aleph 5 wierd voltage p2pversion

[george a]

Hi All

The saga continues

I've now built an A5 effectively point to point -(basically laid out the circuit straight onto perf board and voltages are still much like those of the pcb version.

Q4 & 5 are MPSA42

Q4 has 9.9V across it rather than 4-5 v

Q5 HAS 12.9 v Across it rather than 4-5v - it takes about a minute to reach this level.

The circuit is complete except NO output mosfets are installed.
and C5 is shorted to ground.

Are my wierd voltage readings due to output mosfets not being installed?????

I'm not game to install them to find out in case I lose more of them (5 dead so far)

Also how hot are the 9610's supposed to get ??

Supply Voltage is 36v
and R11 measures at 5.5 V instead of 4-5V - Drain of Q3 measures 3.5V instead of 4V.

Can anybody help please??

[wuffwaff]

Hi,

Q4 and 5 can´t have the right voltages across them cause the fets aren´t installed. The base voltage is normally set by the current through the source resistors.

Drain of Q3 is OK at 3.5V (Vgs and Vg =0V)

Current is a bit high (220ohms, 5.5V gives 25mA instead of 20). You could look at the zener or use a somewhat bigger resistor like 270 ohms wich will give 20mA bias)

Q3 dissipates 0,025A x 27V = 0,675 watt

Q1 and Q2 0,0125A x 36V = 0,45watt

Look at the datasheet for the expected temperature (Rth case to air)

The somewhat high bias can´t be the cause for the blowing of the fets and should normally even sound a bit better.....

You´ll have to check and recheck the wiring of the output stage. Try to start the amp with a variac and/or a lightbulb in series with the mains wiring (this will keep the fets from blowing by limiting the current)

Hope this helps,

William

[george a]

well -- a couple of wires transposed in the output stage....DOH!


all voltages appear OK except for the 9.9V across Q4- is this what should be expected with no power mosfets installed?

Thanks to anyboy who can answer...
Cheers

[wuffwaff]

Hi,

9,9V across Q4 means that all the bias current from Q3 is going through Q1 (25mA x 390 Ohm = 9,75V) wich is possible if the gate of Q2 isn´t connected to anything.

William

[george a]

Thanks Wuffwaff

Looks like time to put in a couple of output mosfets.........


drum roll anybody....?
cheers

[wuffwaff]

don´t forget the variac/lightbulb!

[george a]

Actually I prefer a series of Jacobs ladders to either a light bulb or variac.
Accordingly the power switch should also look like it came out of a power station.

[wuffwaff]

What are Jacobs ladders?

William

[BDP]

C5 is shorted to ground.

Short across C5. This will trick the output into thinking it is a zero output offset and you should see the correct current through Q1.

This won't help testing Q4 or Q5.

PS - This will test the front end with the outputs disconnected.

BDP