Bush AC31 switch on after refurbishment

[tealandsilver]

After many enjoyable days replacing waxy paper capacitors, electrolytics, several way out of spec resistors (including a unblemished open circuit wirewound R15) and several perished wires, came the big moment. Coupled up via an rcd and my newly assembled lamp limiter, I plucked up the courage to flick the switch. On comes the limiter bulb and settles down to a dullish glow. Scale lamp illuminates, speaker emits some noise. Tuning through the bands, zilch. Ah, forgot to stick a wire in for the aerial, now with a small aerial wire I can tune a couple of stations on MW and also SW, none on LW.
Couple of initial faults, volume control incredibly noisy and on tuning there is a spot frequency (mid band) which is very "scratchy" for about a mm of travel. Looking at the variable capacitor the noise occurs with the moving vanes at exactly 90degrees to the fixed vanes and on all bands.

I have a can of Servisol which I was going to spray into the volume pot through a small opening in the side casing. Would you advise doing this?
For the variable capacitor fault, I have inspected it closely and the vanes appear to be true and reasonably clean, would a blast of compressed air (from my garage compressor) cause any issues?

I still need to measure voltages to check and see if further replacement work is required, but am aware I've probably replaced more than I should have, typical nooby error!

Thanks for reading

David

[Sideband]

Well it sounds like you've done everything OK at the moment. Yes it will be OK to use the Servisol but don't flood the pot..A couple of short squirts and work the control back and forth a few time (with the set off of course). Try the set again and if necessary perhaps another short squirt if required. Do the same with the wavechange switch working back and forth again and that may well cure the LW problem.

Worth trying the compressed air on the tuning cap. You might be lucky.

I don't think you've replaced too much. Most radios will need all the waxies changed for reliable operation. As long as you don't randomly change caps in the RF stages, you should be fine.

[Boater Sam]

You are doing OK.
Yes, power off and give the pot a spray and spin, let it dry before applying power.
While the power is off, check the tuning cap with your ohm meter, it only takes a little grit to make a short. If any of the end vanes are slightly bent don't straighten them, it is deliberate to balance the capacitance between the 2 sections. Blow it out certainly. Check the spring or leaf contact that grounds the rotor, if it is making bad contact that will cause a crackle too.

[tealandsilver]

Thankyou for the quick replies. I have checked the tuning cap with my ohm meter and about midway through travel there is an indication of a short. I'm unable to see anything which is causing it and there is no obvious mechanical damage. There is a fair amount of dust inside which looks as if it will blow out so I'll give it a go tomorrow. Have also located the leaf spring for the rotor which could do with a clean. Thinking about it, the radio was stored in a loft for many years so there may be some hidden oxidation where the spring contacts the rotor spindle. I'd imagine that could account for the scratchiness in one position only.

regards

David

[paulsherwin]

Don't expect it to work particularly well with the lamp limiter in circuit, especially if you are using a 60W bulb.

[tealandsilver]

Hi Paul, thankyou for the comment. I had overlooked the effect of the lamp limiter bulb in my excitement of getting something out of the radio! The lamp limiter I built has the bulb live switched rather than the switch being able to bypass the bulb as I preferred not to leave the possibility of a live terminal in the bulb socket when swapping bulbs. Once I've cleaned the tuner cap and volume pot I'll have another go,

regards

David

[tealandsilver]

So, sort of progressing, 2 steps forward one back. Managed to cure the noise from the volume pot by a couple of squirts of Servisol, very pleased as there is no noise at all now from it. Paul's point about operation with the lamp limiter was valid, operates much better without! Blew through the vanes of the tuning cap with compressed air and it cleaned up nicely, disconnected earth leaf from rotor and cleaned it's location slot. However, the "scratchiness" at 90 degrees is still there and more analysis revealed that after 90 degrees I don't receive any stations, annoying as radio works perfectly up to that point. This occurs on all bands. I have checked most of the AC31 posts to see if anyone else had came across this fault but with no joy. One person had a problem with R6 (body spot tip resistor) but mine measures 25K in circuit compared with 22K required with is just outside the 10% tolerance. I have the later model with the tone control fitted. Any advice as to what I check next?

David

[G6Tanuki]

The traditional way to clean-out an old tuning capacitor was to slip a 5-thou feeler-gauge into the gap between each pair of fixed and moving plates. The usual problem with old radios being the buildup of white crusty aluminium-oxide at certain spots on the plates (due to impurities in the aluminium and storage in a damp environment).

Feeler-gauge treatment followed by a blast with an airline usually shifts the 'crust'.

[tealandsilver]

Thankyou for the reply. Your comment re the feeler gauges got me thinking about gaps between the vanes. when I looked very closely at the fixed vanes I noticed that the outermost one has a wear mark. I hadn't appreciated that the moving vanes are mounted off centre to the spindle and one of the moving vanes started to touch the fixed vane at around where I was hearing the noise and where the mark starts. I've very carefully realigned the vane and no longer get the noise on tuning! The tuning capacitor must have had a knock at some time in it's life to account for the mechanical misalignment.
Testing the radio again I can now hear stations on all bands (well only one on LW). I've also rediscovered just how much difference an earth makes to reception! My next task is to start checking voltage levels against the service information to see how they compare. I don't have a signal generator or scope to start checking alignment so I will probably leave as is.

David

[Sideband]

Good work so far! Don't worry too much about alignment...and you certainly don't need a scope for a radio like this. A signal generator is a nice thing to have but reasonably good alignment can be achieved without one....as long as you know what to twiddle! If necessary, some guidance can be given but it probably won't need it.

[tealandsilver]

For reassurance I've now checked out all the valve voltages against the Bush service instructions and all are pretty close to that required with the exception of the cathode voltage on V1 (ECH42) which doesn't appear to be present (should be 3.3V). On one instance I saw 3.9V ac but couldn't get that to repeat. I have cleaned the valve pins (not particularly dirty) but without effect. The radio does appear to be working but I'm not yet convinced that it is as sensitive as it could be. My aerial and earth set up is just a lash up at present, few feet of wire for the aerial and the central heating pipework for earth. I am aware of the potential for an increase in mains interference due to my earth arrangement. Should I be concerned about the lack of cathode voltage on V1?

[ms660]

I haven't got the manufactures manual to hand just the R&TV Servicing one, it looks as though the cathode bias resistor is only switched in when the PU/Gram function is selected, this would be because the triode section of the Mixer/Osc is used as an AF amplifier when that function is selected and would need biasing for Class A amplification.....So, no cathode volts on SW/MW/LW function is normal.

Lawrence.

[Al (astral highway)]

Originally Posted by tealandsilver

My aerial and earth set up is just a lash up at present, few feet of wire for the aerial and the central heating pipework for earth.

A proper long-wire aerial will definitely pull in more stations and you'll likely have less noise, too. Try to get twenty or thirty feet and see what happens.

For MW and LW, it doesn't have to be anything precise, but these are big wavelengths and a couple of feet is tiny in comparison and isn't going to be very resonant.

Bear in mind that there isn't much of anything on LW anyway! I'm not the expert but there's literally nothing where I live. On MW, I pick up a strong local (within 5 miles) radio station and some other relatively local (within 200miles) stations.

But you are geographically in a completely different area and have many other factors influencing your reception that aren't relevant to me!

Nice work, keep it going!

I have in the past temporarily removed ganged tuning capacitors and put them through the dishwasher to give the vanes a good clean. They come up beautifully. And it's quite easy to tweak a vane gap (with suspected bending or near or actual contact of static and moving vanes) with a simple bank card or two if you don't have specialist tools.

[tealandsilver]

Thanks Lawrence.
Looking back at the circuit diagram I hadn't appreciated the change when pu/gram was selected on the cathode.

Good tip Al on tweaking the vane gap, I was very wary and reluctant to start, but discovered that it really was pretty easy. As for a long wire, I have a large garden which lends itself nicely to experimenting. I am contemplating sticking an earth rod into the ground also so that I can compare and contrast.

David