Robert's RMB in for repair

[sobell1980]

I was going to ask, why then aren't all my readings across all the valves higher than quoted.
Thats because it depends on the circuit loading?
My meter is an expensive Snap on meter. It was about £200.00. Sounds like I would be better using original test equipment to save the confusion.

[sobell1980]

It seems pointless to dismantle the set if I'm going to get the same problem measuring voltage across the resistors. Although I could measure the resistance of the resistors .
I took the valve voltages yesterday by wrapping a small wire around the valve pin and inserting the valve. I didn't want to dismantle the set incase it was all a red herring.

[Station X]

Originally Posted by Herald1360

As an extra comment, measuring voltage across the anode or screen resistors themselves will be just as " inaccurate" since the electrode current will see a lower resistance (resistor and meter in parallel) and develop less voltage for the meter to see.

If you're using an AVO Model 7 or similar then I would agree. However the readings submitted by the OP suggested he was using a DMM and he's subsequently conformed this.

Personally I'd check V3's anode and screen resistors, replace if necessary and leave it at that

[Herald1360]

The AVO 7 is rather at one extreme, being only 500 ohms/volt FSD so on its 400V range (say) it's only 200k. A cheap 1megohm DVM will still cause significant errors around a 500k source though- around 30% low whereas with a 10megohm DVM the same measurement will only be around 5% out, near enough not to matter in most valve radio fault finding.

An AVO 8 on 500V comes in at 10megohm too so loading similar to decent DVM though reading resolution not so good.

It's worth noting for pentodes at least (nearly constant current devices) that a simple way to check anode current is to shunt the anode load resistor with a meter on a suitable current range which will give a reasonably accurate figure for the actual anode current.

[sobell1980]

Hello all.

Again, apologies for not getting onto the forum as often as I'd like to. I'm also looking at setting up a proper work area so I can get onto my sets properly with a fixed aerial mast on the workshop. Where we have moved to does suffer poor signal, power line through the garden and even the TV signal is very, very poor.

Anyway, I digress. I've been testing the set in the garage. Slowly the volume fades to nothing. I've located the fault to V4 socket. A very slight adjustment of valve 4 the set bursts into life again. Like you rightly said, corroded iron cracked vskvecdocket pins. Probably from battery fumes in the past. I did change all the sockets on this set years ago. That doesn't mean the sockets I had replaced them with had not suffered the same fate!
Please can I ask the moderators to keep this thread open. I have a couple of sets to complete before I can get onto this one. Namely an Ekco that has been taxing. I just thought I'd bring everyone up to speed on where the set is at, I do appreciate all the help and replies.

Dave

[Chris55000]

Hi!

If Roberts used an Avo 7 to write their service sheet, then the OP can simulate this with a 180K or 220K resistor across the DMM input sockets - if it's a high quality meter as the OP's with shrouded sockets and plugs like most are, clip the resistor to the red prod end with a small croc clip and another one to the radio chassis - the DMM will then read the effective voltage produced by the 180K or 220K across the DMM and the anode/screen resistor acting as a potential divider, replicating the measurement conditions Roberts used! Easy!

Chris Williams

PS!

In the days when Avo 7s were universal, it was common for valve portable service sheets to quote diode/pentode anode and G2 voltages as "very low", a dash or sometimes "test not possible!" There were one or two enlightened makers that did measure these voltages with a valve voltmeter tho', in this case the readings obtained with a modern DMM will be very similar to those quoted on the sheet!

[sobell1980]

Hello all. Slowly catching up on the backlog of repairs. The Robert's will be on the Bench very soon. I've been looking for a B7G valve base/socket for valve 4. I've looked through my spares, I only have the type where the mounting bolt hole centres are 22mm apart. The ones I need are 28.5mm apart. I've been online and it appears that the 22mm type are the only ones available. Can any one help out with a supplier or has one spare?

Many thanks

Dave.

[Phil G4SPZ]

Hi Dave. Only just found your original thread. A suitable valve holder is on its way to you. It's used but it looks OK.

I repaired an RMB for someone a while ago. Two valveholders had been replaced by a previous repairer, the work having been done so neatly that I didn't notice it at first, and when I did spot it, I made the error of assuming that because it looked neat, it had been done correctly. Wrong! On one valveholder, the previous repairer had forgotten to add the wire which grounded one side of the filament. It took me ages to find that one, and it was only after spending some time trying to puzzle out why the filament voltages were wrong on mains power that I deduced, then spotted, the missing wire.

Moral: always check previous repair work, no matter how well-executed it looks!