GK 400RB blowing fuse on start-up

[Blood Mason]

Hi all,
My first post here but I've searched this useful forum many times to help diagnose little problems Ive had in the past.

This time my search has come up short and I am hoping I can pick someone's brains who knows a bit about the old GK 400RB.

I have had this unit for several years and only used it very rarely because it has always been my backup rig in case something goes wrong with my Ampeg.

A few weeks ago my guitarists head blew up mid-jam and he had nothing to play through. Me, having the generous nature that I do, let him use an old metalzone pedal and plug into my 400RB to finish the set.

About 3 songs in the GK blew a fuse mid-song.

A few days later I tracked down a few spare fuses, put one in and it blew straight away upon power-up.

I tried powering up with no load on the speaker outputs and no instrument plugged in and it blew another fuse.

I have checked the output of the transformer when disconnected from the PCB and it seems a stable 40VAC RMS on either side. I have also removed the power supply capacitors and checked that none have gone short circuit and checked the bridge rectifier. All of these things appear to be fully functional.

Unfortunately I am a bit short on test equipment - basically limited to a multimeter!! I don't have a variac. I can take the unit in to work with me and use the CRO but I don't think at this stage I would gain anything. I am currently trying to pick up a CRO, variac, signal generator etc for my little home electronic workshop.

My electronics experience is mainly in lasers and surveying equipment which generally runs on low voltage batteries etc so I am a bit rusty on my power supplies, AC circuits and amps etc.

Can anyone point me in the right direction on what may be causing these fuses to blow? Any general test procedures I should do short of removing every component and testing it.

Any advice at this stage would be appreciated.

I managed to score a service manual and circuit diagram off a generous fellow on talkbass but the service manual itself doesn't mention anything about troubleshooting blown fuses or common problems with components.

Cheers,
Doug

[Jazz P Bass]

GK 400B

I would check the output transistors for a short.

[Blood Mason]

Bingo!!
I was in the middle of pulling these out and testing them when I got your post.
There is a shorted MJ15001.

I have had a look around at my general suppliers and no one keeps the MJ15001 in stock. I can get them but it will take over a week.

Do you think a MJ15003 would be a suitable replacement or would I have to make some mods to the circuit for this to work. I can pick one of these up tomorrow.

The specs seem almost the same except the MJ15003 is rated at 250W instead of 200W so its collector and emmiter current ratings are higher.

If i did this, would I be better off replacing both MJ15001's so that they are matching, or even changing all four output transistors to MJ15003's and MJ15004's?

Cheers for the help.
I'm learning :-)

Doug

[52 Bill]

I would suggest replacing both of the transistors on that side as they need to share the work load evenly. Replacing all four would be alright as well, but at least replace both on the side of the dead one.

[jrfrond]

If you are going to replace all four, and I would, go with the more common and much hardier MJ15024. Change your emitter resistors too.

[Blood Mason]

Hi all,
Thanks for the advice.

OK.... so I replaced all 4 output transistors with 15003's and 15004's respectively.
I also replaced the insulating pads.
I then found I had to replace the 2 driver transistors Q7 and Q10 because they were also shorted.
Transistors Q3, Q4 and Q5 all blew up (literally) along with a few resistors and I replaced all of these.

Now the unit fires up without blowing the fuse

However, there is no output

The "on" LED will not light up but the "boost" LED will.

The speaker box emits a low hum (a 50Hz earthy type hum I think).

Anyone have any suggestions of what I should be looking at next?

Cheers,
Doug

[52 Bill]

Check the + and - 15vdc, low voltage supplies at the regulators and at the preamp chips as well.

GK will sometimes use a small value resistor as a fusible link between the power supply and the preamp chips, so you need to check both places.

[TomCarlos]

Doug et al....

My brother gave me a GK 400RB. See attached photo. I guess it is an original version 1? I dont see anything that says Mark XXXXX. Is this the same amp that you have?

Tom

[52 Bill]

That is a later issue. The first issue amps were plainer looking and had Switchcraft jacks with metal nuts. I think if you look at the back panel there may be a date silkscreened on it or check the pc board for revision markings.

[TomCarlos]

Ok Bill... thanks. I will take a closer look.