Cossor 500 poor on LW

[PaulR]

I have resurrected this set which was my second restoration, having been found on a skip.

I always used a long aerial in the past and it worked reasonably well, but it has always been much quieter on LW than it is on MW. LW has a background hiss with a long aerial and can barely be received at all using the frame aerial. MW wave comes through strongly even with the frame aerial.

I completely re-capped it when I restored it and have now just done the IF alignment which was somewhat out. I have also adjusted C31 to bring LW into line on the tuning scale.

I am sure that it should perform better on LW and I would be grateful for any suggestions as to how I could improve it.

Thanks


Paul

[YT2095UK]

I also look forward to replies on this, I have a 520, very similar cct to the 500/501, and have the same problem although time of day does have a slight influence, and on SW nothing below 8MHz seems to exist.
like yourself, I have done the same with the exception of re capping (it really didn`t need it).

[Steve_P]

This is either the Antenna or Aerial side. Clean the Wavechange switch, valve pins and holder and replace/check C10, C9, C13, C15, C6, C3 and C1.

A capacitor passes less at lower frequencies, so think about it - check C9 and C6 first.

I used the factory sheet, available up top there. Numbers on the trader are different. Here, go for C3 and C6 first.

Cheers,

Steve P.

[PaulR]

Thanks Steve

I didn't change the low value front end caps when I did the set so I will try as you suggest. The set was in poor internal condition when I acquired it and the valve holders were very corroded so I replaced several of them and cleaned the rest. Unlike the rest of the set they were not vey good quality. A forum member very kindly gave me a replacement for the first IFT which was o/c.

I also cleaned the wave change switch, but I will have another go at that.

As a matter of interest it also had an annoying low level hum which is what made me put it away in the end. When I resurrected it yesterday I saw that I had put the replacement dial lamp wires through the same chassis hole as the speaker leads. Re-routing them has much improved this.

I will order some capacitors and post the results.

Thanks


Paul

[kalee20]

I have a Cossor 500, LW is OK although an aerial helps, but SW is really bad. The oscillator almost gives up halfway across the dial. It's not the 7S7 frequency changer valve (replaced), the oscillator coil (rewound), the wavechange switch (bypassed) or any of the R's or C's (all renewed). So it must be a design fault.

Originally Posted by PaulR

As a matter of interest it also had an annoying low level hum which is what made me put it away in the end. When I resurrected it yesterday I saw that I had put the replacement dial lamp wires through the same chassis hole as the speaker leads. Re-routing them has much improved this.

I'm really surprised at this - I have only known hum from dial lamp wiring be a problem when running close to low-level, high impedance circuitry, such as between detector and first AF amplifier valve.

[YT2095UK]

I replaced C9 and 6 and even tried larger values, didn`t make even a Tiny difference, so I replaced them again.
if this helps any, IF on SW I`m at the top end and them wind back slowly I can get stations just below 8MHz, if I start from the bottom end up, I need to get to nearly 10MHz before anything happens.
I think it Might be an osc problem.
removal of C3 (the 75pF) on the LW winding, only makes the sound a little Brighter too, I went to replace it in case it was leaking, but it actually sounds better left out entirely.

[Steve_P]

Have you got the B8A or the Loctal valve version?

Cheers,

Steve P.

[YT2095UK]

mine`s the B8A sort, I learned this to my cost only after buying the 7c6 "equiv" valves for it and, well... you know the rest

[Steve_P]

We've had this before and not cured it. Can't think why.

Change R4 and R3. Also check R17. Try a new rectifier valve too. I wonder if the HT is not quite all it should be?

(Used Manufacturer's sheet. R4 goes from HT to Anode of frequency changer triode, R3 goes from Grid of the Triode to Deck, R17 comes off the O/P transformer to HT line.)

Cheers,

Steve P.

[YT2095UK]

I`ll check into that in the moro, I also have a VHF90A that`s a smoker after 5 mins to see to as well, but yeah, like your C6 9 post, your logic is unasailable (hence I tried it), and it seems from my searching (outside of here) that it IS a common problem amongst these series of sets, 500,501,520.

it would be Nice to crack it here

[kalee20]

Have a look at my own experiences here.

The fact that you get stations on SW when tuning from one direction but not the other, very much indicates an oscillator problem. Tuning to one end allows the oscillator to start, and once running, it keeps going a fair way across the scale. When stopped, you have to tune back quite a way before it restarts again.

[YT2095UK]

I replaced V1 at the front end (RF amp) and it made a slight improvement in overall performance but the problem is still there after now changing All of the valves in the set.
I think I can rule those out for now.
I`m wondering if there may be some way of rigging up a small circuit mechanically activated by the tuning dial cord to stimulate oscillation again as the dial is tumed to lower frequencies, a bit like having a twin carb on a car.

[telstar]

Hi. Have you checked out the modifications shown in the Trader sheet for the variations to the valves used in these models. There are some changes in component values, and I wonder if your particular set required these ?
Regards
Mike

[YT2095UK]

I`v tried both sets of equivs in my set, incl the originals.
you`re quite right though, I have seen specs and it does recommend Resistor value changes for 2 valves if you use them, I didn`t alter the values and just tried it as was as a test for my valves (they only arrived today and I had no way of checking they worked).
the changing of V1 has made the most difference to the set overall, and now I`m using all 62 series.
she`s still my baby regardless of this quirk, but it would be nice to find out Exactly what was causing it, I have a feeling it`s something REALLY simple.

[PJL]

This does not have a true frame aerial like say the ekco u122 or bush dac90. In these the aerial coil is the inductive part of the tuned aerial circuit. Both of these sets perform very weill on the internal aerial.

It uses one of these odd circuits where the aerial is fed into the bottom end of the tuned circuit across a capacitor. Don't fully understand how they are supposed to work and they never seem to anyway. I suspect this has something to do with earthing and perhaps the changes in AC supplies over the years. Try it with an isolation transformer and a proper earth and see what happens?

[kalee20]

It's called 'bottom end coupling' and it does work, though it's not very elegant.

The aerial works into quite a low impedance and a good earth connection makes a lot of difference, so PGL's advice is good!

[PJL]

Kalee20, thanks for explaining what I had suspected but any idea what the impedance to ground is for mains neutral and why or if it might have changed? AC/DC sets requiring external aerials often seem weak on LW and I can't beleive they were as bad when first used.

[YT2095UK]

one of the 1`st things I did when I got this set was replace the mains lead, I used 3 core flex and earthed the chassis, and yes it did improve reception fantastically, I noticed this when I tested for residual current leaks with the set On before connecting it.
I also use a longwire antenna in the back instead of the frame loop that comes with it, it does make a hell of a difference on MW a good 5 fold increase in volume, and only a little bit of difference on SW/LW but non the less there is an definite increase.
I`v also tried combinations of the two, loop and atenna with the longwire attatched.
it`s still poor on LW and non existant bellow 8MHz on SW.

when I get my new workshop set up, i`ll try a copper plate in the garden as a ground, maybe that will be better, currently I`m on a second floor skywalk and the mains earth strap is the best I can do.

[YT2095UK]

I just discovered something interesting, my 1000uH chokes/inductors arrived this morning (related to my homemade chokes thread), and after messing about with them and an amp I made a crude TRF that without a capacitor landed on Radio4 LW.
so I considered putting that in line with my longewire antenna instead of my usual 150pF cap, and it Worked! the signal was at least 2x louder than with the cap or direct to the wire itself.
now I Must point out that the result still isn`t perfect, but maybe these sets could do with an antenna matcher (I think it`s the same as an ATU), and that perhaps it`s getting swamped/saturated with other HF signals around?
I know mine certainly picks up the mouse movements and Monitor noise, and probably many other frequencies generated by a PC without a case on it <blush>.
a Matcher is probably something I should have anyway with a longwire, yet another project for the oncomming winter months me thinks

[Steve_P]

I would also be wondering why SW does not work properly more than that. I do find it hard to believe that they did that from new.

Is the capactor C14 on the SW that critical? Is the HT OK - I mean exactly OK. C17 and R4? C22?

Cheers,

Steve P.