Forte Audio 1A problems... PLEASE HELP!!
I picked up a Forte Audio 1A power amp today and heard it in the seller's setup, and it sounded great. Absolutely quiet, wonderful. I get it home, hook it up, and I have major problems.
First, I now have a hiss and buzz in both channels. Without a source connected, I have a really LOUD buzz in both channels. With a source connected, the hiss/buzz isn't so bad, but it is quite audible at a normal listening level (say, 70dB). It is audible above the music at this level.
Next problem... When I shut the amp down, the right channel dies out shortly after power down, probably two or three seconds. This seems normal for a Class A amp. BUT, the left channel lingers for about 30 seconds playing music, even after the power cord is removed from the wall socket. Additionally, a -3V DC offset is present on the outputs of the left channel with power off. When powered on, no offset.
And, to top everything off, the left channel does not get hot at all!! This one I don't fully get. It plays at the same level as the right, but doesn't get hot at all.
So, does anyone know what this could be? I am just dying to know... I bought this amp with the hopes that I'd get by without having to do any work to it, but apparently I was wrong.
Could the power caps for the left channel be going out?
Please... someone... help. :( I cannot describe how upset I am right now... :bawling:
Re: Forte Audio 1A problems... PLEASE HELP!!
The hum problem sounds like a ground loop between the source and amp. You may find it helpful to read the article on ground loops available for download here.
The other symptoms you describe point to a loss of bias on the left channel output stage. The most likely cause is an open wiper on the bias adjustment pot due to oxidation over the years. If you want to explore this yourself, first unplug the amp and remove the top cover. Note the AC entry point at the center of the rear panel. Keep your hands and tools clear of this area at all times.
Next, locate the left channel PC board attached the left heat sink. You will see 10 large plastic power transistors attached to the sink, each having a corresponding small power resistor soldered to the PC board. Also on the PC board is the bias pot, a plastic part about 1/2" square with a screwdriver port (slotted disc).
Attach a DC voltmeter across one of the aforementioned power resistors and, if the meter is not auto-ranging, set it to measure millivolts (mV). Plug the amp back in, turn it on, and note the DC reading. It will probably be close to zero instead of the normal value for this amp of about 120mV.
Using a suitably sized screwdriver, SLOWLY rotate the bias pot in either direction while watching the results on the meter. If you can get the reading to around 120mV, check the voltages across the remaining nine power resistors and confirm that they give readings at least within 50% of this value.
If this checks out, leave the meter connected, slide the top cover back on tempoarily, and let the amp warm up for about a half hour. Recheck the bias and readjust as needed to get an average 120mV reading across all 10 resistors. You might want to check the bias on the right channel at this point as well, just to confirm that it's in the ballpark.
If you have trouble making a stable bias adjustment, it may be time to replace the bias pot or go looking for another problem, but chances are that this will do the trick. Good luck!
THANK YOU, THANK YOU!
That precisely did the trick! The bias pot was reading 0L on my meter, so I moved it to one side and then back to middle and adjusted it to match the resistance on the other side, fired it up and made a few more minute changes to get the bias to match the other channel. I am now hanging around 130-145mV, while before it was reading 0mV on the left channel.
I am just wondering... Do you, by chance, know what one should expect to pay for a Forte Audio 1A amp?
One sold for 550.00 a while back.
Cleaning out my closet last spring. Sold mine for 550.00. That amp is a bargain at that range. 50 watts of real class A power. And designed by Nelson to boot.
The wiper pot is a common issue now. Those pots are getting pretty old. Move both around to clean the contacts. New pots would be better.
Since my power needs are low, the bias was set artound 100 mv. It never was outputting more than a watt, the full bias was only wasting electricity.
The one I had was quiet, and pretty good sounding. It would be hard to find anything else in that price range that is as good.
These units may not age as well as an AB amp. All those hours cooking may degrade the supply caps or output transistors. That is a small penalty to pay for such great sound.
The Model 1 i had did an msrp of $950 in 1987, don't think you'll be interested in what the price was overhere.(a lot more !)
The 1 had 2 output devices less than the model 1A, and Toshibas instead of Sankens(40/70 watts nominal)
btw, AudioXpress had a 2-piece article for a rebuild of the model 1A in 2003
Thank you all. After warming up, the left channel bias is now sitting a little high at 170mV, so I am going to turn it down a tad. Since my speakers are quite efficient (102dB 1W/m), I may turn the bias down even more. I may try 100mV like Panelhead did.
I feel much better about the price now, too. I paid less than the $550 the other sold for, so I feel I did pretty well.
I will definitely look for the AudioXpress article.
I feel so much better now that it is running and making great music! Thanks all!
Whadya doing with that Forte? I thought you were going to build an Aleph!!
Impulse buy... What can I say? :D
Does anyone happen to have a schematic for this amp?
I had one, also I think there is one in the owners manual. That may be why I got 550.00 for mine, had an owners manual. Also seems like I saw the article in Audioexpress, it may have had a circuit diagram. But it was swapping out the Sankens for some mosfets.
My speakers are 100 dB efficient, it was plenty quiet on them. I just wish it had a single pair of outputs. That is all I ever need.
Also, the 1A is a little better than the 1. The 1A has a fet input. The 1 was bipolar all the way. Did not know about the outputs, seemed the the 1A had five pairs per channel.
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