Dynatron SRX20 FM not working

[Whitespin]

I have acquired a Dynatron SRX 20 radiogram. The radio/amp work fine except for the FM side which appears to be dead. It is a nice piece of furniture and the sound from the record player is superb. My neighbour who is a retired TV engineer has had a look and he thinks it is most likely to be the FM tuner module but without a circuit diagram he can't be cetain. Everthing else checks out ok.

Is it possible to get a diagram and if so are tuner modules available or can more modern ones be fitted. Has anyone any experience with this model of radio.

Regards

Whitespin

[cockshad]

are you sure the model number is correct? the downloadable service data lists a SRX24 transistor radiogram and a few others serial numbers near that,but not 20...btw welcome on board....Tim

[Maestro Maker]

Hi!

Glad the Audio channel works! Maybe I can help re. the radio..

The SRX 20 was fitted to a variety off Dynatron models. You should find the Model number on the rating plate on the back of your cabinet.

Does the AM section work at all?

If there is no FM aerial built into the cabinet I assume you have tried attaching a yard of wire to the FM terminals - its best to try the obvious first!
The FM module may be OK - could only be a fault on the voltage supply to the module.

I may have a circuit diagram for the model.

Cheers

[Whitespin]

Originally Posted by cockshad

are you sure the model number is correct? the downloadable service data lists a SRX24 transistor radiogram and a few others serial numbers near that,but not 20...btw welcome on board....Tim

Hi Tim thanks for the reply. The control panel says

'Stereophonic Tuner Amplifier SRX 20' there are no other markings on the cabinet other than a small gold label on the back with the number (421907) on it.

John

[Whitespin]

Hi Maestro
Thanks for the reply. Firstly there is no plate on the outside of the cabinet only a small gold label on the rear with the number (421907) on it.

On the control panel to the right of the player it says Stereophonic Tuner Amplifier SRX 20

Yes the AM does work and the TV engineer has cleaned the all the swiches (push buttons) and slider audio controls and got the panel lights working. He said he couldn't be sure if it was the actual tuner or one of the periferals around it but it wasn't really his field anyway. With the FM button pressed there is just a very slight hum but nothing else over the whole tuning scale.

There is a simple FM aerial built in and it plugs into the FM aerial socket on the rear of the cabinet giving you the option of using an external aerial if you want.

The Player is a Garrard SP25 Mk 4

Hope this helps to identify the actual model.

Many thanks

John

[Maestro Maker]

Hi John

Am checking for what information I may have. Will post again later.

Cheers

MM

[Whitespin]

Hi MM

Thanks for the update I was beginning to think there were no solutions to my problem.

Regards

John

[Steve_P]

Looking at other Dynatron units from about this time, I notice that the AM and FM IF's and Tuners are different.

So - first question - how skilled are you? If you are skilled, or want to learn, then great. We'll help. If not, and there's no disgrace in that, then somebody will help I'm sure.

Start by measuring the HT to the FM Decoder, FM IF stages and Tuner. If they are there, check the transistor voltages.

If the HT is not there, look back and see where it's failing.

Cheers,

Steve P.

[Whitespin]

Hi Steve
Thanks for the advice. Well I can use a multimeter and solder. I built a single valve radio receiver when I was 14, was a mechanic for a number of years and have built a variety of electronic kits the last being a speed controller for my grandsons R/C electric model car.
I am willing to learn but would need detailed guidance to enable me to identify the various components and the test procedures. If you are willing to help I would enjoy the challenge.

Regards

John

[Maestro Maker]

Hi John

Herewith circuit diagram and layout of the tuner board.
I am doing this remote from home and hope these two attachments will be useable for you.

Please let me know if they download OK and are readable and we can go from there!

Cheers

MM

[Whitespin]

Thanks MM they have printed out fine

Cheers

John

[Sideband]

The main circuit diagram only shows the tuner as a block (FM tuner head), obviously designed to be replaced complete...fine when they were available, not so easy now!

All you can do at present is check that the main 15 volt supply is present (find R429 and check on either end of it). One end will read about 15 volts, the other end slightly lower. If it is not present, then trace it back to TR261. If it is present, then more detailed fault finding will be required but let's try and narrow it down first.

If I'm reading the circuit correctly, it looks like a separate FM I.F is used rather than the more usual combined AM/FM I.F circuit.

Let us know what you find so far.


Rich.

[Maestro Maker]

Hi John
Glad you have the cct. diag etc. OK.
Yes the AM and FM sections are quite separate, the FM having ceramic filter IFs and an IC to do the demodulating etc.
You should look for 15.4 volts at junction of R429 and C434 on the underboard copper side layout - you will find these components marked about one inch to the left of IC401.

Then trace back if needed to the popwer supply chain.
Let me know how it goes - I am out tonight though!

Cheers
MM

[Steve_P]

Also check the +25v line, as it is used to create a tuning voltage for the FM can. Check for volts on R601 (Tuning control).

No tuning voltage, no tuning.

Cheers,

Steve P.

[Whitespin]

Thanks to everyone for their advice.

Right I have extracted the FM board from the body of the unit and it's now hanging on by multiple wires (one group goes to tuner can, others to amplifier side) and I can get at both sides of board. I have located R429 and C434. I have also found a blue/white wire coming from pin 3 on the left of the FM tuner head that the circuit says is tuning volts? This goes directly to what could be the tuner can? along with several other wires from the board. I can't find R601.

Not wanting to damage anything, including myself I have stopped at this point.

Question Am I right in thinking that the negative/earth? wires are black and basically are grounded to the chassis? The FM board has a heavy black cable going from the board to the tuner can? and then is earthed directly to the main chassis. Is voltage testing done by clipping the com lead to anywhere on the chassis range set 200v dc?

Now off out in the garden (where I know what I'm doing) before the boss goes on strike

Cheers

John

[Steve_P]

R601 is the tuning control. Not on the unit, but on the front of the unit.

Black is negative - a croc clip is handy here. The +ve terminal is red, and can be used for measurements.

Garden? What's that??

Cheers,

Steve P.

[Station X]

Garden? What's that??

A place where you put aerials. The bigger the better.

[Whitespin]

Aerial? My wife keeps ours under the sink!

Cheers

John

[Whitespin]

Right I've got 26.9v at R431 and 15.26v at R473 nothing at R429 or C434 I've also got 9.6v at one pin on R470 Apart from those the only other readings are from random resistors on the board and are only 2 or 3 volts.

Unfortunately I've broken off a White/Purple wire that runs from the FM board, along with a yellow wire into the control panel and I dont know where it should go.

Hope some of this makes sense

Cheers

John

[Whitespin]

I've just had another look at the circuit diagram and it looks as if R429 and C434 are fed from the FM switch? It also appears that R438 is also fed from the FM switch (there is no supply there either). There is a supply to the switch (yellow wire) 26.9v but I don't know what colour wires feed out of the switch so I can't be sure if it is the switch that is faulty.

Cheers

John