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Old 3rd Feb 2020, 5:03 am   #1
RetepSnikrep
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Default Dynatron Marlow RG24 refurb

I have just acquired one of these nice quality sets and have reviewed the other threads on the forum about them.
It does work, but before I use it in earnest I want to do some work on it.

I understand no RG24 service document/schematic is a available.

I have ordered some modern caps to replace the 0.01uf ones on the main power amp chassis. I'll do the same on the tuner chassis in due course.

I also have what appears to be an NTC thermistor that has got hot.

There is also evidence of the power transformer and one of the audio output transformers running hot as they have leaked some wax.

General comments and advice would be appreciated. Thanks Peter
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Old 5th Feb 2020, 8:21 am   #2
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Default Re: Dynatron Marlow RG24 refurb

No pointers or comments? OK I will replace all caps and check resistors.
Could do with some info on that possible NTC??
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Old 5th Feb 2020, 11:42 am   #3
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Default Re: Dynatron Marlow RG24 refurb

Hi, to check the mains transformer is OK try running the set without valves fitted for an hour or so (monitoring it). The mains transformer should then only get slightly warm (assuming it is on the correct voltage tap).
Overheating on the output transformer could be due to excessive current in the output stage due to leaky caps

Ed
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Old 5th Feb 2020, 12:09 pm   #4
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Default Re: Dynatron Marlow RG24 refurb

The thermistor is a limiter for the rectifier:

https://www.vintage-radio.net/forum/...d.php?t=130269

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Old 6th Feb 2020, 8:04 am   #5
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Default Re: Dynatron Marlow RG24 refurb

It looks like nobody ever positively identified that strange part that looks like it got hot.

(I don't mean the red thermistor)
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Old 6th Feb 2020, 8:56 am   #6
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Default Re: Dynatron Marlow RG24 refurb

If you're referring to the large black component on the top side of the chassis, that's the mains transformer.
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Old 6th Feb 2020, 10:56 am   #7
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Default Re: Dynatron Marlow RG24 refurb

I think he means the burnt looking component between one of the rectifier pins and chassis.
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Old 6th Feb 2020, 11:17 am   #8
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Default Re: Dynatron Marlow RG24 refurb

The long red resistor is the HT filter resistor.

The component that's connected between one of the rectifier's valve holder tags and chassis is a thermistor which acts as a limiter for the rectifier, as was previously stated.

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Old 6th Feb 2020, 4:48 pm   #9
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Default Re: Dynatron Marlow RG24 refurb

Yes it was the value of that burnt thermistor I was trying to establish..
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Old 6th Feb 2020, 6:00 pm   #10
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Default Re: Dynatron Marlow RG24 refurb

If it's intact try measuring it's cold resistance and compare to some of the thermistors in here....VA1009, VA1010 etc:

http://www.electrojumble.org/DATA/Mu...o%20VA1040.pdf

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Old 15th Feb 2020, 1:34 pm   #11
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Default Re: Dynatron Marlow RG24 refurb

https://youtu.be/hOhyQGG1ZVs

This is now back together working quite well.

However the right channel large bass speaker has some distortion.

I swapped the amp output over and the same speaker/channel still has the distortion, so i'm happy it's the right speaker/s.

I didn't have any 4uf crossover caps, so need to get some and change the crossover capacitor but doubt that's it.

Any ideas? I suppose it could have been overdriven at some stage.
It sounds like cone/driver damage..

Or cone paper edge separation from the speaker chassis.

Damp? What's the best glue for that.

I have seen that problem in my car speakers several times were the cone edge separated from the chassis and gave a similar sort of rattling/distortion.

I've taken it out but can't see anything obvious / loose ....

Well anyone got a spare flux trans RCA 391112 10" speaker
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Old 15th Feb 2020, 5:13 pm   #12
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Default Re: Dynatron Marlow RG24 refurb

I'd take out the speaker and examine the cone. Gently push the cone to see if there is any voice coil rub. It's unlikey to be the Tweeter/s filter cap.
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Old 16th Feb 2020, 9:03 am   #13
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Default Re: Dynatron Marlow RG24 refurb

I have the speaker out and can't see or detect any cone separation around the edges.

I've tried gently pushing the cone in out, there may be some rub, how to cure that?

Even laid on the floor I can hear distortion through it, especially with more bassy stuff.

Failing that anyone got a 10" RCA Flux Trans 3921112 Speaker

Marked 48-802/002 and dated 29/10/62

I need tone generator on the bench to drive it for some testing.
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Old 16th Feb 2020, 9:51 am   #14
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Default Re: Dynatron Marlow RG24 refurb

OK so looking closely at the back of the noisy speaker what do we think about voice coil centering or magnet adjustment using any of the screws circled.

I could easily buy a pair of modern 10" speakers to replace both channels, but fixing this would be nice and preferable.

Impedance?

I'll measure it but am assuming 8 ohms.

I note these are actually 'R&A' speakers, sorry I misread the logo as 'RCA'.
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Old 16th Feb 2020, 10:46 am   #15
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Default Re: Dynatron Marlow RG24 refurb

I must confess to cheating when the bass drivers of my Leak Sandwich speakers started to distort.

I rotated the bass drivers through 180 degrees, played quietly for a month, and that was some years ago now, been absolutely fine.

Measuring the edge of the speaker basket to outer cone surround every 90 degrees confirmed, the cone had sagged.

Worth a try?

Mark
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Old 16th Feb 2020, 11:24 am   #16
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Default Re: Dynatron Marlow RG24 refurb

There is this thread which resulted in a successful repair of a rubbing voice-coil.

https://www.vintage-radio.net/forum/...ad.php?t=90097

Admittedly, the speaker is of different construction with the magnet ahead of the cone and has a smaller diameter, but Mark says he has used this technique on more conventional speakers with the magnet in the normal place.

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Old 16th Feb 2020, 5:05 pm   #17
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Default Re: Dynatron Marlow RG24 refurb

Has the suspension at the back of the cone been checked, it might have come unstuck. Speaker looks too good a condition to scrap.
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Old 16th Feb 2020, 8:51 pm   #18
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Default Re: Dynatron Marlow RG24 refurb

Geoff can you explain that in a bit more detail. ?
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Old 16th Feb 2020, 11:01 pm   #19
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Default Re: Dynatron Marlow RG24 refurb

The corrugated cloth or paper suspension 'spider' attached to the speaker chassis centres the speaker cone and coil. If it comes loose it will rattle and there could be ingress of dust to obstruct the coil.
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Old 17th Feb 2020, 1:07 pm   #20
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Default Re: Dynatron Marlow RG24 refurb

I always thought that those 10" R&A speakers were underspecified by using such a small magnet and a narrow voice coil. They were sourced through bulk buying as R&A was a Pye subsidiary. Also the cone suspension is rather stiff and peaks @ c.90Hz. But my comments did not endear me to the Men in Suits! A pair of new 10" units with a roll surround will make all the the difference.
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