Replacement of OC171 in an Eddystone EC10

[Argus25]

Charles,

....on the topic of the IF and coil cores for alignment I could give some advice, but you probably know all the tricks, but these are my experiences with the EC-10:

It is very important to use a plastic tool, preferably Nylon with a blade at the end shaped to exactly fit the slot in the ferrite slug. Remove an easy to turn slug and craft a tool for the job, rather than just picking a random about right tool. The better the fit the more of the surface area in the slug slot is used and the less chance of it cracking (under no circumstances use a metal screwdriver).

The problem is that the fine rectangular section latex rubber cords, originally placed in the slug threads from new (to stop the slugs moving) melts into a liquid and re-sets as a glue, or it might just turn to powder.

If you attempt to turn a slug with gentle force and it is stuck/frozen, put a drop of WD-40 on it and leave it for 48HRs. Then re-try with small movements to start to loosen it. If necessary repeat that process twice and slowly work it free. Any remains of WD-40 can be washed off with contact cleaner later when the slug is out.

To re-lock slugs, as you know some people now are using Teflon thread tape. The amount of that has to be just right or it can lock and jam a slug too. I don't use it often.

Instead I use very small diameter round (rather than the original square section) elastic rubber cord, which can be obtained by dissecting flat fabric coated clothing elastic, the type of thing that is in old fashioned white underwear waist bands, which contains multiple fine parallel white elastic rubber cords (which are very difficult to get from other sources, but you can buy this elastic from sewing shops, there are a range of cord diameters in that stuff, you need to pick the fabric off it to get it out). This locks the slugs well against vibration but they are also easily rotated for alignment. And the miniature cord is very long lasting and doesn't melt.

Also of note, in the EC-10, there are two positions of the cores in the front end coil array that peak on the correct frequency. In one case the slug position results in tighter coupling with the resonant winding and the coupling winding which has some effect on the gain & bandwidth, it appears that the factory position was that of the less heavily coupled position.

As you know if one of the double tuned IF coils has a damaged slug at one end, it is possible to get it out via the other side when the other slug is removed and use the undamaged slot.

Hugo.

[Argus25]

..also some of the slugs might have hex slots and the correct plastic tool is needed for those.

[unixmanuk]

Thanks for all the useful info, Hugo.

I had my own EC-10 out only a couple of days ago, and it really is deafer than a post on most bands - I live 5 miles from a main high power MW broadcast station, and even that is not particularly loud on it. Most other bands are virtually silent, so reading this may have inspired me to replace all the OC's with the AF178.

I recently restored another (valved) comms receiver with amazing results, so I may have a go at this one now.

I have the diagram around somewhere, but (being lazy! ) how many replacements are required for a complete refurb?

Cheers,

[Herald1360]

Six!

[space_charged]

If your EC-10 is as "deaf" as that then it has some sort of problem. For reference, I can still hear the MW station at approx 700KHz on mine with the RF gain turned down to minimum. On the higher frequency ranges, there is a deafening hiss with the RF gain at maximum at only moderate AF gain. The hiss goes away when I remove the aerial. Its quite a lively set and compares well in sensitivity with my Eddystone 680X.

HF bands are a bit "down" at the moment it seems. This is probably due to rather low solar activity at the moment. There has been discussion also about there being fewer stations on HF these days too.

Having done the OC171 to AF178 replacement I'd very much recommend it. Not only will you be forever free of the dendrite problem, but the replacement transistor out performs the OC171. It has better gain, lower base capacitance and lower noise.

My set had considerable noise audible with the volume control turned fully down. This was found to be caused by the bias resistors (particularly the 15K one) on the base of the first AF transistor (OC71). In a set of this age, all carbon resistors are potential suspects and of course capacitors. Ceramic caps will likely be OK.

I actually managed to hear some SSB transmissions on the EC-10 last night. The BFO on my EC-10 doesn't seem that stable. I'm thinking it might be the variable capacitor. Might try cleaning the spring contacts on the rotating part.

Charles

[space_charged]

Originally Posted by Argus25

..also some of the slugs might have hex slots and the correct plastic tool is needed for those.

More or less decided not to re-align. I remember stuck cores and also cracked cores, specially the hex slot type. These then expand in the coil former and jam tight. I've dug a good few out of television IFT cores in my time. Back then it was easy to get a replacement core, clean the threads and fit the new core. These days, getting replacement cores might be a problem.

It might be interesting to sweep the IF strip and check its response. If there were any sign of double peaks I might change my mind on alignment.

Charles

[AC/HL]

Several off topic posts deleted.

[unixmanuk]

Mine used to be better some years ago, but was never really up to the sensitivity of the valve counterparts. Its performance does seem to have further deteriorated with age, so a refurb of the transistors plus any suspect caps / resistors is probably in order. It's cosmetic condition is better than most though.

It is a bit livelier in the 9 MHz area, but it's always been quieter between 15-30MHz. There is plenty of audio gain though. Probably doesn't help being connected to a rubbish antenna in the garage either. I too recall the BFO not being stable, and the injection level was less than required when receiving stronger signals - microphony was also an issue.

I'll order some new devices and see what happens.

[Argus25]

Originally Posted by space_charged

My set had considerable noise audible with the volume control turned fully down. This was found to be caused by the bias resistors (particularly the 15K one) on the base of the first AF transistor (OC71). In a set of this age, all carbon resistors are potential suspects

The resistor in every EC-10 that requires replacing is the collector resistor in the first audio amp. This carbon comp resistor generates more audio noise than the transistor itself. It is interesting that obviously other resistors in the set of the same type would be doing this too, the noise they generate (which is largely in the audio frequency spectrum) doesn't practically affect the RF/IF stages and doesn't appear at the detector as it is out of the bandwidth of the IF & RF circuits and obviously does not cause enough noise in the oscillator to give any practical problem. That is the only resistor I have had to replace in all my EC-10's. The noise also appears proportional to the resistor's current so it shows up more in the collector resistor at the audio front end than anywhere else, less is generated in the first audio transistor's bias resistors it would seem.

[Argus25]

Originally Posted by space_charged

More or less decided not to re-align. I remember stuck cores and also cracked cores, specially the hex slot type. These then expand in the coil former and jam tight. I've dug a good few out of television IFT cores in my time.

I was able to remove a cracked core (of the type with a hex hole in it). As you point out, if they are cracked in half when the tool is put in them and they are rotated, the two halves expand away from each other, causing the sharp cracked edges to bind in the former's thread making rotation impossible.

However there is this trick: if a hex tool is used, that will normally just slide freely in an un-cracked slug, in this case metal works better, it is coated with some strong epoxy resin (24 hr Araldite), just enough to glue the tool into the slug's hex hole (not enough to get to the threads) this glues each half of the slug to the tool. Then later, when it is rotated, the two halves of the cracked slug do not expand apart and it can be successfully unscrewed.

[Radio Wrangler]

Originally Posted by Argus25

However there is this trick: if a hex tool is used, that will normally just slide freely in an un-cracked slug, in this case metal works better, it is coated with some strong epoxy resin (24 hr Araldite) , just enough to glue the tool into the slug's hex hole (not enough to get to the threads) this glues each half of the slug to the tool. Then later, when it is rotated, the two halves of the cracked slug do not expand apart and it can be successfully unscrewed.

Subtle, and a new one on me.

To think of the time I've spent breaking up broken slugs to dust to get them out.

One for the memory bank!

Thanks,
David

[val33vo]

I know the biasing is out but I have used a BC559 with success.

[Argus25]

Originally Posted by val33vo

I know the biasing is out but I have used a BC559 with success.

Yes, if a BC559 is placed in the radio's front end (Tr1) and one checked the performance of the radio, you would find that the RF gain control was malfunctioning.

Looking at the manual, under the voltage analysis section, the base voltage of TR1 is designed to vary between 1.0V (max RF gain) and 0.1V (min RF gain). Under about 0.55 to 0.6V the silicon transistor's base-emitter stops conducting.

In addition, if you look at the other stages, the base voltages are very low (admittedly with a 20kOhm/volt meter), for TR5 for example it is only 0.7V. Also normally TR5's emitter voltage is 0.4V ( only 0.3V dropped across the germanium transistors base-emitter junction) The 0.4v developed across TR5's emitter resistor determines the emitter current. If a silicon transistor is used the emitter current halves at least.

In short, while silicon transistors can "work" for a quick fix, ideally the bias resistors should be altered to accommodate them. If they are not, the impairment in performance would show up on a formal analysis of the radio's functionality. So it is better to use germanium parts, especially since they are readily available.