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Old 23rd Mar 2014, 7:02 pm   #21
sobell1980
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Default Re: Roberts RMB finally finished but poor intermittent aerial connection.

Hi all,

Just putting the Roberts RMB back together for a test. Regarding the speaker wire connections, does it matter which way they are connected to the output transformer. Speaker wires are black and red, of all the photos I have taken I don't have one of which way the output transformer was connected. Looking at the wiring diagram it apparently does not seem to matter?
Many thanks
Dave.
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Old 23rd Mar 2014, 7:12 pm   #22
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Default Re: Roberts RMB finally finished but poor intermittent aerial connection.

I'd say the polarity of the speaker connections in anything with only one speaker is irrelevant. That of the connections between the O/P transformer and the rest of the circuit is of course critical if negative feedback is employed. Get it the wrong way round and the set will oscillate!
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Old 23rd Mar 2014, 10:14 pm   #23
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Default Re: Roberts RMB finally finished but poor intermittent aerial connection.

Well i have replaced no. 4 valve socket. The set burst into life but after a couple of hours on test it died again. Before the valve holder was replaced i had nothing. After replacing it, on first switch on it was receiving strong stations for a couple of hours. I have switched it off for 30 mins tried again and still nothing. Still have two more valve sockets to replace but it was only ever v4 socket that gave the trouble.
Kind regards
Dave
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Old 24th Mar 2014, 8:35 am   #24
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Default Re: Roberts RMB finally finished but poor intermittent aerial connection.

Had a look this morning. Tried the set again just a volume adjustable hum can be heard. Taken measurements of all the valve bases for anode and cathode. All seem in specification except the anode of valve 3. This tests at 77 volts when the data shhet says it should be 11 volts? I have tested r9 which tests at 0.588 mega ohms. This suggests it's value is ok. Could it be a misprint on the data sheet? All my heaters/ cathodes are testing at 1.32 volts. This is a little low i know but enough to make the valve work. Any input chaps gratefully received.
Dave.
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Old 24th Mar 2014, 8:49 am   #25
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Default Re: Roberts RMB finally finished but poor intermittent aerial connection.

The Trader Sheet votages were measured with an AVO7, which is a low resistance votmeter and this will load the 560k anode load of V3 circuit significantly compared to a DVM.

Ron
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Old 24th Mar 2014, 11:14 am   #26
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Default Re: Roberts RMB finally finished but poor intermittent aerial connection.

If that is the case why don't the anode voltages of the other valves differ greatly as well to what the trader sheet quotes?
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Old 24th Mar 2014, 7:47 pm   #27
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Default Re: Roberts RMB finally finished but poor intermittent aerial connection.

That would be due to Ohms Law. All other valve anodes are fed via transformers with low resistance windings. V3 is the only valve with a high resistance (560k) anode load.

Ron
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Old 24th Mar 2014, 9:14 pm   #28
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Default Re: Roberts RMB finally finished but poor intermittent aerial connection.

Any idea what i should be getting with a digital meter?
Dave.
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Old 24th Mar 2014, 10:43 pm   #29
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Default Re: Roberts RMB finally finished but poor intermittent aerial connection.

My RP4, which is a battery only RMB measures 45 Volts on V3 (DAF91) anode, with the slightly run down battery showing 80 Volts, using a Fluke DVM with a 10M ohm input resistance.

So something between 45 and 60 Volts with a 90 Volt supply is what I would expect. It depends on the mu of the valve and the screen grid volts. In your case, with 77 Volts on the anode (i.e. not much anode current), have you checked the 4M7 screen-grid resistor to see if it has gone high, or whether the screen grid decoupler C17 is leaky?

Ron
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Old 25th Mar 2014, 7:44 am   #30
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Default Re: Roberts RMB finally finished but poor intermittent aerial connection.

My anode voltages for the other valves are about or around 100v. This is right because the trader sheet states the voltages were about 10% higher than the ones quoted in the trader sheet, when they were tested on mains and not battery. So my 77volts on mains supply is about right then for valve 3? With the set switched on, using an insulated screwdriver, if i tap v1 screen grid pin a popping can be heard through the speaker. I can get this to do this before and after c1. Surely this means the circuit from here is ok if this interference can be heard? C17 has been replaced also. I was thinking next of bypassing the wave change switch to the aerial connections to eliminate this. I have the wave change switch chart so this could easily be done. Anymore advice greatfully received chaps. Otherwise i think it is time to invest in a signal generator to make my life easier and find this irritating fault rather than guess work.
Many thanks.
Dave.
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Old 25th Mar 2014, 9:02 am   #31
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Default Re: Roberts RMB finally finished but poor intermittent aerial connection.

Do double check the W/C switch, Dave. The one on mine's been a nightmare!

Nick.
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Old 25th Mar 2014, 9:38 am   #32
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Default Re: Roberts RMB finally finished but poor intermittent aerial connection.

Thanks Nick. I will test continuity across the relevant contacts and may even try to bridge them to totally eliminate it.
What faults does yours give Nick? Mine fired up briefly after replacing v4 socket but now i think that was coincidence. Now i can't get anything other than a clicking in yhe speaker when i tap the grid of v1.. I don't want to give up, i've learned so much from this project. Can't believe yours is still giving you hassle Nick, how frustrating these sets can be!
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Old 25th Mar 2014, 10:53 pm   #33
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Default Re: Roberts RMB finally finished but poor intermittent aerial connection.

Well I have just finished by passing the wave change switch by bridging the terminals that are necessary to select MW. Tuned in and still nothing! Is it still possible those paxolin b7g sockets on the remaining two valves are giving trouble still? If these were bad contacts on the valve pins then why would i get popping at the speaker when probing the grid of valve one? I have gone back and tested continuity of the frame aerial so i know thats good. May be i need to bite the bullet and change these last two valve holders. The two remaining are the CINCH paxolin sort. At least the wave change switch is illiminated even if it was just in the mw position. Would you concur with my way forwards? Thanks for all your help as always,
Dave.

I will post a success story when it is all done and what the final faults were. I shall battle on. Hopefully will be picking up a signal generator soon which will aid my diagnosis.
Dave.
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Old 27th Mar 2014, 7:01 pm   #34
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Default Re: Roberts RMB finally finished but poor intermittent aerial connection.

It appears members have lost interest in my thread on this very lengthy repair, would it be better to start a new thread on the specific problems on this set i am repairing. The title does not really relate to the repair any longer
Many thanks,
Dave.
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Old 27th Mar 2014, 9:17 pm   #35
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Default Re: Roberts RMB finally finished but poor intermittent aerial connection.

Hi Dave,
I can change the title to something more appropriate, alternatively start a new one, with a reference to this one for continuity.
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Old 27th Mar 2014, 9:49 pm   #36
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Default Re: Roberts RMB finally finished but poor intermittent aerial connection.

Hi Bill

Change the name if you so wish, that would be great.

Dave.
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Old 27th Mar 2014, 10:02 pm   #37
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Default Re: Roberts RMB Problems

Not particularly imaginative, but it may rekindle interest.
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Old 27th Mar 2014, 10:06 pm   #38
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Default Re: Roberts RMB Problems

Thankyou Bill.
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Old 2nd Apr 2014, 8:57 pm   #39
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Default Re: Roberts RMB Problems

Dave,

Did you get my PM regarding Saturday?

Phil
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Old 26th Jun 2014, 1:26 pm   #40
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Default Re: Roberts RMB Problems

H i all,
Well all but one of the valve holders have been replaced. I attempted a switch on last night before I start tackling the last valve holder. I thought I might have got something, but absolutely nothing. Must say I am getting tired of this set now! There are a few very low value wax caps left that i thought would be ok that may need replacing? They are to the value of pF and the lemco sort. I just thought I may have got something out of it.*
Occasionally i get absolute radio mirrored from mw onto lw but very faintly, and in exactly the same place on the tuning scale as it would be on mw but on lw. It did this before when i first had it all fully working for a few hours on a bench test. But, even now it only does this intermittantly on first switch on and no other stations on any wave band can be heard.
Any other thoughts chaps on the whole situation? I am going to go ahead and replace the last valve holder and the last few wax caps and see what happens, I'm not holding out for any great results. Any input would be greatly valued. You will be able to see from my previous posts, what has and has not been covered. Many thanks,
Dave.
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