Polytone Mini Brute II Restoration

[SergeyRep]

Help, want to restore the front panel Polytone Mini Brute II, replace the torn gray fabric.

Has anyone had experience removing the front panel? Everything inside is filled with cotton wool, will I be able to get to this panel?

[Cunamara]

It's not cotton wool inside, it's fiberglass insulation- same as in the walls of many houses. when handling it, I'd suggest gloves, a long sleeve shirt with the opening secured and a mask. Otherwise you'll be itchy from the tiny sharp fibers.

I've never tried to take out the baffle from a Polytone cab, so IDK there. However, Polytone probably did not design the cab to accommodate this. It could be glued in, or screwed in or even stapled. Hopefully someone here will know, but you may just have to investigate.

That screening over the speaker is not original, I think.

[SergeyRep]

Yes, the grill mesh is not original, I will make it on a 3D printer. I'm considering another option because of the fiberglass, do not remove the panel, just take out the speaker and replace the fabric. I'm thinking of buying some self-adhesive acoustic fabric. I will also change the capacitor in the power supply and clean the potentiometers.

[SergeyRep]

I did it simply: I filled a syringe with furniture glue and applied it with a needle, then smoothed it out with a hot applicator.

[SergeyRep]

Help, there is a problem, Fuzzy sound. The clean channel began to sound distorted and quiet. Previously, at level 2 the sound was already very loud and clear, now even at 10 the sound is quiet and distorted. I had all the electrolytic capacitors in the power supply and preamp replaced. I also replaced the Molex connectors. I checked the potentiometers, the red switch works. I don't know where else to look for a solution to my problem.

[vintagelove]

Help, there is a problem, Fuzzy sound. The clean channel began to sound distorted and quiet. Previously, at level 2 the sound was already very loud and clear, now even at 10 the sound is quiet and distorted. I had all the electrolytic capacitors in the power supply and preamp replaced. I also replaced the Molex connectors. I checked the potentiometers, the red switch works. I don't know where else to look for a solution to my problem.

Hello, a couple things you can try. First, I would try plugging something with an RCA output into the power amp. Make sure it has a volume control, and that you have it almost all the way down as that power amp is running at full output. If it sounds good, that means the problem is not in the power section. I suspect one of your op amps may be the culprit. I would bring it to a local tech. With very little effort, they can have it back up and running.

[SergeyRep]

Hello, I got everything fixed at local tech, and the amp started sounding even better.
There were several problems. Adjusting the bias of power amp transistors: My power amplifier has two trimmer resistors (photo attached), couldn't find such an electrical circuit. It was necessary to determine the quiescent current, this is the right resistor in the photo. Measured the current - 10 mA (cold 12 mA). Connected an oscilloscope and determined that the pure sound was 4 mA. Set this value of the quiescent current using a trimming resistor. But all these measurements of the resting point must be done on a warmed-up amplifier, otherwise it will not be possible to set the offset point. The next problem: two transistors have heatsinks, the varnish has worn off, and they're touching the case. They needed to be insulated.
I also asked to reduce the background reverb noise: To do this, I soldered a wire to the reverb body and threw it to the common ground - the preamp body, noise has become less. After replacing the capacitors in the preamp, the reverb also became better.
I advise everyone to set the transistor bias point either according to the diagram, or better yet, using an oscilloscope, change the connectors, check the board tracks, and replace all electrolytic capacitors in the preamp and power amp.

Now I want to change the fabric on the front panel, I'll try this, noise reduction coefficient - 0,85 V5777-V5799/145 - Produkte - englisch dekor

[Herbie]

Hello, I got everything fixed at local tech, and the amp started sounding even better.
There were several problems. Adjusting the bias of power amp transistors: My power amplifier has two trimmer resistors (photo attached), couldn't find such an electrical circuit. It was necessary to determine the quiescent current, this is the right resistor in the photo. Measured the current - 10 mA (cold 12 mA). Connected an oscilloscope and determined that the pure sound was 4 mA. Set this value of the quiescent current using a trimming resistor. But all these measurements of the resting point must be done on a warmed-up amplifier, otherwise it will not be possible to set the offset point. The next problem: two transistors have heatsinks, the varnish has worn off, and they're touching the case. They needed to be insulated.
I also asked to reduce the background reverb noise: To do this, I soldered a wire to the reverb body and threw it to the common ground - the preamp body, noise has become less. After replacing the capacitors in the preamp, the reverb also became better.
I advise everyone to set the transistor bias point either according to the diagram, or better yet, using an oscilloscope, change the connectors, check the board tracks, and replace all electrolytic capacitors in the preamp and power amp.

Now I want to change the fabric on the front panel, I'll try this, noise reduction coefficient - 0,85 V5777-V5799/145 - Produkte - englisch dekor

Oh, interesting findings! And very thorough research! Hats off! Nice to see old Polytones well served!

I have a felt-era MBIV and it might need the cap job and bias adjustment. Although there’s nothing wrong with the sound.

My MB had the foam grille but it was quite ”tired”. So I made a new one for it and because I didn’t like the idea that everything is black I chose the fenderish silver grey grille cloth. Not guite authentic… but I like it!

[garybaldy]

Oh, interesting findings! And very thorough research! Hats off! Nice to see old Polytones well served!

I have a felt-era MBIV and it might need the cap job and bias adjustment. Although there’s nothing wrong with the sound.

My MB had the foam grille but it was quite ”tired”. So I made a new one for it and because I didn’t like the idea that everything is black I chose the fenderish silver grey grille cloth. Not guite authentic… but I like it!

I love those cast badges. The foam grill on my diamond tolex MBIII won't support the badge anymore. I have it safely stored away. Your grill cloth is great. I may do the same.

[SergeyRep]

Prepared the surface for gluing new fabric. The old fabric resembles ordinary velour, very thin, the tag is simply glued to the fabric.

[Herbie]

Prepared the surface for gluing new fabric. The old fabric resembles ordinary velour, very thin, the tag is simply glued to the fabric.

Do You know when was Your MBII made? It is red knob era, but the label seems older. Does anybody know what was the grille like in this era, it was not originally steel net, but only thin textil seems weird too. Didn't these have the black foam grille?

Here is my amp with the old and new grille.



The inside of the preamp seems to be exactly the same as Yours.



And I have learned that 137 means CTS speaker and 8113 means that it was made in 13th week in 1981.



If You want, You can substitute the glass wool with modern damping wool. Just remember to find fire resistant version of it!

[SergeyRep]

Do You know when was Your MBII made? It is red knob era, but the label seems older. Does anybody know what was the grille like in this era, it was not originally steel net, but only thin textil seems weird too. Didn't these have the black foam grille?

Here is my amp with the old and new grille.



The inside of the preamp seems to be exactly the same as Yours.



And I have learned that 137 means CTS speaker and 8113 means that it was made in 13th week in 1981.



If You want, You can substitute the glass wool with modern damping wool. Just remember to find fire resistant version of it!

Hi. The potentiometer is dated 1978. I am attaching a photo of the speaker and the label. The original grill has not been preserved, I think it was made of fabric. It was originally like this: Polytone Mini Brute IV

[SergeyRep]

But in this topic it is written: second series (about '74-'80)--soft, black covering; white/gray baffle with no grill cloth/exposed speaker(s); rectangular metal Polytone label on lower right of baffle
Polytone Mini Brute IV
Then maybe there was no grill grid initially.

[SergeyRep]

The project is completed. I applied PVA glue to the panel and let it dry. I cut the fabric to 336 x 372 mm. I activated the glue with a heat applicator. The fabric does not stretch, be sure to make cuts to relieve tension. I glued the emblem with double-sided tape. For now I've installed a metal grill, but later I might make one out of fabric.

[SergeyRep]

Continuation of adventures with Polytone
The weak point of the inductor coil. Ordered a new reverb Accutronics 8AB3C1B , With a horizontal orientation, I will place it at the top, away from the power transformer

[Cunamara]

Nice, looks like it just rolled off the assembly line on its way to the dealer!

Gotta say, those circuit traces are huge compared to modern amps!

[Rodney Gene]

Great work, follow-through and diligence! Looks great. hope you get years of enjoyment from it.

Gotta say, those circuit traces are huge compared to modern amps!

And through hole components - Like old BOSS pedals - used to be incredibly fun to mod pedals, they were roomy and easy to change out parts. Same with 80s rack gear - room to repair and work on.
The last modern pedal I modded (that still can be) were a couple RAT pedals last year. Surface mount components killed the radio star.

[SergeyRep]

Installed a new reverb. I made a shelf and glued a neoprene seal to the reverb. It is located at the top of the amplifier, away from the power transformer. Connected the grounding. There is a hum, if you increase it above 4 it is very noticeable. I sold the old reverb and it was repaired, The buyer told how he repaired it: I had to thoroughly disassemble it to remove the coil. It turned out that both supply wires were damaged. Apparently, they soldered it with active flux and didn't wash it off afterwards, and the copper had degraded.











Stop fixing amp and start playing