Crest CC2800 Help!

[pontiacpete]

I have a crest cc2800 on the bench, R33 and 34 were blown to pieces.
Transformer seems to check out fine, making voltages.
With connector J1 from PT connected and J2 disconnected I can bring it up on a light bulb without a problem/
So I suppose the power supply is working, so far.
When connecting J2, the limiter bulb lights up.
Diode check on the output transistors in circuit show they are shorted.
When I pulled Q119 it passes the diode test. Why is that?

B1, F1and F2 check out good.
Anyone have a suggestion as to where to go?
thanks

[52 Bill]

Diode check on the output transistors in circuit show they are shorted.
When I pulled Q119 it passes the diode test. Why is that?

Because there are 7 other transistors in parallel with it and one of them (or more) may be shorted. You have to test them all until you find the bad one.

[pontiacpete]

I pulled a whole row of transistors Q114-128, each appeared to be shorted between the base and the emitter in circuit but out of circuit they pass the test, no short.
What am I missing here?
Should the output transistors be replaced anyway?

[Jazz P Bass]

Now that the output transistors are removed, what resistance (NOT diode check) do you read from base to emitter, on the actual circuit board?
If you read 243 ohms, that is correct.
Q113 & Q114 each have two B/E resistors in series.

[pontiacpete]

On the circuit board Q114 B/E is 243 ohms, the rest Q116-128 are 22.6 ohms

[Jazz P Bass]

That is a correct finding.
What is wrong with the amp?
In your original post you indicated that R33 & R34 where blown to bits.
Would you be so kind as to indicate what page they are on & what circuit.

[pontiacpete]

R33 and 34 are on the first page, on the far right about half way down. They are Close to the AC input.

When I connect J2(this connector on the schematic is right above R33 and 34, far right of pg 1), and bring it up on a light limiter the amp pulls a lot of current lighting the bulb.

[Jazz P Bass]

Whew!
R33 & 34 are movistors.
They are like an anti surge device. High resistance to start & then they drop to 1 ohm.
If the amp wanted huge amounts of current, yeah they will blow.
So connecting J2 pulls lots of current (+24V, -24V & +6.2V source)?
How about if J1 is connected (High voltage source)?

[pontiacpete]

I don't have the movistors at the moment, they are coming.
So I jumpered the connections

With just J1 connected it starts up fine.

[52 Bill]

I don't have the movistors at the moment, they are coming.
So I jumpered the connections

With just J1 connected it starts up fine.

Then as Jazz pointed out the problem is related to the circuits that use the 24 volt or 6 volt supplies. Reinstall the outputs that you pulled and start by checking the rectifiers and filter caps in the two power supplies connected to J2-B1, C9, C10, C11, C12, etc.

[pontiacpete]

B1 checks out ok, no shorts. All the caps C9-12 and C24 measured in circuit show no signs of a short, they show very high resistance. D4 is ok and R1 and 2 are good.
I'm wondering about F1 and F2, these are fuses right? They measure very low resistance... like a dead short,, like a fuse.

Should i pull these components off the board to check them or is this good for now?

[Jazz P Bass]

F1 & F2 are 2 amp fuses.
Move on.
Is there any way to disconnect the + & - 24 volts (like a header) so you can actually test the bridge circuit?
If not, with the lamp limiter hooked up, see what voltages the circuit is putting out.
Alternatively, check the +24 & -24 to ground.
One or the other may give a tell by the resistance reading.

[pontiacpete]

Reading from the "AGND" (?) the positive side of C12/negative side of C11 to the -/+ sides of B1 , no short

[Jazz P Bass]

O/K, no short.
But what is the resistance of the rest of the 24 volt circuit (+ & -).
We are looking for something that may be be pulling it down.
And do the voltage check.
As you probably figured out already, these Crest amps can be a tangled web.
You need to narrow down what is going on.

[pontiacpete]

It takes a long time to get this measurement thru the caps.
Each side starts at about 200k or so, maybe more.
The meter finally settles down with the neg side of B1 to AGND at 14k and the positive side 22k.

It will be a couple of days before i can power it up. One of the legs broke on the 1943's. So they are coming along with the movistors
I plan to change out the whole row Q114-128. Is this recommended or can I get away with changing just the broken one?