ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Cant help myself.......though I really will need to sell off some of these amps, my girlfriend is beginning to despair every time the postman calls ;~)
This one looks in fairly good shape on the surface............
One of the side panels has had a bit of a moment at some time in its career....
But it is recoverable
The screws securing the lid show signs of removal at some point, but the inside looks untouched....
Lots of dust and cobwebs....
She came with the usual "spares or repair" description.....with the added advice that the lights come on but nobody is at home!.....time for the variac.
Pulling very little current....as the power is applied the power lamp starts to glow as does the overload lamp, both pretty much in unison as the power increases the cut out relay clicks in and the the overload lamp is extinguished.....so it's living up to its description ;~)
I think we need to set up 4400 anonymous! " My name is Craig and I am a 4400olic - I have not repaired a 4400 for 1 day!"
I do have a couple of challenges for you but will wait till you have got this 4400 out of your system!
Peter
Hi Craig
i would be interested in buying one of your refurbished 4400 when you get round to off-loading one.
Cheers
Nigel
You have just made my girlfriend a happy woman......I have three units sitting in my workshop/dining room, I will email you with the pics and description of each and you can choose which one you prefer.
Craig
Ok...had a little play around this morning, with the variac still connected so I could power up gently each time, and discovered that removing plug 11 from PC5 didnt stop the overload relay clicking in, replaced it and pulled plug 12...still the overload relay clicked in, pulled them both and no overload relay. So a little deflated, because I thought this would identify the rogue channel I set to measuring the voltages as I slowly increased the supply voltage. I found that on the right channel the DC voltage measured at the junction of R279 & R279 was negligable.
However the same measurement on the Left channel revealed DC was pouring out.....
So this is the source of the DC thats energising the overload relay, what I dont understand is why when I disconnect the left channel power supply the overload relay still clicks in?.....I tested this out on a working unit and when a power supply plug is pulled from either channel the relay clicks in.....I cant see how this is happening......words og advice required here ;~)
Quick check of the power transistors revealed 0IC102 & 0IC103 where shorted, popped a couple of replacements in and slowly powered up again, overload lamp cam on and stayed on...overload relay remained de energised....so bit of progress.
The connections to the fault switch circuit is not interrupted by pulling P11 and P12.Only the power supply and the outputs are.
To separate the fault switch, you would have to lift R46 or R47 respectively, but as you found out there are other (and much safer!) waysto diagnose problems in the output stage.
Martin
Thanks for that, though I'm still struggling a little with this. The understanding I have of the protection circuit is that the overload relay will be energised by either DC Volts biasing TR16 or by the resistance of R40 going high due to high temperature on the power transistor heat sink.
That being the case why when I disconnect P11 or P12 (but not both) does the overload relay energise, clearly im missing something here?
However pulled OIC100 and OIC101 (tip141) and this is what I found.....
Replaced both with good spares and also changed out all the transistors in the power amp along with C138 and D104/105.....was rewarded with a good looking quiet current reading of around 5mV when I slowly powered up with the variac....no smell of burning shelac or smoke signals
and the other channel..............will test it with a signal tomorrow and if all is well will start to strip and clean and recap.
Hooked up my Ipad and squirted Van Morrison in this morning.....sounds very nice, as they all do. I will replace the "good spare" TIP's with new ones along with everything else....as we now know once an event has occured scars will remain and the "good spares" have been through trauma in a past life which is how they came to be what they are.
Craig,
To me, this amp is just another example of problematic output transistors. I don't see evidence, in your case or mine, where other transistors or components caused the output transistors to fail.
The Texas Instruments TIPs are problematic or the heat transfer from the output transistors to the modular heatsink is the problem and better mating surface is required!
BTW, I got your email, I will revise my bill of material (BOM) for this project send it to you by the end of the week!
Best of success!
Jean
Thanks for that, this will be the last of the BM4400.....I'm about saturated now. Though I would like to make it a thorough job so will replace all the power transistors as opposed to just the destroyed devices, I think the film capacitors will be a nice touch.
Regards Craig
As there is a debate as to the virtues of silicone pads vs mica shields, and as the shields I have removed are still in good shape I'm going with them, they dont look as neat as the pads it's true and they are quite messy.....but they have stood the test of time.
This reminds me of quite afternoons in the office playing tetrus................
I didn't know these film capacitors had no polarity (wont come as a surprise to many) until now.....quite a job getting them to fit, had to be a bit creative here and there.
Moving on....completed work on the pre amp.
Had to replace the stereo decoder chip here, this was confirmed by swapping the board with a working piece when I was getting no FM radio function, replacing the IC corrected the problem....this is the second one of these I have had to replace. Still got a couple of caps to change followed by the pots one at a time.
slowly moving on......