Alba 7070 stereogram

[kirstyd]

Acquired this today [although why I don't know] I was told by its 85 year old owner that it just stopped working and that she now had to listen to The Archers on a tinny digital wireless that she hates. She also told me it had been repaired sometime in the 1980's because one channel stopped working. Once home I had a quick look inside and then powered it up. It came to life for a few seconds on FM radio and then the sound slowly faded and was replaced by a hum the scale lamps also went dim at the same time. It was then that I noticed that the 150 milliamp fuse had been replaced with a 1.5 I can see where someone has fiddled with this in the past by the very bad soldering and a capacitor which to me looks much older than the unit possibly a bodge to get the dead channel working again. I unsoldered the two leads from the transformer to check its output and found just under 9 volts and it's still getting very hot even with no load on it. Could it have just packed up or would there have to be a reason for it to fail?

[Nuvistor]

It’s very likely the transformer have been damaged by a fault, perhaps a s/c rectifier or filter caps or even the output stages of the amplifier.
Have you a current limited PSU you could feed the tuner/amplifier section, see if you have faults in that section that could have damaged the transformer and of course check the recitifier and filter caps.

[SiriusHardware]

According to Radiomuseum.org the details for the Alba '53 chassis' are in "Radio And TV Servicing", volume 1975-1976. Unfortunately I don't have those books, although many forum members here do.

If you think there is a problem with the transformer and you don't have another spare, what is it that follows the transformer - a standard bridge rectifier (or four diodes wired as a bridge) feeding one big smoothing capacitor to produce a single supply?

Check the bridge / diodes first and then if you have a variable voltage bench power supply, you could try feeding DC power straight to the ends of the main smoothing capacitor to see if the radio / amplifier part runs OK that way. If you try that, keep an eye on the current being drawn. If it's sky high or starts low then keeps on climbing up and up, you obviously have a problem further downstream.

Edit: Nuvistor beat me to it this time...

[kirstyd]

well guys i just cobbled together two PP9 batteries as a primitive power supply and i have radio 2 playing nicely with the volume control on max but then it is only getting 18 volts .I have also replaced the very old looking cap and its counterpart on the other channel for two new ones

[Nuvistor]

Well that’s a good start, PSU components to check next. Where in the circuit is the fuse, I presume in the primary of the transformer?

[kirstyd]

yes it is

[Nuvistor]

A large fuse in tbe primary and a S/c in the PSU would cook the transformer. The PSU is probably a simple design , the cost of a rectifier and filter caps will probably be less than a replacement transformer, worth considering rebuilding the PSU if you go ahead with the repair.

Even if the fault is in the tuner/amp, appreciate you have it working with batteries, the rectifier as well as the transformer will have been overloaded.

[Herald1360]

Originally Posted by SiriusHardware

According to Radiomuseum.org the details for the Alba '53 chassis' are in "Radio And TV Servicing", volume 1975-1976. Unfortunately I don't have those books, although many forum members here do.

That took a bit of finding- there's no mention in radiomuseum.org it's in radiomuseum.co.uk!

Anyway it's apparently page 417 in that volume which conveniently appears to be the only one I don't have! Or it's gone walkies

[SiriusHardware]

Oops, sorry about that.

However, it seems Kirsty has narrowed this down to a power input problem so there may not be any pressing need for a full diagram, although I always try to obtain one anyway.

It might still be useful to know (from the diagram) what the AC input to the bridge / resulting DC voltage after the bridge is supposed to be, if anyone can lay hands on their copy.

[TonyDuell]

I do have the appropriate volume of R&TVS on the shelf. I had a quick look this morning and it seems to contain circuit diagrams of everything apart from the power supply -- I could not spot a mains transformer. The tuner, stereo decoder and amplifier diagrams are there

I'd be happy to attempt to scan the relevant pages if you want them.

[Techman]

Yes, I had a look in that book too and can confirm that it shows everything but the power supply. A quick look seemed to indicate that it should be around 25 volts after the rectifier that's not shown.

[TonyDuell]

As I was scanning some other stuff, I did scan the bit of R&TVS on this model. The software I got with the scanner does not produce efficient .pdf files so it's too large to post here but I am happy to e-mail to anyone if they PM me their address.

And yes, it looks like a single -25V (posiitve earth) supply.

[kirstyd]

well the unit has been running for around three hours [on three PP9s one new one and two old ones so when all connected together i am getting around 24 volts ]with no problems at all picking up radio 2 with just a bit of wire stuck in the Ariel socket .I did power up the transformer again with no load on it it got very hot very quickly so it looks like i needs a replacement .The transformer also supplies power to the two scale lamps that are 6.5 volts .Can anyone suggest a suitable replacement ?

[Herald1360]

Something like this would probably do:

https://uk.rs-online.com/web/p/chass...rmers/0504246/

Connect the two secondaries in series to feed a bridge and change the indicator lamps to 12V types and float one across each 9V secondary.

You may find something similar cheaper elsewhere.......

[SiriusHardware]

Can I suggest you try connecting an independent 4.5V DC supply to the wires which would normally be powered from the lamp supply winding on the transformer - you still don't know what took out the transformer - a short or very low resistance on the scale lamp circuit is one possibility.

If you want to keep things original there are a couple of forum members here who can repair / rewind transformers.

If you don't mind deviating, then just use a transformer with a single ~18V-20V output winding and rewire the lamp circuit to use two low wattage 12V automotive (dashboard illumination?) lamps in series so they can be run on the AC-out from the transformer as well, or from the 25VDC out from the bridge if the bridge can handle the extra current. If powering the lamps from the DC output from the bridge, 2 * 12V warm white LED substitute bulbs in series would reduce the current a lot.

[Techman]

I thought I'd have a go at scanning the pages from this book seeing as I've got it to hand and also to have a go with my £2 bargain second hand scanner from the local auction house. They're all at 200dpi and the first one is in straight 'black and white', but I found that this wouldn't produce a good enough resolution for the actual circuits, so the rest I scanned in 'grayscale'. The other problem was that the book wouldn't lay flat on the glass due to the raised edges of the surround, so I compromised by making sure the more important lower part of each page was inside the border of the glass for best focus. Probably better than nothing, so let's see if they'll upload:-

[Nuvistor]

The second photo in the first post shows large transistors, T03 type? Yet the circuit shows AC176 and AC128 as the output transistors.
It’s getiing late for me, what am I missing?

[kirstyd]

thankyou Techman .Just the ticket

[ms660]

Originally Posted by Nuvistor

The second photo in the first post shows large transistors, T03 type? Yet the circuit shows AC176 and AC128 as the output transistors.
It’s getiing late for me, what am I missing?

You're looking at the previous model entry in the book, try the 3rd PDF.

Lawrence.

[Nuvistor]

Thanks Lawrence, knew it would be something silly.