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Old 18th Jan 2019, 6:48 pm   #1
RojDW48
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Default Decca SG188 lash up.

My second one of these nice old things. I pulled it all out to work on because it is difficult to get at anything in its cavernous cabinet and it appeared to have been bodged somewhat. But that turned out to be nothing more than switches in line with the 3 speakers (I wonder why?). All appeared to be in place so I warmed it up gently, connected an old Radford Beaumonde to the central bass output and a couple of little Aiwa speakers to the L & R mid range outs. It sounds really good - 60 years old and the only repair I can see is a 'new' can full of smoothing caps! The only issue is a slight hum in the R/H channel only. Anything obvious I should search for? It has twin 5 stage amps with an EL84 push/pull output.
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Old 18th Jan 2019, 7:05 pm   #2
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Default Re: Decca SG188 lash up.

Very nice chassis there, a little like the US Magnavox and Motorola console amplifiers.

I guess you know that it is a live chassis and the transformer only supplies the heaters. The HT is straight off the live mains? So take care with it on the bench like that.

It's ECL82s in PP by the way.

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Old 18th Jan 2019, 11:09 pm   #3
Edward Huggins
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Default Re: Decca SG188 lash up.

My goodness, that's a revelation. I would not have thought that such a generous top shielded, through chassis, TX would only just be a heater (filament) transformer.
So unlike Decca.
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Old 19th Jan 2019, 1:05 am   #4
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Default Re: Decca SG188 lash up.

Yes - ECL82s - not the ubiquitous EL84s. And - call me Mr Fortunate. Believe it or not - I hadn't focused on the power supply or the twin rectifiers - I have now! Strangely - the mains socket is 3 pin and a previous owner had added a chassis earth (I tried it on and off - it didn't make any difference). I am very careful with these things and I shall be even more so with this one - thanks for the warning!
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Old 19th Jan 2019, 8:04 am   #5
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Default Re: Decca SG188 lash up.

Quote:
Originally Posted by Edward Huggins View Post
My goodness, that's a revelation. I would not have thought that such a generous top shielded, through chassis, TX would only just be a heater (filament) transformer.
So unlike Decca.
How how cheapskate is that. And - 2 x EZ80s all strapped for 1/2 wave rectification. Why would you?

RojDW48
No problem at all. I guess the previous owner just assumed it was isolated because there was a transformer. Not always the case!
Lucky everything was wired with Neutral to the chassis...
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Old 19th Jan 2019, 9:36 am   #6
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Default Re: Decca SG188 lash up.

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Originally Posted by RojDW48 View Post
the mains socket is 3 pin and a previous owner had added a chassis earth (I tried it on and off - it didn't make any difference).
It sounds like it probably has an isolated chassis with internal positive and negative bus.

Mike
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Old 19th Jan 2019, 9:48 am   #7
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Default Re: Decca SG188 lash up.

Could be, but not according to the schematic.
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Old 19th Jan 2019, 10:07 am   #8
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Default Re: Decca SG188 lash up.

I think that was the same as the Decca SRG300 that I had. It certainly had strapped rectifiers and ECL82's in the output. There was a single bass unit, although I thought it had 2 coil windings on it and more bits in the crossovers. The models were around at the same time. It was a live chassis with just a small heater transformer that ran hot.
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Old 19th Jan 2019, 10:25 am   #9
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Default Re: Decca SG188 lash up.

Quote:
Originally Posted by Edward Huggins View Post
My goodness, that's a revelation. I would not have thought that such a generous top shielded, through chassis, TX would only just be a heater (filament) transformer.
So unlike Decca.

Probably because more power usually goes to the heaters than the HT!
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Old 19th Jan 2019, 12:20 pm   #10
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Default Re: Decca SG188 lash up.

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Originally Posted by vidjoman View Post
I think that was the same as the Decca SRG300 that I had. It certainly had strapped rectifiers and ECL82's in the output. There was a single bass unit, although I thought it had 2 coil windings on it and more bits in the crossovers. The models were around at the same time. It was a live chassis with just a small heater transformer that ran hot.
I have been puzzled by the x-over. The circuit diagram shows 4 elements but here there are only 2 and no sign of components having been removed. However it seems to do the job of directing the bass to the centre and R&L mid range to the L&H spkrs. According to the service sheet the balance control acts on the feedback to the cathodes of V1A and V1B - is that normal? I thought balance controls were simpler things than that. Incidentally it doesn't seem to have any effect.
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Old 21st Jan 2019, 12:41 am   #11
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Default Re: Decca SG188 lash up.

TT up and running with no work at all (but I gave it a little service to be on the safe side). The balance control started working with a second squirt of Servisol and a good wind. Found another example of cheapo construction. The screened cables to and from the volume control were only soldered to the ground in the octal plug on one of them (bridged at the control pot can). Sad when your idols turn out to have feet of clay. And the hum in the R/H channel has diminished with some focused cleaning. For all its economies it does sound very good indeed.

Incidentally - can anyone shed any light on my query in the last post? According to the service sheet the balance control acts on the feedback to the cathodes of V1A and V1B - is that normal? I thought balance controls were simpler things than that.
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Old 21st Jan 2019, 9:53 am   #12
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Default Re: Decca SG188 lash up.

Quote:
Originally Posted by RojDW48 View Post
... can anyone shed any light on my query in the last post? According to the service sheet the balance control acts on the feedback to the cathodes of V1A and V1B - is that normal? I thought balance controls were simpler things than that.
Actually a rather good way as there is no additional pot in the signal path.
I mentioned USA console amplifiers before and that is common on them.

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... The screened cables to and from the volume control were only soldered to the ground in the octal plug on one of them (bridged at the control pot can).
That might be by design too, if the screen is not used as a signal path, just for screening, best practice is to only 'ground' one end. It prevents possible ground loop problems.

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Old 27th Jan 2019, 9:18 pm   #13
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Default Re: Decca SG188 lash up.

It was all going so well Several hours running on the bench - no probs. Fitted it back in the box, ironed out problems with the TT. Enjoying a bit of Reggae and fruits of labours when I noticed distortion in the bass speaker (none in the mids)....followed by sizzle, flash and smoke!! Do these symptoms point to anything obvious I should look for when I take the dratted thing apart again ?
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