Roberts RMB Problems

[sobell1980]

I feel we ought to close this thread now. It generates no interest what so ever.
I will let you know my success story.
Dave.

[kalee20]

If you have a strong MW station appearing at the right place on the dial, but faintly, when the set is switched to LW, then it sounds like a problem with the wave change switch. The oscillator will be running as for MW, but I'd suggest that the RF coil will be set to LW. Being not massively selective, a really strong MW station may still be able to break through, and once at the frequency changer grid, the oscillator will give the correct IF because the oscillator thinks it's set to MW.

['LIVEWIRE?']

I have sometimes found that Absolute R. (1215kHz) sometimes can be faintly heard on ca. 275kHz LW on a properly working modern radio cassette recorder. Could this be something to do with oscillator harmonics, since 275 + 940 = 1215, so the 2nd harmonic of 275kHz is twice 470kHz, a common I.F.? Wouldn't explain why the RMB isn't working on MW, though!

[Herald1360]

I think what is happening there is that the oscillator is running at 275+470=745kHz and 1215-745= 470kHz so a strong signal at 1215 getting past the RF tuning and into the mixer will result in a 470kHz output to the IF.

Thinking about it that's just the same as saying that a signal twice the IF above the frequency that a set with a high side LO is tuned to will get through if the RF selectivity is poor. That's just a typical second channel or image response.

[Nickthedentist]

Originally Posted by sobell1980

I feel we ought to close this thread now. It generates no interest what so ever.

I'm interested, it's just that I've got nothing constructive to say, I'm afraid.

Don't give up on it!

N

[sobell1980]

Last valve holder to replace and it looks a nightmare. Been putting me off from doing it to be honest. I tried spraying all the valve holders with some servisol. I fiddled and tipped the valve in the last suspect valve holder. I managed to achieve what i can only describe as tuneable interference, better than a volume adjustable hum. This would crackle and come and go as the valve is moved. Still no stations not even the mirrored absolute radio onto long wave. Lets hope this last valve holder to replace proves fruitful! It feels mighty loose now. The valve could almost tip in its socket by its own weight. It is original to the set the Cinch type. Any more feedback chaps? That would be great. Hopefully get this valve holder changed this week. Other than that i guess it is to invest in a signal generator.
Many thanks.
Dave.

[sobell1980]

Hi all,
Finally braved changing the last valve socket. Again only a MW station can be heard on LW. This faded out and nothing now on any wavebands whatso ever. So all valve sockets have been replaced. The set has practically been rebuilt. All but the very low value caps changed. What are peoples views on it could be a valve breaking down under load? Crackling can be heard when the frame aerial connections are touched with a screwdriver. Really want to sort this one out now.
Dave.

[Phil G4SPZ]

Dave,

It sounds that you need to bring some more sophisticated test equipment to the party, such as a signal generator and a scope. Have you checked the valves themselves, either by testing or substitution?

It is possible that one or more of the Lemco silvered-mica capacitors has become intermittent. These small value capacitors are normally beyond reproach, but I have heard that over time the silvering can migrate away from the riveted connections, leading to intermittent low capacitance faults.

An oscilloscope would enable you to establish immediately that the local oscillator is running and on the right frequency, and if not, you'll have more of an idea where to look for the fault. Once satisfied that the LO is working, you can inject signals, starting with audio at the detector to prove the output stage, then modulated RF at the correct IF frequency at the output and input of the IF stage to establish that it's working properly and to align it if necessary, then finally modulated RF at signal frequency to check the whole set end-to-end and tweak the alignment.

Without doubt you've already done a great deal of work on this set, but you will find that a methodical logical approach will be necessary to finally track down what is probably a single faulty component.

[sobell1980]

Hi Phil,
Thankyou for your continued support and interest in my troublesome RMB. Yes i fully agree with you. I am a Master technician for Toyota, and you don't get to solve problems by guess work. A methodical approach is needed to carefully follow process and the path of the fault and a careful process of elimination will single it out. I only have have an automotive Pico scope at work, no signal generator. Could you recommend one to buy at reasonable cost? Yes I think it has come to the time if i am to progress in this wonderful hobby to expand my kit. Otherwise i'm going to be just getting myself frustrated. I understand ocsillascopes but i feel a first buy would be a signal generator? Any advice on this would be brilliant as i am a bit lost in what to look at. Many thanks,
Dave.

[Phil G4SPZ]

I suggest that a signal generator is a minimum requirement if you intend working on vintage radios (or any radio, for that matter) but for now you can dispense with an oscilloscope. There is a simple way of checking that the local oscillator is running.

Place the RMB next to a transistor portable, right close together. Switch on and select medium wave on both sets. Tune the transistor radio to a clear spot at about 1,100kHz (272 metres) then start tuning the RMB upwards, from the bottom of the MW band. The theory is that you will pick up stray radiation from the RMB's local oscillator on the transistor radio.

The local oscillator will be running at 472 kHz above the frequency indicated on the dial. So, if your RMB is tuned to 500 kHz the L.O. should be on 972kHz. At some point, you should hear a whistle, plop or rustle from the transistor radio, as the RMB's L.O. passes through the point to which the transistor set is tuned.

No signal at all, or a signal that is widely off frequency, points to a fault in the RMB's local oscillator. If you have checked all the DC voltages and resistor values and replaced any waxy condensers in that area, then there's not much left. The small 100pF capacitors marked C1 and C5 on Trader service sheet 1037 are the ones to look at.

[sobell1980]

Hi Phil,
Spent a couple of hours the other night re checking all my voltages etc. Some strange valve voltages but i remember someone on here telling me it was due to the fact i was using a DMM and not an AVO as described in the service sheet. So all seems well. I decided to test the remainder of the very low value capacitors including c1 and c5 which you mentioned to do. Well that will teach me to completely dismiss them due to them being low value and normally ok. And as this site says they are normally all ok or all bad. Well the five left i had not changed are all bad. I have a capacitance test on my meter that is very accurate. I snipped c1 and c5 out. Discharged them, tested them, no reading and my meter will read these very low values. I tested a known good one and others and all read ok and near to the values specified but nothing of c1 and c5. I then went on to the remaining 3. C14, C20 and 21. All failed ! These supply negative feedback. Maplins did not have what i required in stock, i am awaiting for some to arrive in the post i bought from ebay. I will let you know how i get on. I guess this is where the faults were all along with the dodgy selenium rectifier. Any additional input or thoughts greatly appreciated.
Dave.

[Phil G4SPZ]

Fingers crossed!

You will have to do some realignment as these capacitors are in the frequency-determining circuits. But get it working first.

[sobell1980]

Hi all,

Replaced C1, C5, C19,C20 , C21 and C14. Got a strong station briefly not sure what wave band i was on. All went quite again and all that can be heard is audible mains hum through the speaker. I went to adjust the volume and a crackle was heard then an intermmtant tuneable hiss could be heard, which would come and go with the volume pot tapped or moved but would not remain constant. I have removed the volume pot. It should measure 1 mega ohm. From track end to track end i have 1.200 Mega ohms.
opertion of the track starts at 1.200 Mega ohms and for full volume ends up around 300 Ohms. Has this gone high causing my problems? Prodding around this area makes this tuneable hiss come and go. There are no open circuits on the full range of the track. Please bear in mind i am using a DMM and not an Avo as stated in the service sheet. I have stripped apart the volume pot and deep cleaned the track with servisol 10. I understand it may need realignment etc but i should be getting some interference and hiss as i move the tuning condenser. I did however replace the low value 10pF and 100pF caps with the same value, so it should pick up something. Any help or advice greatly appreciated.
Dave.

[robjkmannering]

Are you sure that it is not intermittent valve socket connections? I have a vidor set that crackles and suddenly loses signal. Wiggling the valves cures it for several more hours of use.

Rob

[Phil G4SPZ]

The valve sockets have all been replaced.

The volume control is unlikely to be the cause of your woes, Dave. Have you run through the other tests I suggested in post #50?

[robjkmannering]

Sorry, had not read that. I sold a set of Dx96 valves, the buyer contacted me to say the LO was not running. That had had all new bases also, turned out to be poor contact on the DK96
due to faulty socket....

Rob

[sobell1980]

I do not think the LO is working. Carried out the test in post #50. set my transistor set at both extreme ends of the scale and various quiet spots. I adjusted the valve set through it's entire range. No sqealing or pops. The only thing that did happen was at some points it was as if the valve set was tuning in the donor set but very poorly. The reason i took the volume pot off was because when adjusted i got this tuneable hiss then silence. But testing the track it has no open circuits or if the shaft is wobbled . I just wanted to confirm my readings were not high? Adding to these issues the set has. Readings are in my last post.
Dave.

[Phil G4SPZ]

Tuneable hiss is one possible symptom of a stalled LO. However it sounds like the output stage is working.

You have done so much work on this set that I think you aren't going to get any farther until you obtain a signal generator and inject some test signals to find out which stage is faulty, as I described earlier in post #48. Otherwise you will continue working in the dark.

[Chindit]

Sorry, I've just skim read this whole saga from end to end so I apologise if I have missed something.
Low emission DK96 frequency changer perhaps? I know that the selenium rectifiers in battery/ mains sets are often poor these days and their LT output drops off and if the frequency changer is poor anyway this will cause it to drop out and stop oscillating.
A good DK96 should still work at 1.1 volts LT but a bad one may stop when the LT gets only a little below 1.4 volts.
These battery/mains sets will often have poor valves as they would have seen a lot more use than battery only ones. Just a thought! Colin.

[sobell1980]

Good evening chaps,
Colin, my set has a DK92 as a frequency changer not a DK96. I have just been through all my spares and don't have one. If i remember my LT is around 1.3 volts across all 4 valves but as you say that is no good if they are poor emission. I think the way forward is this signal generator and learn how to use this new tool to my advantage. This will show up where the signal is being lost. If any one could give some advice on the meter i have mentioned and how to use it that would be great. Many thanks,
Dave.