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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 7th Mar 2021, 4:23 pm   #1
wd40addict
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Default Philips' Rocking Crystal Cartridges

In the early days of LPs Philips produced their own record decks and fitted them with what has come to be known as the rocking cartridge. Its unique feature was that the appropriate stylus was selected by rocking the cartridge either one way or the other:

Green = N = normal = 78, Red = M = Microgoove = 33/45.

The problems started in later years when the crystals dissolved to goo and a replacement was required. Eventually Philips produced the GP235 which used a conventional flipover stylus, but still plugged onto the original arm.

I have two decks of this type: one is fitted to a table top radiogram and the other is a standalone deck badged as a Stella. The radiogram came without a cartridge fitted, the Stella had one, but it had turned to goo.

The photo shows the general form of the Philips / Stella deck.

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The observant among you will notice the washers hanging off the back of the arm to give a counterweight. This is to correct the other issue with this deck: there is no counterbalance - the record / stylus bear the full weight of the arm and cartridge! This might have been fine in 1953, but by the time stereo arrived something better was required. Philips must've realised this and made an accessory counterweight, though they are very rare.

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Some years ago I purchased a copy GP235 to get the radiogram going, but was never happy with it due to the stylus pressure which was almost compressing the suspension. For some reason it was also fitted with a lead weight which made things even worse! I eventually managed to remove this by heating the weight with a soldering iron until the glue gave way...

The next photo shows the copy GP235 together with the modified rocking head to be described later. the arrow shows where the weight was removed.

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Seeing a Philips being 'fixed' on TV I wanted to see if I could improve things further, so the counterweight was constructed by bending a PC bracket so it could hook under the arm and then using a bolt to hold an appropriate number of washers. Stylus pressure could thus be reduced to a sensible 4 grams.

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To be continued
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Old 7th Mar 2021, 4:37 pm   #2
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Default Re: Philip's Rocking Crystal Cartridges

As the copy GP235 was stereo I rewired the Stella for a stereo output, the arm already had the necessary wiring. As has been said elsewhere the sound quality from the copy GP235 was not fantastic, although probably good enough for the radiogram. Fed through the Heathkit S-33H in my den it was a bit lacking.

Ebay is awash with Chinese ceramic cartidges and although these are mainly single stylus types there is also a flipover/turnunder variant. I obtained one of these and dismantled the Philips cartidge to try and fit the new unit inside. The photo below shows the idea.

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The arrows show the points of interest: the top pair show where the existing mounting pillars were milled away to give enough depth for the new cartridge. The middle pair shows where the existing side lugs were filed away to be a snug fit around the new cartridge. The bottom pair show where the new cartridge rests nicely on the existing lugs.

To be continued
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Old 7th Mar 2021, 4:45 pm   #3
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Default Re: Philip's Rocking Crystal Cartridges

Without doubt the Chinese cartridge sounds better, but although its stylus is marked LP and 78 talk on the internet tends to suggest it's not true. Putting the stylus under the microscope gave the following picture:

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One is definitely a sapphire, the other could be a diamond, but they are both the same diameter! Comparing with a BSR ST17 you can see the difference:

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Luckily the ST17 will directly fit the Chinese cartridge as it seems to be a copy of an SC11! But mounting in the 'standard' Chinese mounting clip. For this project a clip is not required.

To be continued
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Old 7th Mar 2021, 6:05 pm   #4
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Default Re: Philip's Rocking Crystal Cartridges

Home straight - why does the Chinese cartridge sound better?

Measuring the response of the 'Philips' GP235 Vs the Chinese cartridge with its own stylus and the ST17 gave the following results:

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The Philips copy shows a massive drop in output above 5kHz, the Chinese cartridge shows a peak instead, but it's better controlled. Small differences can be observed between the Chinese Sapphire and the ST17, but nothing significant by ceramic standards.

Some of the Bass rolloff is due to the S-33H which seems to be engineered this way on its 'Gram' input. The S-33H is allegedly optimised for the Decca Deram ceramic cartridge.

I trust these ramblings will be of interest to someone!
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Old 7th Mar 2021, 8:27 pm   #5
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Default Re: Philips' Rocking Crystal Cartridges

Great work there! I have two of those rocking heads awaiting revival.

Fascinating to see the difference between the frequency responses too.

On a related note I have recently used the innards of a chinese Chuo Denshi clone to revive a later Philips GP300 head, will upload some pictures soon
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Old 8th Mar 2021, 8:42 pm   #6
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Default Re: Philips' Rocking Crystal Cartridges

Just a tip regarding the arm weight. On those Philips green or maroon autochange decks with the three curved push bottoms on the bottom right, there is a tiny hole at the rear of the rounded "bubble" at the back end of the arm. Deep in that hole is a tiny screw which controls a vertical spring, hooked into a lug in the arm bracket. Turning that will alter the weight of the arm and cartridge, but be careful, the plastic lug the the spring is attached to is, after all this time, very fragile!

Barry
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Old 9th Mar 2021, 6:50 pm   #7
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Default Re: Philips' Rocking Crystal Cartridges

Thanks Ben!

Barry: interesting that they realised their error on the autochangers...

The cartridge I modified was the AG3016, there's a good chance it's already a replacement for a AG3010. Even back in the day Philips seem to have had a problem with this series of cartridges.

In some quarters the flipover cartridge is known as a 'Banpa'. Measured Output @ 1kHz is approx 400mV @ 5cm/s.

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Old 12th Mar 2021, 10:30 am   #8
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Default Re: Philips' Rocking Crystal Cartridges

I've just bought a NOS SC7M2 with ST17 flip-over. Looks very similar to yours.

Could you say
a) what load was your cartridge running into when you measured the response?

b) ditto tracking weight?

c) did you bridge L&R or is the response one channel?

Thanks
John



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Old 12th Mar 2021, 4:27 pm   #9
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Default Re: Philips' Rocking Crystal Cartridges

Loading was from my Heathkit S-33H amplifier gram input which is designed around the Decca Deram ceramic and uses capacitive feedback and a 2M2 grid resistor. Amplifier was set to mono, so bridged L&R.

As a crosscheck I also tested bridged cartridge directly into the 1M load of my Heathkit IM-48u. This gave an almost identical response except for no bass rolloff.

Tracking weight ~4 grams
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Old 30th Mar 2021, 12:17 pm   #10
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Default Re: Philips' Rocking Crystal Cartridges

Out of interest I just measured a Chinese red/black job. Output was also ~400mV rms @ 1kHz 5cm/s into 1 Megaohm.

Be very suspicious of any figure that doesn't quote how it's been measured. The above figure is normalised to 5cm/s which is the standard quoted for most magnetic pickups.
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