Any Forte Audio owners and fans?

OK, now you've jogged my memory on how it looks inside. There are actually 5 sets of emitter resistors - 10 total. They are the larger gray bodied resistors. They will all measure a little differently. In my case, I measured them all and set bias so the average was roughly right. But I've also read where people didn't even measure the emitter resistor voltages and used temperature as an indication of correct bias. I double checked my temperature (using one of those laser point and shoot temperature gauges) to make sure I was in the ballpark.

Actually, before you do anything, I think establishing a baseline is a good idea. It's helpful to know what the amp seems to be doing before you actually touch or change anything. I'd turn on the amp and let it warm up (you don't need to run a signal to it or even have speakers connected). See how the heatsinks feel. If either are getting too hot right away, turn it off. But I'm curious what the bias conditions are like now and the heat of the heat sinks will give you an idea of whether the bias current is low (cold heatsinks) or high (too hot heatsinks). As mentioned, mine were stone cold but the amp still sounded fine. But I've worked on receivers where the bias setting was too low and it did make the channel sound bad and correctly setting bias make the channel sound normal again.

After you get a baseline, unplug the amp, note the position of the bias pots, exercise the pots then set back to the original position, clamp your probes, double-check everthing, then you can plug in, turn the amp on, and measure the voltage across the emitter resistors. Have your screwdriver ready to adjust the pots. Especially if the heat sinks were too hot when you checked your baseline/current state, you're going to want to turn the bias pot(s) down right away. Turn slowly and observe the voltage change. You're really concerned about absolute magnitude - if the voltage reads negative, you probably just clamped the check points in reverse of the way the current is flowing, but that's no big deal.

I drew green boxes around your two bias pots. There's one on each board. I drew a blue box around just one of the 10 emitter resistors. You would clamp to each side of one (or more if you wanted to measure more than one) of those resistors. Luckily, as you can see, they're pretty easy to access and I'd just clip onto the top one of the pair instead of trying to get to the bottom ones. Since there's only one single bias pot that's going to control the bias current to all of those output transistors, you're not going to be able to get them all spot on anyway - you just want to be in the ballpark.

To keep things simple to start with, I'd just focus on 1 or 2 emitter resistors on each side and see how they measure. I'd set to say 50mV to start with (instead of going to 70mV) and leave it for a bit and see how the heat sinks feel. Should be pretty warm, but you should be able to keep your hand on them comfortably for as long as you'd really like. If you do have one of those laser temperature things, shoot it at the heatsinks and you want to be somewhere around 50C ballpark. Keep in mind that it will take some time for the heat sinks to change temp once you make a change. And even once you get it to where you want it, you should monitor it after further warm up to make sure the values have stayed the same after warm up.


View attachment 977619

I measured each. The left channel is cold and measured 9.8 mv and the right is perfect temp and measures in the mid 20s. I turned the left pot and then measured again. The mv kept on climbing. I tried to turn it back a little but couldn't get my tool to work and now the left channel just measures 0mv. Still cold. Did I mess something up? Turned pot maybe 15 degrees clockwise
 
You exercised the pots pretty good before starting?

15 degrees isn't a huge amount, so i doubt you damaged something.

With the amp off, you may try exercising the pot again really good - end to end maybe 15-20 times, then reset to original position.

Also, how high did the mV climb when you first adjusted it?
 
You exercised the pots pretty good before starting?

15 degrees isn't a huge amount, so i doubt you damaged something.

With the amp off, you may try exercising the pot again really good - end to end maybe 15-20 times, then reset to original position.

Also, how high did the mV climb when you first adjusted it?

OK, turned pot counter clockwise and it came back to life.

Now left measures ~positive 55mv and right measures ~negative 25mv. Both are quite warm so I'm assuming right channel resistors are reversed or something.

Any thoughts?

Thanks!
 
Yeah, seems ok and good sign that they're both warm. I would fiddle with the setting to get them a little closer to each other. Maybe make both 50mV or so. Leave for 10 min and make sure they stay around the same level. If they do, then turn off the amp, disconnect all your probes and such, and fire it back up and see if the distortion is gone.
 
Yeah, seems ok and good sign that they're both warm. I would fiddle with the setting to get them a little closer to each other. Maybe make both 50mV or so. Leave for 10 min and make sure they stay around the same level. If they do, then turn off the amp, disconnect all your probes and such, and fire it back up and see if the distortion is gone.

You've been a tremendous help! I adjusted the voltage to approx 50 on both channels. Heat sinks warmed promptly to relatively equal temps. Almost too hot to touch but not quite. When I get home tonight I'll fire it up and see if problem is solved!
 
It works! You, sir, saved me so much money and heartache! Thanks a ton!

Awesome! Great to hear it. Very happy for you.

Just feel the heatsinks now and then. If they get cool again, you can either exercise the pots again or replace the adjustment pots.

Let us know how it sounds/works in your system.
 
Glad to hear positive reviews. Not many reviews to be found on this amp with a cursory search. Took me some time to find this thread.
I'm running mine with a GFP-555II Preamp (Until I track down a pass pre... Hopefully Forte as well). My front end is a MINTY set of Infinity Kappa 7.1 series 2's, and the model 3 has really opened up the soundstage. The best part is even with the bigger (wider and deeper) sound, it has improved the imaging as well. These two qualities have been tradeoffs for me in the past. Very pleased. Can't wait to find a better preamp! And maybe a second model 3 to biamp.
 
I have just acquired a Model 3 and it is not warm at all, so thanks for the how-to you made, captouch. It going to power my Kef 107s and I hope it will do a better job than my Sansui 9900Z which I feel is lacking a little in the lower region.
 
I put my Forte Model 55 100 wpc class A/B back in the system. I checked the DC offset and one channel was about 5 mV but the other was over 50 mV. The amp boards each have two pots. I e-mailed Jay asking for help in identifying the offset and bias pots. Honestly, I didn't expect to hear back from him, but he did respond. I was able to adjust the offset to very close to 0 mV for each channel. For bias he just gave a heatsink temp to shoot for. My heatsinks are both equal and only about 45 C which is close to what he said should be 50 C or about 50mV across the emitter resistors, so all is good. The only service my amp has had was a bridge diode replacement after it started blowing the main 7 amp fuse several years ago.
 
Note: the photo of my model 3 has gray caps and the 1A has blue caps.

I'm attempting to convert my Forte model 3 into class A mode and need advice. I have a good photo of the Forte 1A which has all the same parts as the 3 and is 50wpc class A. The 3 is 200wpc class AB. Simply going from that photo and a brief reply on a forum from several years back by Nelson Pass, I am trying to accomplish this with limited understanding of electrical engineering. Pass said that if you "tap the transformer for 240V and run it at 120 while increasing the bias" you will have it in class A. So in this photo, the main differences I see are 1) the secondary leads of the transformer are swapped so that they go into those square terminals on the floor of the chassis on the opposite side when compared to mine(note the yellow leads and red leads near the very bottom of the photo, plugged into those square terminals) . 2) the capacitors are wired differently (there's no rail connecting the left and right caps). 3) the entire transformer is flipped upside down (probably due to the leads being short and not being able to reach otherwise). My question: if I wire the model 3 to look just like the 1A (and I mean just like it) and adjust the bias, will I have success? What other recommendations (besides hiring a professional) does anyone have? I have a DMM and and my father is an electrical engineer, but I need advice from someone who's tinkered with these things.

Screenshot_20180304-171514.png 400316-forte_audio_1a__50_watts_pure_class_a__by_neilson_pass.jpg
 
captouch, thanks for that in depth instruction on how to fix the bias on this amp. I picked one up a few days ago and it was stone cold. I followed your instruction and have the heat sinks 47-49C. I exercised and set the pots while checking the mV of a few of the resistors.
Is it imperative that each heat sink be the same temp? The left side is 47c and the right is 48/49. Thanks again, your instruction was detailed and easy to follow.
 
Not to be rude but forte model 3 be able to handle a pair of speakers that dips to 2.5 ohms?
 

Attachments

  • AEF51E4D-DF33-4246-8693-4B274619FE2B.jpeg
    AEF51E4D-DF33-4246-8693-4B274619FE2B.jpeg
    24.4 KB · Views: 6
I had a Model 5, might have been a 6. I probably shouldn't have sold it. At the time it was bested by a new Coda 10.
 
Not to be rude but forte model 3 be able to handle a pair of speakers that dips to 2.5 ohms?
Yes. Both Model 3 and Model 6 handled my Infinity RS-IIb's, which dip down to less than 3 ohms, with no problem.
 
Hello, everyone,
I currently have a model 3 on my bench. Customer complaint is one heat sink is cold, the other warm. Upon removal of the top cover, I only see one pot on each amp board. Most often I see two. One for DC offset, and one for idle current. Thought I would come to this forum to see if there is any information on this. Indeed, no DC offset adjustment, so the single pot is indeed the idle current adjustment, or as is mentioned here "bias". One thing that is not mentioned here is to apply some deoxit to the pots before exercising them. Set the amp on it's back, with the front panel facing up. Put a couple of drops of deoxit fader right on top of the adjustment screws, and let the amp sit this way for a while to let the deoxit soak down into the pot. After this, exercise the pot full turns back and forth 20 times or so. Then set the pot back to it's original setting. Now you're ready to make adjustments as described in this thread. Good luck!
 
I put my Forte Model 55 100 wpc class A/B back in the system. I checked the DC offset and one channel was about 5 mV but the other was over 50 mV. The amp boards each have two pots. I e-mailed Jay asking for help in identifying the offset and bias pots. Honestly, I didn't expect to hear back from him, but he did respond. I was able to adjust the offset to very close to 0 mV for each channel. For bias he just gave a heatsink temp to shoot for. My heatsinks are both equal and only about 45 C which is close to what he said should be 50 C or about 50mV across the emitter resistors, so all is good. The only service my amp has had was a bridge diode replacement after it started blowing the main 7 amp fuse several years ago.
Hi Eickwewg.. i have model 55 to, can you identifying pots bias and offset? My model 55 heatsink seems not equally in the same temp
 
Back
Top Bottom