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Old 8th October 2003, 03:16 PM   #1
qboneus is offline qboneus  United States
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Default Newbie with a Class A amp problem please help

Hello, my name is Tal and I am an audioholic-
Ok now that thats out of the way-

I have a Musical Fidelity a100 amp that I just purchased.
I am having lots of problems keeping this guy running.
This is a timeline of this amp and it's failures last night.

Power on. Volume at 11:00. heatsink temperature 98 degrees F. Wired into NHT SB3's.
15 minutes amp powers off. fan shuts down.power indicator goes out. temp of heatsink 155 deg. F 68.9deg C

8 minutes later amp restarts. fan cycles on.Indicator lights up. HS temp 129deg F 54 deg C

Volume knob reduced to 9:00
8 minutes later amp powers off. Fan,indicator cycle off. HS temp 156 deg F 68.4 C

repeat, repeat
It does this with my Vandersteen 2ci's as well.

Anybody have any knowledge of these amps and failures like this?
any guesses?

I am at a loss.
Thanks in advance for any ideas you may have to share.

Tal
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Old 8th October 2003, 10:04 PM   #2
DrG is offline DrG  South Africa
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Has the amp had any repairs/modifications as far as you can tell?
If so I'd compare the innards with the circuit schematic which is floating around on one of these threads.

I had an A100, no probs. Then I had the A100X - larger case/heat sink and no fan. No probs again.
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Old 8th October 2003, 10:36 PM   #3
qboneus is offline qboneus  United States
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not to my knowledge I will have to ask the previous owner.
I just got it two days ago and didn't really get a chance to check it out until yesterday.

At lunch I tried the following tests:

With the Amp volume at Zero with sources applied to inputs and speaker cables connected after 10 minutes:

front half of heat sink temp-139 deg F
Back half of heat sink temp-143.5 deg F
Left channel 13.7 mv dc offset
Right channel 2.0 mv dc offset

Retest after 10 minutes:
front half of heat sink temp- 150 deg F
Back half of heat sink temp- 157 deg F
Amplifier shut off before dc offset was measured

Sound is good up until amp shuts down.

I am going to test this evening again with sources removed and then again with spk cables removed.

see what happens??
Thanks for your time,
Tal




Thanks for your input.
Tal
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Old 9th October 2003, 06:37 PM   #4
qboneus is offline qboneus  United States
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Further tests revealed a bias current of 1.2 AMPS avg per transistor.
And voltages in +12.9/-13.4??

And it shuts down regardless of the input/Spk cables being disconected.

Still trying....
Tal
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Old 9th October 2003, 08:06 PM   #5
DrG is offline DrG  South Africa
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I wonder if the bias circuitry is healthy? Sounds like thermal runaway causing the protection circuit to kick in... How does the bias current vary from turn-on to shutdown?
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Old 9th October 2003, 08:44 PM   #6
qboneus is offline qboneus  United States
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all of the times we measured it is stayed consistently within the 1.15-1.25 range.
I know that MF was using high current to achieve the class A rating but 1.2 seems huge to me.
My tech said that this was the highest reading he has ever got in thousands of bias measurements over the last 20+ years

Further-shouldn't the Zener's regulate this voltage at +-12v?
we were beginning to wonder if the excessively high current was being helped along by failing zeners.

There are a couple of electrolytic caps that are suspect but would bad caps cause this high of current?

We are still unsure of exactly what type of bias current we SHOULD in actuality be finding?

Any Ideas?

Thanks for your time,
Tal
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Old 10th October 2003, 09:17 PM   #7
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Quote:
Originally posted by qboneus
[B]all of the times we measured it is stayed consistently within the 1.15-1.25 range.
I know that MF was using high current to achieve the class A rating but 1.2 seems huge to me.
My tech said that this was the highest reading he has ever got in thousands of bias measurements over the last 20+ years
Well, I'll like to have your tech over here: my first JLH Amp ran 1.9Amps per transistor @ 22V. My current JLH-Amp runs (now) 2A@28.5V but will run 3A @28.5V at a later stage (with 2 transistors sharing current).

To be more accurate: the JLH uses a dual rail system like MF. The first version is rated about 14W Class A, the second one will produce roughly double that.
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Old 10th October 2003, 09:33 PM   #8
qboneus is offline qboneus  United States
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So do you think this is normal Bias current for this amp?
I am having a really hard time finding any info on this amp.
Any Info helps a lot.
Thanks,
Tal
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Old 4th November 2003, 08:32 PM   #9
qboneus is offline qboneus  United States
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I re-capped and re-resistored the A100 and now she runs right.

The left fan is causing me some hesitation does anyone know where these fans can be bought in the states?

Thanks for your help
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