Another BM4400

[hcraig244]

Ok….I’ve had this amp floating around for  a few years now on the “around 2 it” bench and finally had a look at it….naturally it doesn’t work. At least not completely….one channel is very eratic, switches are not happy and the overload lamp flickers on and off intermittently.

 

[hcraig244]

A look inside gives a clue to its woes…..

[hcraig244]

cobwebs everywhere…..

[hcraig244]

underside look pretty clean however

[hcraig244]

dusted away most o the debis and gave all the switches a blow over with de oxit and left them to soak…..did spot this….looks like a broken track

[hcraig244]

left the deoxit to dry off and had another go…switches all operate much better, tried FM works fine both channels….so does the phono input, got it on test or the day see how it performs, the suspect broken track will need some attention/investigation for sure…nothing is getting hot right now…..so Dillen can I have a capacitor kit please? it’s easier for me to buy from you that trawl the component websites ;¬)

[Dillen]

Service kit etc. here.
And it looks a very nice project.
They are definitely worth it.

Well spotted, that break!

Martin

[chartz]

Hi,

Entirely worth the efforts. Mine has been to service every single day since its repair 10 years ago. The other day we went out for a few days, the 4400 had been left on. No problem at all. Once restored properly the 4400 is reliable and above all, sounds mighty fine. Mine feeds M70s in the lounge.

[hcraig244]

discovered the individual responsible for the cobwebs…at least some of them i’m sure…..

[hcraig244]

disconnected plug P11 and with the help o a little piece of wire connected to r168 220 ohm resistor and gave the suspect track break several hard pokes with a blunt stick, reading never budged…..so as I cant see the underside of the board im going to give it the benefit of the doubt for now….im considering running a piece of wire topside just in case….

[hcraig244]

Received this little lot in the post a couple of days ago…..

 

[hcraig244]

Cheers Martin…….buying one of your kits is by far the best way to obtain the necessary parts for a refurb…I’ve tried buying parts myself and although it can be done I ended up placing orders with multiple suppliers to purchase all the parts….and in some cases there is a minimum order level….so you end up with 5 trim pots of a particular value when you only need 1…..and of course the big output capacitors in the kit are custom made to be the correct diameter to replace those made back in the 1970’s……good job Martin ;¬)

Craig

[hcraig244]

During the outage things have moved on……

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[hcraig244]

still getting used to the new site, as you can see….however the work so ar has not been without its excitement….installed 2 capacitors the wrong way around and had to replace them, schoolboy error! can you spot them?

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[hcraig244]

because i had cut the tails and installed them they wouldnt go in the correct way around….so had to be replaced, will come in useful another day I’m sure ;¬)

[hcraig244]

Hmmmm….looks like we have lost a post or two in the transition, not to worry. Repaced most of the capacitors and decided to power up and check everything is good before pressing on, and to my dismay things had taken a turn for the worse!…..the right side channel still didn’t work and the left side made a sound like a wooden stick poked into a bicycle wheel….sort of a rapid blat blat blat noise, very disconcerting. Had to be something i had done…so had a close look at all the solder connections and found I had been a bit to enthusiastic with the amount of solder i had applied to one of the connections…four points had flowed into one instead of only three…

 

[hcraig244]

In my defence it is difficult to solder under the support that runs above the components in that area…..I’m unsure why that would cause the working let side channel to go blat, blat, blat rapidly….but when i resolved the issue the left channel came bach to life….Martin can you explain?

[hcraig244]

Unfazed by this conundrum i connected up my scope and started to trace the signal path through the right channel….in the past i have connected a signal generator to C200 of the amplifier and checked for the signal at C232 and worked back until i find the signal and replace the defective component….this time i simply plugged an antennae into the set and tuned to a strong signal and using the working left side as a comparison did the same as i would with the signal generator….found this little fellow had failed

[hcraig244]

Was TR210 that had gone down….general purpose transistor BC559….changed it out with a BC558 and was rewarded with a working right channel……love that transistor tester.

[Author]

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