Roberts R200 (OCxx); poor performance.

[m didier]

Hello can anyone with my first transistor radio repair .The radio is a roberts R200.

So far I have recapped it changing all the capacitors including the electrolitics and checked all the other components which seem ok but as its my first time I have checked transistors it may be wrong.

The radio now picks up a few strong station at low volume on LW and MW.
I have also aligned the set using information an Pauls website which improved the volume a little bit. The radio uses OCxx transistors.

Can anyone suggest what to try next?
Martin

[Tim]

Try connecting a signal (from an MP3 player etc.)across the volume control, to check the output stage. If all is well the RF stages are at fault.
The R200 has the ferrite rod quite close to the battery. It is quite possible the coils on the ferrite rod have been moved or even damaged by a careless battery inserter. Check the thin wires to the ferrite coil carefully.
It is possible the OC transistors are faulty, but multimeter tests may not be conclusive, especially in the case of low gain etc. Don't overlook cracked PCB tracks, or wires broken or mis connected by a previous repairer.

[Darren-UK]

Hi Martin,

Could be a number of reasons but I'll go through the simplest and most obvious.

Check the battery is spitting out a good voltage on load...at least 7½v.

Check wavechanger contacts are mechanically good.

Check aerial coil(s) on ferrite rod are not o/c and that wires therefrom are intact. Also ensure coils are correctly positioned on ferrite rod.

Check IFT slugs are secure in their formers. If one is loose and has lateral play you can end up with poor or even non-existent audio.

Test and change if necessary the OC44.

Check biasing ( biasing faults often become apparent after a few minutes of operation from 'cold' ).

Check resistor values around the output stage.

Check voltages around the transistors - data is given on the service sheets for this.

Some of the above checks involve alignment. I presume what you referred to is Pauls info on alignment without using a signal generator ? This is useful and handy info if you lack the required equipment but you really ought to do a realignment using a sig gen before dismissing the problem as a component fault.

Another problem which seems to plague the R200 concerns the little caps inside the IFT cans; difficult to trace and very tedious as you need to open up the offending can.

I would also suggest carefully rechecking where you removed and replaced the caps for any damage to pcb tracks or nearby components due to soldering iron heat.

[m didier]

Thankyou for your suggestions after checking the voltages around the transistors. TR2 voltages seem alot out . The data on the service sheet give emitter 0.5 base 0.6 collecter 7.4 .I get emitter 2.5v base 2.5v collecter 6v .Is this to far out of speck.
Can anyone also tell me the equivalents to oc45 transistor and where to get them from .

martin

[Darren-UK]

Equivs of the OC45 are AC121, AF126, AF133, AF187 and 25B101 according to my equivalents book. The 25B101 is a Japanese jobbie. Off the top of my head I can't say where these can be obtained from accurately but, personally, I wouldn't bother. The OC45 is very common and easily acquired from scrap radios or it's quite likely one of our forum members can supply one. The OC45 seldom gives trouble, unlike the OC44 and OC71 ( that latter nor being relevant here ) although this is only relatively speaking.

From your latest posting, I am assuming voltages around the other transistors are acceptable ? The TR2 voltages you found are way off the mark and I suspect TR2 may have a damaged or shorted junction, possibly due to surge or a short in the (common) emitter circuit. Before changing TR2 you, obviously, need to find out why this has happened. Have you checked the components in the immediate vicinity of TR2 such as R6 ? You said you changed a load of caps, did this include C13 ? It is just possible TR2 has gone duff of its own accord, but very unlikely - suspect the cause to be elsewhere.

I'm assuming you've looked at the other issues which myself and Tim raised ?

[m didier]

Hello, I have been going through your suggestions and the only fault I can find is the voltages around TR2 .I have changed C13 twice just in case and R6 reads bang on.

The only thing I have not done is align it using a signal generator.

Martin.

[m didier]

A quick update, the radio is now working loud and clear on MW and LW but only on one half of the wavebands. Does this mean I will have to align it using a signal generator?

I had replaced R4 with the wrong value resistor, the voltages around TR2 are now correct.

Martin

[Simon Gittins]

Hi Martin,
The tuning capacitor may be faulty (short-circuit over part of its range); this can be checked using a meter with the set off. Clearing such a short requires patience!

Simon.

[Tim]

Well done Martin. Try alignment by all means, and sliding the coils up and down the ferrite rod for maximum signals.
Good luck!!

[Darren-UK]

Originally Posted by martin

A quick update ,the radio is now working loud and clear on mw and lw but only on one half of the wavebands .Does this mean i will have to align it using a signal generator.I had replaced r4 with the wrong value resister ,the voltages around tr2 are now correct.

martin

Where you've got stations loud and clear over medium and long waves, are these stations coming in where they should be on the tuning scale ?

I'm not sure what's receivable in your area but, generally speaking, most of what's still available on MW and is reasonably audible will be found roughly between 250 and 400m. LW has R4 at 1500m, with RTE1 towards the high frequency end and some babbling French station towards the low frequency end. Sorry to talk in both metres and frequencies there

If all is correct regarding what and where on the scale, follow Simon's advice re the tuning gang. I'm assuming Simon is referring, as he's mentioned 'shorting', to fouling veins (or vein). When this happens you almost invariably get a horrible crackle at the loudspeaker. Another cause is incorrect relationship between fixed and moving veins ( ie the airgaps are unequal )but not to the extent where veins begin to foul. This will give the same effect as misalignment of the IFTs and trimmers. A third possibility concerning tuning gang problems concerns 'foreign bodies' causing the signal to go to ground. This could be a tiny blob of stray solder, a tiny piece of wire etc etc. Have a look and see what you can find, it's not always immediately obvious.

Failing all this it's time to look at alignment, including shifting the aerial coils as Tim has just suggested - try this first before you dive into full realignment.

[Superhet Si]

Martin, as you tune does the radio suddenly go pop and die? As the other Simon Says it may me the tuning cap shorting out.

[m didier]

The stations are in the correct place. Most of the stations are between 200 and 400m.

The tuning cap is a bit bent and when tuning in it does crackle a bit so I will have to straighten it up, any suggestions on how to do this?

Martin

[Darren-UK]

Hi Martin,

Defo a prob with the tuning gang then. Two faults can occur here and you need to determine which is relevant to your radio. I'm typing this posting on the assumption that you mean the tuning gang has bent veins ? You actually said the tuning cap itself is bent.

1. One or more veins can be bent through physical damage/abuse/clumsiness. Rectifying this can be tricky and needs much patience. You need, obviously, to find which vein, or veins, is/are bent. Next you need something wafer thin and non-metallic which will slip easily into the air gap between the veins. A strip of, say, polystyrene sheet ( or other soft, fairly flexible plastic such as celluloid) is needed. Tuning gang veins will flex very easily; just slip your strip of plastic between the offending vein(s) and carefully twist it until the vein flexes and you'll find it will resettle itself midway in the air gap. Alternatively use same piece of plastic and carefully bend the offending vein by manipulating it via its outermost edge. DO NOT use anything hard or metallic, plastic is ideal. I have known cases of people using small screwdrivers or knife blades to do this - resulting in scored veins and thus irrepairable damage. You should also make your careful adjustments with the gang closed, as should be obvious from what I've said herein. Take care, take your time. We have had discussions on this subject of damaged gang veins before, so others may suggest alternative methods but the above is what I do myself - usually with success.


2. The moving and fixed veins may be fouling due simply to maladjustment which is often the result of the phantom dabblers work. In these cases you adjust the gang via the screw and locknut on one end of the gang frame. Provided all veins are undamaged you can normally do this adjustment by eye. Do not over-release the locknut and screw though, or you may lose the ball bearings located at the opposite end of the gang spindle.

Bear in mind that if whatever remedy proves necessary, you will likely need to do a realignment of the radio. I recall you saying you did a realignment using Paul Stennings instructions. If so, you've realigned the radio with a faulty tuning gang so realignment will indeed likely be necessary. You may get away with it though if the problem is simply one or two bent veins, let us know how you get on.

[m didier]

Thanks for the replies, it is just a couple of the veins that are bent and I will try to straighten them using the method Darren-uk suggested when I can find something suitable to do the job.

Martin

[m didier]

Thanks for all the help the radio is playing loud and clear ,crackle free

martin