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? |
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. |
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 ? |
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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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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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 |
Re: Very odd fault with Dynatron Radiogram
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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 |
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 John |
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 |
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? |
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.
Chris |
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. |
Re: Very odd fault with Dynatron Radiogram
Does anything else misbehave there?
David |
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 |
Re: Very odd fault with Dynatron Radiogram
A WiFi router sited nearby at the customers premises, perhaps or a plug in WiFi mains extender?
Alan. |
Re: Very odd fault with Dynatron Radiogram
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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? |
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. |
Re: Very odd fault with Dynatron Radiogram
Ok, keep us posted.
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