Decca DRM88 Tingle from chassis and hum thru speaker

[johnny english]

I have just acquired this set and it works on all wave bands lw/mw/vhf but I dont seem to be able to clearly locate the fault. There,s a voltage of 13.5 volts on the chassis. On the audio section there,s a few hunts mouldseal caps which I am replacing but at the moment when I probe around this area the buzzing increases. I have replaced the 2x32uf can with two seperate 33uf caps and all of the high wattage resistors 5w 100ohm are within specification and replaced 2 x1mohm resistors which were both 30% high. The volume and tone controls are working fine. Finally I pulled all of the valves and switched on but the chassis is still giving a tingle. Any advice would be most welcome I am using the trader sheet downloaded from the top right. Ed.

[ukcol]

Hi Ed

Assuming you mean DMR88 and not DRM88, also that you are using the R&TVS data available up top.

I don't think the tingle is a fault. C61 (O.005uF) connected between the neutral end of the mains transformer and chassis could contribute to this if the mains is connected the other way round. In any case I would replace C61 with a class Y type or better still discard it and fit a 3 wire mains lead with the earth wire connected to chassis. You say there is "13.5 volts on the chassis"; with respect to what? If you are measuring between chassis and an earth point with a digital multi-meter there is almost certainly no fault in that area. Even without C61 there will be a tiny leakage current from the mains transformer.

When you probe around the audio amplifier "the hum increases" is a bit vague to respond to. Does the hum vary with the setting of the volume control? Is the hum very loud or quite soft?

I would fit a 3 wire mains lead as I have advised above and check the set out again when you have changed the hunts caps, then report back.

[Nuvistor]

In addition to Colins suggestions.

It would be worth checking the insulation of the main TX, there could be a leak between one of the windings and the interwinding shield.

Unlikely but easy to check and rule out.

Frank

[johnny english]

Thanks for replying Colin/Frank.
I will carry out the modifications you recommend Colin, I had already replaced c61 as it was a tcc waxy regarding the hum it is a soft hum but can be heard at low volume or in quiet passages of music and doesnt rise with the volume control but it is louder than what I`ve encountered on similar radios.
The buzzing around the audio section caps was a hard loud almost magnetic effect when I was checking around this area I was looking for the audio coupling cap. I may need help frank checking the mains tx as I`ve never worked on one before and I dont want to mess things up. As I mentioned earlier the set works fine and is capable of pulling in the stations loud and clear with its own internal aerials. I will report back the conclusions. Ed.

[johnny english]

I earthed the radio and the tingle has disappeared although I think I will fit an iec 3pin connector to the chassis, for the connector on the set is a 2pin using 2 core flat twin flex to which I have lashed a flying lead for the earth.
The hum problem was of my own making because my workshop has been used for storage over christmas my wife let me use the kitchen to work on the set. Due too a tv a dab/fm radio, telephone extension base set and a low voltage anglepoise lamp all using seperate transformers the rfi was immense. I tested the radio out by moving it to another room and the hum was a lot quieter.
So on to the next job the replacement of the hunts caps. Brushing off the dust and muck and cleaning the connections, the smaller sized caps outer casing just fell to bits. I checked the radio was working ok after each replacement cap. By now I had replaced seven caps. One cap left to do only because the outer casing was falling off. This cap c27 300pf 400v, I think is a padder and I have tried to obtain a replacement but all I can see on the web is an lcr polystyrene type from cpc I am wondering if this component would be a suitable replacement. Ed

[Nuvistor]

Unless you confirm it is faulty I would leave it, it probably either mica or ceramic, they can fail but not in any numbers like the paper capacitors.

Frank

[johnny english]

Its a small hunts mouldseal type of which the casing has disintergrated leaving the innards open to the atmosphere. I have left it in place as I did,nt plan too replace any caps around the tuned circuits. Ed.

[Boater Sam]

Is it really a 300pf cap or .003uf? The later is more likly if its a mouldseal.

[Nuvistor]

If you have the correct capacitor C27 is in the feedback for the LO (pin 8 anode to switch). Has Sam states a mouldseal could well be 3000pf (0.003uf) . It could then be a mistake in the circuit diagram.
Frank

[Edward Huggins]

We used to call this tingling the "Skin Effect".
I got so used to it over all those years of servicing that it never really ever bothered me - I usually left it as it was.
All part of the experience so I thougt!

[Neil Purling]

I had a Mouldseal of 3000pf in a Bush DAC70. I used 3x1000pf.
It was making the local osc very reluctant to run, except at the HF end of MW.
The only good Mouldseal is one that is in the bin!

[johnny english]

Thanks everyone for your advice.
On the schematic c56 is indeed marked as a .003uf yet I removed a 390pf 600v so I will replace with .0033uf. I must learn to concentrate more on the schematics. Thanks once again. Ed.

PS. Could a moderator change the model number from drm88 to dmr88 for future reference. Thanks.

[Nuvistor]

I am confused, you noted it was C27 now it's C56, which capacitor is it? On the circuit C27 is 300pf, C56 is 0.003uf.
Apologies if I have missed something.
Frank

[johnny english]

Sorry for the confusion. After sam posted his thoughts I went over the circuit diagram with a fine tooth comb as previously I was just going to swop like for like. I physically came across three 300pf caps but on the diagram only two were listed c27 and c50 on the tuning circuit. On the audio circuit there was an additional 300pf cap, noted on the diagram as a .003uf c56. All of the caps are hunts black mouldseals. Ed.

[Boater Sam]

Don't change any caps that are on tuned circuits unless absolutely essential. They will affect the alignment.

The black and brown mouldseals are normally in the range 0.001uf to 0.004uf, 1000pf to 4000pf or 1nf to 4nf if you prefer.

Anything less than 1000pf in old sets are normally silver mica and very reliable.

[McMurdo]

I'd measure the leakage current to earth to be sure there's nothing more sinister involved.

[julie_m]

If there was a serious leakage problem, your RCD would have tripped; most RCDs in domestic installations are rated 30 mA, but many people can feel much smaller currents than that, even much too small actually to do any harm. (If you connect a 1MΩ resistor between the set's chassis and mains earth, then the voltage measured across that resistance will be 1V for every 1µA of leakage current.)

C61 (0.005 µF -- R&TVS) is connected between one side of the mains supply and chassis. Replace it with 4.7 nF, class Y, and preferrably try to make sure that side is the Neutral. You could even fit a three-core mains lead, at the cost of some originality.

[Nuvistor]

Assuming RCD's in the installation.

Frank

[johnny english]

I measured the leakage current on my dvm ( thanks julie m for the how to) ma-ac range =12ma and ac voltage range = 116 though after 5 minutes or so this rises by 2volts. Now I dont know whether this indicates if I have a problem or not. Ed.

[Herald1360]

12mA of earth leakage is too much! It's unlikely to kill you but it would give you a serious belt if your contact resistance was low enough that most of it went through you!

You really need to find where it's coming from- it's a lot more than would be expected from the usual capacitors to ground or stray capacitance in transformers etc.

Ideally any earth leakage should be well under 1mA and less than about 5mA under any possible single fault condition. (Hence the specs on Y grade capacitors requiring that they cannot fail short circuit and the relatively low max capacitances available)