Forte Model 3 - One channel running hot

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Hello Forum -

I have enjoyed my Forte Model 3 running it into a pair of Magnepan 1.6 with great results. The amp was left one night and since then I noticed the left channel running quite hot (hot to the touch) compared to the right. I have not used it since with the exception to get you theses images. I have not messed with the bias but it looks like that adjustment has slipped (? as visually inspected by the amateur). I have not been able to find what test point I should use to compare the two sides to decide if a bias adjustment is the next step. Note, the IR images is after 5 seconds on time. Any help would be greatly appreciated.
 

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Forte Model 3 left heat sink running hot.

OK, finally got around to getting some voltage drops over the large, high wattage resistors (grey) and comparing hot side to cool side. Please note that I aggressively cooled with heavy duty force air the hot channel (left) and was not able to confirm left side still gets hot-to-the-touch for that reason. I didn’t want to stress a weak component further if is on its way out, I guess.

The voltage drops didn’t seem off to me and the differences were relatively minor between right and left channel. See what you think.

I numerated the resistors 1 -2 - . . . top row, front to back of the amp the starting at 6 -10 bottom row, front to back if that has any bearing in the analysis.

Hope this makes sense and thank you very much for the responses. Looking forward to learning more!!
 

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Official Court Jester
Joined 2003
Paid Member
now I'm realllllly puzzled , too ?!?!?

it's against any logic

can you measure that "hot to touch" with modern thingie - "laser" thermometer?

maybe difference is factually small , but our palms being hysteric .....

as guide , Papa's table attached
 

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PRR

Member
Joined 2003
Paid Member
...can you measure that "hot to touch" with modern thingie - "laser" thermometer?...

His 4th image IS a "laser thermometer". Actually an IR camera. The usual laser-tool measures one spot, the FLIR measures the whole scene at once. It may have a way to put numbers on a specific spot, which may be of interest. However the mellow blue on one side, mild pink on the transformer, and bright red-yellow on the other side, sure shows there is a BIG difference.

Yet not reflected in the emitter(?) resistor voltage drops.

I'd want supply rail voltages. Maybe the hot side is correct and the cool side is not getting full supply voltage.
 
Left channel WAS hot

Thank you very much for all the responses!

I finally have some actual temperature from the heat sinks. In addition, I have resistance and subsequent current values for all the high wattage resistors assuming a measurement is not affected by neighboring components in the circuit. The resistances were measure cold and power off!

Sure looks good to me and indicates there’s nothing wrong with this amplifier. In addition, all this attention has managed to make the problem go away. This would have been quite frustrating if it was not for a lose screw I discovered during a check-all-connection exercise as suggested by Colin123 earlier.
Yes, the amp was re-capped prior to me getting it and the shop must have missed it during re-assembly.

I will hang my hat on that being a fix for now. Both sides run just warm at 42 and 43 C after 5-minute run time. Hand warmers versus bleeding hot!

Please note, the IR images are self-ranging to show that widest spread in color. Earlier images were taken when significantly hotter, but I failed to turn on the actual temp readout function.

I was also speculating that it might be oscillation set this off and that I could test this by shorting the input (?) but I will forgo this for now.

Finally, I noticed that both my thermistors (I think they are) are floating off the heat sink. The paste gave away over time. Any suggestions for best practice to re-attaching them?

Again, thank you for your help!
 

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I am so glad to see this recent post. I have the same problem likely caused by me. In short, I replaced the power capacitors in my model 3, as well as replacing the biasing pots and input IC's (there had been issues stated about them making a popping sound, and one of my channels was doing just that). Turns out, I had to mount the pot in one channel upside down compared to the normal position. This is so I could get a screwdriver to it. My 'assumption' was that it would work CCW as opposed to the normal CW for adjustment. Well, now one channel is right where I like it at 45C, but the other is only slightly warm. Sounds like I may be taking the pot out and finding a way to access it in it's normal position?
 
Forte' 3 Photos

Bear with me. I am not so computer literate as most people.
Ok so these photos are a bit random. what photos would do the most good?

SORRY GUYS, THESE ARE 'BEFORE' PHOTO'S OF THE BOARD REBUILD.
 

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Forte' 3 fuses

I suppose this is progress of a sort. I discovered a blown fuse on the negative rail of the right channel. Turns out that the fuse was a 5 amp fuse, and i am unsure if that is the proper amperage. Seems that an 8 amp fuse was the original size. So, after installing a new fuse, the main fuse blew (8 amp). I stopped there because the universe is trying to tell me something. Any thoughts? FWIW, both rails were seriously drawn down before fuse failure.
 
A little update. I have removed the right channel from the chassis. This is one aspect of this amp that is attractive. With just a few bolts, out comes the channel in one piece, making getting to parts and measurements much easier.
I will start some resistance measurements in the morning.
 
Since the positive and negative voltages check out for each channel at 78v neg, and 78v pos, I will say that the power supply capacitors are installed correctly. For each channel they are in series, but in opposite polarities for each rail.

I reinstalled the right channel, and there is no trouble now with the fuses blowing, or the power supply rails being too low. I am back to one channel running at the correct temp of 43C, and the other being cold. I adjusted the 5K pot to match the left channel (about 2,200 ohms).

FWIW, the input transistors are running pretty warm at about 41C. , and the power input transistors running at about 29C.
 
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