Clarke and Smith GP7 motorboating - UK Vintage Radio Repair and Restoration Discussion Forum

PaulR · 22nd Mar 2018, 4:49 pm

I started a thread asking for a circuit diagram for this record player a little while ago but none seems to be available. It is the one with the ECC83 and 2 x ECL82.

When I bought the record player I found that several of the electrolytic caps would not re-form so I decided to replace them all. It worked after that but not as well as I expected so I went over the resistors changing those much more than 20% out. After I had done the ones associated with the ECL82s I tried it and it seemed ok although I didn't apply a signal to it. I then went on to do those on the ECC83 and when I had finished it started motoboating when I tried it. I assumed that I must have used a resistor of the wrong value somewhere, hence my request for a circuit diagram. In the absence of this Nickthedentist kindly sent me some photographs of his and comparing those with mine I cannot see anything wrong with my work on it.

Can anyone help with sorting it out? I thought this would be a very straightforward restoration but this has proved not to be the case. I am attaching some photos of Nick's and mine for comparison.

Thank you

Nuvistor · 22nd Mar 2018, 5:04 pm

Check the earth screws on the bus bars, if there is corrosion under the tags it can cause instability.

Silicon · 22nd Mar 2018, 7:19 pm

Did you test the new components before you soldered them into the circuit?

PaulR · 22nd Mar 2018, 8:01 pm

Thank you both

The bus bar is secure and I did test the resistors but not the caps.

However. I found that the loudspeaker impedance selector was set at 15 ohms. Changing it to 3.75 ohms seems to have stopped the motorboating. I do now remember turning it at one point and I must have left it in the wrong position. I haven't tried it with a signal yet but I get a healthy buzz when I touch the input pin.

I wouldn't have thought that it would have made such a difference.

Nuvistor · 22nd Mar 2018, 8:38 pm

Is there a feed back circuit from the secondary of the output transformer? Having it set wrong could introduce phase changes or affect the amount of feedback.

It shouldn’t be so sensitive though, well I wouldn’t have thought so.

PaulR · 22nd Mar 2018, 9:23 pm

I am not sure about a feedback circuit, but it feels to be something like that.

I have just put it back into its case and played a record. It sounds a lot better for having the resistors changed. They are probably nearer to the correct values than they have ever been. Such a shame I moved that switch! It would be interesting to know whether it has such an effect on anyone else's player.

PaulR · 23rd Mar 2018, 10:46 am

I tried it again this morning and the turntable only worked intermittently. I took the cover off the switch to find that the "stator" part of the switch had broken into two - another thing I have never seen before!

Does anyone have a scrap Garrard Autoslim - type switch they can let me have? Just the non-moving contact would be all I need.

Thanks

22nd Mar 2018, 4:49 pm	#1
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,233	Clarke and Smith GP7 motorboating I started a thread asking for a circuit diagram for this record player a little while ago but none seems to be available. It is the one with the ECC83 and 2 x ECL82. When I bought the record player I found that several of the electrolytic caps would not re-form so I decided to replace them all. It worked after that but not as well as I expected so I went over the resistors changing those much more than 20% out. After I had done the ones associated with the ECL82s I tried it and it seemed ok although I didn't apply a signal to it. I then went on to do those on the ECC83 and when I had finished it started motoboating when I tried it. I assumed that I must have used a resistor of the wrong value somewhere, hence my request for a circuit diagram. In the absence of this Nickthedentist kindly sent me some photographs of his and comparing those with mine I cannot see anything wrong with my work on it. Can anyone help with sorting it out? I thought this would be a very straightforward restoration but this has proved not to be the case. I am attaching some photos of Nick's and mine for comparison. Thank you Attached Thumbnails __________________ Paul

22nd Mar 2018, 5:04 pm	#2
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Clarke and Smith GP7 motorboating Check the earth screws on the bus bars, if there is corrosion under the tags it can cause instability. __________________ Frank

22nd Mar 2018, 7:19 pm	#3
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,163	Re: Clarke and Smith GP7 motorboating Did you test the new components before you soldered them into the circuit?

22nd Mar 2018, 8:01 pm	#4
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,233	Re: Clarke and Smith GP7 motorboating Thank you both The bus bar is secure and I did test the resistors but not the caps. However. I found that the loudspeaker impedance selector was set at 15 ohms. Changing it to 3.75 ohms seems to have stopped the motorboating. I do now remember turning it at one point and I must have left it in the wrong position. I haven't tried it with a signal yet but I get a healthy buzz when I touch the input pin. I wouldn't have thought that it would have made such a difference. __________________ Paul

22nd Mar 2018, 8:38 pm	#5
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Clarke and Smith GP7 motorboating Is there a feed back circuit from the secondary of the output transformer? Having it set wrong could introduce phase changes or affect the amount of feedback. It shouldn’t be so sensitive though, well I wouldn’t have thought so. __________________ Frank

22nd Mar 2018, 9:23 pm	#6
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,233	Re: Clarke and Smith GP7 motorboating I am not sure about a feedback circuit, but it feels to be something like that. I have just put it back into its case and played a record. It sounds a lot better for having the resistors changed. They are probably nearer to the correct values than they have ever been. Such a shame I moved that switch! It would be interesting to know whether it has such an effect on anyone else's player. __________________ Paul

23rd Mar 2018, 10:46 am	#7
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,233	Re: Clarke and Smith GP7 motorboating I tried it again this morning and the turntable only worked intermittently. I took the cover off the switch to find that the "stator" part of the switch had broken into two - another thing I have never seen before! Does anyone have a scrap Garrard Autoslim - type switch they can let me have? Just the non-moving contact would be all I need. Thanks __________________ Paul