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Old 13th Feb 2019, 4:33 pm   #1
Notwister
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Default Dynatron SRX30 - FM Stereo function repair

Hi all,

first time posting here. I restore vintage equipment but normally from mainland Europe / Japan. Last week I was given a Dynatron SRX30 (D) system which had been stored on an attic for thirty years. It is in pristine condition so I thought I would do a quick restoration to get it running, even though from a 'hifi' perspective it isn't exactly a high flyer.

I was able to procure one page of a schema, using this I have got the amplification & bias running again as the amplifier was dead in the water. The unobtanium Ic's in the tone amplifier seem to work. To me it sounds like a Philips receiver, same layout as well.

There is however one problem I can't solve without the rest of the schema. All radio reception is now working, FM as well. But only in Mono; when I select stereo I am greeted with what I can only describe as a cacophony of sound, distortion etc. I even get the feeling that I am picking up two stations at once, something is pretty severely off here. The stereo light is flashing like crazy when it is on (new lamps).

Of course there is the FM stereo IC; but it seems to give me the correct voltages (around 14v). I could replace it and maybe that IC is the cause and root, but instead of making guesses I would much rather use a schema to backtrack the problem. All electrolytic capacitors have been changed, including the tantalum as a security. Nothing really wrong with the old capacitors, they ought to still do their job.

Does anyone have a schema or has anyone had the same issue?

And maybe this is a stupid question, but without all documentation it's difficult t assess what is design and what it faulty. When I press the FM button, is it by design that the common flywheel doesn't do anything (on mine it doesn't pick up any stations)? I can only get FM reception by pressing one of the 'preset' buttons and turning it. But this might be the design and not another fault.

Kind regards,

Bert
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Old 13th Feb 2019, 9:06 pm   #2
AC/HL
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Default Re: Dynatron SRX30 - FM Stereo function repair

Hallo and welcome,
There are a few threads about this chassis, if you use the search feature above. The only one featuring the decoder, as far as I can see, is this one: https://www.vintage-radio.net/forum/...d.php?t=134438 (post 39)
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Old 14th Feb 2019, 8:55 am   #3
Notwister
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Default Re: Dynatron SRX30 - FM Stereo function repair

Good morning Bill,

Thank you for your reply, I did indeed first search the website for this issue. As for the link you provided, I can't see why the problem would originate there. The tone controls and the IC are in a different path of the circuit. The stereo / mono switch does not touch any part of the tone circuit; it's effects are independent.

Also, the problem has in my experience more to do with demodulation / modulation within the fm circuit. And because that schema is missing from every service manual i've downloaded it is unclear where the fault originates. For instance, it might have it's origin in the 19khz circuit.

Kind regards,

Bert
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Old 14th Feb 2019, 11:46 am   #4
AC/HL
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Default Re: Dynatron SRX30 - FM Stereo function repair

It was the schematic I was referring to, not the text, it's the only place I've seen the decoder part, although I'm afraid it isn't very legible.
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Old 14th Feb 2019, 1:51 pm   #5
SteveCG
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Default Re: Dynatron SRX30 - FM Stereo function repair

I don't have a circuit diagram myself - but looking at the snippet of it (sx30 p23 (1).pdf in the 2017 posting) I was wondering if the two zener diodes in the stereo lamp/ (and presumably power circuit for the stereo decoder IC) regulator circuit might have gone noisy? Do you have an oscilloscope to check for noise across them?

They are a BZX79C16 and a BZX79C12

Zener diodes do have a reputation for going noisy.

Last edited by SteveCG; 14th Feb 2019 at 1:52 pm. Reason: missing bracket
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Old 15th Feb 2019, 8:19 pm   #6
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Default

Hi Steve,

As a precaution because some zeners are prone to breakdown under voltage I have replaced these with modern replacements. But I don't think this is where the problem originates; all voltages check out (based on experience) and are rock solid. I think it might be a problem in the tuning circuit that controls the pilot signal. This is what the manual says:

----------

MPX Decoder Section
Stereo decoding is achieved with the use of an IC-(MC 1310P) -operating in a "Phase-Lock Loop". The only adjustment required for correct operation is to its local oscillator and a test-point is provided (412) so that accurate adjustments can be made. When the incoming signal is modulated with a 19kHz pilot tone carrier, the decoder automatically "locks on" to it and performs the necessary switching operations to re-establish the two stereo channels. As the left and right output signals have some of this switching signal present, it is necessary to filter this out to prevent heterodyne whistles when tape recording and unnecessary speaker currents; the filtering network is included in the BLR201-N module and consists of tuned circuits providing very sharp cut-off above 18kHz. When receiving a "mono" transmission the 19kHz tone is missing, therefore no switching is carried out by the decoder and both left and right hand channels are the same.

---------------

My experience with the decoder IC's of this type is that when they fail, they fail hard. It would simply do nothing and voltages would be incorrect. But what happens when I select the stereo switch looks as if multiple signals are oscillating which throws the stereo decoder off whack. The stereo lamp is flashing like crazy which is logical in this perspective.

But without a service manual for the tuning parts it would all be a tall guess. Of course I have seen my share of tuners but if you don't know what is what in the circuit it is very easy to do more damage.

Regards,

Bert

Hi Bill,

Yes i've seen that schema but it is unintelligible because of it's post stamp size. I've tried zooming in using Photoshop but that wasn't a big help unfortunately. From the looks of it the circuit is rather common but the whole pilot signal circuit is not. And I think that the crux of the problem stems from that section, difficult to see what exactly.

Have a nice weekend!

Bert
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Old 16th Feb 2019, 11:47 am   #7
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Default Re: Dynatron SRX30 - FM Stereo function repair

Bert,

I've had a couple of tuners (W.W Nelson Jones and Quad FM3) using the MC1310P IC and found it a reliably good performer.

If I remember correctly there is a skeleton preset pot which is adjusted to get pll lock. I wonder if the wiper on this pot is making a bad contact? Some pots, like those used in Revox A77 tape-recorders, have proved to be mechanically unreliable with age.
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Old 16th Feb 2019, 6:20 pm   #8
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Default Re: Dynatron SRX30 - FM Stereo function repair

Hi Steve,

MC1310P is a common design, it's interchangeable by several other IC's. They are all pin compatible. But I have seen them fail, especially in Japanese transistor receivers / tuners. The failure is generally a complete lack of stereo as the IC blocks the signal traversal, when it is dead.

There are two trimmers on the board. One is a relatively modern 'closed' trimpot, it is for centering the stereo signal. There is a small bandwidth in which the signal 'locks' as stereo. Using the trimmer I can tune out the stereo signal, which then becomes a mono signal and I get audio. When I move the trimmer back into the stereo bandwidth, the entire signal and audio get's loopy. It's hard to describe, sometimes I feel that I am picking up two stations but that is likely just the warbling effect within the oscillation (or something to that effect).

So, that trimmer appears do work correctly. It's just that when it reaches the stereo bandwidth the signal fails.

Regards,

Bert
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Old 16th Feb 2019, 7:54 pm   #9
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Default Re: Dynatron SRX30 - FM Stereo function repair

I have the rest of the diagram,but in a very bad state.Don't ask me where I found it..
At least it gives the general idea of how the tuner and preamp sections are built.
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Old 16th Feb 2019, 10:03 pm   #10
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Default Re: Dynatron SRX30 - FM Stereo function repair

You can usually rob an FM stereo decoder IC out of a 1980s-vintage boom box; and if you are very lucky, it will even be on a little PCB all of its own.
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Old 18th Feb 2019, 2:00 pm   #11
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Default Re: Dynatron SRX30 - FM Stereo function repair

Bert,

This website has an example circuit for a MC1310P:

http://www.circuitstoday.com/stereo-decoder-circuit

I wonder how similar the circuit is to the one used in your Dynatron?
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Old 18th Feb 2019, 9:39 pm   #12
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Default Re: Dynatron SRX30 - FM Stereo function repair

Hi guys,

I really hate it when something doesn't work that should. So tonight I had some spare time and took the suspect boards for a long test. Good news: the thing is working now, in stereo. And I was correct in assuming there was a problem with the BLR201-N 'module'.

This board (i've attached a screenshot) has four capacitors and two transistors; BC149. All capacitors (6.8uF) were out of spec; I changed them to 10uF. Those BC149 are notorious for all kinds of failures. Changed them to BC550. Then the four pots on there, they were dirty (even though under a green cap). So I marked their position, used a small amount of deoxit and turned them a few times. Always a smart move, but I don't think that was the problem.

Lastly, on the main FM board there are two issues. First the MA8003 is prone to failures and indeed this one was getting hot for no reason. Voltage in was good, just didn't do it's work correctly (intermittent). Changed it to a BC139 (I know, overkill, but no time to check for equivalents). Lastly, the schema of the IC is confused over the capacitor on pin 2; positive or negative side. I changed this to a bipolar capacitor.

For testing purposes I also changed the stereo lighting to a 12v LED using a slightly different path. The 12V LED can handle 14v easily but it actually never goes that far.

So. It works. The tuner itself is nothing special and you need a ****** good antenna to get rid of the stereo noise.

Hurrah!


Thanks for all the help! Now, on to the cassettedeck. It has all the common deck problems and of course the entire fascia has come off ;-). Luckily my wife loves glueing these back on.

Regards,

Bert
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Old 19th Feb 2019, 1:36 pm   #13
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Default Re: Dynatron SRX30 - FM Stereo function repair

Stereo is now working - That's good news !
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Old 20th Feb 2019, 2:06 am   #14
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Default Re: Dynatron SRX30 - FM Stereo function repair

You will no doubt have discovered by now that the FM vari-cap tuning is by pressing and then turning each preset button in turn, with a small indicator moving up and down to show very roughly the frequency between 88 and 108 MHz. If I remember rightly, there is one button which rotates to turn the AFC on and off, with a small red flag that shows when AFC is on. The problem with that system was that the buttons could rotate a little every time you pressed one so they could quite easily go off-tune. The whole preset unit was bought in but I can't remember who from.
On this chassis, the flywheel tuning is for AM wavebands only.
And yes, you need a pretty strong signal to get FM stereo without too much hiss.
That chassis was entirely a Dynatron design, from circa 1972/3.
The amplifier output was 15 watts RMS per channel at 0.1% THD which was deemed to be fairly respectable for its day.
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Old 21st Feb 2019, 9:35 am   #15
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Default Re: Dynatron SRX30 - FM Stereo function repair

Yes, it is clear to me that the flywheel is not for FM; this is an odd choice. I believe the system is from Telefunken as it closely resembles theirs. But it could be from any European producer as they all use roughly the same method. You are right that the way Dynatron implemented the system makes it hard to stay tuned; this is probably why the system has such a large bandwidth. It's certainly not a tuner to use for selectivity ;-).

The amplifier follows a pretty common design for that era. It sounds like an early Philips which is not actually a bad thing. They did not skimp on the amplifier, it is built with good components for the time. It's clear the money was spent on the amplifier. It is an oddity from that era of all-in-one systems; most actually have pretty good amps but are let down by the surrounding audio input components.

It's almost done now, just the cassettedeck. I like the design but will probably donate it to an organisation for the elderly. They love having these machines with a stack of old records. And they don't have the funds to buy them in actual working condition.

Regards,

Bert
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Old 21st Feb 2019, 2:24 pm   #16
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Default Re: Dynatron SRX30 - FM Stereo function repair

That's a really lovely thought, I wonder if something similar would be possible in the UK without the HSE jumping all over the idea and making it impossible to do something nice for our elderly without signing one's life away in case something untoward happened with the system.

Edit- oh, I'm a tetrode now :/ I know it's post-numbers related but I rather liked being a triode! haha
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