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Old 1st Sep 2018, 10:28 am   #1
Spinakerr
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Default Roberts RMB - Checking the basics...

Hi everyone - longtime reader but first time poster on this forum!

My wife picked up a fantastic condition Robert RMB valve set for a handlful of pounds but I can't get any output.

I have only tried it with the mains lead - when plugged in and the volume switch clicked to 'On', there is a very quiet hum from the transformer, no valves glow at all, and there is no output from the loudspeaker.

Items checked so far:

1) Plug (live/neutral) wires in good condition, fuse checked (minimal resistance).
2) All valves have vacuum, are in their correct positions per the circuit diagram and are the right values.
3) The mains/battery switch clicks affirmatively, as does the on/off/volume switch.

Before I delve into the chassis and checking capacitors, is there anything basic I am missing for this set.

It appears to have been serviced in the past 10 years by Fred Kay, who passed away last year, and being in clean condition with no obvious bodges I don't want to damage anything.

My background - all my other amps and radios are transistor based, Sansui and KEF gear from the 60s and 70s. I have recapped speakers, swapped transistors and done basic troubleshooting but not on valve gear.

Here are some pictures:

All advice gratefully received!
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Last edited by AC/HL; 1st Sep 2018 at 2:02 pm. Reason: Images uploaded
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Old 1st Sep 2018, 11:00 am   #2
Nuvistor
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Default Re: Roberts RMB - Checking the basics...

The heaters/filaments in theses valves are fragile, over voltage can destroy them. They can be difficult to see the heaters glowing.

Basic checks, I.e, voltages, if you have the circuit it should give the voltages for you, check the PSU. When checking the valve bases for voltage or signal injection don’t short out any pins, again the heaters are fragile, sorry to labour the point.
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Old 1st Sep 2018, 9:24 pm   #3
Goldieoldie
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Default Re: Roberts RMB - Checking the basics...

Hi
First check the primary of the speaker transformer for continuity .Its located bottom right in your picture .These often fail,especially if the af coupling capacitor has gone leaky.
Replace all wax capacitirs over 1000 pf in value if they have not already been replaced
Also the rectifier fails on these sets
Have you tried wiggling the valves gently ( power off ) to partially clean the pins ?
Best of luck
Regards
Pete
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Old 1st Sep 2018, 10:07 pm   #4
Chris55000
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Default Re: Roberts RMB - Checking the basics...

Hi!

Get the Service Sheet from the link at the top of the page before you begin with the set if you've not already done so!

This set uses parallel filaments so getting it working on mains is a walk in the park compared with the a.c./d.c. series–filament sets that are unfortunately far more common!

I would obtain a good quality 1.5V 'D' size alkaline cell and use this for the filaments for your initial fault–finding!

The first step to take, after making sure the output transformer is intact, connect your LT battery and check the filaments are all lit – note that it's a very dim red that can only be seen in almost total darkness – and if any are out, remove the affected valve and look at the filament tensioning spring towards the top of the valve, if this has sprung up towards the top of the valve it is useless and you need a replacement.

Next, connect the H.T. (the set's internal mains power–pack is ok for this) and switch the set on – you should start to hear some life in the speaker after about three seconds if the valves have adequate emission.

If the set is lively, use a D.M.M. to measure between chassis–metalwork (LT–) and the HT– lead to check the grid–bias applied to the DL94 output valve – this should be –5.5V nominal with respect to the metalwork/LT–, if it's higher than this, the grid–coupling capacitor from the previous DAF91 anode needs replacement ("THAT" cap!) or R12 (390R) has gone high in value, either fault will cause distorted reproduction.

No sound at all, not even rustles, etc., can occur if the high–value anode (1M) and screen–grid (4M7) feeding the electrodes of the DAF91 diode/a.f. pentode voltage amplifier go high in value or open–circuit, simply remove the DAF91 and use a D.M.M. to meter them between the valve–socket pins and the HT+ line connection.

Good–quality metal film resistors can be used for replacement purposes.

The I.F. stage (DF92) rarely gives much trouble, apart from the valve itself or the screen–grid feed resistor.

The Frequency–Changer stage (DK92) can fail to oscillate if the L.T. battery is low or the valve itself is below–par, symptoms of this are simply a dead set except for wave–change switch clicks, faint rustles in the speaker or a "tinkly" noise in the speaker due to natural "microphony" (electrode vibration) in the valve elements!

If you recap the set, use the small yellow axial–leaded Vishay 1813 series of capacitor, they're expensive but the future reliability of the set is well worth the cost – 160V or 250V components will be adequate.

Regarding the mains power–pack, use a 2.5V 0.3A torch–bulb or a 5.6 ohm 3W resistor across it's L.T. output first before trying it on the set's valves in case there's a fault in the primary of the mains transformer causing excess L.T. to be developed!

If you can't get enough L.T. – it should be 1.4V across a 5.6 ohm resistor – the metal bridge rectifier may be in need of replacement – use one of the small square bridge units with a mounting–hole in the middle for replacement, but you'll need to add a small resistor in series with it's + lead to compensate for it's lower resistance giving a slightly higher rectified voltage – you'll need to find the value by trial and error but it will be about 0.5 to 2 ohms.

Use the dummy 5.6 ohm L.T. load first for all your power–pack tests!

Chris Williams
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Last edited by Chris55000; 1st Sep 2018 at 10:19 pm.
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Old 30th Sep 2018, 11:28 am   #5
Spinakerr
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Default Re: Roberts RMB - Checking the basics...

Thanks for the detailed replies - wow what a wealth of knowledge on this forum!

I'm going to read up more on the basics and get the Service Sheet as suggested.

I'm out of my comfort zone on this one so keen to do it slowly and get it right.

Thanks again and I'll keep you updated on progress.
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Old 3rd Oct 2018, 2:59 pm   #6
Phil G4SPZ
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Default Re: Roberts RMB - Checking the basics...

I have repaired many battery valve portables, and defective valveholder contacts have been very common. A single cracked or loose 'pin' can cause the symptoms you describe. It may be that acidic fumes from ageing batteries cause the damage, but if the set has been run mostly on mains, corrosion is less likely.

It's best to inspect each valveholder in turn with a bright light and a magnifying glass. It is possible to replace a single 'pin' if you have an identical valveholder from which to scavenge a new one, but in the long run it's probably better to replace the whole valveholder.

Do check the output transformer primary first, though!
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