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Old 6th Feb 2017, 2:10 pm   #1
Colourstar
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Default RGD RP231 Transistor record player

Out of curiosity I thought I would spend some time on this tatty and neglected record player. It dates from around 1964 and has a transistor amp comprising two OC75s acting as pre-amp and driver for the AC128 push-pull output stage.

The amp works to an extent but is distorted. Taking some voltages provided readings that were all over the place. The first OC75 (pre-amp) had in the past been replaced by a BC154, along with two outsize resistors, all tacked onto the print side of the pcb. I removed all of these and remounted the transistor correctly, along with two more suitably sized resistors. I've changed the electrolytics, which I thought would be past their best, with the exception of C5, for which I don't have a replacement to hand.

Mike's excellent KB Museum website has a copy of the official manual for the player and the link below is provided with his kind permission, although if this flouts any forum rules then of course it should be removed.

file:///C:/Users/760/Downloads/STC_PC1_2_3_4_crts.pdf

The 24v supply reads a shade high at 28v, after which the transistor voltages are all over the place. I've put the actual reading in brackets after the figures from the service info:

TX1 Pre-amp OC75 (replaced with a BC154)

C 11.6v (5.3v)
B 4.9v (2.0v)
E 4.8v (4.1v)

TX2 Driver OC75

C 14.2v (6.5v)
B 2.3v (0.8v)
E 2.2v (0.7v)

And finally the output pair, for which no voltages are noted on the circuit

TX3 Output AC128

C (28.4v)
B (13.2v)
E (13.8v)

TX4 Output AC128
C 12v? (13.8v)
B (0.1v)
E (0v)

I'm not overly familiar with transistor circuits, beyond knowing that the base should be about 0.6v higher than the emitter, which suggests that TX2 might show some promise. Beyond that I'm a little lost, so any help would be appreciated!

Steve
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Old 6th Feb 2017, 2:21 pm   #2
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Default Re: Rgd rp231 Transistor record player

Most of those readings are about half of what they should be. I'd check the resistors.
Are any of the transistors connected to the same heatsink or grounded metalwork?

Odd readings may also be due to cracked print or lifted tag on one of the transistors - had this myself with a player of similar vintage just last week. It looked fine on cursory inspection but was loose.

Had one of those round-headed BC114s fail (intermittent) last month in a Thorn tape machine! Replaced with a BC548.
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Old 6th Feb 2017, 4:51 pm   #3
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Default Re: RGD RP231 Transistor record player

Hi Steve
I have found that the two 50 ohm bias setting trim pots very often go out of spec. They always seem to go high.
The service manual says use 2 ammeters, one across link A and one across link B.Click image for larger version

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Then adjust each for 2.5mA on both meters.
I strongly suggest using 2 meters as adjusting one trim pot alters the value on both.

An alternative way is use 2 DMM set to mV range and measure the voltage across the two 4.7 ohm resistors R22 & R23. This should be approximately 12mV when the current is set correct. Using this method helps prevent the coper track lifting from repeated removal of the wire links.

see also https://www.vintage-radio.net/forum/...6&postcount=30

Mike

p.s. correction of the link to the KB / STC service data; http://kbmuseum.org.uk/other_service...2_3_4_crts.pdf
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Last edited by crackle; 6th Feb 2017 at 4:56 pm.
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Old 6th Feb 2017, 7:26 pm   #4
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Default Re: RGD RP231 Transistor record player

Some very good pointers there. As an initial step I will get the magnifier out and check the print, as the pcb isn't great quality. Those two pots may be under suspicion too.

Could the oddly high figure on the emitter of the BC154 be caused by the biasing being wrong, with the circuit being designed for an OC75? That may need looking at, but I need to get some semblance of normality to the general readings first I think.

Steve
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Old 6th Feb 2017, 7:41 pm   #5
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Default Re: RGD RP231 Transistor record player

Try taking out TX1 to see if this restores the correct voltages to TX2 as it could be TX1 is faulty or round the wrong way. Is the voltage being pulled down or are one of the feed resistors R9, R16 faulty leading to a lower voltage due to high resistance, might explain the higher DC voltage at the rectifier. If you haven't changed C5 try lifting one end in case it is leaky and pulling the rail voltage down.
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Old 6th Feb 2017, 9:22 pm   #6
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Default Re: RGD RP231 Transistor record player

Quote:
Originally Posted by Colourstar View Post
I'm not overly familiar with transistor circuits, beyond knowing that the base should be about 0.6v higher than the emitter, which suggests that TX2 might show some promise. Beyond that I'm a little lost, so any help would be appreciated!
Remember that that 0.6V is for silicon transistors. For Germanium it's more like 0.1-0.2V.

Any transistor where the base is lower than the emitter is not functioning correctly for some reason- either it's faulty or something else round it is.

The voltages around TX1 in the service sheet don't make any sense unless they're significantly affected by the meter used to measure them, but I would expect a silicon device should work fine there- its collector current would be a bit lower than for the OC75 but not seriously so.
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Old 7th Feb 2017, 8:49 pm   #7
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Default Re: RGD RP231 Transistor record player

Quote:
Originally Posted by crackle View Post
I have found that the two 50 ohm bias setting trim pots very often go out of spec. They always seem to go high.
Out of interest I removed the 50 ohm trimmers on the RGD- one reads 318 ohms, the other a whopping 694 ohms! No wonder the thing sounds sick. It looks like this may be the heart of the problem as I can't find any out of tolerance resistors or dodgy caps.

I shall source some new trimmers and go from there I think.

Thanks again to everyone for the contributions so far.

Steve

Last edited by Colourstar; 7th Feb 2017 at 8:55 pm.
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Old 8th Feb 2017, 7:14 am   #8
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Default Re: RGD RP231 Transistor record player

Hi Steve
Before I realised the trim pots were out of spec. I spent literally hours trying to set them. Then after a day or so the amp was distorting again.
I strongly advise using two meters to set the new ones up.

Mike
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Old 9th Feb 2017, 4:31 pm   #9
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Default Re: RGD RP231 Transistor record player

Hi Mike

Will do!

I see that Maplin have some ideal 50 ohm presets, so I'll grab a couple when I next have the chance and report back.

Steve
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Old 13th Sep 2017, 8:18 pm   #10
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Default RGD RP231 Revisited

After a lengthy hiatus I've only just got round to fitting the replacement trimmers to this tr*ns*storized record player. With two digital multimeters I set the trimmers to give approx 12mv across R22 and R23 as suggested by Mike (Crackle).

The amp sounded reasonable but after a short while a nasty niff reached my nostrils and I discovered that the secondary winding on the motor was cooking up and had become too hot to touch. In common with several other record players, this supples a voltage which is rectified to provide a DC supply for the amplifier.

Even with the amp supply disconnected the winding still heats up. Out of interest I measured the resistance of the winding. It reads 4.6 ohms against the 3 ohms stated in the service info. Does the evidence suggest shorted turns?

Steve
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Old 13th Sep 2017, 9:41 pm   #11
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Default Re: RGD RP231 Revisited

Hi
When you disconnected the amp, did that also disconnect the rectifier and smoothing capacitor from the motor winding?

Mike
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Old 14th Sep 2017, 7:32 am   #12
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Default Re: RGD RP231 Revisited

Quote:
Originally Posted by Colourstar View Post
It reads 4.6 ohms against the 3 ohms stated in the service info. Does the evidence suggest shorted turns?
Those figures suggest meter lead resistance rather than shorted turns!

As said, a test with the overwind isolated is needed for shorted turn confirmation.

If it's really an overwind, removing it shouldn't be too difficult. A small separate TX could replace it, if it's too awkward to rewind.
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Old 14th Sep 2017, 5:44 pm   #13
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Default Re: RGD RP231 Revisited

Also remember that shaded pole motors (which I assume this is) naturally run hot -- the copper 'shading loops' are effectively shorted turns.
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Old 15th Sep 2017, 7:54 pm   #14
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Default Re: RGD RP231 Revisited

Hello all,

I disconnected the secondary motor winding from the input side of the rectifier so there's no power to the amp at all.

The winding gets so hot the binding tape around it starts to make a bubbling noise. Somewhat alarming!

Certainly an alternative power supply could be rigged up if the present arrangement is beyond help.

Steve
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