Forte Model 3 Bias

Zero Cool

Member
Paid Member
2004-09-20 6:10 am
MN
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
 
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! :)
 
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
 
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.:):)
 

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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?