Sound Project TA6000 FM Stereo problem

[PaulR]

I bought the above receiver a few weeks ago for next to nothing. Mine is labelled "Pye" but I think they were made by Philips.

It is really well made and looks good although it is just too wide to fit a standard 19" shelf.

It sounds really good but FM stereo is iffy. The indicator usually does not light even on strong signals but occasionally it does light even on weaker signals. Stereo isn't working when the indicator does light. Turning the selector switch round back and forwards several times seems occasionally to make stereo work.

If the receiver looked to be poor quality I would just consign it to the loft but it seems a shame not to attempt to get it working as it looks so good. I appreciate that FM stereo is complex and not easy to repair but I was wondering whether anyone who knows about these things could suggest anything I could try to sort it out. I found a circuit diagram at the Radio museum.

[Edward Huggins]

I think these were built by a Philips subsidiary in Scandinavia. There were EKCO versions. These are well well designed and built with above-average FM performance. Look for dry-joints or track-breaks on the Decoder board.

[PaulR]

Thanks Edward, I will do that. It sounds great even on FM with no stereo. I am just listening to some records on it.

It's just a pity it isn't 1/2 inch narrower as it won't fit into my hi-fi unit

I have just remembered I have a Philips PM6455 on a shelf. I can look for the stereo response using that rather than an aerial

Edit - I have just seen that I said in my first post that stereo doesn't work when the lamp is lit. I meant to say that it doesn't work when the lamp isn't lit - i.e. it isn't a simple problem with the lamp

[PaulR]

I have looked over the boards and there is nothing obvious to see. The soldering is shiny and looks above reproach. It has been stored in damp conditions and the underside of the board was very dirty. I cleaned if off with a small brush and IPA but it hasn't helped the situation. On further examination the FM section seems to have some sort of intermittent fault as the signal strength meter dips then recovers.

The leads connecting the boards together use the "wrapping method" rather than being soldered so I think I will try soldering them to ensure a good connection.

The switches are unusual (to me at least) as they appear to be ranged along a long shaft which runs in a case along the top of the board. Unfortunately it isn't possible to access this easily as the board containing the tuning cap and dial pulleys is mounted on top and so the tuning cord would be disturbed by removing it. I have squirted in some switch cleaner as best I can. I am attaching a couple of photos to explain this better.

[Goldie99]

The switches are pcb mounted rotary slide switches - the long shaft simply transfers the rotary motion from the front control knob to a small gear mechanism within the switch, translating the rotary motion to operate what is actually a slide switch.

Unfortunately, the internal geometry is such that it is almost impossible to get cleaners to the contacts, even using spray cleaners. Depending upon what the switches actually control, e.g., if them seem related to the fault(s), you may need to remove & completely dismantle them, to clean them properly.

Alan

[PaulR]

Thanks Alan.

It may well be the switches in view of the conditions under which it seems to have been stored. However, I can see myself getting into all sorts of trouble if I remove that tuner board in order to access the switches. I think I will try soldering the connections just in case that is the problem and if not I will put the covers back on and put it in the attic.

[PaulR]

I managed to get the board holding the tuning caps off. This was made easy because a bracket to "park" the main tuning cord pulley/drum is provided. I wondered what it was for when I first took the cover off then it dawned on me that it was to hold the the drum to keep the string in place whilst the board is loose. It is to the right of the tuning caps in the second photo above. Quite a thoughtful addition.

I haven't managed to diagnose anything because no amount of flexing, tapping or rapidly turning the switch control makes any difference so I am now thinking that there is a faulty component somewhere. When I apply a constant signal from the Philips PM6455 signal generator the signal strength meter keeps dipping briefly and the stereo beacon only lights if I repeatedly turn the switch off FM and back again. It goes off when the tuning indicator dips and tends not to come back on again. Even when on if I switch either channel off on the signal generator the sound still comes through on that channel so there is very little stereo separation.

The complexity of the circuit is much beyond my experience I am afraid.

[PJL]

Wire wrap connections are as good or better than solder joints.

Have you tried the 6455 on a known working stereo receiver?

This uses a PLL stereo decoder chip but the circuit does not indicate which one, can you identify the part number?

There are voltage values for the pins of the stereo decoder chip on the circuit, you could check these.

[PaulR]

No, I have never actually used the 6455. It just looked nice and I have a soft spot for test equipment! My latest acquisition is a Philips PM 5324! I will check it against another set.

In the meantime here are the pin voltages (it's a HA1137W)
1 1.96
2 1.95
3 1.96
4 0
5 0.3
6 5.19
7 4.9
8 5.56
9 5.51
10 5.57
11 9.67
12 0.01
13 1.61
14 0
15 4.78
16 0

15 stands out as it is grounded according to the circuit diagram

[PaulR]

Actually pin 15 isn't grounded on the circuit diagram. It is floating on the PCB. I think the circuit diagram shows it as 0v but it is difficult to maker it out conclusively

[Goldie99]

My radio diagnostic skills are almost certainly worse than yours, so I'll leave that to others. In the mean time, it may be worth a look at the HA1137W datasheet - that also has typical circuits / voltages etc., that may be useful.

https://datasheetspdf.com/datasheet/HA1137W.html

Alan

[PaulR]

Thank you, Alan

[PJL]

That IC is the FM IF and Demodulator. It is the stereo decoder chip we are looking for. If you search in google with the part number it will normally find a match and tell you the function.

[PaulR]

Apologies, I didn't read your message properly. The chip is an LA3350.

Voltages

1 12
2 3
3 5.72
4 6.54
5 6.67
6 10.37
7 0
8 0.49
9 6.47
10 0.39
11 2.2
12 2.14
13 1.9
14 2.15
15 2.13
16 2.9

Thanks

Paul

[PaulR]

PS Pin 6 drops to 0.7v to light the beacon when the stereo signal is detected.

[PaulR]

I have managed to sort it out. It was potentiometer. I had tried moving them slightly but it seemed to make no difference. This afternoon I decided I had nothing to lose so I squirted switch cleaner on them and turned them from end to end, mentally noting where they had been set. One made a difference so I worked on it and it is much improved now. I probably should replace it with a new enclosed type.

I have ordered the rather unusual 3 pin IEC 240 plug for the aerial socket.

[PJL]

Congratulations on fixing it. You are referring to the trimmers? One of them probably sets the decoder oscillator free run frequency, it needs to be close to the correct value to phase lock with the incoming signal.

[PaulR]

Thanks! It was a potentiometer rather than what I would think of as a trimmer. I couldn't make it out the number very easily. It was R3** but the number was crowded out by other components. I need to take the cover off again as the pointer isn't reading correctly after I removed the tuning cord drum so I will try to see what it was.

[PaulR]

It was R351

[PJL]

That maybe setting the stereo mute on low signal but I cant find a datasheet for the LA3350.