Distortion On Pre War Defiant MSH920 Radio. - UK Vintage Radio Repair and Restoration Discussion Forum

12jslater · 19th Jun 2017, 8:18 pm

I have been using my pre war defiant everyday for the past 3 months since its restoration, it is a proper work horse of a set and when I switched it on yesterday the audio had distortion in a similar way to when audio peaks. Or like when a microphone try's to record with to high of a volume input.

It has been totally recapped and 2 of the resistors were replaced in the restoration.

Help would be appreciated,

Thanks, Jake

Silicon · 19th Jun 2017, 10:18 pm

Have you tried an external speaker?

I don't have a schematic but it seems to be a 'short superhet'.
Valves are FC4, ACVP1, AC2/PenDD and a UU4.

12jslater · 19th Jun 2017, 10:20 pm

Yes. I tried one of my hi fi speakers and still the same sound.

Steve_P · 19th Jun 2017, 10:49 pm

I have not got the circuit to hand either - sorry. But does this set have a Gram input. If it does see what happens when you feed a signal in there.

If it still sounds bad look around the AC2/PenDD. The manual is not up top - I just looked - but the voltages are here: http://www.r-type.org/exhib/aaq0304.htm

I hope the Output Transformer is OK!

If it is OK then the diodes in the AC2 may be bad but please look at the IF valve - ACVP1.

If you are listening to Radio 4 then try other stations as R4 tends to have sibilance problems.

Cheers,

Steve P.

Leon Crampin · 20th Jun 2017, 9:38 am

Check the soldering on the top cap of the output valve - on the valve itself. These joints often fracture due to thermal fatigue as an output valve runs hotter than an RF stage and there is no flexibility in the connecting lead to relieve stress. It's very hard to spot a broken joint and an eyeglass may be necessary.

A poor connection here will give distortion as the grid potential drifts around, upsetting the bias. As the Mazda valves are tough, they nearly always seem to survive this treatment, and normal operation is restored on resoldering the joint.

As it seems to take about 70 years for these joints to fracture, I have not designed any countermeasures.

Leon.

19th Jun 2017, 8:18 pm	#1
12jslater Pentode Join Date: Nov 2016 Location: Wolverhampton, West Midlands, UK. Posts: 117	Distortion On Pre War Defiant MSH920 Radio. I have been using my pre war defiant everyday for the past 3 months since its restoration, it is a proper work horse of a set and when I switched it on yesterday the audio had distortion in a similar way to when audio peaks. Or like when a microphone try's to record with to high of a volume input. It has been totally recapped and 2 of the resistors were replaced in the restoration. Help would be appreciated, Thanks, Jake

19th Jun 2017, 10:18 pm	#2
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,171	Re: Distortion On Pre War Defiant Radio Have you tried an external speaker? I don't have a schematic but it seems to be a 'short superhet'. Valves are FC4, ACVP1, AC2/PenDD and a UU4.

19th Jun 2017, 10:20 pm	#3
12jslater Pentode Join Date: Nov 2016 Location: Wolverhampton, West Midlands, UK. Posts: 117	Re: Distortion On Pre War Defiant Radio Yes. I tried one of my hi fi speakers and still the same sound.

19th Jun 2017, 10:49 pm	#4
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Distortion On Pre War Defiant Radio I have not got the circuit to hand either - sorry. But does this set have a Gram input. If it does see what happens when you feed a signal in there. If it still sounds bad look around the AC2/PenDD. The manual is not up top - I just looked - but the voltages are here: http://www.r-type.org/exhib/aaq0304.htm I hope the Output Transformer is OK! If it is OK then the diodes in the AC2 may be bad but please look at the IF valve - ACVP1. If you are listening to Radio 4 then try other stations as R4 tends to have sibilance problems. Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

20th Jun 2017, 9:38 am	#5
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,874	Re: Distortion On Pre War Defiant MSH920 Radio. Check the soldering on the top cap of the output valve - on the valve itself. These joints often fracture due to thermal fatigue as an output valve runs hotter than an RF stage and there is no flexibility in the connecting lead to relieve stress. It's very hard to spot a broken joint and an eyeglass may be necessary. A poor connection here will give distortion as the grid potential drifts around, upsetting the bias. As the Mazda valves are tough, they nearly always seem to survive this treatment, and normal operation is restored on resoldering the joint. As it seems to take about 70 years for these joints to fracture, I have not designed any countermeasures. Leon.