Mullard MAS183 whistles - UK Vintage Radio Repair and Restoration Discussion Forum

telstar · 28th Sep 2016, 2:13 pm

I am working on my Mullard MAS183 (similar in circuitry to the original Philips 206a, but with octal tubes). The oscillator is now working, but the stations are accompanied by instability (whistling) when tuned and is difficult to separate. The trimmers have been tampered with and following alignment procedures does not improve the situation. I can not see any broken coils in the tuner section. Anyone had any dealings with these models, please?
The I.F Transformers are sealed and 470kHZ gets through o.k. Maybe the I.F filter might be out of alignment in the front end, but that looks to be sealed.
Cheers
Mike

PJL · 28th Sep 2016, 3:16 pm

Usual problem is the valve metalisation on the ECH35 or EF39. Even if they look intact, the connection can fail which is a wire that extends a short distance out of the top of the base.

Boater Sam · 28th Sep 2016, 5:15 pm

Quick test, fix an earth lead to a cut open coke can and drop it over the valves one at a time, you will soon see which one it is.

lloydwells · 2nd Oct 2016, 6:24 pm

Weld through ( copper ) primer quite nifty for recoating valves

crackle · 2nd Oct 2016, 11:35 pm

I have had this as well on different sets, if it is not a problem with valve screening as mentioned, check the decoupling capacitors.

Mike

telstar · 3rd Oct 2016, 2:25 pm

Hi Mike All the wax capacitors have been replaced with modern yellow types. The ECH35 valve has been replaced by a C.V equivalent. The EF39 seems to have good contact with chassis, but I suppose I could use a 6K7 and see if this works. I hope to post a photograph of this radio (it is larger than the permissible 200 kb allowed on this site.
Cheers
Mike

Voxophone · 3rd Oct 2016, 3:40 pm

I just had this problem on a set in which I substituted a known good EF39 and performed an IF alignment. I was able to fix the problem by de-tuning the IF slightly - i.e. broadening/flattening the peak.

PJL · 3rd Oct 2016, 5:10 pm

Does the metalisation of both the EF39 and ECH35 show continuity to pin 1?

telstar · 4th Oct 2016, 2:51 pm

Hi. Yes there is good continuity from the metallisation of both EF39 & ECH35 to their respective pin 1 terminals. In this case the F.C is a better grey colour (CV1347)which seems to have survived better than the red colour.
I have managed to receive some weak stations (volume control at maximum), and that occurs when you put a finger on the top cap of the frequency changer then the whistling disappears. The front end must be working in order to do that task.
Unfortunately, I will have to re-string the dial cord assembly as it has come loose.

PJL · 4th Oct 2016, 4:12 pm

Your finger is adding capacitance to earth and preventing oscillation. I would check capacitors particularly the screen grid decoupling and you might try adding a 0.1uF across the HT.

telstar · 5th Oct 2016, 2:07 pm

Hi. All the wax capacitors have been replaced in this radio. These stations that are received are tuneable but weak.

PJL · 5th Oct 2016, 2:14 pm

If it is IF instability then the AGC voltage will be very low. Try measuring it across the AGC filter capacitor, C6 in trader 478 for the 206A.

telstar · 5th Oct 2016, 4:58 pm

Hi. I will look at the voltage at the a.g.c capacitor and find out the result. Also, I notice that when touching around the second I.F transformer, a buzz is heard in the loudspeaker.
Mike

telstar · 8th Oct 2016, 2:08 pm

At long last I have traced the fault to the front end. There were two capacitors that had been incorrectly installed. When there were stations received before, they sounded like the effects single side band radio broadcasts produced.
It now receives m.w stations. I have the dreadful task of rewinding the cord drive.

Mike

telstar · 4th Jan 2017, 3:34 pm

Hi. Actually the problem has reappeared, the whistling actually comes when tuning between stations. I might try adjusting the I.F Filter and see if it has any effects.
The drive cord works perfectly now as I have found that the cord used by builders is really useful on this kind of job.

ms660 · 4th Jan 2017, 3:47 pm

Noting the points in frequency terms between the whistling (beating) when tuning might give a clue.

Lawrence.

28th Sep 2016, 2:13 pm	#1
telstar Octode Join Date: Dec 2003 Location: Leeds, West Yorkshire, UK. Posts: 1,593	Mullard MAS183 whistles I am working on my Mullard MAS183 (similar in circuitry to the original Philips 206a, but with octal tubes). The oscillator is now working, but the stations are accompanied by instability (whistling) when tuned and is difficult to separate. The trimmers have been tampered with and following alignment procedures does not improve the situation. I can not see any broken coils in the tuner section. Anyone had any dealings with these models, please? The I.F Transformers are sealed and 470kHZ gets through o.k. Maybe the I.F filter might be out of alignment in the front end, but that looks to be sealed. Cheers Mike

28th Sep 2016, 3:16 pm	#2
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Mullard MAS183 whistles Usual problem is the valve metalisation on the ECH35 or EF39. Even if they look intact, the connection can fail which is a wire that extends a short distance out of the top of the base.

28th Sep 2016, 5:15 pm	#3
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: Mullard MAS183 whistles Quick test, fix an earth lead to a cut open coke can and drop it over the valves one at a time, you will soon see which one it is.

2nd Oct 2016, 6:24 pm	#4
lloydwells Heptode Join Date: Aug 2016 Location: Dumfries, Scotland, UK. Posts: 550	Re: Mullard MAS183 whistles Weld through ( copper ) primer quite nifty for recoating valves

2nd Oct 2016, 11:35 pm	#5
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: Mullard MAS183 whistles I have had this as well on different sets, if it is not a problem with valve screening as mentioned, check the decoupling capacitors. Mike

3rd Oct 2016, 2:25 pm	#6
telstar Octode Join Date: Dec 2003 Location: Leeds, West Yorkshire, UK. Posts: 1,593	Re: Mullard MAS183 whistles Hi Mike All the wax capacitors have been replaced with modern yellow types. The ECH35 valve has been replaced by a C.V equivalent. The EF39 seems to have good contact with chassis, but I suppose I could use a 6K7 and see if this works. I hope to post a photograph of this radio (it is larger than the permissible 200 kb allowed on this site. Cheers Mike

3rd Oct 2016, 3:40 pm	#7
Voxophone Hexode Join Date: Oct 2015 Location: Sunderland, Tyne and Wear, UK. Posts: 340	Re: Mullard MAS183 whistles I just had this problem on a set in which I substituted a known good EF39 and performed an IF alignment. I was able to fix the problem by de-tuning the IF slightly - i.e. broadening/flattening the peak.

3rd Oct 2016, 5:10 pm	#8
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Mullard MAS183 whistles Does the metalisation of both the EF39 and ECH35 show continuity to pin 1?

4th Oct 2016, 2:51 pm	#9
telstar Octode Join Date: Dec 2003 Location: Leeds, West Yorkshire, UK. Posts: 1,593	Re: Mullard MAS183 whistles Hi. Yes there is good continuity from the metallisation of both EF39 & ECH35 to their respective pin 1 terminals. In this case the F.C is a better grey colour (CV1347)which seems to have survived better than the red colour. I have managed to receive some weak stations (volume control at maximum), and that occurs when you put a finger on the top cap of the frequency changer then the whistling disappears. The front end must be working in order to do that task. Unfortunately, I will have to re-string the dial cord assembly as it has come loose.

4th Oct 2016, 4:12 pm	#10
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Mullard MAS183 whistles Your finger is adding capacitance to earth and preventing oscillation. I would check capacitors particularly the screen grid decoupling and you might try adding a 0.1uF across the HT.

5th Oct 2016, 2:07 pm	#11
telstar Octode Join Date: Dec 2003 Location: Leeds, West Yorkshire, UK. Posts: 1,593	Re: Mullard MAS183 whistles Hi. All the wax capacitors have been replaced in this radio. These stations that are received are tuneable but weak.

5th Oct 2016, 2:14 pm	#12
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Mullard MAS183 whistles If it is IF instability then the AGC voltage will be very low. Try measuring it across the AGC filter capacitor, C6 in trader 478 for the 206A.

5th Oct 2016, 4:58 pm	#13
telstar Octode Join Date: Dec 2003 Location: Leeds, West Yorkshire, UK. Posts: 1,593	Re: Mullard MAS183 whistles Hi. I will look at the voltage at the a.g.c capacitor and find out the result. Also, I notice that when touching around the second I.F transformer, a buzz is heard in the loudspeaker. Mike

8th Oct 2016, 2:08 pm	#14
telstar Octode Join Date: Dec 2003 Location: Leeds, West Yorkshire, UK. Posts: 1,593	Re: Mullard MAS183 whistles At long last I have traced the fault to the front end. There were two capacitors that had been incorrectly installed. When there were stations received before, they sounded like the effects single side band radio broadcasts produced. It now receives m.w stations. I have the dreadful task of rewinding the cord drive. Mike

4th Jan 2017, 3:34 pm	#15
telstar Octode Join Date: Dec 2003 Location: Leeds, West Yorkshire, UK. Posts: 1,593	Re: Mullard MAS183 whistles Hi. Actually the problem has reappeared, the whistling actually comes when tuning between stations. I might try adjusting the I.F Filter and see if it has any effects. The drive cord works perfectly now as I have found that the cord used by builders is really useful on this kind of job.

4th Jan 2017, 3:47 pm	#16
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Mullard MAS183 whistles Noting the points in frequency terms between the whistling (beating) when tuning might give a clue. Lawrence.