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Philips N4308 output stage fault

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colourstar
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Hi all

High time I said hello again!

I have a Philips N4308 tape recorder which has a problem with its onboard amp. There is no output as such - just the faintest trace audible with an ear to the speaker. This isn't surprising as the voltages are considerably awry. I wonder if anyone can help me understand what might be going on in this circuit?

As a bit of background info, ever since I've owned this machine (and prior to this fault) it has always had the habit of emitting the occasional loud single 'pop' from the speaker either during playback or when just switched on and idling. Whether that's a clue or not I don't know, but worth a mention.

Here are some voltages, with the expected first and the actual in brackets. I've attached a scan of the relevant part of the circuit, although we'll have to see whether or not it's legible!

AD187

B 1.6v (0v)

C 13.25v (25.5v)

E 1.47v (0v)

AD162

B 13.25v (25.5v)

C (To chassis)

E 13.4v (25.5v)

AD161

B 13.6v (25.5v)

C 25.0v (25.5v)

E 13.5v (25.5v)

Hmm something of a pattern there! The 25.5v (actually 25v on the circuit) is a supply voltage derived from X1 and X2 (BY126) and after arriving at the collector of AD161 it then appears at the collector of the AC187 and both base and emitter of AD162 without dropping so much as 0.1 of a volt despite the various resistor networks. I'm not sure how this can be?

It would be great to get some advice.

Thanks everyone,

Steve

n4308cct

Mod Note: Replaced Steve's diagram with a clearer version and added his voltage readings in red. Also uploaded the Philips N4308 service manual to the data library for those that wish the full cct to either follow along or assist Steve.

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Topic starter Posted : 11/05/2020 8:49 am
Nuvistor
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T6 could have a base emitter short, that would drag the base voltage down, T6 not conducting will raise it collector volts. R571 could also be open circuit and remove T6 base voltage stopping it conducting.

If T6 is on a heat sink that is bolted to the chassis, it could also have a short base to the case, not uncommon with this type of transistor. If it is on a heat sink bolted to the chassis try removing it from the heat sink for a few seconds and see if the voltages stabilise.

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Posted : 11/05/2020 9:43 am
Cathovisor
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Agree with Frank - there's a DC path from the centre point of the output pair to the base of the AC187 (DC stabilisation) so that should be above the emitter by about 100mV: if it was o/c there's a potential divider formed by R570/571 and the potential would be higher. From those readings it looks like the base of the AC187 is shorted to chassis?

This is one of those cases where a good DMM trumps an AVO for measuring the subtle differences in B-E potentials in germanium devices.

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Posted : 11/05/2020 10:57 am
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Cathovisor
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One thing I'd add - there's a German eBay seller who specialises in germanium devices but also, a caveat; should it prove to be the output pair, make sure you get a matched pair. You might still be able to get replacement mica washers for TO-66 devices too.

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Posted : 11/05/2020 11:15 am
colourstar
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Excellent- thank you both. Funny how you can look at a circuit for ages and just not see something. I really excel at that! Sometimes you just need a fresh pair (or pairs) of eyes on the subject. The AC187 is in a heatsink screwed to the chassis. With it detached and floating free the output stage comes back to life so I hope that the AD161/2 pair have survived the ordeal and a new AC187 will put matters right. The voltages are now within about 1v of where they should be. I suspect the loud pops I mentioned were a warning sign that something was dying.

I've always had a soft spot for these Philips machines. We used to have one at our primary school and it looked much more streamlined and impressive than the old Wyndsor valve tape recorder we had at home. I still think the styling is very smart and they work well if you are prepared to put in the effort of renewing the belts etc (having had the pleasure of removing the gloopy mess of the old ones).

Steve

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Topic starter Posted : 11/05/2020 1:11 pm
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Cathovisor
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Steve,

I've PM'ed you a link to the chap selling those devices, but I also note Langrex here in the UK have a plentiful stock of the devices.

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Posted : 11/05/2020 1:48 pm
colourstar
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@cathovisor Great stuff. Thanks Mike, you are a gent!

Steve

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Topic starter Posted : 11/05/2020 1:54 pm
Nuvistor
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Glad it was straight forward, the pops and clicks could be external, fridge, boiler, etc. A new transistor will prove whether it was the AC187.

Check the replacement, it will have been in stock some time so could have the same tin whisker problem.

I don’t think there is an article on the forum about tin whiskers, Chris please replace this link if there is.

Some reading.

https://www.vintage-radio.info/whiskers

 

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Posted : 11/05/2020 3:09 pm
Cathovisor
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To the best of my knowledge the tin-whisker syndrome only affects jelly-encapsulated transistors, Frank - AC187s are epoxy-encapsulated (they're actually big enough to be output devices in their own right, the AC187K/188K are used in my Grundig cassette recorder). Crashes and bangs also affect silicon/epoxy transistors - see my thread on the Sony Earth Orbiter I repaired (and anyone who's enjoyed Lokfit failures).

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Posted : 11/05/2020 3:12 pm
crustytv
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Posted by: @nuvistor

I don’t think there is an article on the forum about tin whiskers, Chris please replace this link if there is.

Hi Frank,

there was indeed a thread on Vrat way back in 2012. The thread also links to Mark's article on his website on vintage transistors.

Vrat Link on tin whiskers.

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Posted : 11/05/2020 3:21 pm
Nuvistor
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@crustytv

I did look Chris, but obviously not very well. Must try harder. ? 

I am stumped, I can understand whiskers growing through the jelly but not through epoxy, so anyone know the failure mechanism of the AC187 breaking down to its case? 

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Posted : 11/05/2020 3:27 pm
jjl
 jjl
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I've come across the odd AC128 with an internal short to the case before. As the AC187 has the same type of construction to the AC128, I would expect the same thing to be seen with the AC187.

 

John 

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Posted : 11/05/2020 6:01 pm
Cathovisor
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Now, are AC128s glass/jelly or glass/epoxy construction? It's been a long time since I took a hammer to one! Now I'm not sure if AC187s are epoxy-filled... ? 

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Posted : 11/05/2020 6:08 pm
Cathovisor
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The more I look into it, the more I'm beginning to think that AC187s are not epoxy filled - they're much older than I thought. Radiomuseum has them listed in devices from the early 60s to the mid-80s.

Oh well, comments about assumptions and failure come to mind here...

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Posted : 11/05/2020 6:24 pm
Cathovisor
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Posted by: @nuvistor

I am stumped, I can understand whiskers growing through the jelly but not through epoxy

As hinted elsewhere, I thought the AC187 was a much newer device than it actually is so mea culpa, Frank. ? 

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Posted : 11/05/2020 6:31 pm
crustytv
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Posted by: @cathovisor

I'm beginning to think that AC187s are not epoxy filled

I have two in stock and can assure you they are clear epoxy filled not jelly. I gave it a prod under the scope and it did not flex, solid as a rock.

ac187

 

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Posted : 11/05/2020 6:57 pm
Cathovisor
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@crustytv

Exactly the type used in my little Grundig along with its friend the AC188 ? - I always thought that was a very neat solution to the heatsinking problem. But isn't the bottom piece the glass substrate the actual device is mounted onto?

(Witness Mike going into full self-doubt mode here.)

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Posted : 11/05/2020 7:05 pm
crustytv
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Posted by: @cathovisor

But isn't the bottom piece the glass substrate the actual device is mounted onto

manu

 

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Posted : 11/05/2020 7:16 pm
Cathovisor
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Have a look at the web page devoted to how the AF11x is constructed. In those the leadout wires pass through a glass disc and then the lump of doped germanium is mounted onto that.

The PDF linked to https://nepp.nasa.gov/whisker/anecdote/af114-transistor/index.html shows the construction nicely.

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Posted : 11/05/2020 7:31 pm
Nuvistor
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@colourstar
When you have the recorder working with a good transistor, try the trick on the faulty transistor of shorting cbe together and connecting a charged capacitor across case to the leads, it can sometimes clear the short.

See Marks article for details in the post from Chris.

 

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Posted : 11/05/2020 7:53 pm
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