Roberts RMB Problems

[ColinB]

Well spotted - no circuit diagram here...

[sobell1980]

Right, bridge rectifier built last night. 4 diodes. All cathodes facing away from transformer and facing towards L17. I have soldered in exactly according to the service data sheet. Two cathodes join at one point and solder on to a tag on a board which later arrives at L17. Then two anodes join at one point and then go to the negative of c26. The pair of low tension wires then join between the cathode and anodes opposing each other in the diamond shape. I received just shy over 1.4 v at my heaters but then the bridge rectifier i built got very hot. Too hot almost to touch. I Checked my current draw in series between the bridge rectifier and L17, ithought it was drawing too much current. It is drawing 250 milli amps or thereabouts. I removed my negative wire from c26 and the bridge rectifier. I tried it again, it did not get hot at all, my current draw was the same and i had 1.6v at my heaters. The diagram clearly shows a negative connection to ground at c26. I followed it to the letter as you described. The diodes i am using are 1N 4007 831. I am sure they could cope with this voltage and current draw as they are identical to ones i used in the half wave rectifier beforehand. A little bit baffled now. I was so chuffed when i got the correct voltage at the heaters, it is just getting to hot now. Any help gratefully received.
Dave.

[Phil G4SPZ]

Hi Dave,

The 1N4007 is a 1 amp diode and should be adequate, although diodes in rectifier circuit, especially with large values of reservoir capacitor, are called upon to supply very large peaks of short-duration current. This means that their stated rating may not be sufficient.

It's not quite clear what's happening now. You first got 1.4 volts and the rectifier got hot, then you got 1.6 volts and the rectifier stayed cool, am I right? I presume the radio is still working, i.e. no filaments have blown? I can't explain why the diodes got so hot, but you said that the diodes ran cool when you disconnected the negative wire from C26. This suggests that C26 is faulty, passing too much leakage current. I know you said you have replaced C26 but you need to check again that it is in good condition and the right way round - these electrolytics are polarised and it's vital to get the polarity correct.

1.6 volts sounds about what I would have expected from a silicon diode bridge, but it's a bit too high for comfort on those valve filaments, so as we know the current (0.25A) and the voltage to be dropped (1.6 minus 1.45, say, i.e. 0.15V) so by Ohm's Law we can calculate the resistance of a series dropper as (0.15/0.25) or 0.6 ohm. It's wattage will be 0.15 x 0.25, or 0.0375 watts.

In my resistor box I have some 1 ohm 0.25 watt resistors. Two of these in parallel will make 0.5 ohms, and as a dropper this will leave you with 1.475 volts at the filament, virtually perfect. Wire the resistors between one end of the transformer winding and the top of the bridge rectifier, as this will help relieve the diodes of a little stress.

The resistors are small enough to fit in the space available, and I can send you a couple in the post if you send me a PM and remind me of your address.

[Nuvistor]

hi Phil,
I am probably wrong but should that not be 0.025A giving 0.15/0.025=6 ohm?
Frank

[Phil G4SPZ]

No, Frank, Dave's measured the LT current at 250mA (see his post #82) which is a quarter of an amp or 0.25A. 0.025A is only 25mA.

[Nuvistor]

ok, I was thinking the valves were 1.4v 25ma, I should have read the posts from the beginning. I will do that now.

[Phil G4SPZ]

'Livewire' explained the situation most succinctly in post #74. The Roberts RMB uses the earlier 91/92 series of valves with 50mA filaments.

[Nuvistor]

I have read post 74, understand now and I was completely off the mark, I will go and stand in the corner now.

[Phil G4SPZ]

I've done the same thing, many times - glanced quickly at a posting and dashed off a reply, having put two and two together and made five! I try not to post anything nowadays until I'm absolutely sure of my facts

[sobell1980]

Originally Posted by Phil G4SPZ

"You first got 1.4 volts and the rectifier got hot, then you got 1.6 volts and the rectifier stayed cool, am I right? I presume the radio is still working, i.e. no filaments have blown? I can't explain why the diodes got so hot, but you said that the diodes ran cool when you disconnected the negative wire from C26. This suggests that C26 is faulty, passing too much leakage current. I know you said you have replaced C26 but you need to check again that it is in good condition and the right way round - these electrolytics are polarised and it's vital to get the polarity correct.

Yes, 1.4v with negative of bridge to negative of C26 it got very hot. To the point of smoking, which is how i noticed. Remove the earth from c26 it ran cool and i had 1.6v.

I have removed c26. Firstly the polarity is 100% correct before i snipped the wires. I then discharged the capacitor, tested it on the capacitance tester on my expensive DMM and it reads 2200 uF.

I dont think the resistors will make it run cool enough as it was so hot, unless you are suggesting to leave the earth off and fit the resistors Surely it will not be smoothing the LT properly then? Perhaps it is as you say, the diodes can't deal with the high rated capacitor? I was going to order a proper bridge rectifier anyway as my homemade one looks untidy and cumbersome. It was only built to prove it worked etc. I have seen some rated up to 10 amps and 1 KV. That should cope with it?

The cap C26 is as you know, brand new. It has literally only been powered up for testing the set a few dozen times. I also went to the expense of buying well known branded F and T electrolytics. This is all very unusual. Let me know your thoughts on what i have put across regarding a higher rated bridge rectifier etc, or to leave the neg off to c26 and fit resistors? Thanks for your continued interest Phil, and everyone else.
Dave.

[Phil G4SPZ]

By leaving the negative lead of C26 disconnected, you are effectively disconnecting the reservoir capacitor. The diodes are then seeing a higher impedance load, as the choke L17 appears as a high reactance to the ripple component of the full wave rectified pulses of DC coming from the bridge rectifier.

The diodes you have used are rated at 1 amp, and that's an absolute maximum DC current. When forward biased, each diode will drop about 0.6 volt so it will be dissipating 0.6 watt, and these diodes are physically small, so they'll run warm. The current pulses into C26, however, will only last around 2.5 milliseconds per 10 millisecond half-cycle, and in that time the diodes have to pass enough current to recharge the capacitors to their peak voltage. So if the average current drawn from the smoothing network C26-L17-C25 is 250mA, the peak current through the diodes will be seven or eight times that. In other words, your 1N4007 diodes are operating above their absolute maximum current ratings.

I would suggest using something like 1N5400 3-amp 50 volt diodes or an encapsulated bridge rectifier like this one from Maplin. You don't need a high peak inverse voltage rating, so your 1kV would be overkill, and 50 volts is more than adequate as the actual PIV across the diodes will be less than 5 volts.

Incidentally, you probably paid well over the odds for branded electrolytic capacitors - they seem to be targeted at the audio market, where high price is deemed to equate with good quality sound - and any normal, decent capacitor would be electrically identical.

By the way, why did you replace the original metal rectifier in the first place? Had it actually failed? If not and you still have it, you could try putting it back...

[ronbryan]

I can confirm that the LT heater supply on the RMB Trader sheet 1037 is incorrectly documented.

The actual circuit is a centre tapped transformer, with the centre tap returned to chassis. A selenium bridge rectifier is fitted, with the two transformer AC inputs connected to the ~ connections and the DC output taken from the +. The - output is however not connected, so the bridge only functions as a bi-phase rectifier. I think there is a small degree of adjustment to the DC voltage by selecting tapping points on a length of resistance wire on a tag strip near the rectifier.

Ron

[Phil G4SPZ]

That's a very useful bit of information, Ron, many thanks! So much for Trader service sheets...

Dave, try disconnecting the wire from the right-hand corner of the bridge rectifier, where the anodes of the two diodes join, from chassis, then reconnect C26 and see what happens. See if you can find the tapped voltage adjuster shown in Ron's photo.

[Herald1360]

That would explain the heat! Removing the negative bridge connection would restore the circuit to original. However, it now sounds like the original 1N4007 incarnation with two diodes was correct so why the low voltage?

[Phil G4SPZ]

Originally Posted by Herald1360

...it now sounds like the original 1N4007 incarnation with two diodes was correct so why the low voltage?

Yes, that's baffling me too. The forward voltage drop of a silicon diode is generally assumed to be lower than that of a selenium rectifier, and it's common to have to insert a resistor to drop the excessive voltage produced by the diodes, not the other way round. The photo in post #92 clearly shows a coiled-up length of resistance wire which may have something to do with it. Until Dave comes back with the results of his latest tests, we'll just have to be patient

[sobell1980]

Hi chaps,
I have no such resistive wire on my rmb model. I have checked continuity from the output of mr2, (my bridge rectifier i built) and L17. My reading is zero ohms. So no resistive wires there. I have viewed the photo and i do not have that coiled wire or tag strip on my set. Perhaps the design was changed and it is only on so many sets, not all. I think i am going to go with the plan of, fit the encapsulated bridge rectifier. This means i will remain with 1.6v LT supply. Then fit two 1 ohm resistors in parallel between mr2 and l17 to bring the voltage to 1.4v or there abouts. It maybe i have to fit the resistors after L17 as this is where i measured my 1.6v at a point on R3.
Kind regards.
Dave.

[Nickthedentist]

Dave,

I'm very excited to read that you got signs of life with a 1.5V cell. Keep at it.

N.

[sobell1980]

Thankyou Nick,
Your encouragement is greatly appreciated. It has been at times a painful project to the point of walking away and not even wanting to look at the damn thing. And then of course it will need completely re -alighning. So i will be learning about that too i guess.
Dave.

[Phil G4SPZ]

Originally Posted by sobell1980

...This means I will remain with 1.6v LT supply. Then fit two 1 ohm resistors in parallel between MR2 and L17 to bring the voltage to 1.4v or thereabouts. It maybe I have to fit the resistors after L17 as this is where I measured my 1.6v at a point on R3...

It's by no means certain that you'll get 1.6 volts again, Dave, so when you have fitted the new bridge rectifier - and remember to leave the negative (-) terminal of it disconnected - ensure that C26's negative connection is going direct to chassis. Then measure the DC voltage across C25 as accurately as you can, and let us know the result. Don't run it for too long if the voltage exceeds 1.5 volts.

[sobell1980]

Hi chaps,
I picked up the Bridge rectifier from maplins that Phil recommended in one of his posts. I've got a bit of tidying up to do with the set before i proceed any further. I have bits of extra wire soldered in and a home made bridge and the awkward LT smoothing can to place neatly back in. So it will be probably be sunday before i can give you my results. Work commitments and family etc. Just want to go over the bits i have had to add in as extra such as the cell holder i soldered in meeds to be removed. I shall get it nack to standard again and move on with the next step. Be in touch Sunday woth my new bridge soldered in and my readings. Many thanks all.
Dave.