Having reently been asked to repair one of these Professional HiFi Amplifiers with a defective mains on/off switch (one pole totally o/c), which I've now replaced, I found that the BR82 bridge rectifier was s/c across 3 of it's 4 terminals-this in turn had caused the 5A(T)fuses in series with the transformer secondary to blow. I've now obtained a replacement rectifier and 20mm ceramic fuses of the correct type, and checked the secondary voltage on the transformer, which is 28-0-28vac, measured with my DMM. This would give nominally 40-0-40vDC, which, to, me is about right. As there do not appear to be any other faults, I'm wondering what caused the rectifier to fail in this way. On another forum someone suggested that the faulty on/off switch may have caused this to happen, but, despite almost 50 years of working with electronics, I cannot see how an open circuit connection between one side of the mains supply and the transformer primary could possibly do this, since, surely, no current can flow in any circuit unless both sides are connected? If I'm wrong, someone please enlighten me!!
Your symptoms suggest a problem with the load fed from that supply, does the amplifier still work?
Does it keep blowing fuses?
Does it keep blowing fuses?
If the on/off switch arcs continuously, it will take out the primary of the transformer, without a doubt and not affect the secondary.
I have had the bridge rectifiers on the VX-150 and the 900 fail before and with the excess current, being practically a short circuit on the secondary, it is likely to overload an old mains switch.
New switch and bridge will affect a long term cure, in my experience.
I have had the bridge rectifiers on the VX-150 and the 900 fail before and with the excess current, being practically a short circuit on the secondary, it is likely to overload an old mains switch.
New switch and bridge will affect a long term cure, in my experience.
Point taken,, but could arcing not produce high frequency high voltage on secondary side and this could bourn the bridge rectifier
I didn't think about HF-HV on the secondary, amplitude, the phenomenon you describe, JSE, but, having checked the transformer and replaced the switch, I'll fit the new rectifier, monitor the current drawn by the load, and check that the amplifier works O.K., which, A.F.A.I.K, based on my earlier checks, it should, then it should be a case of 'job done!'
The transformer will not pass HF very well. It was designed to be most efficient at between 40 and 70hZ the peak voltages at switch on/off will be well within the bridge rectifier's 200volt 10Amp rating.
All I can add to the above is that, at least under no-load conditions, the transformer is O.., with the expected secondary voltages present. Although i didn't make a note of them, and the amplifier is some distance away, so I can't check the figures, from memory, the DC resistance of the primary winding is about what I'd expect from a large toroidal Tx.
I have now fitted the new bridge rectifier, and, after having checked the A.C. through the rectifier (ca. 400mA quiescent), fitted 2 new 5A(T) Ceramic bodied fuses, besides replacing the on/off switch with the correct type supplied by MAJ Electronics of Kingswinford (+44(0)1284 278646), put the amp. on test, so far for only about 15 minutes, and it is working well. The only other thing I've had to do is spray some Servisol 10 into the function switches on the rear panel, as these were slightly crackly. The VX-150 will go on a longer 'soak test' before it's returned to it's owner, but it looks like a case of 'all's well that ends well' The only other comment I'll make for now is that I found the new switch very difficult to press into the panel cut-out despite the fact that it is identical in every way to the failed one. The panel is maybe 0.125" (over 3mm) thick, and, with the fixing lugs on the new 'snap-in' switch being stiff that could explain why. Anyone have any tips as to how to more easily fit these switches?
If you spray a little silicon polish, (Mr Sheen), on a cloth and wipe the panel cutout with it, that works well in most cases.
Reading how the post progressed, there was an awful lot of over thinking going on.
As I said, keep it simple and to the point. It is only an amplifier after all.
Good to read it is now fixed.
Final thing,note the position of the quiescent current pots, give them a twiddle and put them back to where they were. That will knock the dust off the wipers.
Reading how the post progressed, there was an awful lot of over thinking going on.
As I said, keep it simple and to the point. It is only an amplifier after all.
Good to read it is now fixed.
Final thing,note the position of the quiescent current pots, give them a twiddle and put them back to where they were. That will knock the dust off the wipers.
Didn't think of using Mr. Sheen or similar, Jon-next time I will do so. I'l check the Iq. pots., as you suggest. Might be a good idea to do that with any pre-set pots. generally come to that. As you may know, those in the HH are good quality Piher or similar enclosed types, but there could still be an ingress of dust into them. As a general rule, I use enclosed pots. to replace the skeleton pre-sets used in a lot of consumer electronics products.
If the smoothing caps are old and starting to go short it could overload the old switch and rectifier.
Sometimes things don't fail catastrophically and just slowly go bad.
Sometimes things don't fail catastrophically and just slowly go bad.
They check out O.K., Nigel, but maybe I should change them as a precaution. In the meantime I've found that the slide switches on the input PCB didn't respond to Servisol treatment, so I'll have to replace those, assuming I can still get suitable types
Switches with part Nos. SK43003VG5 & SG43001-GB would seem to be suitable types, but I cannot find a supplier on either UK eBay, CPC (a leading UK Parts supplier) or Cricklewood Electronics(a London-based parts stockist) Does any forum member know from where these may be obtained?
I have since found that I can obtain the switches from MAJ electroic, so will order from them. On an unrelated point, the DC offset on this amp. after several minutes running, is 6-7mV on one channel and 11mV on the other, which, to me, is very good, as it should be.
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Since my last post, I've basically completed the repairs to the VX-150 (new on/off switch, rectifier, secondary fuses, and repairs to the i/p PCB, mainly replacement of intermittent slide switches.) On test all functions are working, though I did wonder why, when I connected a CD player, via a twin RCA socket to stereo jack plug, I was only getting music out of one speaker, no matter how I set the slide switches. Perusal of the circuit diagram seems to suggest that, in order to hear the CD player or other stereo source through both speakers, I would need two adapter leads, each of which consisted of an 0.25" Stereo jack plug and two RCA plugs, so that, in any setting of the slide switches the sound would be amplified through both channels of the VX-150. For someone like me, more used to domestic stereo audio equipment, the input switching arrangements on the 150 appear at first to be quite complex, although much of it is obviously to do with the fact that, besides the stereo jack i/ps, there are XLR and Octal sockets, plus Balanced, unbalanced, and stereo/mono options.
Do you mean the T/E/U switch??
In domestic hi-fi use, leave both on "U", unbalanced. This skips a stage you do not need, goes direct to the gain control.
Attachments
Yes, PRR, that is the switch to which I referred. I had traced that highlighted path through the diagram, so could see how a signal from one of the i/p gets to the o/p stage of the 150. As you know there is an identical switch in both channels, which is why I came to the conclusion that, in order to hear audio in both speakers from, e.g., a CD Player, the LH channel o/p of the player would need to be taken to (say) CH1 i/p, and the RH channel o/p to CH2 i/p of the HH Amp. Further perusal of that diagram shows that in the 'T' position, the Octal, Jack, and XLR connections are simply paralleled, whilst in the 'E' position i/p signals are fed through IC1. This summary, which I trust is correct, may help others unfamiliar with this type of professional amplifier.