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Beomaster 4000 addiction in the making

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Craig
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Craig posted on Wed, May 4 2016 8:08 AM

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


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Craig
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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.

 


Craig
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Craig replied on Wed, May 11 2016 7:34 AM

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 ;¬)

Søren Hammer
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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 Yes - thumbs up

Vinyl records, cassettes, open reel, valve amplifiers and film photography.

Craig
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Craig replied on Thu, May 12 2016 7:37 AM

I'm wondering now if I should buy a signal generator too in order to trace this fault.....

Craig
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Craig replied on Thu, May 12 2016 9:26 AM

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?

tournedos
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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:

  • Computer audio outputs are typically not galvanically isolated, which may result in ground loops and hum
  • If the output is a combined headphone / line level socket, the output impedance can be pretty much anything
  • Because of the unknown impedance and "0-100" volume settings, you won't have much idea of the real output level into a given external device unless you just try and measure it. If the device you're troubleshooting is dead, test & set the output level with some other similar device/input that works.

Keeping these in mind, they can be just as useful for troubleshooting as a "real" signal generator.

--mika

Craig
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Craig replied on Thu, May 12 2016 1:40 PM

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 ?

Søren Mexico
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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.

Craig
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Ok.....got my scope up and running


Craig
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downloaded a free signal generator for the Ipad and got that hooked up also,


Craig
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started to work through the circuit from the tape2 input checking both left and right channels for the 4khz signal I was injecting.......


Craig
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Craig replied on Sat, May 14 2016 10:57 AM

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 ;¬)

Craig
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Craig replied on Sun, May 15 2016 3:35 PM
Got back to it today........removed both transistors TR41 and TR52 and replaced them in reverse, powered up and still the right channel works fine and the left is still not playing ball. Well at least TR41 is eliminated from the list of potential suspects.
Craig
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Measured these readings on the scope first the good channel............


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