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Old 10th Oct 2017, 8:49 am   #1
GrimJosef
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Default Blaupunkt 14.922 radiogram - is it meant to be a boom box ?

I'm currently working on a Blaupunkt Arkansas radiogram. Blaupunkt used Arkansas as the name for a range of valve 'grams over the years. This is one of their last ones, from the mid 60s I believe. The main chassis is labelled type 14.922. I haven't been able to find any documentation for this but it looks essentially identical to the pictures of the type 13.922 and it has the same valve line-up, except that the 14.922 also has a second magic eye - an EAM84. (This is fitted as an FM stereo indicator but bizarrely it seems never to have been used as the plug-in stereo decoder (optional ?) hasn't been fitted to this unit.)

Anyway, my question relates to the audio stage - a single ECL86 in each channel. The DC voltages on the valves measure close to the values in the 13.922 data sheet but when I connected the set to my workshop speakers I was struck by the way it boomed. So I plugged an audio analyser into the pickup (PU) input and measured the frequency response of each channel into a resistive dummy load with the volume control rotated to full. The results in dB vs frequency in Hz are shown for the left and right channels in orange and blue on the graph. Sure enough there's some treble roll-off and a great big bass boost peaking around 100-200Hz. No wonder the music sounds like it's being played down a length of sewer pipe.

A couple of explanations occurred to me*, but thinking about them I can't see that either holds water. So could it just be that the set is actually meant to sound like this ? Or should I be looking for a fault in both channels ? Or is there another explanation that I haven't thought of ?

I could just pile in and start removing capacitors and measuring them. But as you can see from the redrawn 13.922 circuit, which shows one channel of the audio section, assuming the 14.922 is the same there are 18 capacitors, excluding the ones which are smoothing the HT rail. So that would be quite a big job and it would be a bit embarrassing to have undertaken it only for one of the more experienced folks on here then to come on and say "Oh yes, if you'd asked me I could have told you that all 60s radiograms sound like that !".

Cheers,

GJ

*Thoughts I've had include

1. The amp has been tuned to make up for serious shortcomings in the gram's speakers. But when I listened through those speakers the music sounded just as boomy. And there are sockets for external speakers which mute the internal ones. The set wouldn't sound decent through those if the audio spectrum was seriously distorted.

2. The amp includes some compensation for the records' RIAA recording characteristic. But the measured frequency response isn't a very good match to the RIAA curve, which is shown in black on the graph. Also the same amp is used for the internal radio signals and for internal tape input, neither of which needs RIAA correction. The only electronic difference is a deep-ish bass roll-off provided by a 3nF/1.5Mohm RC filter on the PU input. Also the gram was originally fitted with an Acos GP73 crystal cartridge and with relatively high-impedance loading the signal from this will 'self-integrate' which will approximate the necessary correction automatically https://www.vintage-radio.net/forum/...d.php?p=300251.
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Last edited by GrimJosef; 10th Oct 2017 at 9:05 am.
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Old 10th Oct 2017, 9:04 am   #2
paulsherwin
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Default Re: Blaupunkt 14.922 radiogram - is it meant to be a boom box ?

I can't really comment on this specific model without listening to it, but German luxury radiograms generally did have a boomy sound because that's what customers liked and associated with luxury. The vast majority of people had no exposure to high fidelity reproduction and had no interest in accuracy. To some extent it was a generational thing.
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Old 10th Oct 2017, 10:26 am   #3
G8HQP Dave
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Default Re: Blaupunkt 14.922 radiogram - is it meant to be a boom box ?

The circuit diagram seems to show that someone went to a lot of trouble to tailor the frequency response, so it was intended to sound like that.
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Old 10th Oct 2017, 11:54 am   #4
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Default Re: Blaupunkt 14.922 radiogram - is it meant to be a boom box ?

I'm a bit puzzled about some things on the circuit diagram:
Is switch "c" supposed to be changing from radio to tape, or from PU to "radio/tape"? The switch contacts seem to indicate the latter.
Does the volume control really have two tappings? That would be a pretty rare item, I would think.
What does switch "a" do?
Is switch "b" really meant to be two switches in series and if so, why?
Colin.

Last edited by ColinTheAmpMan1; 10th Oct 2017 at 11:57 am. Reason: More questions.
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Old 10th Oct 2017, 12:06 pm   #5
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Default Re: Blaupunkt 14.922 radiogram - is it meant to be a boom box ?

Looking at various Arkansas schematics a volume control with two tappings is common.

Lawrence.
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Old 10th Oct 2017, 12:35 pm   #6
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Default Re: Blaupunkt 14.922 radiogram - is it meant to be a boom box ?

Yes. It's clearly been rather carefully and expensively engineered to sound in a particular way. I'm just surprised they went to all that effort to make it sound this way, which is why I was wondering if a capacitor or two could have drifted horribly.

Cheers,

GJ
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Old 10th Oct 2017, 12:47 pm   #7
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Default Re: Blaupunkt 14.922 radiogram - is it meant to be a boom box ?

The two channels are so close I think it would be unlikely that the same parts have failed in the same way on both channels. I would have expected more of a difference even with failed parts in both channels. Not impossible I suppose.

Perhaps designed for this type of music. https://youtu.be/G9UqZOv8-OY
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Old 10th Oct 2017, 12:58 pm   #8
Edward Huggins
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Default Re: Blaupunkt 14.922 radiogram - is it meant to be a boom box ?

There's some interesting circuitry after the OPTX secondary. My feeling is that it was consciously designed this way and somehow complemented the 4 speakers in the radiogram's cabinet.
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Old 15th Oct 2017, 5:50 pm   #9
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Default Re: Blaupunkt 14.922 radiogram - is it meant to be a boom box ?

I seem to remember reading somewhere that these radiograms were favoured by the Caribbean community back in the day because they had this specific sound apparently it was great for playing reggae records .
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Old 15th Oct 2017, 6:44 pm   #10
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Default Re: Blaupunkt 14.922 radiogram - is it meant to be a boom box ?

I was about to say that kirstyd. I seem to recall Lenny Henry saying that on Radio 4. I doubt they manufacturers had Lee Perry recordings in mind at the time, but it's interesting all the same.
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Old 15th Oct 2017, 6:48 pm   #11
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Default Re: Blaupunkt 14.922 radiogram - is it meant to be a boom box ?

Yep in the Manchester area at least, Blue Spot's were sometimes spotted...One Step Beyond...

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Old 18th Oct 2017, 7:31 pm   #12
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Default Re: Blaupunkt 14.922 radiogram - is it meant to be a boom box ?

I have serviced a number of these radiograms, mostly for people from the Caribbean community, they sound great playing reggae.
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Old 18th Oct 2017, 9:38 pm   #13
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Default Re: Blaupunkt 14.922 radiogram - is it meant to be a boom box ?

Thanks for all the info folks. I've spent a bit more time with this and made some progress, but I'm not out of the woods yet I fear.

The first thing to say is that the tonal properties of the amp depend quite strongly on the position of the volume control. Measuring the results with the volume at full might not reflect its 'typical' behaviour.

The second, and maybe more important, is that the signal fed back from the network at the output is affected by the position of switch A. The two positions of this switch are labelled 'Hi-Fi' and 'Sonor' (presumably for Sonorous or something like it). I made the measurements with the switch in what I think is the Hi-Fi position. But I've since discovered that the switch contacts were very badly oxidised and may not have been making good contact. I've cleaned them now, to the best of my ability, and the switch's behaviour does seem more reproducible. But I'm not convinced it's working properly. Somewhat to my surprise the sound seems less coloured in the Sonor position, although there is clearly still a good deal of bass boost and some treble roll-off. In the Hi-Fi position things sound worse. If the volume control is moved away from either end of its travel a good deal of hum appears (the same on both channels, as far as I can tell by ear). I did start to trace the circuit but the components are spread across at least three separate pcbs, none of which is easy to extract and two of which have one side hidden by other parts of the set. So far the circuitry around the bass, volume and treble controls seems to be as I've drawn it for the 13.922 model, except I haven't been able to locate the 470pF capacitor shown shunting the 1Mohm pentode grid-leak. More importantly I haven't found the inductor which is shown attached to switch A. I really need to sort out what's going on with that switch before I can be sure that all is well.

I tried Bob Marley's 1975 'Live' from The Lyceum album. It wasn't too bad on Sonor, but it was a bit grim on Hi-Fi.

Cheers,

GJ
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Old 18th Oct 2017, 10:40 pm   #14
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Default Re: Blaupunkt 14.922 radiogram - is it meant to be a boom box ?

This has been covered before but maybe not with such a forensic look at the circuitry. As others say, these machines seemed to bridge a cultural gap in terms of being attractive to Carribean people settling in the UK, having apparently, the right look and sound for the Reggae culture and [presumably] that of Germany!

They certainly didn't seem to be as prominent in other parts of the UK. The Cocktail Cabinet aspect itself [of some models] may also have been quite significant although it's certainly not exclusive to Blankput. I think it may have been Lenny Henry who referred to having somewhere to keep the Rum as well, which can be as staple a part of Carribean daily life as, say, whisky possibly is in Scotland or perhaps Gin in the Home Counties! Not everyone makes purely technical buying decisions as we all know from another current audio thread.

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Old 18th Oct 2017, 11:00 pm   #15
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Default Re: Blaupunkt 14.922 radiogram - is it meant to be a boom box ?

Blaupunkt had a reputation as good as Grundig for well engineered radios and radiograms using loudspeakers with good excursions to reproduce a tailored audio response. The tailored response, whilst not quite to fletcher - munson curves, was well designed and can be modified by use of the bass and treble control.

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Old 19th Oct 2017, 5:05 am   #16
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Default Re: Blaupunkt 14.922 radiogram - is it meant to be a boom box ?

I have some OPT's off a Blaupunkt gram pretty similar to yours, which use a ECL86. From memory they had an odd Z ratio and extra secondary winding's. I was unable to figure out how it was supposed to be connected to get the correct anode load.

Pretty sure mine had a tapped tone control as well as quite hefty bass speakers, far bigger than other gram's.

The gram was very well engineered with some very complicated doings round the tuning section allowing the AM arial to be moved on the horizontal axis. What I'm trying to get at is that the tone altering/shaping circuitry could be either well engineered and thought out, or a bit of seemed like a good idea at the time. Bit like some of Philip's Prof Brainstorm like idea's.

Andy.
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Old 19th Oct 2017, 6:33 am   #17
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Default Re: Blaupunkt 14.922 radiogram - is it meant to be a boom box ?

Going back to GJ's first post and the plotted responses, the graph seems to invite looking at the difference between the plots and the black line which I take is the RIAA equalisation curve?

In this case there seems to be more bass boost and more treble cut than I'd ever want.

But, with a pizo-electric cartridge, is RIAA appropriate? They usually get flat amplification, so the plotted responses look even worse.

I was just thinking that the cocktail cabinet was a good idea, anyone listening to that would want to be seriously drunk, when a memory surfaced. Wasn't there a preamp design (transistor) in WW by Bailey/Burrows/Quilter that loaded a piezo cartridge into a low-z, and then applied bass boost to compensate for the resulting roll-off?

Is something like that going on? An audio analyser will probably be a much lower z source than this amp is intended to work with. I think you need to create a drive network to model the characteristics of the original cartridge to get plots that give the real picture. There's probably interaction with the cabinet and speakers too.

I now have this mental image of the upper management of Blaupunct deciding on the wanted tone of this beastie as it bounces around their board room playing oompah band music flat-out as they also test the cocktail cabinet...

Blau-punk'd

David
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Old 19th Oct 2017, 7:48 am   #18
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Default Re: Blaupunkt 14.922 radiogram - is it meant to be a boom box ?

Quote:
Originally Posted by kirstyd View Post
I seem to remember reading somewhere that these radiograms were favoured by the Caribbean community back in the day because they had this specific sound apparently it was great for playing reggae records .
Absolutely spot on! In the late 60's I used to help out with service calls for a long gone radio chain a branch of which was in Stockwell Road Brixton London SW9.

The Blue Spot radiogram was bought exclusively by this community, as you say for the general boomy sounding bass.

It had a particular sound that did not sound 'right' on general records.

I had one of the models, the one with the cocktail cabinet, in my shop at Colliers Wood for many years and the community loved it. On closing the shop I presented it to one of my reggae customers, a bus driver from Merton Bus garage [AL]

He became quite emotional having owned one many years before. Happy Days! John.
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Old 21st Oct 2017, 4:37 pm   #19
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Default Re: Blaupunkt 14.922 radiogram - is it meant to be a boom box ?

Looking at the full schematic, available on Google, there is no RIAA equalisation to the cartridge which is loaded by 1.3m ohms suggesting a standard crystal cartridge is used.

Further, when the fixed tone buttons viz Normal/Solo, HiFi/Sonor and Normal/Piano are released, the only tone correction is through the volume control which follows normal european practice. One interesting point is what I believe is the use of positive feeback, aka Philips, for the treble control.

All in all just what was expected in a high quality product of its day marketed at those who enjoyed German big band sound. As for the Carribean audience, maybe they too had heard live music and listening to sound through a Bush radiogram lacked interest or excitement even if the measured frequency response was technically better at 1 metres on axis.

This reminds me when my son bought a Tannoy 603 speaker expecting Tannoy Eaton sound only to find it didn't. Extensive use of equalisation helps but at the end of the day it is down to physics.

Chris
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