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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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8th Feb 2019, 2:38 pm | #21 |
Dekatron
Join Date: Nov 2009
Location: Lincolnshire, UK.
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Re: Alba 7070 stereogram
I've caused the confusion by copying double pages at a time. If you're not used to these books then it's not always realised that no page space is wasted and each piece of service information just follows on from the previous one.
I could have copied each page separately, but I wanted to hit it in five shots and keep it all in one post rather than eight and over two posts. The other more important reason (excuse) was that to copy single pages would have meant trying to hold down half the book on the scanner to stop it from falling off under its own weight and would have needed a three handed job to do this, and operate the computer keyboard and mouse at the same time, so it was easier to just plonk the book on top and leave it there. I don't usually post scans to the forum, but just take photos of circuits with an old camera which generally takes just moments to do. The card comes out of the camera and into the computer - job done. I think in this case that the scans have probably come out better than photos would have done, despite having the less relevant top part of all the pages slightly out of focus. As for the burnt out transformer on the set being discussed - we've been here before unfortunately. There's been at least a couple of cases in the recent past where a mains transformer has burnt out supplying a transistorised amplifier and it's ended in frustration and tears each time. Twice I've advised that the amplifier unit be tested FIRST by coupling up some 9 volt batteries in series, as not everyone has a bench power supply for the purpose, and each time this advice has been ignored. However, kirstyd must have been reading this advice in those other threads (just guessing) and has done the correct thing and made that test using batteries, so well done, and you've proved that the basic electronics are all good. It might be worth putting an ammeter in series to check current draw from the batteries and report the reading to see what members think, also to monitor the current from first switch on to see if it tends to increase over time with leakage as the output transistors warm up. It's strange that the service information doesn't contain the power supply, either it was a mistake, or it wasn't available at the time for some reason, or perhaps it was considered too mundane to be included and take up valuable page space, who knows! The problem as regards the transformer in those previous cases mentioned is that from what I remember, the power requirement voltage wise was only a couple of volts different from what's required here and it was a surprise to me that an appropriate transformer is just NOT available to buy at a price that would make the repair worth while. A previous repair attempt mentioned ended up with wrong transformers being purchased, also possibly wrongly connected or faulty rectifiers being used coupled with not testing the amplifier on batteries first as I'd suggested and it all ended in disaster. So to sum up - did the transformer just fail on it's own or has something else caused it to fail? If all the rest of the electronics including rectification and smoothing etc. prove to be good, then can a correct transformer that's readily available and at a sensible price be advised by anyone this time? We don't want the previous disaster I mentioned to happen again. The real answer in all these cases would be to get the original transformer rewound. Glad the service information is useful to you, kirstyd, and well done for doing the 'battery test' - just check the current draw if you can. Last edited by Techman; 8th Feb 2019 at 3:07 pm. Reason: Some typos spotted! |
8th Feb 2019, 4:34 pm | #22 |
Dekatron
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Re: Alba 7070 stereogram
Not your fault Techman, I should have looked more closely.
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8th Feb 2019, 5:30 pm | #23 |
Dekatron
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Re: Alba 7070 stereogram
As regards the PSU circuit, I suspect it was just forgotten about.
But the next entry in the same book is another Alba chassis and that does include the PSU (transformer, bridge rectifier, smoothing capacitor). The transformer is tapped for the scale lamps as described here. I suspect it's much the same circuit. If you need it, then either Techman or I could scan that bit of that diagram for you. |
8th Feb 2019, 7:42 pm | #24 |
Heptode
Join Date: Mar 2014
Location: Leeds, West Yorkshire, UK.
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Re: Alba 7070 stereogram
close up of the transistors. There's four of them
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8th Feb 2019, 7:51 pm | #25 |
Dekatron
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Location: Newcastle, Tyne and Wear, UK.
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Re: Alba 7070 stereogram
Did you try running the scale lamps to see if the scale lamp circuit is OK? The lamps will be intended to run on about 6VAC so it would be advisable to use a lower DC voltage (3V or 4.5V) to try running the lamps. The lamp circuit will have to be disconnected from the transformer winding when you do this, obviously.
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8th Feb 2019, 8:07 pm | #26 |
Heptode
Join Date: Mar 2014
Location: Leeds, West Yorkshire, UK.
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Re: Alba 7070 stereogram
Hi Techman Yes i had read about using batteries to do this test .I have had it running all day [on the batteries]with no problems at all however the unit has been using a pair of my own speakers from an old music centre .Today i have been looking at the ALBAs own speakers [you can actually tale out the screws and remove the backs on them quite a novelty these days]inside i found that the wire had obviously been yanked at some point causing one of them to come loose and worse still touch the other connection causing a dead short.I also found the same fault on the other speaker but at the plug end this time [ what with the wrong fuse and these wires touching each other could this have caused the transformer to burn out? ]I spent half an hour soldering all the relevant wires back where they should be .Whilst doing this i noticed that whoever had attempted to do this repair before me had soldered the wires to the wrong points which would have caused the speakers to be out of phase incidentally the cable on the speakers is just black [no white line indicating + .I may have found a suitable Transformer removed from a Ferguson music centre .Its on its way down here from Newcastle as soon as it gets here i will post again
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8th Feb 2019, 8:09 pm | #27 | |
Heptode
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Re: Alba 7070 stereogram
Quote:
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9th Feb 2019, 6:18 am | #28 |
Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
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Re: Alba 7070 stereogram
Have you tried disconnecting the lamp supply wire from the mains transfomer, and (with the lamps in place) connecting about 4.5V DC between that wire and the 'common' end of the transformer secondary winding to power the lamps? If there's a short-circuit in the lamp wiring it could damage the transformer.
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9th Feb 2019, 11:02 am | #29 |
Heptode
Join Date: Mar 2014
Location: Leeds, West Yorkshire, UK.
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Re: Alba 7070 stereogram
Will try that while I am waiting and let you know what I find.
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9th Feb 2019, 12:06 pm | #30 |
Dekatron
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Location: Leominster, Herefordshire, UK.
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Re: Alba 7070 stereogram
Shorted speakers would certainly cause excessive currents to be drawn- in fact you've been very lucky the output transistors have survived it!
The AD161/2 pair is good for up to about 10W output, depending on the detail in the design and should produce a perfectly acceptable noise with reasonable speakers. There were some classic Mullard Modules- LP1173 ISTR which gave a good account of themselves. You want to end up with about 24V dc on the power rails, a bit lower won't hurt though will reduce ultimate grunt but any higher is pushing your luck.
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11th Feb 2019, 9:52 pm | #31 |
Heptode
Join Date: Mar 2014
Location: Leeds, West Yorkshire, UK.
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Re: Alba 7070 stereogram
Hi guys
Well the transformer arrived this morning .It has two outputs as you can see in my photos 24 volts is just perfect and the unit has been running well for about three hours with nothing showing any signs of stress [nothing getting warm never mind hot].The other winding is supplying 16.6 volts a little two much for the 6.5 volt 3 amp scale lamps .what would be the best way to remedy this problem .Possibly beefier bulbs perhaps ? or is there a better alternative . |
11th Feb 2019, 9:53 pm | #32 | |
Heptode
Join Date: Mar 2014
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Re: Alba 7070 stereogram
Quote:
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11th Feb 2019, 11:10 pm | #33 |
Dekatron
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
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Re: Alba 7070 stereogram
As a matter of interest, have you tried measuring both the AC voltage out from the transformer and the DC voltage across the main smoothing capacitor while you have the unit running? Normally, an AC voltage of 'X' into a bridge rectifier and smoothing capacitor results in a DC voltage output of 'X plus a significant amount more'. Make sure the DC voltage is what you thought it would be. It may be higher than you expect.
If the DC output voltage with this transformer is acceptable, then for the lamps perhaps the easiest solution would be to change the bulbs to 18V bulbs, same physical shape and same wattage as the originals, but otherwise keep the wiring the same - assuming they are wired in parallel, not in series. |
11th Feb 2019, 11:19 pm | #34 |
Heptode
Join Date: Mar 2014
Location: Leeds, West Yorkshire, UK.
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Re: Alba 7070 stereogram
Will do as you suggest and let you know what i find
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12th Feb 2019, 9:19 am | #35 |
Octode
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Re: Alba 7070 stereogram
I think my warning would be a little stronger than SiriusHardware's.
You need to see 24 volts DC on the smoothing capacitor. (The big can in your pictures). Any more may harm the whole unit. You have a 24 volt AC out from the transformer, once rectified and smoothed, by the big cap, you could see 30 to 34 volts DC. (You have to multiple the AC voltage by 1.41 to estimate the DC voltage.) That might be more than the voltage rating of the capacitors in the set and worse may be exceeding the working voltage of the output transistors. All the circuits I see for AD161/AD162 pairs use 20 volts, with 24 volts DC maximum... Alan
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12th Feb 2019, 11:28 am | #36 |
Dekatron
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Re: Alba 7070 stereogram
Yes, that's what I was getting at, of course.
If the DC voltage is too high, and it probably is, then Kirsty could always try running the unit on the 16V winding, assuming that winding can also supply enough current for the needs of the unit. With 16VAC in, the DC voltage will be a bit safer and closer to 24VDC, maybe a bit less. It would be a pity to wreck the unit through overvoltage after getting this far. |
12th Feb 2019, 1:45 pm | #37 |
Dekatron
Join Date: Nov 2009
Location: Lincolnshire, UK.
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Re: Alba 7070 stereogram
This was exactly the problem with the previous record players that I mentioned, in that a correct transformer couldn't be sourced that would give the end DC voltage required after rectification and smoothing.
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12th Feb 2019, 10:04 pm | #38 |
Heptode
Join Date: Mar 2014
Location: Leeds, West Yorkshire, UK.
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Re: Alba 7070 stereogram
running the unit on the 24 volt winding i am getting 30 volts DC off the rectifier .Using the 16 volt winding i am getting 20 volts DC with no noticeable change in volume or sound quality so perhaps it would be better to use the 16 volt winding using the other to power the lamps ?
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13th Feb 2019, 2:07 am | #39 |
Dekatron
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Re: Alba 7070 stereogram
You won't notice any big difference at 20V on the amplifier unless you turn the wick right up- you'll hit clipping somewhat earlier on the volume control. Roughly speaking max output power will drop by about 30%, so if it was 10W before, it'll be about 7W now. A couple of watts is normally plenty for domestic listening so the lower voltage won't matter and the amp will be nicely understressed.
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13th Feb 2019, 10:12 am | #40 |
Dekatron
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
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Re: Alba 7070 stereogram
Yes Kirsty, I think we would keep on using the 16V winding for now. Transformers with 18V output may not be common but transformers with 9V + 9V output used to be very common, maybe look at using one of those with the two output windings in series?
Now that you have a workaround so you can enjoy the unit working, maybe consider getting a price to get the original transformer repaired? It does no harm to ask. |