Clarke and Smith GP7 cartridge

[PaulR]

I have one of the above record players which has its original Acos GP91-3 SC cartridge.

I thought I would try a new BSR SC7M which I recently bought. I was surprised to find that the sound level is very low and upon researching it I have discovered that the Acos is a high output cart. I would have thought that with two stages of amplification a medium output cart would have been more appropriate. I wondered whether it is something to do with balancing the sensitivity with the mic input.

Has anyone successfully used a medium output cart with one of these record players? The best circuit diagram I can find is here https://www.vintage-radio.net/forum/...1&d=1333488230

[Techman]

It's a bit strange. Perhaps the old Acos cartridge is a little down in output due to age (although I've got one in a player and it still performs well) and the BSR cartridge is in fact faulty - have you tested it in anything else?

Edit: although you say that the Acos is the original.

[Edward Huggins]

Yes, the Acos GP91-3sc was always going to be the wrong cartridge for this record player so it is not the original. As you say with all those stages of amplification it's designed for a medium-output cartridge. There must be some loss of gain in the AF stages of the amplifier. Maybe low DC, coupling/cathode caps or valve/s being down. Ideally you can cheaply buy one of those Dentsu Clone "Red" Chinese stereo cartridges (in your case wired for Mono) and these do perform very well.

[PaulR]

I assumed that the Acos was original. It was the one fitted when I bought the player. There is even a sticker saying that the Acos cartridge fitted will play stereo records. I have replaced some electrolytics and several resistors but not the other caps as they are all Mullard mustard ones.

I actually have two BSR cartridges and they are the same. They are both fitted into standard Garrard heads so I will try them in another player tomorrow. I will see whether I have some spare valves to try. I have been using the mic input with a Rogers SFM tuner and there is plenty of gain on that.

[PaulR]

I wonder whether the history might be that it was originally fitted with a mono cartridge then upgraded as part of a mass upgrade of schools record players by the local authority. For whatever reason they decided on this cart and decreased the sensitivity of the amp in order to fit it. That might explain the sticker drawing the user's attention to the fact that stereo records could be played on it.

I will compare the amp to the circuit diagram.

[kalee20]

I remember us having a C & S record player, from new (bought via Merioneth Education Authority about 1967), this had a Garrard SP20 record deck and I seem to recall an Acos GP91-1SC cartridge. I don't believe it was the 3SC.

There's certainly a difference in gain between the Mic input and the record player - the mic didn't go via the tone control networks.

The circuit diagram looks almost correct, but I can't remember an EF86 being in there! As far as I know, it was the GP7 amplifier that was fitted, EZ80, 2xECL82, ECC83 and that's it!

[PaulR]

This had an SP20 when I received it but I had an SP25 going spare so I fitted that. I have just had a look for any dated components but cannot find any. Very mysterious!

[Edward Huggins]

Originally Posted by kalee20

I remember us having a C & S record player, from new (bought via Merioneth Education Authority about 1967), this had a Garrard SP20 record deck and I seem to recall an Acos GP91-1SC cartridge. I don't believe it was the 3SC.

There's certainly a difference in gain between the Mic input and the record player - the mic didn't go via the tone control networks.

The circuit diagram looks almost correct, but I can't remember an EF86 being in there! As far as I know, it was the GP7 amplifier that was fitted, EZ80, 2xECL82, ECC83 and that's it!

The ACOS GP91-1sc would have been the correct cartridge for the amplifier in this. An ACOS GP-93sc would have overloaded the first stages.

[PaulR]

Thanks all.

I am assuming that it should basically follow the circuit diagram above except that the EF86 is omitted leaving the output from the tone controls together with that of the mic volume control to be connected to the control grid of the first ECL82.

In fact I have found that there is a resistor in the region of 500k in the lead from the gram volume control to the grid, pin 2, of the ECC83 and the grid leak resistor is also in the region of 500k instead of 1m. I may be wrong but I assume this would form a potential divider and attenuate the output from the cartridge by around 1/2. There is also something odd regarding the connections for the mic socket but I am still figuring that out!

Nickthedentist kindly sent me some photos of his a while ago in a PM but I seem to have had a clear out of older PMs. I will contact him to ask if he still has them and can forward them again

[PaulR]

Thanks all.

Mine is also the GP7 version omitting the EF86. I thought I had found the reason for the attenuation of the gram output as there is a 470k resistor in the lead from the wiper of the volume control and the anode leak resistor is also 470k instead of 1m shown on the circuit diagram. I assumed that this was acting as a potential divider and halving the voltage from the cartridge. However, it seems to be standard as Nick the Dentist kindly sent me some photos of his amp shown in this thread https://www.vintage-radio.net/forum/...d.php?t=145011 and the same resistors appear.

Back to head scratching!

Sorry, I thought I hadn't submitted the previous post and sent this one instead as I had found the photos. If a moderator could delete post 9, I would be grateful.

[PaulR]

The card which says the Acos cart will play stereo records has become detached from the woodwork and I can see from the printing on the back that it is probably part of the box which contained the cart now fitted.

I have been playing it again with the BSR cart and the output is actually acceptable at full volume - I would have expected it to raise the roof at that. The sound is rather better than with the Acos. I wonder now whether it has always been like that and someone bought the replacement cart to get more volume out of it in a school hall.

The circuit isn't like the one above. Following the leads the one from the wiper of the mic volume control seems to go to the same connection as the cathode resistor of the gram section of the ECC83, there is nothing else connected to that tag. How it actually works is rather beyond me, but the mic input at least works very well. Looking at Nick's photo his seems to be the same. All rather beyond me!