Bose 1801 (1974)- Restoration

[aokman]

Hey guys thought i would share my latest restoration job on here, its been going on for a while actually but i thought you might like to see some engineering from the 1970's and what these beasts are capable of will post rest of the content shortly

Yes believe it or not BOSE did have good products at one point this is about the peak of their workmanship. With a massive power capability and the transformer from a power station its perfect for music or doing some spot welding

POWER OUTPUT:

RMS continuous at 8 ohms, at or below rated distortion, 20Hz to 20kHz 500 Watts (250/CH)
RMS continuous at 4 ohms, at or below rated distortion, 20Hz to 20kHz 800 Watts (400/CH)
Total Harmonic Distortion: at or below rated power from 20Hz to 20KHz 0.5%
Intermodulation Distortion: at or below rated power, any combination of two frequencies, 20Hz to 20kHz 0.5%
Hum and Noise: 100dB
Basic Amplifier Input Sensitivity and Impedance: 1.5V - 50k ohms
Damping Factor: 40

GENERAL:

Dimensions:
Width: 18"
Height: 7 - 3/16"
Depth: 18 - 1/2"
Weight, unpacked : 38KG

Initial impressions we good so far, hooked the unit up and it powered up, no input has been connected but DC offset on L - 136mV R - 98mV.

List of things to carry out so far:

Refinish front fascia
New knobs for front panel
New lighting
Speaker protection circuits to be fitted
Recapping
more to come...

Current supply capacitors are 14,000uf @ 100v - Drop in direct drop in replacement of exact size 47,000uf @ 100v...

Have to say this is the most hefty amp ive ever worked on, it makes my 2275 look likes a fetus.

[aokman]

Catchup 2

I set out refinishing the front fasica, i straightened out the border bars that were bent ready for refinishing. I have removed the perspex and taken it back to 800 then back to 1200 ready for polishing...





I gave the amp a run in once i confirmed the readings from the terminals and it works fine, however it does have a hum in the right channel. On first look the right amp module has been replaced and is IMMACULATE. The left channel looks original (bit worse for wear) and has had field modifications for power resistor flaring up and a few other issues near the zener diode? I am looking over a modification page i found http://www.dcx2496.fr/Bose_1801_mods/index.html which has some interesting upgrades made also.

Having said all that i'm amazed that Bose could build such an amp, seeing its chassis with 3-4mm thick aluminum all around and then there are the heatsinks...they make great grab handles trying to lift it on the work bench.

[sonicj]

wow, thats a monster amp! was it intended for commercial use?

i can't tell for certain, but that resistor on the bottom of the board looks like it has cold joints. i see 3 or four more that look questionable, but could just be the lighting or flux residue.

are you going to do the plexi polishing yourself?
-sj

[slateraptor]

Who needs solder when you can bolt your transistors directly to the PCB.

[aokman]

wow, thats a monster amp! was it intended for commercial use?

i can't tell for certain, but that resistor on the bottom of the board looks like it has cold joints. i see 3 or four more that look questionable, but could just be the lighting or flux residue.

are you going to do the plexi polishing yourself?
-sj

Yeah the soldering is atrocious on that board, its an old revision with hot fixes all over place and im slowly cleaning it up and repairing tracks, posting more info today as ive done quite a bit on it...

The amp was built for home use but mainly to drive a set of Bose 901 speakers which had 9 drivers per enclosure and were rated for 10-450w RMS @ 8ohm

Polished the plexi myself with sandpaper and metal polish believe it or not

[aokman]

Small update - im finally back from holidays and i managed to get some work done on the Bose

Removed the rear plate assy, desoldered all the connections to replace the bad power cable and also i am also replacing the power cord with a IEC connector instead so i can use any cord i want Bose certainly chose a very thick cord for a 2kw max load.

Bose suck at service manuals and there are components missing from the list left right and center, not to mention duplicate names? Silver Mica caps have arrived to replace all those nasty ceramic caps

Looking over the amp boards, it seems that the newer amp module has metal film resistors fitted compared to the older one which is definitely carbon films. Either way im rebuilding both modules to mirror each other.

[aokman]

After much frustration with the Bose service manual i threw it aside and started taking apart one of the amp boards, i removed the heatsink coupling and drivers and stripped it of all resistors and some diodes/caps

The service manual is so wrong with its parts listing that its not funny, values are all wrong and some parts dont even exist! I recommend no-one pay attention to anything on it when it comes to resistors. I pulled every one from the board and metered them out followed by looking up their rating values. Ive compiled a list here for you and a pic of the resistors i removed, feel free to let me know if you can identify any of them such as their power handling etc as i cant work out the last few and ill update the list...







Have started standing down the front fascia / realigning the bezel plates when have deformed over time im assuming, also removing the dings from the edges and trying to give a smoother finish... Also removed the front plate from the amp chassis, removed the VU meters and starting rebuilding them, both needed realignment and new foam as they are falling down into the housing.









Heres now the front faceplate is looking at the moment

[Rudy7373]

Interesting! I have a Bose 1801 I bought new in around 1974. It still works however I am missing one output transistor in one channel. The amp was damaged in a fire some years ago and I restored it except I could not get a replacement device for the one damaged. The amp still functions almost as new, a little less output potential in the one channel. I cleaned it up and it really looks new. I don't use it much however. It is so heavy, when I received it years ago it came by Road Way express semi!!!

[aokman]

there were quite a few options for the output's they were replaced with the MJ15011 later in life, 1 module has the new ones the other has the older version. Aparently not interchangable according to the service manual however there hasn't really been many changes to the amp design... I have a batch of MJ15024's to replace all of them on both modules.

[Rudy7373]

The amp still functions. At  that time when I fixed it, the devices were $18 each and all had to be replaced in one channel. The originals were not available, so I have one missing. I keep the amp for sentimental reasons.

[aokman]

fair enough certainly is a unique amp to own, it will be in my long term collection when its finished though its costing quite a bit to restore. 28 output drivers cost a bit 

[Rudy7373]

I actually have the original shipping box up in my attic. The amp had several strong straps around it so you could lift it out of the carton. I think I originally paid about $1,100 for it in 1974 dollars! Oh, never lift it by the front panel!!!

[Rudy7373]

Quick question: Is the MJ15024 a direct replacement?? It looks like Digikey has them for $4.56. I would imagine one would have to replace all in one channel. I do remember replacing all the current balancing resistors in the one damaged channel on mine. At the time Bose wanted $18 for a replacement and I would have had to replace all 14, I think it was.

[aokman]

Not completely sure on it yet, they are similar in application except the 15024 is rated for 16amp 250w while the 15011 was 10amp 200w. There are changes some characteristics aswell but we will see how they go once assembled, i am replacing all at once anyway

[Rudy7373]

It would seem the higher rating would be OK. I think it said a frequency limit of 4 Mhz, which is fine. I guess it depends on the other characteristics. This is only an audio application so one would think anything close should work, but all have to be the same to balance the load.

[ciccio]

I'm pretty sure  that you can safely replace MJ15011 with MJ15023, but my experience of 30 years of amplifier repair suggest:
1) replace all devices in one channel - save the others for spares.
2) if there is a bias adjustment pot (I do not remember if Bose had one, the trimpot on the PCB could be for bias or for offset adjust) adjust it for minimum bias, then wait for the amplifier to arrive at working temperature before touching the trimmer and readjust it carefully for minimum crossover distortion (check the output waveform  with a very small signal (1 V output) at about 10 kHz, on a  4 ohm load (it can be a small power resistor).
Stop rotating the trimmer as soon you see the crossover distortion disappear.
Leave the amplifier on, without signal and load, and check the heat sinks temperature (with your hand).
If the temperature goes up on one channel, reduce the bias setting on it.

[CraigP]

I was enjoying this thread, I wonder if you've made it any further on this old Bose unit?

[T4P]

Ceramic caps ... nasty ? Wait you are one of those audiophools . Really .

[SeanB]

Can probably be used, but planar and EPI transistors are not the same. I have had some where if you use a replacement the improved transition frequency causes it to oscillate, as the original output devices were so slow that they made a low pass filter, so the new devices needed an extra base emitter capacitor to roll them off. Old and current 2N3055's are a case in point, the original planar device would struggle to deliver high gain at 10kHz, while the EPI one would work up to a few MHz with no problem. Not too good if you had a 360 degree point in the feedback loop at some point over 50kHz, then it would get hot or burn out tweeters for no apparent reason ( Here's looking at you Phase Linear).

[saturation]

Kudos, aokman for the good work and detailed photos.  Most impressive amount of work.  Where did this amp comfe from?  The corrosion on the aluminum and rust suggests it was in a fairly humid environment, even near the ocean.

[aokman]

I was enjoying this thread, I wonder if you've made it any further on this old Bose unit?

more coming soon i have finished one channel sorry for the delay but its quite a project restoring it and its hard to get the time from other things lol

I dont really have a problem with ceramics however the best amps i have used in the past have used silver mica caps instead so i took the opportunity to change them over to something a bit better.

The amp was sitting in a shed for 30yrs i think, was in very bad condition overall with all inputs / outputs rusted out. Pots dried up, to its credit though the amp fired straight up with no issues on either channel. However there was a significant different between channels due to 1 being replaced mid life, my goal is to have the 2 channels matching by the end

[stelios21]

I've just bought a 1801 in dire(?) state and set to restoring it so I've read your posts here and in OCAU with great interest.
Seems you're doing a great job!
Funnily enough my item suffers from the same power cord fraying as yours!
Upon opening it up to attempt to replace the power cord found a number of things.

• it appears one channel completely failed at some point as there are signs of extreme heat that caused the paint to disappear from one of the metal panels and black suit deposited on the top cover. Not good...
• Also the molex(?) connector is blackened too
• The damaged channel board has been replaced with one marked Rev. 5 on it.
• One of the main PSU caps. has been replaced at some point (can't tell which one but both are Sangamo, same size one with white jacket,  one with clear jacket).
• 4 ceramic resistors half soldered across(?)  the delay relay (possibly one end came detached at some point)
• mains earth disconnected from the chassis!!!

Thankfully, all cabling is intact.

The other channel board has the following changes that I can see:

• 3 output transistors replaced with ones marked 2n3442
• all sorts of component changes
• a resistor soldered on the back side
• about a quarter of the copper tracks heavily and really sloppily tinned
• blackened molex(?) connector (I guess the board had been swapped to the other side at some point? as the mating end of the connector is more blackened on the other channel)
• only 3 original screws left - the rest are all sorts and have probably damaged the threading on the heatsinks / case
• phono socket and plug missing - phono cable directly soldered to board

Now this board has no rev. marking on it (so does that make it the original version?), no component markings / drawings on the parts side and generally inferior parts quality (although I can't tell if this is due to them having been replaced at some point).

So my conclusion is, at some point one of the channels failed, possibly taking one of the psu 'lyitcs with it and was replaced.
The other one had a number of repairs done by someone who couldn't care less (I think the amp was used in a PA system).
One of the boards is probably original the other is probably revision 5.
One of the mods carried out includes adding power resistors on one side of the mains transformer (still to identify their purpose & function).

My aim is to restore this with quality components but keeping as close to the original as possible.
I'll probably start by locating transistors, carbon resistors etc. that match the originals.
Later on, I'll attempt reforming the psu caps.
I think I'll also go with replacing the ceramics with silver mica or polystyrenes though. They just look naff - original or not.
I would like to clean the boards too and remove the sloppy tinning carefully (hopefully it's not there to repair track damage underneath)

I'd be keen to hear of any advice and find out how your restoration is coming along. If finished, are you pleased with the sound? Made any additional changes ?

Best regards

Stelios