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Old 24th Feb 2018, 5:27 pm   #1
tigger449
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Default Dynatron HFC51 lost channel

Hi,

I've just started up my Dynatron HFC51 for the first time in a couple of years and it's got no left channel output from either the radio or phono. I've swapped the speakers and leads around without success.

Can anyone give me a clue as to where I should look for trouble first?
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Old 24th Feb 2018, 5:48 pm   #2
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Default Re: Dynatron HFC51 lost channel

Any fuses blown, I cannot remember if the channels have seperate fuses?
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Old 24th Feb 2018, 6:06 pm   #3
Edward Huggins
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Default Re: Dynatron HFC51 lost channel

This is a very common problem on this chassis and usually means voltage testing/trouble shooting on that channel. Best to just double check the Headphone socket as well. Is the OP confident to investigate further?
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Old 24th Feb 2018, 6:20 pm   #4
tigger449
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Default Re: Dynatron HFC51 lost channel

I've checked the fuses on the pre-amps and both are fine. It's a HFC55 if that makes any difference - don't know why I thought it was a 51.
On turning the volume to max with the right speaker unplugged, there's a very faint sound - crossover? There's also a click from both speakers when turning off the main switch...

Last edited by tigger449; 24th Feb 2018 at 6:30 pm. Reason: More info
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Old 24th Feb 2018, 8:21 pm   #5
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Default Re: Dynatron HFC51 lost channel

Worthwhile cleaning the switches and controls, it could be as simple as that.

I don’t have a circuit and it’s 40 years since I last saw this unit, not much help.
You have one good channel, if the cleaning doesn’t help I would use signal injection or an oscilloscope plus voltage readings, you should be able to locate the faulty stage.
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Old 25th Feb 2018, 9:48 am   #6
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Default Re: Dynatron HFC51 lost channel

An audio signal injector would be useful, or just try touching the connections on the volume pot with a screwdriver hold the metal part, switch to aux or gram turn up the volume to max, you should get a healthy buzz from both speakers. Be careful keep away from the mains side, see what you get let us know.

Mick.
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Old 25th Feb 2018, 3:10 pm   #7
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Default Re: Dynatron HFC51 lost channel

I've given the switches a good spray of Servisol and checked for any obvious loose connections but no improvement. There is the usual crackle from both speakers when switching on the mains and when sliding the volume control so something is clearly getting through. I think I'm at the stage where I need hands on assistance!
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Old 25th Feb 2018, 3:25 pm   #8
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Default Re: Dynatron HFC51 lost channel

Schematic here.

Costs a couple of quid.

Cheers,

Frank
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Old 25th Feb 2018, 5:03 pm   #9
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Default Re: Dynatron HFC51 lost channel

Thanks, Frank!
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Old 25th Feb 2018, 5:10 pm   #10
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Default Re: Dynatron HFC51 lost channel

If you are getting a crackle from the volume control that suggests the amp after the V/C is working, if you get the manual, post the part of the circuit from the output of the radio/ phono to the V/C.
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Old 26th Feb 2018, 7:41 pm   #11
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Default Re: Dynatron HFC51 lost channel

Here are the relevant pages from the Dynatron service manual...
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Old 26th Feb 2018, 7:50 pm   #12
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Default Re: Dynatron HFC51 lost channel

Check the voltages on the faulty channel 483 IC and the input and output capacitors, one feeding pin 2 and one feeding the top of the volume control.

Hopefully it will be one of those two capacitors.
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Old 26th Feb 2018, 8:01 pm   #13
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Default Re: Dynatron HFC51 lost channel

Is this the one with the little black modules, if so there is a post on here somewhere about them, I must have replaced loads in the distant past, of course no longer available now.

Mick.
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Old 26th Feb 2018, 8:05 pm   #14
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Default Re: Dynatron HFC51 lost channel

Sorry Mick, I have forgotten what the inside of these music centres look like. The IC’s if that’s what you mean will be near the volume and tone controls and will be black with perhap 14 pins, the capacitors also near the controls could be a variety of colours, Callins made black ones, Philips/Mullard made blue ones.

I would check the capacitors first.
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Old 27th Feb 2018, 12:43 am   #15
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Default Re: Dynatron HFC51 lost channel

I've had some experience in repairing these so here goes:

I take it you have a multimeter, you will need it.

Please note that sometimes these modules when they fail get very very hot.

Connect the negative to ground, and with the positive lead measure the voltage on each of the output transistors collector (The heatsinks) You should get a reading of around 42V on one and half that on the other (approx 21V).

If you get this reading on both amplifier modules, then you can assume that are likely to be ok. Run them for a few minutes to ensure they are not getting hot.

If either (or both channels) are faulty then proceed as follows:

Replace all 7 transistors in the faulty module and the 1K preset.

Remove R209 and R207, and swap them over.

The original transistors are no longer available so substitute with these

BD130Y is available or use 2N3055
MA3008 use MA 8003 (MA3008 may be a misprint)
ME0412 use BC204
C1115 use ME8002
C1131 use 2N5322

They are available from Cricklewood electronics.

Once you have replaced these parts, remove the fuse and put in a 10R 2W resistor.

Set the preset to minimum.

Set the volume control to minimum

First connect your voltmeter to measure the voltage between ground and each of the heatsinks and switch on. You should get as indicated above, roughly 50V to 60V on one and half that on the other. if you dont switch off there is something wrong. Check the heatsinks to see if they are getting warm, if so again switch off.

If you are getting 1/2 supply voltage on one heatsink and it is cool, proceed to the next step.

Connect your voltmeter across the 10 ohm resistor, switch on.

The quiescent current is 16mA which through a 10 ohm resistor will give you 160mV

If it is lower than this then gradually turn the preset clockwise to obtain this reading.

It is is higher than this, replace R207 with either a 2.7K or a 3.3K and then measure the voltage again, your aim is to set the preset for around 160mV.

When you are happy with this, run the amplifier for some time with your meter across the resistor and check the voltage across the resistor hasn't risen or if it has by very little.

The reason for swapping the resistors around is because the original transistors had slightly different characteristics which affected the bias and therefore the quiescent current.

When you are satisfied that all is ok, switch off, remove the 10R resistor and fit an F1.6A fuse.

Good luck.
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Old 27th Feb 2018, 9:57 am   #16
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Default Re: Dynatron HFC51 lost channel

I may be wrong but I thought that we had established that the output stages were OK, I was suggesting that the pre amp ic /module may be at fault, this was a very common fault back in the day. Mick.
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Old 27th Feb 2018, 10:02 am   #17
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Default Re: Dynatron HFC51 lost channel

Has the OP tried the simple test described in post #6?
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Old 27th Feb 2018, 3:53 pm   #18
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Default Re: Dynatron HFC51 lost channel

As one channel is apparently working normally, the O.P. can use that for making voltage comparisons which might reveal the fault on the other channel, or at least provide some clues.
I am not certain if this unit is the type that has the moulded plastic "tone " module with wire 'pins', (and they do go faulty,) I have some N.O.S. ones, and can provide him with one at P&P cost only (when I find them!), if required. Tony
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