Forte Model 3 Three sounds blown in only a small frequency range

[FIXED] Forte Model 3 Three sounds blown in only a small frequency range

I've owned a Forte Model 3 (three) for about 10 years or so and just love it. I'm a lucky guy... They were tearing down a house and let us go through it and take anything we wanted. On a shelf in the garage, there it was... All dusty and waiting for the next lucky owner. I asked if I could have it and they said yes. When I originally powered it up, there were problems. I don't remember everything but the right side was MUCH cooler than the left. I found a local guy that worked at Audio Research in Minneapolis and he fixed er up. I recall, he adjusted the bias and some other things. He was VERY impressed by this amplifier. So am I!! It's wonderful. It's been purring for years.

Now, recently, it starting to show it's age. The guy that repaired it last time is no longer working on electronics and has switched careers. He won't work on it. After browsing these forums for a few weeks and have found that Nelson Pass is also an active member, I immediately joined. I've searched this forum for schematics for the Model three and everyone says it's essentially the same as the Model one. I opened up the amplifier and looked around (not touching anything) and I'm positive I can repair this myself and feel the pride like you all do with the upgrades and repairs being done by yourself. I'm excited!!

Here is what I know:
When the amp is cold (I know, let er warm up) and now when its also warm, the right channel sounds like it's blown in only a small frequency of the range. It sounds like it's within many voices and certain higher mids. The bass and highs sound fine. I was initially thinking it was the speakers (Klipsch Forte II's) so I switched out the speakers to Klipsch Heresy II's and the problem still exists. The Forte also have new Crite crossovers in them. I'm pretty sure it's the amp. Now it seems like it's in both channels...

I'm ready to dive in. But wait!... I'm not 100% sure what the benefit of a class A amplifier at 50 watts is vs a class A/B amplifier at 200 watts. With that said, and most people going through the process of converting these to class A, I think that may be worth the work if it improves the sound stage and overall quality of the sound. I can only get my Adcom GFP-565 to a bit less than 1/2 volume so maybe class A would be beneficial.

On a scale of 1-10 for electronics knowledge, I'd say I'm a 2 :(. But I am so impressed with how you all help each other here, I'm willing to learn. Bare with me!!

Can anyone tell me what I should look at to determine why I'm getting the blown sound out of the amp. Since I'm a beginner (sorry in advance), but I'm very technical (In IT for 25+ years) and careful, I can follow instructions and learn as I go.

What should I look at to determine why voices make the speaker sound blown. I have a meter.
What caps should I replace and where should I buy them? Sounds like the 4 LARGE ones and I'll bet some on the output boards?
When running this amp, the temperature of the heat sinks are barely warm to the touch. I'm reading they should be much warmer. I have a hand held laser temperature meter and should be able to set the bias with that.
Should I convert this to class A? I love the sound so much of this amp that I'm afraid that changing it may alter my love for it. Hopefully, for the better :)

I've attached some pics. When responding, please guide me like I'm 5 years old. that will not only help me, but others that run across this thread.

Here is a link to a photo album of all the pics of this amp.
Forte Model 3 Three Amplifier Photos - Google Photos

Thank you in advance for your help!!!
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@Zen Mod

Ok. Very nice!!! So I tested. Results are below. On the bad side of the amp, one of the resistors showed 0. And then it went to a higher measurement. then the top leg came off. I was figuring it was a cold solder joint but now I know.

Meter set to VDC 200mv
Left side (good side)
Top Row: 43.3, 50.1, 43.3, 41.1, 47.8
Bottom Row: 34.8, 41.1, 47.7, 30.9, 71.1

Right side (bad side)
Top Row: 66.0, 56.4, 50.3, 0, 39.5
Bottom Row: 47.5, 46.6, 47.2, 38.9, 42.2

Here is a picture of the broken one: Shared album - Ed Rockwell - Google Photos

Do all the values look good except for the one that came off?
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gone mobile...

Ok. I have not dismantled transistors from a heatshik. I may be able to find it online but a short explanation would be great. I can follow instructions. I've built and upgraded many PC's with CPU's. This looks a bit different but can figure it out.

I will attach previous pics to posts when I get home tomorrow.

Official Court Jester
Joined 2003
Paid Member
big ones - just unbolt and you're good - no special process with putting them back , because they have silycone pads , no goop

smaller ones - also unbo;t , but you need to clean them , mica pad and heatsink from old goop and apply new, when putting everything back ( at least , by pictures it seems they have white goop applied)

idea is to dismantle bad channel pcb and inspect all solder joints and renew them; if in doubt , just suck old solder and solder again .....

post pics of copper side , when you are in process

one thing more - I hate to see bolt directly on transistor case , without any washer ..... if bolt length permits - put both big washer and split washer , on each ttransistor
Sounds good. Are there any caps on that board or other components I should verify or replace? I know caps come in different sizes/shapes so I figure since I'm in there, may as well do them. I can't identify every component :( yet :). But I will check every solder joint and fix any that are suspect.

Expect a response later today. Just woke up and have some chores to do before I get to play with toys.

Here are photos of the copper side (as you call it). I'll inspect closer as well. Should I replace/verify anything else before I fix the resistor and put washer/lock on the transistor case and button it back up? I'll move on to the other side and verify the soldier joints there as well.

I found 3 other cold solder joints that I repaired as well.

Per request, see attached pics.


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Official Court Jester
Joined 2003
Paid Member
don't forget to clear old/put new thermal goop to smaller transistors , and do the same for these thermistors (blob with long legs) , straighten their legs a little to be sure that they're in contact with heatsink when everything is in place

after montage , check mid pin of each big transistor for shorts with heatsink

if good , power up , preferably with Bulb tester , for first time


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ZM has all good advice.

Did the large transistors have 'goop' on the insulators? (looks like it) What type of insulators are they?

edit: If I were you I'd not worry about going too deep replacing things. Get it going then worry about that.
All the caps look film and silver mica (aren't they?). If so, I'd leave them at least for now....
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Thank you ZM for your help!!! All went as planned. Since I work in the computer field, I had heatsink compound. Used that on the small transistors and the thermistors (the thingy with legs that was just leaning against the heatsink).

The large transistors I didn't add any compound to. If there was some on them, it may have been part of the silicone pads.

On the right (bad) channel, I think I fixed something like 4-5 cold solder joints. On the left (goodish) channel, I fixed 3-4 (I think) bad solder joints.

I had a lightbulb ready to go but... Forgot to put it inline. But, I was very careful and remembered what I had to take apart (with lots of pics).

I hooked it back up and she just sang like the old days!! I listened for a while with a critical ear and did not notice any issues.

I really really appreciate the help! I'll be donating to the forum shortly :)

Now... the one question I have... Still wondering if I should convert her to Class A. Currently, I can never listen beyond 4 on the volume anyway. Will this amp in Class A get better than it already is?

Maybe this is a question for @Nelson Pass? If you guys think I should start another thread for this, just let me know. I can do that :)

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