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Old 3rd Dec 2017, 12:02 am   #1
Michael Maurice
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Default Very odd fault with Dynatron Radiogram

I've been servicing a Dynatron RG98 radiogram with a 1375 chassis.

I've serviced the turntable and replaced the belt on the tape deck. I also had to replace the cartridge and fitted an AT95.

The entire radiogram without the speakers is built on a large wooden plinth. The radiograms speakers are plugged into LS1 as are mine when it came into the workshop.

Now the problem.

When selecting DISC (records) there is a loud motorboating sound. Its only there when disc is selected and its loudness is affected by the volume control.

Not being able to sort it out in the customers house, I brought it home.

HOWEVER

It behaved impeccably in my workshop, running off the mains and not the isolated supply.

So I returned it to the customer where again it made its motorboating noise.


I tried the following:

Unplugging the cartridge holder makes little effect

Unplugging the wiring loom from the deck does alter the noise somewhat.

Grounding the inputs makes no difference

Putting a 470nF/275V~ X type capacitor makes no difference

Putting various capacitors across the supply rail in the phono stage makes no difference.

So I've brought it back to the workshop and guess what, No noise at all.


The only difference between the customers home and mine is the electricity supply and the the loudspeakers.

So what is going on and what can I do about it?
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Old 3rd Dec 2017, 12:29 am   #2
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Default Re: Very odd fault with Dynatron Radiogram

The different speakers. Both channels?
Are we talking transistors here? If so the output stage is oscillating possibly due to duff caps and your speakers in the shop load or unload it enough to prevent it.
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Old 3rd Dec 2017, 12:53 am   #3
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Default Re: Very odd fault with Dynatron Radiogram

Hi,
just a couple of random thoughts !
Is the earth ( ground ) connection good at the customer's house ?
are the speakers nearby to the radiogram ?
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Old 3rd Dec 2017, 1:54 am   #4
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Default Re: Very odd fault with Dynatron Radiogram

You have probably investigated this - but back in the 70s I had a similar problem with a Rogers Ravensbourne amp - intermittent motorboating on the MM disc input. It turned out to be a cracked resistor in the phono stage. There were 4 sensitivity settings selectable by prodding combinations of 2 buttons. Prodding enthusiastically, of course, resulted in the problem disappearing temporarily!
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Old 3rd Dec 2017, 6:40 am   #5
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Default Re: Very odd fault with Dynatron Radiogram

Stepping back a bit could this be acoustic coupling between the speakers and the gram pickup? This would account for it being site dependent.
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Old 3rd Dec 2017, 9:29 am   #6
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Default Re: Very odd fault with Dynatron Radiogram

Speaker impedance?

It will affect the current the amplifiers take, and thus the loop gain of an unwanted feedback path created by an iffy decoupling capacitor.

When you're close to the edge on oscillation, the sensitivity to various influences canbe amazing.

David
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Old 3rd Dec 2017, 1:43 pm   #7
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Default Re: Very odd fault with Dynatron Radiogram

Quote:
Originally Posted by Boater Sam View Post
The different speakers. Both channels?
Are we talking transistors here? If so the output stage is oscillating possibly due to duff caps and your speakers in the shop load or unload it enough to prevent it.
Both channels are affected in exactly the same way, it is a transistorised amplifier, can try different speakers later.

Quote:
Originally Posted by dragonser View Post
Hi,
just a couple of random thoughts !
Is the earth ( ground ) connection good at the customer's house ?
are the speakers nearby to the radiogram ?
No idea about the earth connection, but removing the earth has no effect. The speakers are built into the cabinet so very close by.

Quote:
Originally Posted by RojDW48 View Post
You have probably investigated this - but back in the 70s I had a similar problem with a Rogers Ravensbourne amp - intermittent motorboating on the MM disc input. It turned out to be a cracked resistor in the phono stage. There were 4 sensitivity settings selectable by prodding combinations of 2 buttons. Prodding enthusiastically, of course, resulted in the problem disappearing temporarily!
Unlikely, I did a lot of prodding both in the customers home and the workshop.

Quote:
Originally Posted by JohnBHanson View Post
Stepping back a bit could this be acoustic coupling between the speakers and the gram pickup? This would account for it being site dependent.
Absolutely not, the noise is still there whether playing a record or not or when the cartridge is removed from the deck. Its also there when the inputs are grounded.
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Old 3rd Dec 2017, 2:03 pm   #8
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Default Re: Very odd fault with Dynatron Radiogram

More random thoughts ! Wonder if it could be a microphonic capacitor in the disc channel ? unlikely if both channels are affected, I know but perhaps possible if due to dielectric deterioration. Is the main voltage rail different in the two locations, due to mains variations ? Are speakers different inpedance so poor decoupling is becoming apparent ?
Good luck.

Ken
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Old 3rd Dec 2017, 2:18 pm   #9
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Default Re: Very odd fault with Dynatron Radiogram

What type of selection method is used I.e. mechanical or electronic?
It would be interesting to see circuit diagram.
Cheers
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Old 3rd Dec 2017, 2:45 pm   #10
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Default Re: Very odd fault with Dynatron Radiogram

I think I can say with certainty that the fault resides in the phono pre-amp stages.

Its loudness is controlled by the volume control, ie at min no noise at max very loud.

Its not mechanical because removing the pick up cartridge shell, does nothing to it.

If you ground the output where it goes into the switch, it goes away on each channel that you ground.

Selection of input switching is mechanical
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Old 3rd Dec 2017, 3:09 pm   #11
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Default Re: Very odd fault with Dynatron Radiogram

A couple of thoughts that might help you track it down :

Can you find a pair of small-ish speakers that work OK with the unit in your workshop? If so take them back with the unit to the customer and try them there. If it doesn't motorboat with those, then the fault is presumably speaker-related either de-stabilising the amplifier feedback loop or mechanical feedback to some part of the preamplifier.

Do you have a variac? Try it in your workshop with (sensibly) high and low mains voltages. Can you get it to motorboat?
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Old 3rd Dec 2017, 5:09 pm   #12
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Default Re: Very odd fault with Dynatron Radiogram

Is the customer's place near a powerful radio transmitter or sitting underneath a mobile phone base station? If there was a radio interference problem, the pickup preamp is the place I'd expect it to be audible. Short of turning up at the customer with a spectrum analyser, sticking 47pF or so across the inputs to the pickup preamp might be worth a try.

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Old 3rd Dec 2017, 6:15 pm   #13
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Default Re: Very odd fault with Dynatron Radiogram

So far I've tried it on different power supplies in the workshop, both isolated (no earth) and standard mains, with no noise whatsoever.

Chris, I was beginning to wonder the same, will have to try it.
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Old 3rd Dec 2017, 6:21 pm   #14
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Default Re: Very odd fault with Dynatron Radiogram

Does anything else misbehave there?

David
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Old 3rd Dec 2017, 8:58 pm   #15
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Default Re: Very odd fault with Dynatron Radiogram

Just re-read your 1st post, and other posts, so just to recap, if you don’t mind.
It runs on “isolated mains” at customer. Could you clarify please.
You removed chassis, still on its wooden plinth? Or did I misunderstand!
So at your workshop - it works ok - even on different mains - the difference is:
Not in original cabinet; not original speakers; not on same mains - voltage / form factor?
I tend to think the problem is in the preamp stage.
It’s an interesting fault and I will be interested know the outcome.
Cheers
John
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Old 3rd Dec 2017, 9:20 pm   #16
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Default Re: Very odd fault with Dynatron Radiogram

A WiFi router sited nearby at the customers premises, perhaps or a plug in WiFi mains extender?
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Old 4th Dec 2017, 12:35 am   #17
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Default Re: Very odd fault with Dynatron Radiogram

Quote:
Originally Posted by John10b View Post
Just re-read your 1st post, and other posts, so just to recap, if you don’t mind.
It runs on “isolated mains” at customer. Could you clarify please.
No it doesn't it runs on normal mains. Its my workshop that has two supplies, one isolated with no earth and one 'direct' mains wired with earth.


Quote:
Originally Posted by John10b View Post
You removed chassis, still on its wooden plinth? Or did I misunderstand!
So at your workshop - it works ok - even on different mains - the difference is:
Not in original cabinet; not original speakers; not on same mains - voltage / form factor?
The 'chassis' consists of a large wooden plinth which houses the turntable, the cassette deck, the amplifier, the output stages and power supply. This is removed from the cabinet leaving only the speakers and FM aerial behind. The mains is or should be standard. The customer lives around 15 miles away.


Quote:
Originally Posted by John10b View Post
I tend to think the problem is in the preamp stage.
It’s an interesting fault and I will be interested know the outcome.
Cheers
John
I'll let everyone know as and when I've solved it.
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Old 6th Dec 2017, 1:37 pm   #18
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Default Re: Very odd fault with Dynatron Radiogram

This may be not the case here, but I once had a similar problem that was intermittent with a 1980s Amstrad Tower system. It turned out that the long cassette deck playback/record switch (on the cassette PCB) was faulty.

It may be worth giving the selector switche(s) a blast of Servisol Super 10 if you haven't done so already.

Have you also checked that the pickup wiring to the record deck is okay and intact?

Last edited by Fidelity Fan; 6th Dec 2017 at 2:05 pm.
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Old 7th Dec 2017, 4:28 pm   #19
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Default Re: Very odd fault with Dynatron Radiogram

The problem occurs when the cassette is switched off, so I think we can safely rule out the rec/play switch.

Pick up wiring to deck is good, It wouldn't have worked at my workshop if it wasn't.

HOWEVER

Since last posting, one of the output amplifiers has developed a fault, which I'm trying to repair so we'll have to wait until thats done.
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Old 8th Dec 2017, 10:44 pm   #20
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Default Re: Very odd fault with Dynatron Radiogram

Ok, keep us posted.
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