Forte Model 3 Bias

[Zero Cool]

I have a Forte Model 3 on my bench that has bad Bias trimmers, I replaced them with 20 turn pots as everything i have read says the bias is very touchy. this definitely made the bias much easier to precisely adjust BUT, the Owners/Service manual states 70mv across the .47 ohm emitter resistors. but at 45-50mv after 20 minutes the heatsinks are both at 50deg C! I didn't dare push it past that point without getting some clarification.

Any advice?


Zc

 

[Zero Cool]

I figured SOMEBODY would have something to say?? I can't imagine this thing running at 70mv of bias, it would be way too hot I think??

the sinks run at 50deg C now. was this amp supposed to run hotter than that?


Zc

 

[Zen Mod]

if thingie is having anything with Pa , you know that 45-50C is magic figure ...... more than exact Iq value

 

[RobertS61]

Zen Mod is right as usual.
NP did not find my 56 degrees with my SA/1's any problem.
Fifty degrees Celcius is just fine.

 

[Badbrad]

I have a model 3 running very cool as well. I'm ready to rebias it but would love a bit of help. Does anyone have a schematic of a model 3 or is willing to help a bit. Have meters, a digital scope & ready to go... just want to be careful.

 

[cumbb]

No problem to run these amps with lower bias. As you like;-)

 

[Nelson Pass]

As posted reply to Pass Labs thread, the Model 3 schematic is virtually identical
to the model 1, mostly supply and bias difference.

 

[cumbb]

Forté Audio Model 3: caps and schematics?

Forté Audio Model 3: caps and schematics?

 

[Badbrad]

Thank you Mr. Pass. I just wanted to make extra sure I didn't let the smoke out of my Forte. I have no spare amplifiers kicking around right now.

 

[Badbrad]

OK Forte lovers and audiophiles. Can anyone give me a scientific, not an artsy answer to the differences I should hear running my Forte Model 3 with high or low bias voltages? I listen on a set of Magneplaner 1.7is with a REL T9i sub. Thanks in advance.

 

[Badbrad]

Hi, you seem to know lots about these Model 3 amplifiers. Mine is working fine, but with 30 year old electrolytic caps in the power supply. Is there any reason I wouldn't want to bump them up to 47,000uf 63V from the 22,000uf 45v? I guess the power switch might not like it too much, but any other issues I should consider?

 

[hrad]

I could use some advise. I've been enjoying my forte 3 after Stasis 2 blew. I've noticed it is always a lot cooler and when I power it off about 20 seconds later there is a loud pop in one speaker. I noticed the led on front turning off about that time. Is there a large capacitor that takes that much time to dissipate? Could it be faulty? Will this damage speaker? If I have it serviced what should be replaced and recommended parts?
I know this is a lot to ask, thanks for any help!

 

[stereohifi]

Model 3 power transformer primary

Is all Model 3 existing are all with double primaries, 0-115V and 0-115V ?
So the unit can be converted to euro 230v or what I intent to do on 120v here in N. America, turn down the DC voltage and increase the bias to have like Model 1 a 50 watts in Class A.
I have an eye on a Model 3 I like to get but like to know if all power transformers were from the the same supply or for Canadian/US electrical codes needed to be different?
Of course not without have a maintenance done on main boards to replace bias pots and check everything else like caps....
Thanks

 

[stereohifi]

Forte Model 3 updated

Finally did the bias pots replacement with 5kohms 15 turns Bourns type.
Rechecked mica and thermal paste on transistors.
Checked also Mepco/ Centralb 24000uF/45 v power supply caps, Values and ESR are very good surprising!
Installed some decoupling 0,1uF/100v on each.
They keep their charges for hours with any load on them, no leakage...
Reinstalled everything,and slowly variac and gets +/- 75Vdc that feeds the power boards.
Tested fine 0,02% THD at 1 watt and max power got measured 250 Watts per ch. both driven at 400Hz.
Bias is solid and plays around 40mV per emitter resistors legs (0,47 ohms) output transistors.
Waiting for a laser temperature reader to fine tune but now for 2-3 hours it keeps warm and cosy.
No noise and it sounds very good, will install it in my hifi setup to hear all of his sweetness.

 

[Captouch]

Finally did the bias pots replacement with 5kohms 15 turns Bourns type.
Rechecked mica and thermal paste on transistors.
Checked also Mepco/ Centralb 24000uF/45 v power supply caps, Values and ESR are very good surprising!
Installed some decoupling 0,1uF/100v on each.
They keep their charges for hours with any load on them, no leakage...
Reinstalled everything,and slowly variac and gets +/- 75Vdc that feeds the power boards.
Tested fine 0,02% THD at 1 watt and max power got measured 250 Watts per ch. both driven at 400Hz.
Bias is solid and plays around 40mV per emitter resistors legs (0,47 ohms) output transistors.
Waiting for a laser temperature reader to fine tune but now for 2-3 hours it keeps warm and cosy.
No noise and it sounds very good, will install it in my hifi setup to hear all of his sweetness.

Timely post for me, as I was thinking about doing the same thing. Do you have a P/N for the Bourns trimmers you used?

 

[stereohifi]

Bourns 5Kohms 15turns trimpot for bias orte Model 3.

Hi, you may find the same reference in digikey.com or mouser.com .



3006P-1-502LF Bourns Inc. | Potentiometers, Variable Resistors | DigiKey

 

[Captouch]

Hi, you may find the same reference in digikey.com or mouser.com .



3006P-1-502LF Bourns Inc. | Potentiometers, Variable Resistors | DigiKey

Thanks very much!