Bandmaster AB763 Restore Question

[GregS]

I just got a blackface Bandmaster AB763. I haven't tried it out yet as I want to replace the two wire AC cord with a properly gounded 3 prong cord/plug first.

The inside appears to be all original with nothing replaced/modded. The 470 Ohm screen resistors seem to have undergone heat stress - one is burnt and cracked. I figure I'll replace these with 5W types.

Also I checked the resistance of the 1500 Ohm grid resistors under the screen resistors. One is 1700R which I think is ok but the other (the one under the cracked screen resistor) is about 2100R. Is it ok for it to have drifted this far out of spec or should I replace it?

Thanks,

Greg

[Tom Phillips]

...Also I checked the resistance of the 1500 Ohm grid resistors under the screen resistors. One is 1700R which I think is ok but the other (the one under the cracked screen resistor) is about 2100R. Is it ok for it to have drifted this far out of spec or should I replace it?...

My recommendation is to replace both 1500 Ohm grid resistors while you are at it. The actual resistance value change won't make much difference but it appears that they are showing evidence of stress. Why take the chance that one will fail open and damage a power tube?

Regards,
Tom

[GregS]

Ok. Thanks.

Is there some trick to getting the strain relief apart in order to remove the old power cord? If there is so far I haven't found it.

Greg

[Bruce / Mission Amps]

Ok. Thanks.

Is there some trick to getting the strain relief apart in order to remove the old power cord? If there is so far I haven't found it.

Greg

There is a special tool for this... I think most of the builders and repair folk here have one but a medium sized pliers is usually big enough to squeeze the livin' piss out of the thing until you can pull it out.
Look at how the tension strain relief cut out can be squeezed into the actual holder and cable (making it a little smaller in diameter) and you'll see how it works.
When you are done, make sure your ground wire is the longest one of the three, even if it means curling it up a bit.
You want that ground wire to be the last wire still connected to the chassis if the power cord pulls out and the hot unprotected 120v wire is flying around in the chassis!
Also, note the most of the Fender aux power plugs on the back of the chassis have the 120v "hot wire" soldered onto the wrong blade of the socket... make sure your black wire is soldered to the smaller one not the wider one.
Wide and white is neutral.

[Bruce / Mission Amps]

I just got a blackface Bandmaster AB763. I haven't tried it out yet as I want to replace the two wire AC cord with a properly gounded 3 prong cord/plug first.

The inside appears to be all original with nothing replaced/modded. The 470 Ohm screen resistors seem to have undergone heat stress - one is burnt and cracked. I figure I'll replace these with 5W types.

Also I checked the resistance of the 1500 Ohm grid resistors under the screen resistors. One is 1700R which I think is ok but the other (the one under the cracked screen resistor) is about 2100R. Is it ok for it to have drifted this far out of spec or should I replace it?

Thanks,

Greg

As mentioned already, since you are there, just replace the 1k5 resistors with 1/2 watters while you can.
You must replace the 470 ohm resistors too..... but with 6L6s, you don't need anything over a 2 watt 470 ohm resistors and the standard resistors is actually only a 1 watter.
I'd use a 2 watt metal oxide 470 ohm resistor there.

[GregS]

Thanks for the info. I'm assuming the strain relief won't be reusable after I do that but that's not a big deal. Thanks for the tips on the wiring as well.

Greg

[GregS]

As mentioned already, since you are there, just replace the 1k5 resistors with 1/2 watters while you can.
You must replace the 470 ohm resistors too..... but with 6L6s, you don't need anything over a 2 watt 470 ohm resistors and the standard resistors is actually only a 1 watter.
I'd use a 2 watt metal oxide 470 ohm resistor there.

OK thanks. I'll save the 5W ones for something else. For the 1K5 grid resistors would you recommend metal film or carbon film?

Greg

[Tom Phillips]

.. I'm assuming the strain relief won't be reusable...

It should be reusable.

...would you recommend metal film or carbon film?

If that discussion starts back up it will consume all the available bandwidth. I say that you can use either type as long as its a good quality part. I'll leave it at that.

Cheers,
Tom

[GregS]

If that discussion starts back up it will consume all the available bandwidth. I say that you can use either type as long as its a good quality part. I'll leave it at that.

I was thinking more in terms of what type would stand up to heat better rather than their sound.

Thanks,

Greg

[Gibsonman63]

Strain Relief

I was able to re-use mine on several of my amps. I used a little vasoline, then held it in place with a large pair of needle nose pliers and tapped it into place with a small hammer.

It is still not easy. You have to hold it just right and tap it just right to make it work, but it is do-able.

[Regis]

Strain Relief Tool and replacement strain reliefs

[Tom Phillips]

$16 is a good price.
I'm not sure if it's the same quality tool but I know that I paid over twice that for my strain relief tool many years ago. It was one of those things that I put off getting for a long time but now every time I use it I'm sure glad I have the proper tool.

For anyone who does not have the specific strain relief tool I'd recommend channel lock pliers as the next best choice.

Tom

[GregS]

I used a giant pair of pliers that I could apply a lot of force with and also had a good grip. Luckily, they were also bent in a way that allowed me to get at it easily. It came out no problem. However, putting it back in with a significantly thicker 3 wire cord wasn't easy. I ended up trimming some of the outer insulation of the cord where the strain relief fit over it. I also filed the leading edges of the strain relief to allow it to enter the chassis hole a little easier.

I think I'll invest in the proper tool for the future. Thanks for posting that link.

Greg