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Hi folks
I am having a little difficulty repairing a Marshall VBA 400 bass head amplifier.
Initial problem was that it was blowing the output tubes.
Problem is I have no H.T voltage.
On the back board I noticed a dry joint on one of the thermistors Ir1 or IR2.
Also TR1 was shorted TIP30c.
So I sorted these issues out.
Now I have voltage from the mains input to the L.T transformer.
No mains voltage going to the H.T transformer.
There is a relay before the the stand by switch that must close in order to supply the mains voltage to the HT transformer. This is not closing. I have replaced this still no joy.
The relay is also related to the small circuit containing the TIP30Cwhich goes to the speaker outputs.
There could still be a problem here which is not allowing the relay to close.
I have 50 or so volts on the collector of the TIP 30c, no voltage on the base or emitter.
Been at this for hours trying to find broken traces faulty components dry joints. Still no luck.
Is anyone familiar with this amp or this issue that could lend a hand or even if I could understand the circuit better that would be a help??
If your looking at the attachment direct your attention to the red x at the top middle of the page.
Also bottom left is where the TIP30 circuit is also relating to the relay.
Schematic attached also.
Regards
Joe
I am having a little difficulty repairing a Marshall VBA 400 bass head amplifier.
Initial problem was that it was blowing the output tubes.
Problem is I have no H.T voltage.
On the back board I noticed a dry joint on one of the thermistors Ir1 or IR2.
Also TR1 was shorted TIP30c.
So I sorted these issues out.
Now I have voltage from the mains input to the L.T transformer.
No mains voltage going to the H.T transformer.
There is a relay before the the stand by switch that must close in order to supply the mains voltage to the HT transformer. This is not closing. I have replaced this still no joy.
The relay is also related to the small circuit containing the TIP30Cwhich goes to the speaker outputs.
There could still be a problem here which is not allowing the relay to close.
I have 50 or so volts on the collector of the TIP 30c, no voltage on the base or emitter.
Been at this for hours trying to find broken traces faulty components dry joints. Still no luck.
Is anyone familiar with this amp or this issue that could lend a hand or even if I could understand the circuit better that would be a help??
If your looking at the attachment direct your attention to the red x at the top middle of the page.
Also bottom left is where the TIP30 circuit is also relating to the relay.
Schematic attached also.
Regards
Joe
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