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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 10th May 2023, 2:19 pm   #1
kirstyd
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Default Dynatron HFC 64

When I first powered this thing up only one channel worked. Removing the record deck allowed me to see that one of the amplifier units inside had a blown fuse. Replacing the fuse caused the dead amp to work for a few seconds then the fuse blew again. Incidentally it's the one in the teak cabinet with two doors underneath and the Goldring deck. Any ideas where to start guys? The sound from the good side is very good. Good bass and crystal clear treble.
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Old 10th May 2023, 2:37 pm   #2
ben
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Default Re: Dynatron HFC 64

Sounds like something is drawing too much current, probably one transistor is leaky, though it's unusual that it worked at all. Have you got the diagram?
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Old 10th May 2023, 2:46 pm   #3
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Default Re: Dynatron HFC 64

You will almost certainly will need the Service Manual which includes the schematics, a very comprehensive manual is available here :-

https://www.service-data.com/section.php/10489/1/hfc64

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Old 11th May 2023, 9:15 am   #4
Michael Maurice
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Default Re: Dynatron HFC 64

What tends to happen is that the components around the bias system seem to change value including the transistor itself.

What I do is to remove the fuse and solder a 10R resistor across the fuseholder. Connect a voltmeter across the resistor, set the preset control to minimum and switch on.

You are looking for a current through the resistor of around 10mA which will show up on the meter as 100mV.

The tolerance is 6 -16mA which equates to 60 - 160mV across the resistor.

If this reading is much more than that, replace the pot VR208, the transistor T203 and the resistors R207 and R208.

If you still cant get the current correct, change the values of R207 and R208 until you can.

Once you've got that correct, run the amplifier and check that the current stays within spec.

Once you're sure that its okay, remove the resistor and replace the fuse.
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Old 11th May 2023, 6:20 pm   #5
kirstyd
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Default Re: Dynatron HFC 64

I now have the service info for this. Thank you Michael I will try and wade my way through your instructions.
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Old 24th May 2023, 12:06 am   #6
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Default Re: Dynatron HFC 64

Been away for a few days so not been able to look at this until today. I had a close look at the blown fuse with a magnifying glass and discovered that it's the wrong one [160mA] according to the service sheet it should be 1.6 amps. I put in the correct fuse and the thing worked but the transistors on the heatsinks got hot [it was on for less than a minute . I switched off immediately.
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Old 31st May 2023, 9:51 pm   #7
kirstyd
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Default Re: Dynatron HFC 64

Had another look at this today and started by checking some resistors [mainly the ones Michael mentions in his thread ] R207 and 208 were way out of spec so i replaced them and then i thought i would power up before going any further .the results were promising instead of the two big transistors getting very hot they now only get warm so i decided to check some more resistors . I found two more that were way out so i replaced them as well and switched on again. this time the transistors stayed cold .But i then noticed the transistors on the other amp were warm [not hot] so i tested the same resistors on that one .they were also way out .I replaced them and now everything is running cool . Ive had radio 2 on for two hours and all is well everything is still cool . I have noticed that the two amps are not the same [one has nine transistors the other only has seven ] I would have thought they would have been identical or maybe someone repaired it before by simply sticking another amp module in .Anyway it seems to be ok now [fingers crossed]
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Old 1st Jun 2023, 8:32 am   #8
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Default Re: Dynatron HFC 64

Well done getting it sorted ,nice units these .Mick.
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Old 1st Jun 2023, 9:02 am   #9
kirstyd
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Default Re: Dynatron HFC 64

yes it does sound very good . I have cleaned the sliders and the other switched and they are now quiet when adjusted but the FM presets are noisy .I haven't tried to clean them at all .Can i use switch cleaner [Servisol] on them or is that not a good idea
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Old 2nd Jun 2023, 10:55 am   #10
vinrads
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Default Re: Dynatron HFC 64

I would say servisol would be ok ,I tend to use a silicone based cleaner for the sliders this stops them jamming. Mick.
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