HELP .... Aleph5

[mefinnis]

My Aleph5 has been working fine for quite some time, however I was irritated by a slight buzz from the toroid, so I decided to replace the 650VA unit with 2 x 500VA units.

After turning-on the mains fuse blew, then on repeat power-up the left channel was gone. (I had to do some drilling and I think a filing may have found its way under the PCB).

Anyhow, I'm now at the hair-pulling frustration stage. Basically I get about 3-4V under the positive rail at the output.

Removing the PCB so all of the output devices are free they all check OK (measuring Vgs per Nelson's article). I have replaced all other semiconductors on the boards (even though they measured OK in situ ....... I was getting desperate!)

My questions, for those with better EE theory than me (which is >90% of the list):

1. Is it possible for the output devices to be blown but still give a correct reading on Vgs using the set-up in Nelson's article?

2. Can anyone think of another reason we might have 30V at the output?

Thanks

Mark

PS: And yes .....that slight buzz really wasn't that bad

[wuffwaff]

Hi Mark,

I´m feeling verry sorry for you .

If you´ve got 30 volts plus on the output the fets are probably allright (otherwise it would be 34 volts)

If you have replaced all other semiconductors you must check the sense connections from the active current source. When this is disconnected in any way you will get your 30 volts.

hope this helps,

william

[mefinnis]

The "thot plickens" ......

Measuring circuit voltages:

Z5 = 9V / R11 = 5.2V
Q1/Q2(S) = -5V to ground (cf +4V)
dV across R14 = 0

Checking R14 = open circuit !!!

I have never seen a metal film resistor blow without any visible sign of stress externally.

Checking other resistors, R19 is also open circuit.

Off to the shop tomorrow

You see something new every day!

mark

[Eric]

Mark, sorry to hear of your troubles! While I was working on my a40, I managed to toast several Mills wire wound resistors that showed absolutely no outward signs of trouble. When they failed, they formed an open circuit also. I never thought to check them until I tracking down the source of some distortion...

[Audiofanatic]

Hi Mark,

Sorry to hear about you amp.
Have you try to put the 600VA transformer back in?

I have no idea what could be the problem.

I hope you'll find out soon.
All the best.

Audiofanatic

[Duck-Twacy]

Damn, that is bad luck, such a nice amp.

Maybe time to start building and AlephX??

[mefinnis]

Thanks for those who replied. All is now well after 4 days of hell!

I ran around in circles unable to make head nor tail of the problem and wound up at the point where I had floating DC at the output, ranging from -2 to +3V.

All of the input voltages were OK - I-source / diff pair / R19.

Output FETs Vgs = OK. dV across the bias resistors also varying erratically over time, like the dV at the output. It was as though the sense circuit was having intermittent trouble picking-up input.

could there be a dry solder joint, or could there be a microscopic flaw in one of the PCB tracks.

In absolute desperation I resoldered every joint and tinned every track on the PCB so it is now a shiny silver ......... CURED !!!!!

I was about 5 nanoseconds from making a whole new PCB from scratch

cheers
mark

PS: Yeah ...... I've been watching the X-threads with great interest. Though, I'm cured from tinkering for a little while, I might just sit back (with a nice glass of red) and listen to some music

[Audiofanatic]

Mark,

Good for you! I was shure you would be able to repair it.

I'm happy for you. (I'll take a white one tonight)

Audiofanatic

[TjongKristian]

Hi Mark,
I am just curious, does the amp better after you tinned the PCB or is it the same? What kind of Solder are you using? Silver? THanks.

[Mark A. Gulbrandsen]

Yea, how does it sound now? I have been toying with the idea of doing that same thing to my 2's..... or possibly throwing the PCB away completely and giong P to P.
Mark