Hacker Radio:Motorboating - RP25

[Bonky28]

First Post:

Motorboating - RP25. S/n 4380 (so one of the earlier ones)

(Please treat me like an idiot as my knowledge of electronics is very limited. I do have a meter and sig gen but no ‘scope. I can just work Ohm’s Law that’s all!))

FM reception is excellent.
MW and LW show ‘motorboating’ - being more sensitive to this type of instability when the bass control is turned up.
I understand that the first port of call is the 5 -pin plug on the amp board. This has been wiggled, and cleaned to no effect. Swapping the amp board for another from a fully working RP18 still shows motorboating. The centre pin (earth) is well-connected to the chassis earth.
The batteries show no-load 17.73V and the quiescent current for the whole radio (not just the amp board) is 19mA (a little high?)

Earthing the outside (can) of L5 on the T306 board results in the motorboating howl disappearing -but I cannot tune in any stations.

Please can anyone help?

Would a simple ‘scope help diagnose problems? If so, can anyone recommend a simple and inexpensive one?

There are many similar threads on the Vintage radio site which I’ve printed off - but the advice there doesn’t seem to work with my stuff.

Many thanks,

Richard (Sheffield UK)

[crackle]

Try replacing C21 350uF (manufacturers schematic) AM circuits decoupling electrolytic.
And if there are any other electrolytics marked "Callins" replace those as well, they are famous for being bad.

Mike

[Bonky28]

Thank Mike;will do

R

['LIVEWIRE?']

Please note that 350uf is not a 'preferred value' Modern capacitors (and resistors) come in ranges 1, 2.2, 3.3, 4.7,5.6, 6.8, 8.2, and their multiples. The nearest easily obtainable value Electrolytic Capacitor will be 470uf, though you may be able to get a 330uf one. Either will be a suitable replacement. Electrolytic caps. have a wide tolerance , maybe + or - 20% of their nominal value. Use Caps. whose voltage rating in equal to, or greater than, that of the original. Be aware that electrolytics must be connected the correct way round. Most modern ones have a line of '-------' to indicate which is the Negative pin or lead. (Apologies if any of the above is stating the obvious)

[Bonky28]

Thanks everyone. Update:
C21 had already been changed, as have all the transistors on the 502 board (to 2N3906s I think. C13 (Vishay) looks new...but I don't know if it's original.
Needless to say, the motor boating is still there. Any more ideas...please!
BW

Richard

[paulsherwin]

Replacing the AF117s in the AM IF stages with 2N3906s isn't a straightforward thing to do and it may be causing the instability. You can sometimes get away with it, but silicon transistors have very different characteristics and can be too lively without other component changes. Do you have any germanium RF transistors that you could try?

[Bonky28]

Thanks Paul.

What would be a better substitution? (Pref one where I don't need to adjust the biases etc).

Thanks again,

Richard

[Cobaltblue]

Hi Richard

I just checked one of my RP25's and the can of L5 is earthed.

I am wondering if there is an internal short inside the can or something on the board and someone has insulated the can to resolve it?

The example I have in front of me has tape on the upper and lower side of the L5 can presumably to prevent it shorting to adjacent components.

Pure speculation on my part but it seems odd the can not already earthed.

Cheers

Mike T

[paulsherwin]

There are lots of threads on subbing AF117s, but as a quick summary:

The classic sub is the AF124-7, which are all the same and identical to the AF11x, but they are a bit pricey nowadays. Most Ge RF transistors seem to be OK and the cheapest ones seem to be ex USSR from Russia and the Ukraine, though I've not used them personally. The last AF11x AM IF strip I did used some ex equipment 2N1307 switching transistors, but they aren't cheap if you have to buy them NOS.

Maybe a forum member would be prepared to sell you a few suitable types?

See https://www.vintage-radio.net/forum/...d.php?p=807721

[Bonky28]

Thanks Paul.
I have: 2N1307; 2N3094; AF124-7; AF127: BF450; GT322 (Russian) as substitutes on my list. As you say, some are expensive!
BW
Richard

[paulsherwin]

The 2N3904 is an NPN transistor, and the BF450 isn't really any more suitable than a 2N3906 for this application. There is the 2N1309, which is a higher frequency version of the 2N1307. Checking eBay quickly the cheapest options at present seem to be the GT322 or the AF125, though you never know what's going to turn up.

AF124-7 means AF124, AF125, AF126 and AF127. These are all identical.

Things have been complicated by some of these transistor types being popular with guitar fuzz pedal builders, which has pushed up prices.

[ukcol]

Originally Posted by Bonky28

The batteries show no-load 17.73V and the quiescent current for the whole radio (not just the amp board) is 19mA (a little high?)

Although not necessarily the case with your set it is always a good idea to eliminate the batteries as the cause when faced with a transistor radio that motorboats when you turn the volume up.

I would expect the no load voltage of two good PP9s in series to be higher than the 17.73v that you are getting. I have just removed a partly discharged PP9 from a set and its off load voltage is 9.65volts.

Measure the battery voltage with the set running and motor boating, if that voltage dips a couple of volts or more, try fresh batteries.

[crackle]

I wonder if C21 was fitted the right way round, the last one I changed was a Daly make.

Mike

[Bonky28]

Originally Posted by Cobaltblue

I just checked one of my RP25's and the can of L5 is earthed.

I am wondering if there is an internal short inside the can or something on the board and someone has insulated the can to resolve it?

The example I have in front of me has tape on the upper and lower side of the L5 can presumably to prevent it shorting to adjacent components.

Pure speculation on my part but it seems odd the can not already earthed.

Thanks Mike,

If I earth (chassis earth) the can on L5 AND all the other coil cans (separately)on the 502 board then the noise disappears BUT I can't pick up any stations.

I'll investigate further (after the rugby!)

BW

Richard

[Bonky28]

I hope I haven't given you (and others) a wrong impression here. When I said the 'can' I wasn't talking about the large aluminium box that covers the board but the outside metallic parts of the coils themselves.

Thanks

R

[Bonky28]

Update: Swapping the batteries for new ones (19.2V no load) made no difference. Using the old ones again, as soon as the howl started the voltage dropped from 17.2V to 15.98V.

As far as I can see only the outer metallic part of L5 is earthed so am a loss to understand why, when I do connect it directly to chassis earth I can pick up no stations - albeit the howl disappears. There is few few Ohms resistance between chassis earth and the L5 screen.

C21 seems soldered in correctly.
Ho Hum! (Literally!)

Thanks everyone for all the advice so far ; much appreciated.
Richard

[Bonky28]

I have a mind to swap some of the smaller, non-electrolytic caps. (C22;C20:C17) on the 502 board.

Am I right that they are all ceramic types?

Thanks,

Richard

[Andrewausfa]

I think you should leave them until you've found the cause of your motorboating. Otherwise you'll be chasing your tail trying to fault find by replacement and possibly introducing new faults.

Have you checked and re-checked every new connection you've made and the transistor voltages against the Hacker list? Any dry joints on the pcbs? Highlighter pen, circuit diagram and confirm everything by marking it off as you trace the circuit. Just an idea.

Andrew

[Radiocruncher]

Good advice from Andrew. It’s very unlikely that the non electrolytic caps would be an issue unless it’s dry joints. It sounds like the set has been got at. Have you cleaned the FM switch with switch cleaner as that could cause issues? I had to take one apart recently and clean it with emery paper as switch cleaner didn’t do the job. If all else fails it could be worth changing back to AF12x transistors. Worth checking the circuit diagram to make sure no resistors have been changed when they swapped to silicon if you are considering going back to germanium. They are available but as has been said they can prove expensive. I have some GT322B transistors that you can have at cost to try as that would be the cheapest option. PM me if you want them.

Good luck

Graham

[Bonky28]

Thanks Andrew and Graham and others.


I have used switch cleaner. I don't think any of the resistors have been changed, but I think I ought to check them. Thanks for the offer of the GT322bs. If all else fails I'll email you - thanks!

Could any of the components on the switchboard be at fault here?

BW
Richard