Resistor Value Issue

[EdGizzard]

Hi All,

I'm in the process of trying to fix this limiter and believe the dead Resistor in the picture (R3) to be the culprit.

But according to the manual (see attached) R3 should be a 15 Mega ohm 1/2 Watt Resistor. Which i believe is brown green blue. I make the actual resistor in the picture orange black brown 300 Ohms.

Have i missed something?

Should i replace whats there or get a 15 Mega ohm replacement?

Thanks so much guys
Ed

[Sideband]

I'd make sure you've identified the correct component first! What makes me slightly suspicious is that if it is a resistor, it doesn't look like the others fitted (it might have been changed previously of course). There isn't a 300 ohm resistor listed in the components either. So firstly make sure it IS R3 (which looks like the grid leak for the two pentodes). If so, that could NEVER be 300 ohms!!

Either someone has fitted a wrong value or it isn't R3!

[m0cemdave]

The circuit diagram is not clear, but that looks like the release time constant. Obviously 300 ohms is much too low.

Maybe a previous owner has been trying to change it, and got the resistor value wrong?

I'd try a 15M, and check the value of C1, which also doesn't look original.

That's quite a classic piece of equipment...

[Uncle Bulgaria]

Have you tested it with an ohmmeter so you know it's really dead? I had a recent issue with a resistor supposed to be several Mohms which looked to have the colour code of something orders of magnitude smaller.

It turned out the colours had simply faded. It wasn't dead either, just the meter I used first couldn't accurately measure that many millions of ohms. It could be that with another test method that resistor is actually a kosher 15M.

Or it could be that it's not R3, so do check that first!

[Diabolical Artificer]

Use 3 10 meg R's if you haven't a 15M to try & get it going: two in parallel in series with the third.

That split PCB might be an issue too, there could be ground issues from one board to another.

Andy.

[Craig Sawyers]

I was fooled two days ago measuring a dropper chain in a voltmeter. There were 7 off 10Meg and an 8.2Meg in series - so 78.2Mag.

Got out my trusty Fluke 87V DVM, a current product.

Open circuit!

I checked that the common terminals between two resistors were shorted - yup. Checked each resistor - all correct (but out of tolerance). But end to end - open circuit.

Got the manual out - the Fluke 87V has a maximum of 50Meg!

So it is easy to waste half an hour before RTFM.

Craig

[pmmunro]

Sorry if I attempting to teach egg-sucking for elderly maternal relations but the component in the photograph looks "glassy" to me. Could that part be a diode?

PMM

[TowerRadio]

Or a capacitor. Les

[dave cox]

PMM has it, 'tis CR3, listed as a diode!

[ms660]

The component in question which is shown disconnected in the photo does look to have been in the position of R3 so far as I can make out from the board component markings, which is in parallel with C1 (0.15uF 200V) according to the schematic, both of which are in the grid circuit of XV1 and XV2 the 5749 valves.

Lawrence.

[EdGizzard]

Thanks Guys.

Just to confirm the detached component in the picture is indeed a resistor and is in position R3 on the schematic. The markings on the board are very clear so I can confirm that.

The blue cap in the picture (C1) is in line with schematic and is the correct value.

Its worth mentioning that this unit has been in service for around 10 years with no problems till now. So that 300 ohm resistor or whatever value it is has been there all this time.

The 2 pentode valves do not light up (observed before i detached the resistor R3) when turned on. This lead me to this part of the circuit.

I’ll order 15 MEGA OHM and solder it on there and see what happens. Unfortunately I don’t have the correct values to wire a couple in parallel.

I'll report back. Thanks Guys

[EdGizzard]

Hi All,

So I've changed the resistor to a new 15 M Ohm. But unfortunately it has not fixed the issue.

The valves in sockets XV1 and XV2 do not light up. Any ideas of where to start would be much appreciated.

Thanks
Ed

[Uncle Bulgaria]

Not "lighting up" implies the filaments are not receiving current, or are open circuit. You should investigate there first. The circuit diagram in your initial PDF is too low resolution for me to read, but I can't see how your 15M can have anything to do with heater problems.

[EdGizzard]

Thanks for ruling out the 15M resistor. I'll try and track down a better quality CD. I'll also post some detailed pictures. Thanks

[ms660]

Originally Posted by Uncle Bulgaria

The circuit diagram in your initial PDF is too low resolution for me to read

A better copy

https://bh.hallikainen.org/uploads/GatesM5546a.pdf

Looks like XV1 and XV2 heaters are DC fed, maybe check for any breaks to or in the supply circuit including the 2.2 Ohm resistor (R65)

Lawrence.

[Keith956]

Not sure but looking at the circuit it appears that XV1/XV2 have a separate heater supply, which is rectified DC from the bridge rectifier CR1-4. Or am I reading it wrong?

In which case, if this pair of valves don't 'light up' it might be worth checking if that supply is functioning - I'd test the output of the rectifier first.

[EdGizzard]

Ok I've checked these diodes. Picture attached.

Measurements using a MM in diode mode. Diodes tested in circuit.

CR1 – to plus + 7v + to plus - 7v
CR2 – to plus + 7v + to plus - 7v
CR3 – to plus + 7v + to plus - 7v
CR4 – to plus + 7v + to plus - 7v

I guess all four are all faulty?

What modern equivalents would you guys recommend to replace these?

I’ve found what appears to be the original here:

https://www.electronicsurplus.com/1n...icon-rectifier

Thanks everyone.

[PJC58-Hythe]

Before you suspect the diodes have you checked R65 2.2R resistor and the valve heaters? You need to test the diodes out of circuit. If it is the diodes something like this would be fine https://uk.rs-online.com/web/p/schot...ifiers/7514843

[Uncle Bulgaria]

But what is the heater voltage? The symptom is no visible glow in the valves, so check the voltage on the heater pins first. If nothing there, check the rectifier output voltage. Or the other way around, whichever is more accessible. Do that before unsoldering anything like the diodes in order to test them out of circuit.

[PJC58-Hythe]

It's easy to remove the valves and test heater continuity and with them out it's also easy to test R65. If both those are OK then it makes sense to remove the diodes and test them.