VE301W Radio help

[Station X]

JUSTDAMO has sent me this radio for repair. It arrived last week and thanks to being packed well it didn't suffer any damage in transit.

Today I separated the chassis from the cabinet and made a visual inspection.

This set was manufactured by Tefag. It has has a double pole mains switch which I suspect may not be original.

There is a crack in one lower corner of the Bakelite case, which Damo agrees was pre-existing.

The tuning dial is distorted. This can generally be corrected by applying gentle heat from a heat gun to soften the material. It needs to run reasonably true, as the tuning knob "pulley" engages with its circumference giving a crude slow motion drive.

As the picture shows there is absolutely no point in testing, or attempting to reform the smoothing capacitors! The can will need restuffing.

[Station X]

Work on this set has got way ahead of my reporting it in this thread. A few more observations:-

The tuning capacitor is very loose on the chassis. It's secured by screws with their heads under the chassis which screw into tapped bushes rivetted to the tuning cap. The heads of these screws won't be accessible until the capacitor block has been removed.

The valve holder for the rectifier is cracked.

Someone has cut the speaker leads halfway along their length. I could understand someone without a soldering iron cutting them close to the speaker, but halfway along their length?

As previously noted the tone correction capacitor is missing with only the leads remaining.

[Herald1360]

Would that crack actually be a problem?


Cutting the speaker wires half way along would make them easier to rejoin using choc bloc or similar non solder method. Might even be safer than risking damage to tags at either end even if soldering is a rejoining option.

[Station X]

Like I said. The repair work is way ahead of my reporting it.

[Station X]

With the visual checks out of the way it was time to get out the DMM and start testing components. All these tests were done without the need to apply mains power to the set.

WOUND COMPONENTS

When I repaired my own set I made notes of the resistance of all wound components. These notes were useful for comparison purposes when checking the present set.

All wound components tested OK. I did have a heart stopping moment when checking the resistance of the inter-valve transformer primary winding which appeared to be open circuit. A careful look showed that one of the thin wires had broken away from a tag on the capacitor block, the break being barely visible. Testing to the end of the wire showed that the winding was OK. I'll reattach it once I've restuffed the capacitor block.

The high impedance speaker winding tested OK, with a reassuring crackle when the meter leads were attached.

I also took the opportunity to check the insulation resistance of the mains transformer using a Megger. It tested OK.

[Station X]

VALVES

My valve tester is out of action at the moment, so I checked the valves for heater continuity and inter-electrode shorts using a DMM.

V1 REN904

Almost all the metallisation was intact. The envelope was firmly attached to the base. The heater had continuity and there were no inter-electrode shorts.

V2 RES164

The envelope has become detached from the base. There was a lot of debris, presumably adhesive, rattling around in the base. The heater had continuity and there were no inter-electrode shorts.

V3 RGN354 (RECTIFIER)

The envelope was firmly attached to the base. The heater had continuity. There was a full short from the heater to the anode.

The anode in this valve is of rectangular cross section wrapped around the heater and in close proximity to it. I assume the heater has drooped and contacted the anode.

I'll proceed with the repair, using the RGN354 from my own set for testing purposes. If all is OK I'll try a silicon replacement. It's up to JUSTDAMO whether he sticks with silicon or purchases a new valve.

[Techman]

Originally Posted by Station X

V3 RGN354 (RECTIFIER)There was a full short from the heater to the anode.

This is the problem with some of these old directly heated valves. There's nothing to stop a heater string breaking away and touching the next nearest electrode, which unfortunately in this case is the anode. I had a similar thing recently with a PX4, but perhaps luckily with these valves there's a nice control grid mesh nicely wound round it to keep it away from the dreaded anode. I managed to burn the errant string away on this particular valve, which continued to work on the remainder of its heater. I don't think this will be possible with your rectifier as the heater may be in just one length and burning it clear may cause it to go completely open circuit. I suppose at the end of the day it's a bit of a risk with a rectifier with nothing else such as a grid between anode and cathode/heater from stopping a dead short situation, although there's the same risk even with a replacement directly heated valve, so you may have nothing to lose in trying to remove the short in the old valve.

[Station X]

Thanks Techman.

I will try blowing off the short. Were this successful I'd be concerned about subsequent failure and its effect on the rest of the set.

[JUSTDAMO]

Silicon is fine by me

[Station X]

Time for some actual repair work. I decided to start by restuffing the capacitor block.

Detaching the capacitor block from the chassis without unsoldering any wires was a doddle, as I'd already made the tools to do this job. However detaching the paxolin lid from the metal casing proved more difficult then with my own set. I had to resort to inserting the jaws of thin nose pliers between lid and casing, progressively crushing the pitch until the lid came free.

The resulting mess shows how futile it would have been to attempt to reform the caps in the block. The metal case was put on one side to be renovated later.

An interesting detail difference from my own set is that this capacitor case has an earthing strap rivetted to it.

[Sideband]

...it also shows the danger of powering these old sets without doing basic checks. That capacitor block may well have been the cause of the rectifier demise. Extra work for a simple check....

[Station X]

With the capacitor block removed it was a simple matter to tighten the screws securing the tuning capacitor to the chassis. They were very loose, which can only have been down to vibration as the capacitor block has clearly never been removed until now. These sets have the tuning capacitor rigidly fixed to the chassis with no insulated or flexible mountings.

The pictures show the capacitor block lid before and after being cleaned up ready for fitting new capacitors.

[Station X]

Before fitting new capacitors to the capacitor block's lid I tested the set's resistors. The absence of the capacitors results in fewer shunt paths and capacitors which need to be charged by the meter before a stable reading can be obtained.

Predictably the two 2M resistors had drifted up in value to around 2.5M. However, given their positions in the circuit, this hardly matters.

The 700R resistor is connected in series with the negative side of the HT supply to provide grid bias to the output valve. It had drifted up in value to 740R. This will reduce the output valve's anode and screen currents. I'll see what effect this has on the set's performance.

The 0.1M (100k) screen dropper resistor measured 0.17M (170k). It will be replaced before I power up the set.

All other resistors were within tolerance.

[Techman]

Originally Posted by Sideband

...it also shows the danger of powering these old sets without doing basic checks. That capacitor block may well have been the cause of the rectifier demise. Extra work for a simple check....

Actually I don't fully agree with all of that statement.

I fully agree that with something that old a proper visual check is a must before even thinking about applying any power. Even I wouldn't think of applying power to a capacitor block looking like that, and I'm one for getting the mains on at an early stage to get the state of play with a set. I find that I just have an instinct as to whether a set will take the mains without disaster or not - it's almost as if the set speaks to me!

The bit I'm not sure about in that statement is that the capacitor block may have caused the demise of the rectifier. How can a shorted capacitor block cause a directly heated filament to short to an anode? Believe me, I have seen standard rectifiers melt their anodes (full wave indirectly heated type) which then short together (anodes, that is) with the glass of the valve getting so hot that it's actually started to melt out of shape and suck itself in - of course the transformer doesn't survive in this situation. However, I can't see that this can cause the fault with the valve in this particular case.

Now the following is just my 'opinion' and may not necessarily be fact, but I think that the fault with this valve has been caused by a jolt, probably in the post. The fault that I mentioned in the previous post where I had a filament string dangling and shorted to the grid, was with a valve that was sent to me through the post. It came from a very reliable source and was fully tested and working before it was sent, but had a damaged and dangling filament string when it reached me. The bottom line is that these valves don't travel well in the post and need special attention to packing, even then it's not guaranteed to survive. If they're in their original box with just cardboard, then they need removing from the box and literally suspending in bubble wrap and the box flat packing separately, unless the box is large enough for extra packing and bubble wrap, then in bubble wrap around that with a large outer box for the whole lot. The above is just opinion on my part, though.

If I were to offer Graham any advice at all, it would be to let the owner of the radio obtain an original replacement valve himself and go with the solid state rectifier option for the purpose of the repair - it seems that the sets owner is happy for this anyway, so not a problem in this case.

[Leon Crampin]

I encountered a similar rectifier problem with the UU3 in my 1936 Murphy A30C. One of the cathodes had slipped axially in its mica support - but banging the valve carefully brought it roughly back into position and resulted in the valve testing OK with no shorts.

The UU3 is an attractive original valve which I wished to retain, but endangering the irreplaceable mains transformer was an unacceptable risk. My fix was to simply wire a 1N4007 diode in series with each of the UU3 anode connections, hidden well under the chassis within suitable sleeving to the valve base terminals.

The UU3 soldiers on and has done so for a few years now. The early Mazda valves were almost indestructible...

Leon.

[PJL]

The RGN354 has horizontal anode and filament wires - madness.

[CambridgeWorks]

Graham, somewhere I have a 3 pin rectifier, iirc a Philips?? Is it worth me looking it out, just for cost of postage.
Rob

[Station X]

Yes please Robin.

[CambridgeWorks]

Will look around tomorrow and get back to you.
Rob

[CambridgeWorks]

Hi Graham, It is a Mullard DU10, but tests extremely low unfortunately. CT160, Vf 4v, Ia 60mA settings as book. See picture.
You can still have it if needed?
Rob