageamp/ vintageamp@earthlink.net best of luck, tony Quote Link to comment Share on other sites More sharing options...
Marvel Posted October 23, 2004 Share Posted October 23, 2004 Lynnm, You have done quite well in biting your tongue! "Without merit" NOT -- he makes valid points in trying to determine what is wrong. Yes, Class A amps run warm to hot -- they are always biased on, to get around the crossover distortion in the transistors. I also have an amp with large cooling fins. Depending on where it is, they get warm or not at all (it isn't running in class A). Normally, extended listening at 90db and it is still at room temp. ...but something has indeed changed. Best of Luck to you. All points made have been valid, and it may be slipping into A/B somehow as Tony has pointed out. Quote Link to comment Share on other sites More sharing options...
mikebse2a3 Posted October 23, 2004 Share Posted October 23, 2004 shiva thats good info from tony so try some of the contacts he suggest I wasn't able to connect to the address for Jon soderbegt for some reason. also checking for the class operation switch is a good idea. I do remember forte building a class AB version of this amp so I'm thinking there want be a switch though. good luck mike Quote Link to comment Share on other sites More sharing options...
Shiva Posted October 23, 2004 Author Share Posted October 23, 2004 I want to thank all for taking the time to post and for trying to help me mend my little amp. I will see what Nelson or John Soderberg have to say. Thanks again. Quote Link to comment Share on other sites More sharing options...
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