Roberts R600 speaker problem - UK Vintage Radio Repair and Restoration Discussion Forum

Andy Day · 20th Nov 2007, 9:46 pm

Hi folks, more of a hint than a request for help here. I've just done the audio section of one of these sets, replaced the high value resistors, output and driver transistors.

I wasn't happy with the sound quality so I disconnected the input end of the volume control and injected an audio signal to plot the frequency response - which apart from a peak at 150hz was substantially flat.

I was surprised to find the fault was the speaker, which resonated 'orribly at exactly 175hz, a loud harsh rattle! Can't see any obvious mechanical faults, and I think any remedial intervention would mess it up even worse, so I swapped it for a good'un from a scrapper.

One of those rare instances when a bit of test equipment was useful!

geofy · 21st Nov 2007, 12:37 pm

Andy

Unless the speech coil has become detached, it is not possible to remove the magnets to take a look, though this would rattle at an frequency, you could try putting a thin coat of PVA flexible adhesive around the outside edge of the cone to damp excessive movement. If you are junking the speaker anyway you have nothing to lose by trying this. I have repaired torn and completely detached cones using this adhesive or polyurethane types. Also repaired the thin wires that sometimes go open circuit due to excessive movement, going into the speech coil on a few occasions rather than junk a none working speaker.

Geof

Phil G4SPZ · 22nd Nov 2007, 9:46 pm

Quote:

Originally Posted by Andy Day

I was surprised to find the fault was the speaker...

I had to replace the speaker in my R200 for exactly the same reason. Are Roberts speakers prone to this?

Phil

Nickthedentist · 24th Nov 2007, 12:14 pm

Quote:

Originally Posted by G4SPZ

I had to replace the speaker in my R200 for exactly the same reason. Are Roberts speakers prone to this?

The R200 ones are Celestions. The only ones I've had to replace have been ones that have been damaged by damp. The condition of the rest of the set was an indication of previous storage conditions; peeling Rexine, delaminated ply, light corrosion on the chassis and speaker basket etc. etc.

Nick.

Andy Day · 24th Nov 2007, 7:53 pm

Hi, the speaker in this model is a silver elliptical marked "Rola" whoever made them. The magnet is swaged onto the frame before the voice coil and cone are glued in, so there is no chance of getting at anything from the back. Access would involve picking away the edge of the cone and the dust seal which goes over the cone end of the voice coil, and it just doesn't seem worth it! My main interest was that the fault was so well shown by a frequency response test, not something I would have normally tried first.

PS. The speaker is otherwise perfect, no damage from damp or any other agent.

Nickthedentist · 24th Nov 2007, 8:08 pm

Quote:

Originally Posted by Andy Day

Hi, the speaker in this model is a silver elliptical marked "Rola" whoever made them.

Rola and Celestion were one and the same essentially at that time

20th Nov 2007, 9:46 pm	#1
Andy Day Pentode Join Date: Aug 2005 Location: Hemel Hempstead, Hertfordshire, UK. Posts: 154	Roberts R600 speaker problem Hi folks, more of a hint than a request for help here. I've just done the audio section of one of these sets, replaced the high value resistors, output and driver transistors. I wasn't happy with the sound quality so I disconnected the input end of the volume control and injected an audio signal to plot the frequency response - which apart from a peak at 150hz was substantially flat. I was surprised to find the fault was the speaker, which resonated 'orribly at exactly 175hz, a loud harsh rattle! Can't see any obvious mechanical faults, and I think any remedial intervention would mess it up even worse, so I swapped it for a good'un from a scrapper. One of those rare instances when a bit of test equipment was useful! __________________ The older I get, the faster I was!

21st Nov 2007, 12:37 pm	#2
geofy Retired Dormant Member Join Date: Jun 2006 Posts: 1,798	Re: Roberts R600 speaker problem Andy Unless the speech coil has become detached, it is not possible to remove the magnets to take a look, though this would rattle at an frequency, you could try putting a thin coat of PVA flexible adhesive around the outside edge of the cone to damp excessive movement. If you are junking the speaker anyway you have nothing to lose by trying this. I have repaired torn and completely detached cones using this adhesive or polyurethane types. Also repaired the thin wires that sometimes go open circuit due to excessive movement, going into the speech coil on a few occasions rather than junk a none working speaker. Geof

24th Nov 2007, 7:53 pm	#5
Andy Day Pentode Join Date: Aug 2005 Location: Hemel Hempstead, Hertfordshire, UK. Posts: 154	Re: Roberts R600 speaker problem Hi, the speaker in this model is a silver elliptical marked "Rola" whoever made them. The magnet is swaged onto the frame before the voice coil and cone are glued in, so there is no chance of getting at anything from the back. Access would involve picking away the edge of the cone and the dust seal which goes over the cone end of the voice coil, and it just doesn't seem worth it! My main interest was that the fault was so well shown by a frequency response test, not something I would have normally tried first. PS. The speaker is otherwise perfect, no damage from damp or any other agent. __________________ The older I get, the faster I was!