Philips 4307 Low Volume

[DMcMahon]

While doing the finger hum test, also try changing the Track switch positions.

David

[DMcMahon]

Head cleaning is best done using IPA (Isopropyl alcohol /Isopropanol) generally using a cotton bud or soft rag, some people prefer Methylated Spirits, cleaning the heads several times even if the heads look clean.

The rubber Pinch Wheel can be cleaned with IPA but do not over wet it, spittle also can be used, cleaning a dirty Pinch Wheel will not improve the sound level.

David

[dazzlevision]

Originally Posted by 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?

Don't use switch cleaner, as it also has a lubricating function - not good! You should clean this off - use IPA below.

Isopropyl Alcohol (IPA) is safe on the tape heads, metal tape guides, "rubber" pinchrollers and the metal capstan (the tape is gripped between it and the pinchroller).

I use cotton buds moistened with IPA and gently wiped across the heads, etc. If the tape path is heavily contaminated with tape oxide, you may need several applications - with clean cotton buds each time.

The N4307 will have a low level audio output on one of the DIN sockets at the rear, which you could connect to the tape input of an amplifier, to see if the audio off the tape is getting to the stage just prior to the driver/power output stages (i.e. just before it get to the volume control).

[keef531uk]

Fortunately I have some IPA so I've done as recommended and used several cotton buds to clean the tape heads and wheel. I've attached a photo - they look pretty clean to me but I'm no expert. When I press play the mechanism to the left of the rubber wheel in the photo moves toward what I assume is the tape read head. This makes it difficult for me to get my finger in the gap. I could pull this back slightly and I tried with a cotton bud but there is no audible sound when doing this. I also connected my DIN cable to my amplifier tape input and the sound out of the amplifier is very low just like out of the speaker. When this was working previously, the sound was fine, so both the speaker and the DIN output are affected by the fault. With regard to measuring voltages – unfortunately there are no voltages stated on the circuit diagram so I’m at a bit of a loss as to know what the correct voltages should be.

[DMcMahon]

Voltages for most of the transistors are shown on the schematics in the N4307 ERT & Manufacturers Service documentation, that is available as purchaseable Instant downloads from here - https://www.service-data.com/

For the finger hum test I meant touching the soldered connections at the rear of the Record/Playback head. Am not familiar with the N4307 so do not know if these particular heads actually have the soldered connections at the rear of the heads or if the heads have integral wires coming out of the heads and wires connecting downstream somewhere.

David

[dazzlevision]

Originally Posted by DMcMahon

For the finger hum test I meant touching the soldered connections at the rear of the Record/Playback head. Am not familiar with the N4307 so do not know if these particular heads actually have the soldered connections at the rear of the heads or if the heads have integral wires coming out of the heads and wires connecting downstream somewhere.
David

The N4307 & N4308 record/playback head has a metal screen over it, so the soldered wiring at the rear of the head is possibly inaccessible.

[Sideband]

Take the screen off.....single screw. Replaced a fair few heads on these 50 odd years ago when at Philips.....

[keef531uk]

Thanks again for everyones help - it is very much appreciated! I’ve removed the tape read head cover and placed my finger on the soldered connections at the back of the read head and I get no hum either from the speaker or through the DIN cable connected to my amplifier. Does this suggest the pre/power amplifier circuit is faulty? I will download the service manual tomorrow as recommended and check the voltages.

[keef531uk]

Okay, feeling a bit stupid now I paid for and downloaded the ERT and Maintenance manuals and found the section with the table of transistor voltages. I paid attention to the diagram indicating which terminals were the BCE for each transistor and measured the voltages on the soldered side of the PCB whilst in play mode. For transistors T5, T6 & T7 the voltages were all within 10% of the stated values in the table. As I wanted to double check, I turned the unit over and attempted to measure the voltages again on each of the transistor ‘pins’ where I could be more sure I was definitely measuring at the correct spot. However in doing this I accidentally created a short between T7 and the transistor case which made a small flash and appears to have blown the glass 1500ma / 240V fuse. I've had a look for a replacement fuse but there appears to be slow and fast acting fuses and I'm not sure which one to get? Clearly I'm not sure if I have done any further damage as yet. What a muppet I am!

[dazzlevision]

That's unfortunate, but the PCBs in these recorders are very compact with very thin print tracks and tightly packed components.

The failed fuse will have either an F or a T stamped into one of the two end caps. F = quick blow and T = anti-surge.

[keef531uk]

Thanks dazzlevision. Having got my magnifying glass out I think it's actually a T500ma / 240V fuse so I've ordered a kit and will cross my fingers the fuse has protected the circuit.

[DMcMahon]

Originally Posted by keef531uk

I’ve removed the tape read head cover and placed my finger on the soldered connections at the back of the read head and I get no hum either from the speaker or through the DIN cable connected to my amplifier. Does this suggest the pre/power amplifier circuit is faulty?

The head finger touch hum test is not a scientific test but as long as you were in playack mode, had the volume wound up high, touched all connections on the head and tried all positions of the track switch, then to me it does point to a fault upstream in the pre/power amp circuitry as opposed to an open circuit/dead PB head.

If you have not already done it, then clean and exercise the track selection switch with the switch contact cleaner.

David

[keef531uk]

Hi everyone, discouraging news I'm afraid Having replaced the fuse, when I turned the unit on I got a loud hum from the speaker and then smoke started to appear from the bank of four resisters labelled R578, R581, R579 & R582 on the manufacturers circuit diagram. I turned the unit off and I suppose I could change the transistor that I was testing when I accidentally created the short but clearly now I don't know what else is damaged. As always any thoughts would be gratefully received?

[DMcMahon]

They are the emitter resistors respectively for T7 & T6.

I would first remove T6 & T7 and recheck their diode junctions to see if still good, also with the transistors removed check that the paralleled R578/R581 & R579/R582 measure around 4 Ohms.

David

[vf_marshall]

I do have a fair amount of N4307/8 parts including a spare board (maybe minus an AC187/01) etc, plus a fully working 4307 and a 4308 if you need any components measuring. I've restored around 15 of these over the past few years, and the AC187/01 screwed to the underside of the pot support bracket needed changing on all but two of them. faults ranged from loss of sound altogether, through intermittent loss and on a couple, the vibration caused by changing the track selector was enough to loose sound altogether.

[keef531uk]

So I finally got round to de-soldering the transistors and the NTC thermistor (R576) so I could test them disconnected from the circuit. I believe what I have found is that everything checks out OK except for transistor T7 which, whilst OK between the NP junctions, appears to have a short between the collector and emitter. Is it possible this could be the cause of the overheating and the smoke coming from the bank of resistors as mentioned in my earlier post? I have ordered two spare AC187/01 transistors so once they arrive I can try replacing the one with the short. Whether this was the original problem with the low sound will remain to be seen.

[keef531uk]

So I've now had chance to replace the faulty T7 transistor and re-solder the other working transistors. This appears to have fixed the fault I caused when testing the transistors in situ as there is no overheating anymore. However the original fault, i.e. the very low volume is still there. I'm at a bit of a loss now as to whether it's possible one of the transistors is, for want of a better expression, "semi-faulty" or whether the problem lies elsewhere?

[Ambientnoise]

I vaguely recall a similar (unbalanced volume) problem on (I think) the same model. It was caused by the “loudness” tapping on one of the volume pots being open circuit, probably where the tap joined the main track. Repair attempts failed so I disconnected the tappings on both channels to restore balance. Sorry, it’s a bit vague

[Ted Kendall]

Originally Posted by keef531uk

So I've now had chance to replace the faulty T7 transistor and re-solder the other working transistors. This appears to have fixed the fault I caused when testing the transistors in situ as there is no overheating anymore. However the original fault, i.e. the very low volume is still there. I'm at a bit of a loss now as to whether it's possible one of the transistors is, for want of a better expression, "semi-faulty" or whether the problem lies elsewhere?

Are the small signal jobs Lockfits? In which case proceed with suspicion.

[keef531uk]

I have now used my last spare AC187/01 transistor and replaced the T5 transistor mounted in-between the tone and volume potentiometers. I had high hopes this would fix the low volume issue but alas it has made no difference. I checked that using my DIN cable into an external amplifier that the fault was still there and it wasn’t just a problem with the tape unit’s loudspeaker and yes, the problem is the same via an external amplifier. Correct me if I’m wrong, but does this rule out an issue with the volume potentiometer as I can still hear (low) sound from the external amplifier whether the tape unit’s volume is turned up to max (where I can just about hear sound through the tape unit’s speaker) or turned down to zero (when I hear nothing from the tape unit’s speaker).

I’m wondering if the next candidate for replacement is the T6 transistor which is an AC188/01. I don’t have one of these so would have to order one. Would it be worth me swapping any other components so I can order them at the same time?

Thanks to everyone who has offered feedback – I really appreciate your help!