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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 8th Dec 2019, 9:49 pm   #21
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Default Re: Philips AG4126/7 blown thermistor

I agree, it seems like the transistors are fine.

The thermistor looks burnt in the photo, but is it?

There's a resistor in parallel with it, so there should at least be something in the way of sound at the speaker.

Have you checked that resistor?

Have you checked the speaker and its connections?

If you're relying on the output from the record deck cartridge, just remember that this is an old Philips unit which are known to pack up. What are you using for audio input to the unit if not the record deck pickup arm cartridge playing a record?
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Old 8th Dec 2019, 10:05 pm   #22
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Default Re: Philips AG4126/7 blown thermistor

As others have said, it will sort of work without the thermistor. You haven't actually said if there is any sound at all.....and you can't 'grind down' a disc ceramic capacitor to replace a thermistor....a thermistor is a type of resistor so it's never going to work.

So is there any output at all?
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Old 8th Dec 2019, 10:09 pm   #23
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Default Re: Philips AG4126/7 blown thermistor

The thermistor looks burnt in the photo, but is it?

Here is a picture of the bits I took out....

History is that I bought this many years ago from a jumble sale. The unit worked very faintly, with the volume control not functioning. So I put it away, and 20-odd years later it surfaced in the garage.

So I thought that I would make a start on it....

I opened up the unit to investigate, unsoldered the amp unit and removed it, and found the blown thermistor. I could re-assemble it as it is and check it with a feed from the deck - but while the amp is out I might be able to check some more? And replace the thermistor?

Note that I mentioned earlier that the parallel resistor solder joint looked a bit dry to me as I removed the thermistor. Might that have caused the blow on its own? Would it be ok to replace with the 150 ohm item I found? This is what I'm wondering before replacing it....
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Old 8th Dec 2019, 10:20 pm   #24
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Default Re: Philips AG4126/7 blown thermistor

....and you can't 'grind down' a disc ceramic capacitor to replace a thermistor....

Never said that you could. I just mentioned that I had heard of doing that to lower the value as an example of a possible physical alteration of a component - but I didn't think that sticking a blown thermistor back together would be practical..... my background is vintage radio control equipment, and this is a technique from that field.
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Old 8th Dec 2019, 11:27 pm   #25
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Default Re: Philips AG4126/7 blown thermistor

Ah, things make more sense now you've told the full story, although it's difficult to know just how much to put in a first post.

Replace the thermistor or compromise with a substitute - someone pointed you to the data on it in a previous post, so you should be able to come up with something, even if it's just another resistor for the time being. It could be that the parallel resistor was dry jointed causing the thermistor to fail. Then feed it with an audio signal and see what happens. I don't know this particular model and I couldn't find a picture of it on line (although there is an original copy of the full service information by Philips for sale at this very moment for around a fiver posted if you do a search), but depending on whether it uses a crystal or ceramic head, you may find that if it's the crystal type that it's duff. If you've got good practical skills you can probably adapt one of the very cheap Chinese black and red ceramic cartridges to fit in the old head. One way or another I think you'll get it working.
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Old 9th Dec 2019, 10:27 am   #26
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Default Re: Philips AG4126/7 blown thermistor

I've found thermistors at 120 ohm and 150 ohm - will try a 120 and see how it gets on.

If no joy I suppose that I'm into replacing all the electrolytics. Which will be hard, given that direct replacements are going to be hard to source. Is there a known place to go to for these?
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Old 9th Dec 2019, 10:57 am   #27
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Default Re: Philips AG4126/7 blown thermistor

You won't need direct replacements. Anything that will fit of similar or higher capacitance should be fine. Low value ones may be replaced by film types if there's room. Max voltage rating will almost certainly be higher for a similar physical size but as long as it's less than about ten times the actual circuit operating voltage that's fine too.
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Old 9th Dec 2019, 12:02 pm   #28
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Default Re: Philips AG4126/7 blown thermistor

It is unlikely that all the electrolytics will be bad - it's perfectly possible that they're all fine. You will need to fault-find methodically if you're going to get anywhere.
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Old 9th Dec 2019, 1:08 pm   #29
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Default Re: Philips AG4126/7 blown thermistor

Thanks to everyone for the comments and assistance - I'll wait until the new thermistor is delivered and report back...
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Old 13th Dec 2019, 11:40 am   #30
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Default Re: Philips AG4126/7 blown thermistor

Now we have a thermistor fitted, and the amp produces a hum. High when the volume is up - low when it's turned down.

But when I put a record on, no signal comes through. Does that mean the cartridge has had its chips?
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Old 13th Dec 2019, 12:00 pm   #31
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Default Re: Philips AG4126/7 blown thermistor

Very likely, especially if it's the original Crystal Cartridge. A 'buzz test' at the cartridge terminals will prove whether or not the amplifier is working.
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Old 13th Dec 2019, 12:22 pm   #32
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Default Re: Philips AG4126/7 blown thermistor

I've tried a simple test - taking the cartridge off and touching the contacts in the arm. I get a buzz on two contacts, and lowered noise levels on two others. So the wiring to the amp seems fine.

With the cartridge on, rubbing the stylus produces no noise. Looks like a dead cartridge. It's one that's built into a narrow tapered headshell - an AG3228. I can't find one for sale, but I can find an AG3310, which looks similar. Do you think that would be a reasonable substitute?
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Old 13th Dec 2019, 2:42 pm   #33
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Default Re: Philips AG4126/7 blown thermistor

Before you go any further, do adjust the idle current of the final stage with R22.
Do open up linkage "A" and "B" and measure the current there. It should be 9.5mA - 10.5mA according to the schematics.
Changing the thermistor will change the idle current.

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Old 13th Dec 2019, 5:03 pm   #34
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Default Re: Philips AG4126/7 blown thermistor

Do open up linkage "A" and "B" and measure the current there. It should be 9.5mA - 10.5mA according to the schematics.

Thanks for pointing that out. Did it, and found it at around 28ma. The lowest I could get it to go was 17ma. That's with a nominal 120 ohm thermistor, which, annoyingly, I did not test before putting in. The pot R22 is at full stretch. Do you think I ought to add another resistor in series?

Pot is 2k, so a 1k resistor across A-B might bring things into spec?
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Old 13th Dec 2019, 5:40 pm   #35
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Default Re: Philips AG4126/7 blown thermistor

Hmmm. The amps there are dependent on the volume setting. Low the amps are 0 - mid-range the amps are around 10mA, full they are around 30-40mA.

Am I doing something wrong?
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Old 13th Dec 2019, 5:52 pm   #36
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Default Re: Philips AG4126/7 blown thermistor

Put your finger on the transistors and ask yourself are they getting too hot.
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Old 13th Dec 2019, 6:16 pm   #37
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Default Re: Philips AG4126/7 blown thermistor

Warm, but not hot by any means.
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Old 13th Dec 2019, 6:27 pm   #38
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Default Re: Philips AG4126/7 blown thermistor

I suspect that the 9-10mA figure is what should be measured when the other voltages in the circuit are as specified? I can't see any other setting ....
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Old 14th Dec 2019, 7:15 pm   #39
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Default Re: Philips AG4126/7 blown thermistor

Quote:
Originally Posted by Dodgy Geezer View Post
Hmmm. The amps there are dependent on the volume setting. Low the amps are 0 - mid-range the amps are around 10mA, full they are around 30-40mA.
"Before you go any further, do adjust the idle current of the final stage with R22."

Just to confirm - idle current setting is with no audio.
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Old 14th Dec 2019, 11:17 pm   #40
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Default Re: Philips AG4126/7 blown thermistor

Quote:
Originally Posted by buggies View Post

Just to confirm - idle current setting is with no audio.
Does that mean with no signal and volume set to low? Because the volume setting seems to drive the current between 0 to 40mA depending on how it's set...
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