bose 1800 amp

[playthatbeat]

So, we use a Bose 1800 amp in the studio to move the PMC AB1 monitors.. I have been very very pleased with the sound of this, it can emulate a small nightclub for me, making work all the more, er, immersed! Anyway, a couple of weeks back, i hear some crackling, like bad static, a bad power connector or something, i pay no notice, as it happens, especially in the cyber rat's nest that is my playpen, but, it's followed shortly after by THE biggest squeal You have ever heard. Like horrific, much more Square then just feedback, the speaker cone is STANDING out from it's surround on the left, with the LED meter showing 100% in the left..

NO automatic cutout from the amp, no effect from turning the amp level down on that channel - so it took me a few seconds to work out it was actually the amp, and kill the power.. Result is a dead driver, a Dynaudio 24 W 100 – 8 OHM.. No joke.. New driver for the PMCs will be a pain, but http://www.hifisound.de can get the swapout from dynaudio organised, as that's the way they do it - You return Your broken one, and they swap it out.

So, to the amp.. the service dosc here: http://electronics-diy.com/pdf/Bose_..._Amplifier.pdf on page 27 explain what happend..

Subject: Resistor Failure Date: 09/19/1977
Remarks:
We have found that the resistor R3 on the 1801/1800 amplifier PCB's have shown a higher
failure rate than normal. Studies reveal that the resistor normally runs at about 150-170°F. The
etch layout at R3 is not sufficient to provide adequate heat sink for this component, and the
resistor eventually changes value due to the heat.
This changing value causes the resistor, capacitor C2, and the zener diode CR15, to fail. The
symmetrical circuit, R2, C2, and CR6 all have sufficient heat sinking capability on the etch
layout and does not cause failures.
To improve the reliability of the boards, R3 should be changed from a carbon resistor to a
ceramic resistor of the same value. The ceramic resistor should be elevated slightly above the
PCB surface to provide adequate air ventilation for self cooling purposes. The new ceramic
resistor P/N is 108676-302.
In addition, R2 should also be changed to a new value. The resistor will be changed from 3kW
to 8.2kW, P/N 104064-822. This resistor is carbon, and due to the etch layout, need not be
elevated.
This modification must be preformed on all PCB's brought in for repair, regardless of the actual
failure.
Bulletin #1

So. That happend to me. See pics..

Should we fix it? HELL YEH. 1977 this baby is from! Weight: 7 stone, but, it SOUNDS amazing. And it' SILENT.. no stupid fans.. No horrible switcmmode PSU hash, this is WORTH fixing..

Just sayin'

[playthatbeat]

So to fix this, and hopefully have it back to life again, i need to replace the following on the amp card, and probably do the other card too:

R2 - replace with 8.2k carbon 1/2w 10% [2021 - Vishay 8.2kΩ Wire Wound Resistor 5W ±5% AC05000008201JAC00 RS Stock No.:683-1077 Mfr. Part No.:AC05000008201JAC00]
R3 - ceramic 3k 2w 10%
R7 - 10k 10% 1w
CR15 - 1n5425 zener diode 15v 1/2w [not available 2021 - Alt: ON Semiconductor, 15V Zener Diode ±5% 5 W Through Hole 2-Pin 017AA-01 RS Stock No.:463-280 Mfr. Part No.:1N5352BG]


C8 - 200pf disc cap 200v 10%
C3 - axial 14000uf electrolytic 100v

These are all shown in the service manual, and are either part of the 2nd service bulletin on the unit, or are toast because of the fault.

My unit has a too high serial number to have NOT had these fixes done at the factory, but has a low enough serial number to be still using the old power transistors, so IDK how together it actually is.. Who cares, the VERY parts mentioned by the service bulletin re: resistor failure are failed here, so i'm going to do the brave thing and rebuild it with fresh parts, inspect the rest as best i can then try plugging it in again!!! lol..

[playthatbeat]

SO, rebuilt the broken amp card, all good, bar the 1n5245 zener diode, which seemed a bit small and shitty compared to the one that was blown, but hey, same rating, same part number, let's see how that goes!

Amp has been on gentle soak for an hour+ now, no issues so far..

See piccy for a look at the new parts in place, and a look at the OTHER card, which must have failed previously, as the parts have been changed at some time, and there is evidence of scorching around the holes in the board..

So, job done.. FYI, the mouser part numbers in 2005 were: 512-1n5245btr , 791-rc1-103kb , 75-tva1412 , 660-mo2ct631r302j , 75-562r10tst20 for the repair, and 588-of822je for the fix to R2..

[Edit 2021 - You should uprate all these parts to the biggest You can fit in the space, and shop around - i Used eBay and RS]

I'm not sure when the other channel 'went' but it DID die, and was repaired to bose spec, but the strange thing is the channel i just repaired was not done in the same way - different shaped resistor in R3, i'd say the original one untouched, but R2 HAD been changed on BOTH cards for the 8.2K! Who knows? Oh, BTW, the serial number of this unit is in the mid 2000's, making it a rev 2, or maybe rev 3 (final) so it's possible some of the mods were done at factory..

I Am worried about using this again in studio, after the damage it did to my monitors, but the Peavey i have to fill in is nasty sounding - very harsh and bright - and has 2 FANS in it! like wtf? so i really need more ambient background noise? i think not..

We will see..

[playthatbeat]

Wow, that fix lasted 10 years of being hammered with all manner of noise!!! OK, so here We are , same fault. This time i caught it before it killed my speaker. Looks like the same issue. SO, big chunky ceramic resistors like the one used in the 'factory' are nowhere to be found at the usual new parts places, so hit eBay and there are plenty there.. most are rated a 3.3k +/- 10%.

The Diode i used must have been a bit weedy, so i'm uprating that also, and same for the 8.2k resistor. I edited the post above to reflect this.

I think the board itself is still a little ropey due to the damage caused the first time it failed. It really needs a total cleaning, and some reinforcing of the tracks that were toasted.

I'm also a bit suspicious of the ratings for the parts in the replacements, so i'll be over-specing these to fu*k, 1/2w diode and 3w resistors are all now 5w. It looks like some solder has vanished from one end of the Diode also - surely i did not forget to solder it??? It was the heat. Yes, Defo the heat, not me.

Oh yeh, i attached the manuial - the link above still works, but hey, the internet is changing, it might not be there in another 10 years when this thing blows up again.

Did i mention that the caps in this thing are a big as beer cans?

[playthatbeat]

Still working on this - did the resistor and some diode replacements, but the high voltage on the line persists.. Enough to toast a test speaker :/ some excellent tips here: https://audiokarma.org/forums/index....-again.584844/ so i will be following this guy's mod scheme, and trying again..

More digging reveals this unit is an 1800 Mk.1 - this is very old, and the later editions included FANS for cooling. How lame is that?