Advice on replacing AF201C

[Nukeybrown]

I've searched for a replacement which comes up with NTE160. Can anyone suggest anything other than NTE160 as not really readily available. Its also coming up with AF127. Or is a AF201C just a AF201?
I'm getting transistor headache. I have got a couple of AF178

[ukcol]

The Russian GT322B may be a good substitute and is available quite cheaply on ebay.

[Nukeybrown]

Circuit

[Herald1360]

2sa386?

[paulsherwin]

It seems to be a Siemens RF transistor from the AF117 era. What is the circuit actually doing? I'm a bit puzzled that it is using a mixture of Ge and Si transistors for no obvious reason.

I agree with Colin that the usual RF Ge candidates are likely to work. An AF178 should work if you have some to hand - certainly worth trying.

Are you sure the originals are bad?

[Nukeybrown]

Its taken from a blaupunkt pop 70 radio paul. Dont want to post the rest of the schematic as may be in copyright stuff. Not sure if the ones in are bad but theyre looking like likely problem. Going to inject signals tomorrow and see what comes out. Issue is the set doesnt put anything out on fm/mw/lw basically any radio stuff but the phone input is working fine so no issues at the amp end.

['LIVEWIRE?']

If it's any help, Mullard Data Books list the AF139 as a 'comparable' device to the AF201 (they don't mention the 201C) AF139 is a TO-72 transistor 'for use as a mixer/oscillator at freqwuencies up to 860Mhz' with a fT @12v of 560Mhz. This suggests the 139, and , by implication, the 201 (for which I don't have any data) was intended for use in UHF tuners, so why Blaupunkt used it in the circuit shown in post #3 I've no idea. No doubt a GT322B would work in the Pop 70 circuit,which is AM &/or FM.

[paulsherwin]

The 'C' is likely to be a gain grouping.

[Herald1360]

http://alltransistors.com/transistor...ansistor=21788

[paulsherwin]

That data claims the ft is only 100MHz, which seems unlikely, though not impossible of course. If true it make it even easier to sub.

Towers agrees with 100MHz.

['LIVEWIRE?']

I didn't have my copy of Towers handy when I posted my comments, and, as noted, the 1976 edition of the Mullard Data Book, from which i quoted, lists the AF139 as being 'comparable' to an AF201.

[mhennessy]

The BC108 is the first transistor in the stereo decoder.

The AF201 devices are working at AM and FM IF frequencies, so need to reach to 10.7MHz.

They are not in a package that is known to suffer from whiskers. I'd want more convincing before assuming they are faulty. Some DC conditions would be a good start.

I'd sub them with an AF124/5/6/7. For testing, even a recently-zapped AF11x would do. The oft-recommended AF139 is somewhat different and often doesn't work well in my experience - I've had more luck with PNP Si types. Towers and others can only be a general guide, naturally.

[Nukeybrown]

Only reason im looking at them is im not getting anything through the amp side from any radio bands, only getting phono input coming through amp side

[mhennessy]

Yes, I understood that. And it's entirely possible that the fault is caused by a transistor. But don't rush in - especially as it's possible to damage perfectly good transistors by removing them. Once you've done the obvious (switch cleaner, visual inspection, etc), measurements and interpretation is far better than randomly swapping parts

[Nukeybrown]

Thanks mark, just trying to work out what would take out all fm/mw/sw and fm to the amp.

[mhennessy]

In roughly this order, dirty switches, dry joints, bad connectors, failed passive components, failed semiconductors (unless they are AF117-types or Lockfit).

Most of the radio circuitry is shared by both FM and AM (MW, LW, SW). In FM, there is an additional FM front end, plus the stereo decoder in this set. So, there's plenty of opportunity for a single fault to take out the whole radio section.

I know I sound like a cracked record, but understanding how it should work is key to fixing it when it doesn't. Random part swapping is not a valid approach for something complicated like a radio, especially transistors, given that they came last in the above list. Start by measuring the DC conditions to see how they compare to the numbers given on the diagram. Then if they are OK, try signal injection, as you suggested earlier...

But usually, when an amplifier isn't amplifying, it's because the DC conditions are wrong. That's why so many people are good at repairing radios with just a multimeter

[Nukeybrown]

I have already checked dc conditions at the af201c transistors mark but need to go over them again. The 1.1v on off the top of my head v703 tramsistor isnt present. Going to check the other 2 tonight and inject a signal and try and track it back.

[mhennessy]

In which case I'd start with C801. That's the supply decoupling capacitor for the IF stages, and there should be a health voltage across it - perhaps 14V or thereabouts. If missing, check it's not gone short, then go upstream (to the right). But if present, look more closely at the conditions around V703 and V704.

V704 is a BF243. I've had those fail in the past, but only very occasionally. But let's see some voltages before jumping to any conclusions.

Good luck!

[Nukeybrown]

Result mark , the track from c801 was broken, now connected and we have stations etc. Only issue i need to sort not is the background noise over both channels , its low but not mains, its more like bacon crackling in a pan.

[Phil G4SPZ]

Originally Posted by Nukeybrown

...like bacon crackling in a pan.

Something's frying, then... sorry, seriously, intermittent rustling or crackling sounds can be caused by many things, including a faulty decoupling capacitor. These are fitted at various points along the supply rails. Does the level of the background noise vary with the volume control setting? If so, the fault is before the volume control, and if not, it's in the audio stages.