Newbie with a Class A amp problem please help

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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
 
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.
 
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
 
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
:boggled:
 
qboneus said:
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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