Pye FenMan II - FM Bass Distortion - UK Vintage Radio Repair and Restoration Discussion Forum

Jim - G4MEZ · 14th May 2007, 10:06 pm

I'm currently in the process of restoring my FenManII (a supurb set) and I notice a little distortion on FM. This distortion only affects deep bass as heard on typical Radio 2 pop music. Listening to Radio 3 and 4 it is absolutely perfect, no distortion whatsoever.

Radio 1 shows the same distorted bass. I proved the audio amp by playing a typical pop songe through the Gram input and it came out sounding fine across the range with nice tight and well defined bass.

I've tried replacing the EABC80 Discriminator and the EF85 Limiter with no improvement. For sake of completness I also tried another EBC41 but without improvement.

I also notice that FM tuning is very critical, just a tad either side of the right tuning point and the audio is very distorted and ragged.

I cannot for the life of me work out why only the bass is distorted. Is this due to bandwith limitations in these older receivers and an artifact of heavy compression used on R1 and R2? If so, I would have thought the treble would have suffered first.

A similar, although very much less pronounced effect is also observed on my Bushes (64 and 81s).

Any thoughts? This one is really making me frown

paulsherwin · 14th May 2007, 10:13 pm

It's because the modulation levels are so high.

You can fit resistors across the IFTs to flatten the frequency response - this usually helps somewhat. You can experiment with values to get the best distortion / selectivity / sensitivity compromise.

Paul

Radio_Dave · 17th May 2007, 10:30 am

Quote:

Originally Posted by Jim - G4MEZ

I also notice that FM tuning is very critical, just a tad either side of the right tuning point and the audio is very distorted and ragged.

I've just been playing with one of mine and I can't say that I have the problems, you describe, with the tuning. The bass sounds OK (to me) as well

Hopefully others can offer suggestions

David

Aub · 17th May 2007, 10:54 am

Hi,

Are you sure you're getting a good signal? A poorer signal may cause the tuning to be more critical.

best regards

Aub

Jim - G4MEZ · 17th May 2007, 12:21 pm

Thanks all! Much appreciated

Quote:

Originally Posted by Aub

Hi,

Are you sure you're getting a good signal? A poorer signal may cause the tuning to be more critical.

best regards

Aub

Yes, a real thumping signal Aub. I'm only a few miles from the Bow Brickhill relay and the TI is fully deflected. The tuning criticality is no different when tuned to Oxfod, which is around half the signal strength of Bow Brickhill.

Steve_P · 17th May 2007, 3:17 pm

Wonder if the I.F. is OK and on frequency. If not, realingn the I.F. Cans and see if that improves it.

If you get the Service Sheet, it can be done with an A.M. Generator. Get the I.F. Right and see what happens.

Cheers,

Steve P

PaulR · 17th May 2007, 4:53 pm

Quote:

Originally Posted by Steve_P

Wonder if the I.F. is OK and on frequency. If not, realingn the I.F. Cans and see if that improves it.

If you get the Service Sheet, it can be done with an A.M. Generator. Get the I.F. Right and see what happens.

Cheers,

Steve P

I had a simiar problem with a Bush VHF61. The tuning was so "fine" that any drift as the set warmed off put it right off the station. Realigning it cured the problem.

Paul

yestertech · 17th May 2007, 6:12 pm

To put your mind at rest Jim, I have noticed this on more than one set, despite full alignment and "nulling out" the discriminator setting until exact on each of them
It's always the same, high modulation + heavy bass = distorted AF.
I don't know the technical reason, but I ASSUME it's a product of much higher FM deviation levels than would previously have existed, coupled with extra processing at the transmitter ( optimod et al ) . Capital ( 95.8 London ) is the worst offender, Radio 3 and 4 are always crystal clear by comparison. In the old days the IBA would monitor over-deviation and give the offenders a slap, but I doubt that anyone knows or cares these days !!
Andy

Tim · 17th May 2007, 10:58 pm

I have also noticed bass distortion on Radio 2 over the last few days while listening on my R707, (and on the radio in the works Vaxhaull Combo,but that's a rubbish radio anyway). I thought it might be due to the new power supply I had just treated the Roberts to, but it happens on battery too. Backing off the treble control helps of course.

Keith · 18th May 2007, 11:44 am

ISTR fitting 4K7 resistors across the IF's in my Bush VHF62. It's generally OK although I still occasionally hear bass distortion. Is there any mileage in stagger tuning the IF's I wonder?

Steve_P · 18th May 2007, 1:06 pm

Possibly a good idea. I might try it and let you know.

But, I had a Unitra Radiogram here with a similar fault. It was the O/P Valve. I'd changed THAT capacitor, and although it sounded OK on AM, it had this form of distortion on F.M.

The first thing I did was change the FM Valve - No Joy. But it sounded OK at the demod end. The output valve had been damaged slightly, and a new one put things right.

It's Push-Pull on a Pye. I'd feed in a signal from the Gram input and scope the output. Or just fit 2 x replacement EL84's and try it. Also the rectifier might be past it's best. Just a few volts drop.

When this happens, I put the old valves back on the shelf with a note attached, because in something else they might be (often are) OK.

Just a thought!

Cheers,

Steve P

14th May 2007, 10:06 pm	#1
Jim - G4MEZ Tetrode Join Date: Feb 2006 Location: Newport Pagnell, Buckinghamshire, UK. Posts: 95	Pye FenMan II - FM Bass Distortion I'm currently in the process of restoring my FenManII (a supurb set) and I notice a little distortion on FM. This distortion only affects deep bass as heard on typical Radio 2 pop music. Listening to Radio 3 and 4 it is absolutely perfect, no distortion whatsoever. Radio 1 shows the same distorted bass. I proved the audio amp by playing a typical pop songe through the Gram input and it came out sounding fine across the range with nice tight and well defined bass. I've tried replacing the EABC80 Discriminator and the EF85 Limiter with no improvement. For sake of completness I also tried another EBC41 but without improvement. I also notice that FM tuning is very critical, just a tad either side of the right tuning point and the audio is very distorted and ragged. I cannot for the life of me work out why only the bass is distorted. Is this due to bandwith limitations in these older receivers and an artifact of heavy compression used on R1 and R2? If so, I would have thought the treble would have suffered first. A similar, although very much less pronounced effect is also observed on my Bushes (64 and 81s). Any thoughts? This one is really making me frown __________________ Jim G4MEZ

14th May 2007, 10:13 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,968	Re: Pye FenMan II - FM Bass Distortion It's because the modulation levels are so high. You can fit resistors across the IFTs to flatten the frequency response - this usually helps somewhat. You can experiment with values to get the best distortion / selectivity / sensitivity compromise. Paul

17th May 2007, 10:54 am	#4
Aub Nonode Join Date: Mar 2005 Location: Nuneaton, Warwickshire, UK. Posts: 2,039	Re: Pye FenMan II - FM Bass Distortion Hi, Are you sure you're getting a good signal? A poorer signal may cause the tuning to be more critical. best regards Aub

17th May 2007, 3:17 pm	#6
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Pye FenMan II - FM Bass Distortion Wonder if the I.F. is OK and on frequency. If not, realingn the I.F. Cans and see if that improves it. If you get the Service Sheet, it can be done with an A.M. Generator. Get the I.F. Right and see what happens. 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...?

17th May 2007, 6:12 pm	#8
yestertech Nonode Join Date: Dec 2002 Location: Coulsdon, Surrey, UK. Posts: 2,135	Re: Pye FenMan II - FM Bass Distortion To put your mind at rest Jim, I have noticed this on more than one set, despite full alignment and "nulling out" the discriminator setting until exact on each of them It's always the same, high modulation + heavy bass = distorted AF. I don't know the technical reason, but I ASSUME it's a product of much higher FM deviation levels than would previously have existed, coupled with extra processing at the transmitter ( optimod et al ) . Capital ( 95.8 London ) is the worst offender, Radio 3 and 4 are always crystal clear by comparison. In the old days the IBA would monitor over-deviation and give the offenders a slap, but I doubt that anyone knows or cares these days !! Andy

17th May 2007, 10:58 pm	#9
Tim Dekatron Join Date: Jan 2003 Location: Bradford on Avon, Wiltshire, UK. Posts: 3,310	Re: Pye FenMan II - FM Bass Distortion I have also noticed bass distortion on Radio 2 over the last few days while listening on my R707, (and on the radio in the works Vaxhaull Combo,but that's a rubbish radio anyway). I thought it might be due to the new power supply I had just treated the Roberts to, but it happens on battery too. Backing off the treble control helps of course. __________________ "Nothing is as dangerous as being too modern;one is apt to grow old fashioned quite suddenly."

18th May 2007, 11:44 am	#10
Keith Heptode Join Date: Jan 2003 Location: Tewkesbury, Gloucestershire, UK. Posts: 693	Re: Pye FenMan II - FM Bass Distortion ISTR fitting 4K7 resistors across the IF's in my Bush VHF62. It's generally OK although I still occasionally hear bass distortion. Is there any mileage in stagger tuning the IF's I wonder? __________________ Keith Yates - G3XGW VMARS & BVWS member http://www.tibblestone.com/oldradios/Old_Radios.htm

18th May 2007, 1:06 pm	#11
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Pye FenMan II - FM Bass Distortion Possibly a good idea. I might try it and let you know. But, I had a Unitra Radiogram here with a similar fault. It was the O/P Valve. I'd changed THAT capacitor, and although it sounded OK on AM, it had this form of distortion on F.M. The first thing I did was change the FM Valve - No Joy. But it sounded OK at the demod end. The output valve had been damaged slightly, and a new one put things right. It's Push-Pull on a Pye. I'd feed in a signal from the Gram input and scope the output. Or just fit 2 x replacement EL84's and try it. Also the rectifier might be past it's best. Just a few volts drop. When this happens, I put the old valves back on the shelf with a note attached, because in something else they might be (often are) OK. Just a thought! 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...?