Philips 4307 Low Volume

[keef531uk]

I have inherited a Philips 4307 reel to reel tape recorder from my late father. Having replaced the belt I now have the problem that even with the volume turned up to maximum, I can only just make out what is playing on the tape the volume is so low. I have seen elsewhere that this is likely a problem with so called ‘tin whiskers’ on the AC 187 & AC188 germanium transistors? However, having exposed the circuit board, I can’t tell which these transistors are. I would very much appreciate advice as to whether this is likely the cause, help to identify the transistors – see the attached photograph and advice on where to obtain working replacements please?

[paulsherwin]

The output transistors you mention are on the bottom left of your photo, bolted to a heatsink.

However, it's unlikely that tin whiskers are responsible for your symptoms (though not impossible). The most likely cause is dirty or oxidised switch contacts. Have you tried cleaning these with contact cleaner?

[keef531uk]

Thanks Paul. Forgive my ignorance but which switch contacts do you mean? Not sure if it helps but I have tried using a DIN cable as I would like to record the tape audio to my computer using Audacity and this experiences the same issue. I should also mention that initially the problem was intermittant, sometimes the volume was fine and then it would go low, however in the last few days it has been permanantly low.

[Welsh Anorak]

Also it might seem obvious, but don't forget to make sure the heads are scrupulously clean.

[paulsherwin]

The fact that the problem is intermittent is very suggestive of dirty contacts.

There will be a multi contact switch somewhere which switches between record and play, and that is the primary suspect. You should also clean the insides of the volume and any other rotary controls.

The standard switch cleaner for many years was 'Servisol Super 10', now renamed 'Kontakt 10 Super'. Other brands and formulations are available. Don't use ordinary WD40 unless you are really desperate, as it makes a mess.

https://www.ebay.co.uk/itm/234602283567

[keef531uk]

Superb, thank you so much for the help! I'll obtain some of the recommended switch cleaner and give that a try and post back the outcome.

[paulsherwin]

You could try just exercising the record/play switch vigorously to see if that helps. It may not clear the fault completely, but if it improves things it will confirm that the switching is the problem.

[dazzlevision]

I have restored quite a few N4307s and N4308s.

Very low audio output can also be caused by tin whiskers inside the Germanium transistor bolted to the control panel (AF output sage driver). It's just visible in the OP's photo - TS6 type AC187/01.

[paulsherwin]

If so, you may be able to confirm it by unscrewing the bolt and lifting the transistor off the heatsink so that the case is isolated. Don't run it for very long like that as it may get hot, but it should be OK for a few minutes while you check.

[Silicon]

The Electrolytic capacitors covered in a transparent blue plastic have a good reputation generally.

However the small ones may have dried out after 50 years and may not be performing as they should.

The switches are the long green plastic objects with a row of metal tags on the top.

[keef531uk]

Hi everyone, by way of an update, I now have a can of Super 10 switch cleaner and have tried spraying the switches and operating the play and record buttons. I did this last night and again this lunchtime and unfortunately it has made no difference to the sound. It also looks like the intermittent nature of the fault has turned into a permanent fault as I have not heard sound at full volume for several days now. If I listen carefully, I can make out what is playing on the tape but it's very low even at full volume. I've just seen the additional posts regarding the transistors and I'll give these suggestions a try and post back. Thanks to everyone for your help!

[keef531uk]

With reference to the attached photograph, I've now had chance to unscrew each of the transistors from the heat sink and gently tap each of them in turn. Unfortunately this has made no difference to the low volume. As you can see in the photography the markings on the transistors are somewhat unclear to me so I have numbered them 1 to 4 in the image for easier reference. I'm at a bit of a loss as to know what to do now. I have a digital multimeter however I'm not sure what voltage levels I should be looking for between the collector/emitter/base junctions. Does anyone know what voltages would indicate normal vs fault conditions? Any further advice would be gratefully received.

[ben]

I have repaired a number of these units and can confirm that the driver transistor is often to blame. As suggested above undo it so it floats free, then re-test. You can sometimes get away with making up a separate heatsink for it, as the can is what is often shorting.

[keef531uk]

Thanks Ben - forgive my ignorance but which one is the driver transistor or is it not one of the four transistors I have highlighted?

[DMcMahon]

As stated in Post 8 the output driver is type AC187/01 and is bolted to the control panel. This is T5 situated between the volume and tone controls (closer to the volume control).

From your photo in Post 12 you have something between what looks like the volume and tone controls, you have numered it #4 but due to the poor quality of the photo I cannot identify it as a transistor, it also looks like there is a cable connecting to it ??

Note that there is another AC187 (T7) , one of the complementary push pull output pair that is bolted to the heatsink.

David

[keef531uk]

Thanks David. I have de-soldered the transistor between the volume and tone dials which is #4 in my previous photo. As per the attached image, when I turned the transistor round it is indeed an AC187/01. Does anyone have any recommendations as to where to obtain a replacement? Is the only option to source an original that may well fail again or is there a modern alternative?

[Sideband]

Is it faulty? You don't say if you have actually checked it out of circuit (which is the best way). You can use the 'diode test' mode on your meter (if it has one.....most have) for checking the junctions. You can use the resistance range to check for leakage/short between BCE and case. There should not be any connection between BCE and the case.

It would be a shame to change it just on speculation when a replacement may not make any difference....

[keef531uk]

Thanks Sideband - I was actually having the same thought and I've just learned how to test a transistor using the diode function on my multimeter . So having done some tests, I can confirm there is no short between BCE and the case of the transistor. Given that this is an NPN transistor, when testing with the diode function, I receive a small voltage with the common lead of the multimeter on what I deduce is the base terminal and the positive lead on the collector and emitter in turn. If I switch the polarity I receive overload on the multimeter display for the same tests. Based on my elementary understanding, I believe this is indicating the transistor is working OK? Given this test result, I re-soldered the transistor and performed another tape playback test but no difference in the volume level. I tested the remaining transistors in situ, both in terms of a short to the case and using the diode test function and they appear to check out. I'm now totally stumped as to how to proceed from here. Once again, any suggestions would be gratefully received?

[DMcMahon]

Have you cleaned the heads as per Post 4 ?

It gets difficult now, I would start with checking the transistor leg voltages, comparing readings to the values (where shown) shown on the schematic to see if any sigificant differences.

If you are able to touch the Record/Playack head connections with your finger, try touching the connections while in Playack mode with the volume turned up, should be able to get loudish hum response from the speaker (from at least one wire)

David

[keef531uk]

Thanks David. I did spray some switch cleaner on the heads and then wondered if I’d done something stupid! Is there a best practice way to clean the heads? Also the large rubber wheel on the head assembly looks a bit grubby but I’m nervous about using solvents and harming the rubber?