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
I seen this on ebay and couldn't resist another challenge, was sold as left channel down and some "reddish tints" on the metal work. Haven't powered it up but had a look inside and spotted this
Looks like someone in the past has had a go, notice the replaced capacitor and the odd pair of series resistors in position 570 on the left channel. looking at the values of both resistors (one at 170 ohm and the other at 180 ohm) they have been used to replace the original 330 ohm component. I don't see any outward signs of frying or damage.....however I have contacted Martin and requested a full capacitor kit and a recommended set of components for this rogue channel......I will post pics as things develop.
Ok...
Been a while but I've finally got my hands on an old dual beam oscilloscope! its a vintage CRT job that lacks lots of modern day refinements but I'm hoping to use it to follow the incoming signal on the functioning right channel and compare this at the same points on the failed left channel. Tested the scope with its own 1v ptp square wave test signal and both channels work ok...however it came without a probe so I have ordered one up.....should be interesting ;¬)
Craig: Ok... Been a while but I've finally got my hands on an old dual beam oscilloscope! its a vintage CRT job that lacks lots of modern day refinements but I'm hoping to use it to follow the incoming signal on the functioning right channel and compare this at the same points on the failed left channel. Tested the scope with its own 1v ptp square wave test signal and both channels work ok...however it came without a probe so I have ordered one up.....should be interesting ;¬)
Real dual beam scopes are a treat to use. My Philips scope is it's (heavy) weight worth in gold when checking for faults or anomalities
Vinyl records, cassettes, open reel, valve amplifiers and film photography.
I'm wondering now if I should buy a signal generator too in order to trace this fault.....
Just had a look on the internet, seems there are some free signal generator software packages available to download.....does anyone know if they are any good?
I assume you are talking about signal generator software that runs on a regular computer? I suppose all of them do what it says on the tin, but there are a few caveats:
Keeping these in mind, they can be just as useful for troubleshooting as a "real" signal generator.
--mika
Thanks Mike
Is there any reason I cannot hook the computer audio output to my oscilloscope and set the signal up prior to injecting into the amp? maybe .5 volt 1khz ?
I was playing around with this software scope/signal generator before I bought my digital scope
Collecting Vintage B&O is not a hobby, its a lifestyle.
Ok.....got my scope up and running
downloaded a free signal generator for the Ipad and got that hooked up also,
started to work through the circuit from the tape2 input checking both left and right channels for the 4khz signal I was injecting.......
I was able to trace this signal, varying the amplitude via the volume slider, and varying the frequency via the sig generator all the way to the base of both transistors TR41 and TR52.....however I am loosing the signal at the wiper of the emitter bias trim pot 569 on the left channel, the signal is still good at this point on the right channel 479. So my fault is upstream of the bias on TR52.....unfortunately our garden gate fence post has been blown down in the wind......so I will have to leave it for a short while. I will carry on asap......any advice would be welcome ;¬)
Measured these readings on the scope first the good channel............
Then the not so good channel, the reading where taken from the emitter idle voltage trimpot wiper, the bad channel looks all chopped up and ragged....I also noticed a faint buzzing from the transformer.....any ideas ?
A buzzing or noisy Beomaster 4000 transformer is not uncommon. I had one that was noisy to the point it made the Beomaster unusable. Beolover recently repaired one with that problem. The solution was a new manufactured, toroid replacement. It turned out well for him but be aware, it isn't cheap like buying replacement capacitors.
sonavor
Yes Sonavor I have seen Rudi's BLOG regarding this exercise, and your right it is quite an expensive enhancement. I myself noticed a leak on one of my big smoothing capacitors last night as I was exploring my failed channel, the big 5000uf 80v piece....I have some replacements so will swap all three out this evening, I don't think this is my issue however. I will continue to pursue this by exchanging each component from the right channel to the left channel until I bottom it out.....I have received the recap components for my third 4000 unit and also for my pair of beovox 3800 speakers.....but I'm enjoying this challenge too much right now to let it go and drop onto another job. As I get better at this I have now noticed that the signal I am picking up on the base of TR52 (the failed channel) is not a healthy signal at all and in fact looks more of a half wave form, where the working channel exhibits a true sine wave at this point........I will persevere ;¬)
Ok....got the power supply capacitors changed out and confirmed everything was back to where I started prior to detecting the leaking cap, powered up ok and still gives the same readings on each channel.....I will now systematically swap each output transistor from the good side to the bad side and see what happens.
progressing through the output board swapping the transistors over one at a time and checking for the healthy signal at the wiper of the emitter idle voltage pot on both channels I find that in each case the right channel continues to deliver a healthy signal with the transistors from the left channel.......until I reach left channel TR53, when swapped with TR42 on the right the right channel fails to give the signal and it appears on the left channel.....great I thought, cracked it! however after about 10 seconds the signal disappeared....so now I have lost both signals....powered down to check connections and discovered TR53 was very hot.....swapped both transistors back and the signal reappears on the right channel as previously however I now check the original TR53 and it is getting hot.....so my problem looks downstream of this component.
i'm enjoying this good mystery Craig. I did a similar troubleshoot with analog scope/same iPad app on a Rotel preamp. Found the lost right channel at a cracked solder joint.
Finally.............after what has seemed like a lifetime I have bottomed this out, 2 failed components TR52 and TR54 on the left channel. Two failed components caused me considerable confusion.....I suspect TR54 went down and in turn burned out TR52, I'm probably wrong there but not to worry. One thing I have learned is that replacing capacitors on a working machine and fault finding down to component level are two completely different things....this has been a war of attrition, and all for a machine I will be keeping for spare parts for the two healthy units I already use.........Well now I can get on with replacing the capacitors on Steve's (sounds heavenly) 4000 and also the same on the two beovox 3800 speakers. I will start a thread on the speakers exercise in due course...... ;¬)