Dynatron Elan Repair Guidance

[OldTechFan96]

Hello,

Recently I bought a tatty Dynatron Elan that I found at a local market. It is missing its FM aerial and its speaker. It looks to have been messed about with.

The potentiometers and switches were all given a blast of switch cleaner.

The first thing to remedy would be the floating orange wire from the negative side of the on/off switch. It can be seen on the right hand side of the photograph. Does anybody know where it goes to?

I have the trader sheet from the service data CD.

Thanks

[ukcol]

Hello

The switched negative (orange wire) is common to 5 components. See diagram below. Given that the orange wire is so short it should be possible to work out where it came from and double check with a DMM for continuity to those 5 components.

[OldTechFan96]

Thanks ukcol, I think that I've got it.

Having a good look at the PCB and the circuit diagram I have came to the conclusion that the orange wire goes to the point marked on the photo that I've attached.

I noticed that each side of the switch has two wires coming from it. One the red side one wire went to the positive of C23 and the other went to the collector of TR7. On the black side one wire went to the negative of C23. Following the circuit diagram suggests that the orange wire must go to the collector of TR8.

[ukcol]

I see what you are saying.

I would solder the orange wire to the post you have identified in your picture, and then as a double check, make sure you read about zero ohms between that post and the appropriate connection of each of those 5 components.

[OldTechFan96]

I've done some continuity checks between the post and the five components you mentioned apart from the headphone socket as the wires look to have been cut off at some point.

The DMM beeps and reads 0.5 ohms. So I think that there is continuity in the right places?

[ukcol]

Excellent.

BTW. The earphone socket is designed to cut out the speaker when an earphone is plugged in. If the socket wiring has simply been disconnected there will be no fed to the speaker and hence no sound, so check that out.

[Radiocruncher]

Strangely enough I have one of these out of the chassis at the moment so here’s a picture that may help.

Regards

Graham

[OldTechFan96]

Thanks Radiocruncher, that confirms the position of the orange wire. Great videos, by the way!

Once connected, the radio worked poorly on MW/LW and not at all on FM. The AM IF module (Mullard LP1156) and the FM tuner unit (Mulard AE02119) all contained AF1** series transistors.

The AM module was sorted first. Two AF117 (IF amp) and one AF115 (mixer-oscillator) transistors were replaced with BC557b. The only electrolytic capacitor in the module (10uf, 16V) was replaced.

The module was reinstalled in the radio. The replacement transistors worked a treat! I could pick up all of the usual stations on MW and LW.

The next step will be replacing the transistors in the FM tuner unit. It contains one AF115 (mixer-oscillator) and one AF178 (RF amp).

Are AF178 transistors known to be a problem?

[Radiocruncher]

Thanks for the compliment and glad you like the videos. Well done on getting the radio working again. The AF178 should be fine as it’s in a different can than the AF115.

Good luck

Graham

[OldTechFan96]

I've replaced the AF115 in the VHF tuner unit. It was a bit of a job to remove the board. A series of photographs and diagrams are all ways worth making when removing a board with lots of wires coming from it.

There was still no FM when I turned the radio on so more investigation is needed.

HT on pin 7 of the VHF tuner unit was only 1.9V and should be 8.5V. I'll investigate by measuring voltage back from pin 7. Since the voltage on pin 8 is correct I think that R1, C2 or C3 could be at fault. All three components look to be difficult to get at.

Also, the on/off switch is broken internally, which is a shame.

[OldTechFan96]

On one side of R1 (1K ohm) there is 12.7V and on the other side there is 1.9V.

I removed R1 assuming it was high. It was a bit tricky to remove because of the wires and switch-bank in the way. It measured spot on at 1007 ohms! Therefore it is not the culprit. I'll do a more through check of the components along the HT line to the VHF tuner.

Maybe the voltage at one side of R1 should be higher than 12V?

[OldTechFan96]

I've tested the components along the HT line and they are all OK. All the resistors are spot on. I've marked the tested components on the pictures below.

I'm stumped at the moment and don't know how to proceed.

Any ideas for what is causing low HT?

[Radiocruncher]

It looks like you have gone through it well. What transistor did you use to replace the AF115? I had a similar problem once and it turned out that I had the leads the wrong way around on an AF125. If you need help then I will hook mine up and check the voltages in that section.

Regards

Graham

[OldTechFan96]

The transistor I used in place of the AF115 is a BC557. I have double checked its position and it is installed correctly.

I'd be thankful if you could check the voltages in your Elan.

Thanks

[OldTechFan96]

I've drawn a diagram of the voltages along the HT line.

It is odd to think that there is a 10V drop from one end of R1 to the other.

R1 is spot on as it measures 1k ohms.

Does anybody have any ideas?

[OldTechFan96]

I've been having another look at the Elan and its no FM fault. I have been able to contort the VHF tuner section in such a way that lets me probe around while the radio is on.

I have removed and tested TR1, 2 and 3 in the VHF IF section. They all test good.

I measured the voltages around TR1, 2 and 3. They all measure correctly except from the collectors of TR1 and 2. They should measure 0.2V. I think that I'm measuring 0V.

I hooked up the signal generator (Heathkit IG-102) to the VHF aerial and set it to output 10.7MHz with a modulated audio tone. The audio tone came through once the signal generator warmed up. (Surely this confirms that the VHF section is receiving OK?)

My first plan was to probe with my oscilloscope each side of both detector diodes.

Expectations: A modulated FM signal carrier before the detector and an audio frequency after the diode.

I have photographed the waveforms I saw on my oscilloscope.

I'm not 100% clued up about what waveforms I should be seeing on a VHF tuner so I don't know if what I'm seeing is quite right.

If I switch off the signal generator off nothing can be seen on the AF side of the detectors. This made me think that the detectors were O/C. I checked them out of circuit and they were fine.

Anyway, does what I'm seeing look about right?

Thanks

[ronbryan]

With an FM signal fed to the IF chain, you would expect to see the demodulated audio across C12, which is fed from the tertiary winding on the IFT and a steady DC voltage between ground and stabilising capacitors C15 or C16.

Ron

[crackle]

It sounds as if your BC557 is not working in the FM mixer/oscillator position.
My suggestion would be to Zap the AF115 which you removed, test the transistor to check it is working, then replace it and see if the FM is working on the radio.
If it is then you know the fault is with the silicon transistor you used to replace the original germanium one. You can sometimes sub a germanium transistor with silicon but it does not always work in FM tuners.
My suggestion then would be to get some AF125 transistors to replace the AF115 in the FM tuner.

Zapping an AF11x transistor; If you have not done this before it is quite simple and I have found it works in at least 9 out of 10 cases.
Remove the transistor from the circuit and solder the E B C legs together. Set your bench supply on to 30 volts and connect one terminal to the soldered E B C legs, connect the other terminal onto the shield wire. Then tap the transistor gently on the desk turning it around as you do so to try and catch any tin whiskers about to short.
When done remove the bench supply and unsolder the legs and test the transistor.

Mike

['LIVEWIRE?']

Personally, as I've remarked before, I would simply replace the AF115 with either an AF125, or, if, it is suitable for VHF use, a GT322. The latter might be easier to fit, but I don't have data to hand.

[OldTechFan96]

I will have a look into sorting TR1.

I have tested two AF115 transistors and they both have E to C shorts. Will 'zapping' clear this short?

My bench PSU only goes to 20V. I will try zapping the transistor.