Bush AC model PB12

[longneck90]

Hello everyone. I would be thankful if one of you experts could give me some help or advice. This Bush radio had no signal when I bought it. I discovered, (among other problems) that a wire from IFT 2 had been taken to the wrong tag. Now, the radio is 'motorboating' and whistling. I have changed all relevant capacitors to no avail. I haven't changed the capacitors in the IFT can, mainly since everything looks perfect in there. (I removed it to have a look.) I have aligned the IFT 2 with the Signal Generator and there is no interference when I am doing that. But when I remove Generator, the interference is still there. I have adjusted the IFT 2 without the Signal Generator, where the adjusters end up in a completely different place, and I am able to remove the 'motorboating' and other interference, and listen to all the stations. I have changed C35, which is between IFT 2 primary and Pin 4 V3, and runs through R17 to earth. No change. When I disconnect R17 the interference stops, and I can receive stations. I tried a replacement Resistor where R17 was. No difference. If I connect a flylead to the IFT 2 tag, where it is connected to C35, and leave the other end of the flylead unconnected to anything, the interference either stops or reduces greatly. The electrolytic capacitors have been replaced. I would be delighted with any help or advice which might help me solve this problem. All the best. John.

[jonnybear]

Interesting one, you say there has been tampering with the connections, to various parts of the set, what you need to do is sit down with the circuit and meticulously check all connections are correct, if you say wire was to the wrong tag ( which wire photo would help) it is possible if there has been someone in there that they could have transposed the wires to IFT2 ie the connection going to R9/C25 and the anode of V2. The other thing, is the metalic coating of the Valves intact as this can cause all sorts of problems? especially ef39. If the coating has come off wrap some tin foil around it make sure it is connected to pin 1, hope this is of help.
John

[Wellington]

Hi John

From the component numbers you use, it looks like you're working with the Bush service instructions. Good move on not changing the capacitors in the IFT can - these are described as 'silver mica' capacitors in the service instructions and unlikely to be duff.

Rather than focussing on the area around IFT2 and V3, perhaps (if you haven't done so already) check some of the voltages on the valves throughout the set and compare them with those quoted in the service instructions (pg.7). This might indicate if a problem in a different area of the set is causing the symptoms.

My gut feeling is that the motorboating and whistling may have different causes. Leaving the whistling aside, the motorboating may be caused by an open circuit decoupling capacitor somewhere. This set seems to have a few of these - C34 (2uF HT decoupler) and C22-25. A bad solder joint on any of these components could also be a possiblity. Also, the circuit shows that screened leads are used to carry the freshly-detected audio signal to the top cap of V3 - it might be worth checking that the screening is still sound and connected to chassis.

The only other suggestion I have is: do check your work around the smoothing and reservoir capactitors, especially that the connections to chassis are sound.

Originally Posted by longneck90

I have changed all relevant capacitors...

Which ones did you class as 'relevant'? Was one of them C37?

Like John, I am also curious about the wire from IFT2 being taken to the wrong tag - how did you find that out?

Good Luck.

[HamishBoxer]

How many more wrong wires if the "phantoms" been there first?

[longneck90]

Thanks to johnnybear and wellington. The circuit manual I have is the 'Bush Radio Service Instructions'. The EF39 is indeed minus some of its original wrapping. The 'wrong' wiring from IFT 2 turned up because I closely followed the wiring diagram, and thought,'that wire shouldnt be there. It was directly connected to earth. I did replace C37, and all the capacitors which are underneath the panel where C37 is sited. The panel is connected by about 7 or more wires, which have to be removed to get access to underneath the panel where C37 is. What is very puzzling is that when a station is tuned in, reception is perfect, yet, turn the tuning dial one side of the station and violent disturbance ensues. For now I will go back and look at all the joints that I soldered, and examine the circuit and valve voltages more closely. Thanks for all your information. All the best. John

[PJL]

You need to follow John's advice and try wrapping the EF39 with some aluminium foil. They are notorious for causing instability if the metallisation is missing or disconnected.

Measuring voltages whilst it is unstable will not tell you much as it causes the AGC to turn on which changes all the operating conditions.

[Herald1360]

Originally Posted by longneck90

What is very puzzling is that when a station is tuned in, reception is perfect, yet, turn the tuning dial one side of the station and violent disturbance ensues.

That would be consistent with instability caused by poor valve screening. When a strongish station is tuned in, the agc will wind back the gain making the feedback caused by the lack of screening insufficient to cause oscillation.

When tuned away from the signal, the gain rises until oscillation starts.This is seen as a signal by the agc which then winds back the gain. Depending on agc time constants and probably other non linearities this may result in lower level oscillation or oscillation that stops and starts repeatedly. This will cause all sorts of whistles and motorboating effects as the set is tuned.

[paulsherwin]

One more vote for the EF39 screening being at least part of the problem. As has been said, you can bodge it with kitchen foil.

[longneck90]

Hello to everyone and thanks for all the information. This morning I have traced the circuitry around, and connected to IFT2. It is OK. Strangely, when I disconnect C35, the interference stops and I can tune in stations clearly. When I reconnect C35, interference resumes. I checked the voltages on the valves. V1 P3 147v, P4 79v, P6 76v. V2 P3 110 v, P4 131 v. V3 P3 76v, P4 3v, P5 1v. V4 P2 1v, P3 204 v (119 without signal). P4185 v, (194 without signal) I will try to get a good EF39. Sorry I wasn't clear about 'relevant' capacitors etc. In the Bush Service Instructions there is a list of AVC, decoupling and bias components, to be checked if there is no signal. I changed or checked them all. I will now go and study the information that you have all provided, and thanks for ell the help. I'll let you know the outcome, which may take some time. All the best, John.

[longneck90]

Hello to you all. Success. I have another Bush PB12. It has an undamaged (ie, coating) EF39. I tried that and the radio is going well now. I have re-aligned IFT2 and there is no 'motorboating or interference. I was just wondering whether there is any special paint or something similar that can be painted on to those valves to rectify the problem. Anyway, I very much appreciate all your helpful information. Thanks a lot. All the best. John.

[Herald1360]

There is special paint, but it's expensive and doesn't look original anyway, so you may as well just wrap some foil round the valve, then wrap a few turns of tinned copper wire round the foil so as to push it into the "groove" between the envelope and the top of the base, twist the wire to retain it and connect one end to chassis.

Elegant it ain't but it works and is simple to remove if you get a replacement valve with good metallising.

[longneck90]

Hello Herald 1360, Thanks for your advice. I'll try wrapping tin foil around it like you said. All the best. John.