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Old 17th Oct 2014, 7:05 am   #101
sobell1980
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Default Re: Roberts RMB Problems

Morning chaps. Ok, I managed to squeeze an hour in last night on the rmb. Tidied all the wiring up and got the set back together and removed my soldered in test leadscetc. Soldered in my new bridge rectifier. Here are my findings. With the new bridge rectifier soldered in and omitting its negative connection i'm getting 1.73 volts at c25. figuring i need about 1.45 v here is what i worked out.
1.73v- 1.45v equals 0.28v. 0.28/0.250 amps equals a resistance needed of 1.12 ohms to drop that voltage to 1.45v. If my maths is correct, 2x 2.20 ohm resistors soldered in parallel will achieve 1.10 ohms resistance in the circuit giving me my required 1.45v LT voltage. This is assuming the current draw of the heaters is the same with the new bridge in. I did not measure the current draw again as i assumed it would be the same or there abouts. Your input would be gratefully received as always. I can zip up to Maplins then later and purchase some resistors and get this set put under test and aligned over the weekend.
Many thanks.
Dave.

Last edited by sobell1980; 17th Oct 2014 at 7:08 am. Reason: Added more text.
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Old 17th Oct 2014, 7:31 am   #102
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Default Re: Roberts RMB Problems

Your calculations are correct, Dave, and the voltages you quote seem perfectly reasonable.

If you have any difficulty obtaining the correct resistors, let me know in a PM with your postal address and I'll drop a couple in the post to you.
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Old 18th Oct 2014, 11:08 am   #103
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Default Re: Roberts RMB Problems

Thanks Phil, you have been most kind. My wife is picking me a pair of resistors up this morning from maplins while she is out. Many thanks for your kind offer of help though. Also for your time and patience with me and the rmb. When I come to align the set, would it be possible to use my Avo meter to see the output? It is the only analogue meter I own. How do I set the meter up exactly. This will be my first full alignment I have ever done. So sorry for the simple questions. I will be following the service data, but any pointers would be most welcomed.
Many thanks,
Dave.
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Old 18th Oct 2014, 4:02 pm   #104
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Most strangest thing occured today on the RMB. I soldered in my new bridge rectifier as described in post #101. Today i fitted the two resistors in parallel to bring the LT in spec. I went to test the LT voltage with the resistors fitted and my reading was 1 volt. I thought, how on earth? It was then i noticed and heard the mains transformer getting extremely hot. Too hot to touch. I switched off and double checked my wiring. All seemed okay. I removed the encapsulated bridge and built my own again, omitting the negative connection as before. Switched on, it did the same again, transformer far too hot. I completely snipped the two LT wires coming from the mains transformer secondary so they were out the circuit and under no current loading. I switched on again, same again, transformer too hot to touch. I then snipped the HT secondary windings out the circuit. Switched on, transformer stayed cool. I did notice though that some one had soldered the HT secondary to MR1 which is correct. MR1 had been bypassed by an inline diode which was also soldered to the post of MR1 where the HT secondary would have originally been soldered to. With no wires attatched to MR1 i measured from the posts to chassis and had continuity of mega ohms. So in effect the post of MR1 was been used as a tag to solder the replacement diode to, but if this was high resistance maybe this was causing a high current draw? Hope i make sense? I left the HT disconnected, i soldered back in my encapsulated bridge rectifier, and the parallel resistors for the LT circuit. Switched on and left my meter on the LT circuit, measuring 1.35 v at the LT side and the transformer remained cool for the whole time. The voltage remained steady. I measured the current draw and this was around 250 milli amps. Nothing got hot, so all good for the LT side not causing the transformer to run hot. I then soldered the HT circuit back in. The secondary i soldered to the replacement inline diode but not to the post of MR1(selenium rectifier) as was before. I soldered the negative connection of the HT to chassis. Switched on checked all HT voltages, all ok. left it switched on with my hand on the mains transformer and it remained cool. So something on the HT side was causing the transformer to run very hot. The heat build up inside the mains transformer caused high resistance and the LT to drop as it warmed up. Does anyone think MR1 being used as a solder post was causing my problem. This is the only thing i have not reconnected, it seems to have made a difference. I have been moving the chassis about alot so unless something was touching and now it isn't? But it seems to much of a coincidence that i have disconnected MR1 from the circuit and now all is well!!
Any thoughts? Many thanks.
Dave.
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Old 18th Oct 2014, 9:01 pm   #105
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Default Re: Roberts RMB Problems

Hi Dave,

You do seem to have suffered from a temporary short across the mains transformer's HT secondary winding. Whatever the reason, you seem to have solved it for now, so leave the original MR1 isolated. I have to say that you're only getting 1.35 volts LT, which is still perhaps a little on the low side, but as long as your DK92 is still working reliably I should leave things as they are. Where exactly did you connect the two paralleled 2.2 ohm resistors into the circuit? Do a soak test for a few hours to see if all continues to operate satisfactorily.

Quote:
Originally Posted by sobell1980 View Post
When I come to align the set, would it be possible to use my Avo meter to see the output? It is the only analogue meter I own. How do I set the meter up exactly?...
The answer is yes, you can use an Avometer to monitor the output as you align the receiver. Which model Avo have you got? The Model 8 is perhaps the best as it has a 2.5 volt AC range, whereas the Model 7 has a 10 volt AC range. The set-up is simple, just connect the meter on its lowest AC range directly across the speaker terminals. When you inject your modulated signal from your signal generator, this will produce a single audio tone from the loudspeaker, and the Avometer will register the AC voltage present. You aren't looking for a particular value, only a peak reading as you bring the circuits into alignment. Viewing the needle of an analogue meter is far more reliable at detecting the peak than, for example, listening to the sound level itself. Even the old Avo Model 7 will give accurate readings right down at the bottom of the scale. The higher up the scale the pointer moves, the more easy it is to see the point at which the peak occurs, but it's not difficult to judge.

When aligning the set, turn the volume control up to maximum and use the smallest signal generator input you need to obtain an audible output or a meaningful indication on the Avometer. Keep reducing the sig gen input as the circuits come into alignment. If necessary, you may need to disconnect the 'speaker to avoid being driven mad by the whistle, but don't forget to connect a substitute load resistor across the output transformer secondary. I have to say I've not found this to be necessary, and I simply leave the speaker connected, but I do use an electronic analogue AC voltmeter which is more sensitive than an Avometer, so I can do an alignment without deafening myself.

Aligning a set is quite rewarding and you should find a worthwhile improvement in performance when you've finished. Shout up if you run into problems. You've come a long way with this RMB, the winning post is in sight...
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Old 19th Oct 2014, 10:28 am   #106
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Default Re: Roberts RMB Problems

Thanks as always Phil. I now have a low pitced hum constantly through the speaker. It does not alter with volume. This presented itself after i stripped the volume switch apart to clean the wiper and track. It was not there before this. Twiddling the volume pot shaft and rotating it could get the hum to intermittantly vanish,but it would soon return. It then tended to dissapear nearly completely at full volume. But now,no amount of "fine tuning" the volume pot rectifies
the low pitch hum. It now remains constant. It is not motorboating, neither does it sound like mains hum, it sounds lower than that. Is it possible my volume pot is the cause?The volume pot also contains the on/off switch for the set. It is a log type and i think 1.8k ohms. Obviously i can't align the set with this racket coming through the speaker. All the caps in the set are replaced, including the mains smoothers. Could it be a bad earth/negative connection somewhere perhaps so the decoupling caps can't ground fully? I had this problem on a Hacker RP31, nightmare to find. Just seems likely as the volume pot as it only occured after this repair.
Many thanks,
Dave.
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Old 19th Oct 2014, 10:32 am   #107
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Default Re: Roberts RMB Problems

It's 1M, not 1K8

Try shorting the wiper to earth - I suspect the hum will go.

Does it test OK with a resistance meter, out of circuit?

Have you got another 1M log pot that you could lash-up as a test?

Good luck,

Nick.
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Old 19th Oct 2014, 1:13 pm   #108
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Default Re: Roberts RMB Problems

Shorted wiper to earth. Kills the radio station but not the hum. Yes the track tests ok resistance wise. I tested it several times when it was apart for cleaning. I don't have another pot to test it with I'm afraid.
Dave.
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Old 19th Oct 2014, 1:39 pm   #109
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Default Re: Roberts RMB Problems

In that case, the hum is probably on the LT or HT lines. Try operating the set from dry batteries - if the hum goes, investigate all your work around the rectifiers and smoothers. Battery valves are particularly sensitive to hum on the filaments.

It's odd that every time you fix one fault, another crops up. That set is jinxed...
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Old 19th Oct 2014, 5:52 pm   #110
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Right, all thats sorted, turned out to be poor earthing of one of the mains smoothing caps to chassis. Few! A new wire soldered to chassis from the large mains cap negative connection sorted it.

Ok, now for the alignment. I have followed the service sheet to the letter for the IF alignment. That all went well apart from L12 core is seized but i achieved high output across my meter from L15,14,13. I could hear the tone volume increase across the speaker and fade away in harmony with what my meter was reading. I could not use my Avo 7. It needs looking at and i believe new batteries. So i ended up using my expensive DMM. It gave a steady reading inline with what i am doing and also has a bar display along the bottom which also helps to read it rather than fluctuating numbers. So i got the IF sorted. I moved onto MW RF and ocsillator section. It tells you to lay your sig gens leads near to the frame aerial and and adjust L7 for maximum output. This is where i am struggling. I have laid the sig gens leads by the mw frame aerial. I have set the cursor on the set to 550 metres, set the sig gen to 545 K cycles. I am adjusting L7 and get nothing on my meter or sounds from the radio. I then moved on to adjust c37 and c32, I get the same. I have no stations on MW. On lw i have radio 4 but appears to be slightly out on the scale. I can also receive on LW Absolute radio as loud as you like which is a MW station. I have had this before with this set, this mirrored station on longwave, but it was ever so faint. Mind you, now the set is up and running it may have been louder before hand anyway with this mirrored station. Am i doing something wrong with the RF alignment ? I have my meter set up across the speaker side of the OP transformer, set volts a/c. I have the sig gens leads by the frame aerial, but no output on my meter, tuning hiss when i am adjusting etc. Any help gratefully received.
Dave.
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Old 19th Oct 2014, 5:57 pm   #111
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Default Re: Roberts RMB Problems

So close now! Just to hear something out of it is brilliant. Disconnection of MR1 has stopped the mains transformer over heating. Had the set on test for 3 to 4 hours. I have also fitted some different value resistors in parallel for the LT heaters. I am now getting a steady 1.45 v. Much better. Just this alignment to sort now if anyone can help with my post #110. I think from replacing every cap on the set, the alignment now will be way out. Look forwards to your valued input. Many thanks,
Dave.
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Old 19th Oct 2014, 8:56 pm   #112
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Default Re: Roberts RMB Problems

Dave I think I would go back to the first stage of MW alignment. If you get nothing when you adjust L7 then the first thing is to increase the signal generator output or put the wires VERY close to the frame aerial. Getting a MW station on LW suggests the local oscillator may be a long way out of tune. If there is complete lack of signal on the low frequency end of the MW band and silent over the band there is not much to be gained by adjusting the capacitors.

Have you tried varying the frequency of the signal generator with the set tuned to the 550 metres position? It is possible that L7 is a long way out of tune. I don't have the circuit for the set but I am guessing that L7 is the local oscillator coil. Once you get sound from the generator you can move from there.

I have had this problem a few times when the alignment was way out. Once I get a signal I then adjust the signal generator frequency towards 550 Kc/s until it is very weak and adjust the coil to make it stronger. It is a lopsided way of alignment but I have usually been able to end up eventually with the signal generator at the correct frequency. The alignment can then follow the instructions.

It all depends on whether you can get a response from the signal generator on MW. If not there is another problem but the fact that you get something on LW makes me think you might not be unlucky with another jinx appearing.
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Old 19th Oct 2014, 9:19 pm   #113
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Default Re: Roberts RMB Problems

If you look at the circuit diagram, Dave, you will see that L7 is the adjustable inductance which sets the local oscillator frequency on medium wave, in conjunction with trimmer C37 and fixed capacitor C9. No stations on MW suggest a fault in (a) S8, (b) C9, or (c) S11. I think you have been previously advised to check and double-check the wavechange switching.

As you now have an oscilloscope, and now that the set is working after a fashion, you should be able to examine the waveform appearing across C35. You know that long wave is working, so set the radio to LW and tune in Radio 4, then put the 'scope probe on the top of C35. You are expecting to see a local oscillator sine wave at roughly 672kHz. Adjust the Y gain to give a meaningfully-sized trace on the screen, then adjust the timebase speed until you can see the waveform. You will probably need to adjust the trigger level to lock the trace. Once locked, look at the timebase setting. This will be calibrated in terms of microseconds per centimetre (us/cm) and 1 us/cm equates to 1MHz. 10us/cm equates to 100kHz, and so on. Make sure the fine timebase speed control is set to "CAL". You should be able to work out, by measuring the number of cms that one whole cycle occupies on the screen, the approximate frequency of the local oscillator, using the fact that frequency = 1/time per cycle. In general terms, the oscillator should run at 472kHz higher than the frequency to which the set is tuned. So to receive Absolute Radio on 1,215kHz, the L.O. should be running at 1,687kHz.

My guess, as I have previously opined, is that the L.O. is wildly off frequency on MW, or not working at all.

The fact that L12 is seized may be part of the reason for the problem. The frequency changer stage produces two main outputs, the difference (between the LO frequency and the signal frequency) and the sum (of the two). The wanted output is the difference frequency, and the purpose of L12/C3 is to accept this frequency by presenting a high impedance at resonance, and reject all others. Have you replaced C3? If so, you'll definitely need to free off L12 and peak up L12/C3 at 472kHz.

If you have replaced C9, check its value and connections.
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Old 26th Oct 2014, 2:50 pm   #114
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Default Re: Roberts RMB Problems

Hi Phil and everyone else following my posts.
sorry for delay in replying, it was a busy working week.
I carried out the scope tests. Attatched are pictures of my scope readings. I managed to lock the trace with the trigger level. However I don't have fine time base cal setting? I tuned into radio 4. From c35 this is what is displayed. If you could go through the maths with me again please and help me interpret the readings. Time base was set to 10 microseconds on both pictures. Voltage was set to 5volts and then 2 volts.
Many thanks
Dave
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Old 26th Oct 2014, 6:25 pm   #115
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Trying to upload the photo's from my camera phone the right way.
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Old 26th Oct 2014, 6:26 pm   #116
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That's better, you might be able to read the trace now. Many thanks.
Dave.
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Old 26th Oct 2014, 7:06 pm   #117
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Default Re: Roberts RMB Problems

Dave,

VERY roughly, the period of your waveform seems to be 1½ divisions on the graticule. At 10us/division, that makes the period ~15us or 15x10-6 seconds or 0.000015 seconds.

f=1/t so the frequency is 1/0.000015 = 66666Hz or just under 67KHz.

Could that be right? I might be wrong - haven't played with a scope for many a year now!

Nick
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Old 26th Oct 2014, 7:21 pm   #118
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This is assuming I have the scope set up correctly! That's why I have included a picture of the whole scope so you guy's can see the settings and what I'm working with. Thanks Nick. If you could break down the math for me? Still not quite understanding some of it. Many thanks.
Dave
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Old 26th Oct 2014, 9:06 pm   #119
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Default Re: Roberts RMB Problems

Surely the timebase has been set to 5us/div, not 10us/div - it looks that way to me anyway.

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Old 26th Oct 2014, 9:08 pm   #120
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Default Re: Roberts RMB Problems

Good point! It's 5us/div in one, and 10us/div in others.

But the displayed waveform is also different (period = 0.75 divisions, or 1.5 divisions respectively), so I think my sums still hold.
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