Polytone Mini Brute

[Doc Stitches]

Help......

Polytone Mini Brute lll / lV / M-PA blowing fuses as soon as it is turned on.

1 Do I need to use slow blo fuses?

2 Is the 15 in. Gaus speaker worth reconing? Model 5840

3 Where are the output transistors?

Thanks
Doc Stitches

[52 Bill]

Help......

Polytone Mini Brute lll / lV / M-PA blowing fuses as soon as it is turned on.

1 Do I need to use slow blo fuses?

2 Is the 15 in. Gaus speaker worth reconing? Model 5840

3 Where are the output transistors?

Thanks
Doc Stitches

Usually there are two fuses, one power fuse on the back panel and one speaker fuse inside the amp on the power amp chassis. The power fuse is usually a slow blow, the speaker fuse a fast blow. The back panel is usually marked with the rating and type of fuse.

Most Gauss speakers are big dollar replacements. Look up the model on-line and see what they are selling for. If the repair price is cheaper then an equivalent replacement then you decide.

The power amp is separate chassis located on the bottom of the cabinet. It usually is held in by 4 screws on the bottom of the amp. You will have to remove the speaker and the back panel to access it.

Most of these that I have seen that are blowing fuses are usually either shorted output transistors or a shorted bridge rectifier.

[Doc Stitches]

Bill

I am having trouble sending the pictures. How is your imagination/memory?

Are these three little black guys ( with cooling fins ) the possible output transistor culprits? The one on the far right test different than the other two. With black lead on chassis ground only 1 pin shows continuity. The other two have readings on 2 pins.

Should I change all 3?

What are the 2 round silver colored parts attached to the big black cooling fins?

Thanks again
Doc Stitches.

[Steve Conner]

The "2 round silver colored parts" are probably the output transistors. If you were looking for them, you found them! Do they have any part numbers printed on there?

[Doc Stitches]

Steve.

Numbers are the same 2N5631

[52 Bill]

The "2 round silver colored parts" are probably the output transistors. If you were looking for them, you found them! Do they have any part numbers printed on there?

Steve is right, the outputs are TO-3 metal cased transistors. They are mounted by 2 screws on the 2 vertical finned heatsinks at each end of the chassis. Probably MJ802 and MJ45?? (whatever the compliment to the 802 is).

If the three little guys are mounted to the circuit board, they are the drivers.

Unscrew one of the outputs and pull it out from the socket that it was screwed into. There will be a thin mica washer insulator that separates the metal case from the heat sink. Don't crack it or lose it as you will need to re-install it later.

Place the transistor pins up, in front of you. The pins are not centered from top to bottom of the case. Orient the case so that the pins are horizontal and above the center line of the case. Take your meter and set it to diode test or resistance. Place one lead on the metal case and touch the other lead to the pin on the right side. The case is the collector and the right pin is the emitter. You should get no reading between these two pins. If you do, then the transistor is shorted and must be replaced. The pin on the left is the base. You should get a low reading from the base to the emitter and to the collector in one meter lead orientation. When you reverse the meter leads you should get high readings on these pins.

Check the second transistor the same way. If you want, you can try to test the transistor without removing it from the circuit. Just touch your meter to the case and to the socket pins instead of the transistor pins.

If you look at the circuit board there will be a small square molded black box with 4 leads coming out of the bottom. This is the bridge rectifier. Use your meter to test for any shorts between any of the leads. The meter will usually read low and then slowly rise as the meter tries to charge the filter caps. If you get a really low reading across two leads, that diode may be shorted.

Edit: I just saw your post, both are the same number (NPN)? How old is this amp? Was it working with these in place?

[Doc Stitches]

I am getting a pin-to-pin reading from one transistor. Excellent test instructions.

Any chance my local electronics shop might have this part? Looks like Motorola part.

Thanks
Doc Stitches

[Doc Stitches]

CORRECTION they are different numbers thanks 2N5631 and 2N6031

Does anyone know the part number I need to order and best place to get it?

The schematic I have is not very clear/specific.

Also there are 2 bridge rectifyers one large one and one smaller one 121 and one 71A.

I GET READINGS FROM ALL 4 LEGS ON THE LARGE ONE BUT I ONLY GET READINGS FROM 2 LEGS ON THE SMALLER ONE.

Thanks again

[52 Bill]

CORRECTION they are different numbers thanks 2N5631 and 2N6031

I thought so, one is an NPN and the other is a PNP. If you check on-line, you should be able to find someplace that stocks these parts. But if there is a problem finding one, then substitute another part number with similar ratings. That circuit is not that critical and almost any transistor will work in there.

I don't stock those parts, so if I was working on this amp, I would just replace both outputs with a new set of complementary transistors, like an MJ802 and an MJ4502.

Also there are 2 bridge rectifyers one large one and one smaller one 121 and one 71A.

I GET READINGS FROM ALL 4 LEGS ON THE LARGE ONE BUT I ONLY GET READINGS FROM 2 LEGS ON THE SMALLER ONE.

I don't understand what you're saying here.

A bridge rectifier is just a pre-wired combination of 4 diodes in one package. If you look at the schematic for a lot of solid state amps, you'll see the rectifier circuit as 4 diodes drawn in a diamond shaped ring or bridge.

If you think of home plate and second base as the bottom and top of the ring, there will be a diode that connects second base to first base and another that connects home plate to first base. The positive ends of each of these diodes meet at first base. When an ac voltage is applied to home and second (or the negative ends of the diodes) the two diodes only allow the positive half of the cycle to pass, creating a positive voltage at first base.

On the left side of the bridge the diodes are wired with the negative ends meeting at third base, so the negative voltage is created there.

When you test these with your meter, you want to see that: 1-the diodes are not shorted (allowing current to pass in either direction) and 2- that the diodes are not open (allowing no current at all to pass).

Place your meter's red lead on one of the four wires of the bridge, let's call this home plate. Now place the black meter lead on the wire representing first base. You will either get a low reading (somewhere around 0.7) or a high reading. As long as you don't get a really low reading like zero, continue the test by moving the second meter lead to the wire to the left of home or third base. Again you should get either a high or low reading, just not zero.

If all is good so far, reverse the polarity of your meter leads and retest the same way. What you should find is that where you got a low reading you should now get a high reading and where you got a high reading you should get a low reading.

Now move the red meter lead to second base and take readings from there to first and third. Again, you should get either a high or low reading and again when you reverse the meter leads the readings will flip-flop.

[Doc Stitches]

Thanks for the detailed help/homework. I appreciate it.

so.... could a bad bridge rectifyer cause an amp to blow a fuse as soon as it is switched on ?

[Don Moose]

could a bad bridge rectifyer cause an amp to blow a fuse as soon as it is switched on ?

Absolutely - a shorted diode would let the negative swing yank down on the positive rail (or connecting the positive swing straight to GND).

Hope this helps!

[Steve Conner]

Hi guys

Those transistors are obsolete.

The 2N5631 is NPN, I'd recommend the MJ15024 as a replacement, and the MJ15025 for the 2N6031.

The MJ802/4502 have a lower voltage rating than the original devices: they're 90V parts and the originals were rated 140V.

The MJ15024/15025 are about the beefiest transistors you can get, 250V and 16 amps. The MJ15003/4 are the 140V versions, you might be able to get them cheaper.

I forgot to mention, a reconed Gauss speaker seems to go for about 120 bucks on Ebay. You could maybe stick in an Eminence instead, or the 15" driver that Warehouse Guitar Speakers sell for bass. Henriksen's Jazz Amps use Eminence Beta series for guitar and Delta for bass.

It won't sound quite the same, but if the Polytone's cabinet is sealed (no ports) then nothing too awful should happen.

[Doc Stitches]

Hey Bill.
The 2N6031 is the shorted resistor. Do I replace it with the MJ4502?
Thanks
Doc

[Steve Conner]

Even if only one of the transistors blew, I suggest you replace both of them anyway. The other one could be damaged and would fail soon.

Replace the 2N5631 with a MJ15003(or MJ15024) and replace the 2N6031 with a MJ15004(or MJ15025)

[52 Bill]

Even if only one of the transistors blew, I suggest you replace both of them anyway. The other one could be damaged and would fail soon.

And when two outputs are randomly replaced, sometimes the operating points are different enough to cause premature failure and or sound problems. Some circuits are more sensitive to this than others.

Replace the 2N5631 with a MJ15003(or MJ15024) and replace the 2N6031 with a MJ15004(or MJ15025)

I'd suggest that you replace both outputs with a new set of whatever type you want or can get. Just be sure to watch the polarity of the transistors when re-installing, making sure that you replace the PNP in the correct socket and the NPN in it's correct socket.

[pecosjoe]

I know this thread is old... but I have to replace my output trans.. I bought the replacements suggested by Steve.The repair man took out the old ones (didn't put them back)and did not mark which side they go back to. So now I'm giving it a go. How do I know which side the PNP or the NPN goes on?? Help.Thanks

[52 Bill]

The short answer here is that the NPN is supplied by the positive power supply and the PNP is powered by the negative side.

Please be aware that there may be other problems as well, like bad driver transistors, open ballast resistors, etc.

Make certain that you use an insulating washer between the heatsink and the transistor case, as well as heatsink compound.

I'd also suggest to use a light bulb limiter when you first power it up, to save your new parts if there are other circuit problems.

[pecosjoe]

Thanks Bill, I will give it a try.

[Steve Conner]

Well whoa, doesn't the Mini-Brute have a compound pair output stage where the transistors are the other way round? Here are the schematics, that may help you to figure out which transistor is which.

POLYTONE SERVICE INFO