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Audio & Hi-Fi 1970/1 HMV 2025 Record Player (Yuck)

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Cathovisor
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Posted by: @jskinner97

So am I looking in the output stage? 

I'm not sure - I'd certainly be looking at what's common to both amps.

 
Posted : 15/06/2022 8:59 pm
Jamie
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image

@cathovisor I wonder if even a dry joint could cause it…

 

this is something I’m scanning the diagram for but frankly there seems to be very little in common. Both channels are entirely separate, very odd indeed…

 

I wonder if the socket could be arsing about . Maybe it has a limiting effect? Say if you wanted to record onto tape etc for obvious reasons you wouldn’t want feedback 

 
Posted : 15/06/2022 9:02 pm
Jamie
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Specifically I’m thinking R3,4,5,6

 
Posted : 15/06/2022 9:08 pm
PYE625
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Is it possible to isolate the supply to either channel and power them seperately? It could be a fault in one channel causing excessive current drain from the PSU. You could power them by an external bench PSU being careful to limit the current in case there is a possible short in one of the amplifiers.

The only way to find this is by process of elimination of each section of the whole item.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 15/06/2022 9:16 pm
PYE625
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Maybe a safer option to save damage to any expensive AC128's etc by the possibility of too much current from a bench PSU, would be to simply power the whole thing via a low wattage mains lamp limiter. This should give an indication, along with voltage measurements, of a possible breakdown of a transistor causing the supply to drop resulting in low volume.

Looking at the DIN socket in the above diagram, it is hard to see how it could be causing the fault on both channels at the same time simply by powering up/down.

 

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 15/06/2022 9:46 pm
Jamie
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Posted by: @pye625

Maybe a safer option to save damage to any expensive AC128's etc by the possibility of too much current from a bench PSU, would be to simply power the whole thing via a low wattage mains lamp limiter. This should give an indication, along with voltage measurements, of a possible breakdown of a transistor causing the supply to drop resulting in low volume.

Looking at the DIN socket in the above diagram, it is hard to see how it could be causing the fault on both channels at the same time simply by powering up/down.

 

I was wondering if it may cut the audio from the speakers for some reason or rather... perhaps to a lower level to record onto tape maybe? Otherwise the OP may be too high for a tape recorder... that's my thinking anyway. 

 
Posted : 16/06/2022 9:39 am
Jamie
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Ok so, the set will not operate without the gram plug plugged in… the gram motor has 4 wires going to it (two white blue and brown) one thing I noted is removing the two white wires I gain full amplitude briefly before it goes to nothing (presumably this is the regulator detecting nothing!)

 

one white goes to the fuse the other goes to the rectifier. Apologies if I’m rambling somewhat… The motor seems a tad intermittent, could this be causing excessive draw? Only problem is the set refuses to allow me to disconnect it, presumably the regulator needs to detect some resistance to operate. 

I believe the two white wires are 18V AC the motor runs continuously 

83D379AF E0C3 4213 AA4E 19892CDE44BA
5925031E 0281 4554 9AD6 76C40F98720C
37DCF08E 79E0 4D52 B06E CE611800F4EF

 

 
Posted : 16/06/2022 11:50 am
Cathovisor
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The two white wires are the 18V overwind on the motor as you correctly deduced and as it has a tape input, the motor runs all the time to allow this. My first stereo record player was a Bush SRP58 that had no such niceties so when the deck shut off, power to the amplifiers was removed! 

Do you have, or can you borrow, an oscilloscope? I'm still favouring instability as being the issue. As an alternative, can you measure the current the amplifiers are taking with no signal? This might be significant. 

 
Posted : 16/06/2022 3:50 pm
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Jamie
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Posted by: @cathovisor

The two white wires are the 18V overwind on the motor as you correctly deduced and as it has a tape input, the motor runs all the time to allow this. My first stereo record player was a Bush SRP58 that had no such niceties so when the deck shut off, power to the amplifiers was removed! 

Do you have, or can you borrow, an oscilloscope? I'm still favouring instability as being the issue. As an alternative, can you measure the current the amplifiers are taking with no signal? This might be significant. 

No scope, but I can do some current measurements. At "quiescent" it should be 42mA... Where's best place to measure this from? Another thing i've noticed, the metal "grounding" shield around the 4 OP transistors appears to be positive, instead of negative? Could the entire "ground" be inverted? 

 
Posted : 16/06/2022 3:53 pm
PYE625
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Posted by: @jskinner97

 Could the entire "ground" be inverted? 

It certainly is with respect to the set's PSU. Look at the rectifier circuit and the positive output is the ground. This is normal.

You could insert the multimeter in series with one of the AC feeds from the motor and look at the AC current, this will be an easy first impression of current draw. I would expect the total current something under 100mA at rest.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 16/06/2022 4:11 pm
Jamie
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@pye625 Ahhh I see, thats fine then. I will get some readings shortly

 
Posted : 16/06/2022 4:13 pm
Cathovisor
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@jskinner97 I just had a look at the photos you uploaded - I always thought it really tacky, sticking mains on a PCB!

 
Posted : 16/06/2022 4:28 pm
PYE625
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Mind you, if there was a serious short, F1 would likely blow as this is rated at 500mA.

Well, you might hope it should !

F1 is a great place to break the circuit and measure the current.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 16/06/2022 4:32 pm
Jamie
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Posted by: @pye625

Mind you, if there was a serious short, F1 would likely blow as this is rated at 500mA.

Well, you might hope it should !

F1 is a great place to break the circuit and measure the current.

Across the fuse (with the fuse removed) 33mA. 10mA less than the normal “rest” 

With the fuse in circuit measuring across the fuse, merely 5mA!

 
Posted : 16/06/2022 5:15 pm
Jamie
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C1 & C2 are fine. As is R29. R30 is missing on my circuit board. 

R16 is fine, R15 is fine 

I also removed the neon, wondered if that could be drawing too much but nope. 

 
Posted : 16/06/2022 5:58 pm
PYE625
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Oh so the 42mA is total current.... I assumed per channel.

This is becoming mighty strange then. All voltages check out ok I guess?

The fact that the current draw is 10mA down might suggest part of the circuits are not being supplied.

Looking at the balance control, if the wiper was not making good contact, this might cause some odd results. But I think you should carefully check all the published voltages on the circuit at every stage.

 

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 16/06/2022 5:59 pm
Jamie
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@pye625 my circuit is different so it’s hard to say. But on both VT1 (even tho I have TWO VT1’s!) I only have 2.2V so I’m missing the 7.25V reading if that’s any use 

 

balance control is fine 

 
Posted : 16/06/2022 6:12 pm
PYE625
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Well that is a start, missing or low voltage on both the collectors. Where does this supply come from and what is the base and emitter voltage?

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 16/06/2022 6:16 pm
Jamie
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Also just checked R11 & R12 which are in series with each other both within tolerant at 24k and 25k…

 

the base and emitter voltage is correct 

 
Posted : 16/06/2022 6:18 pm
Jamie
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R14 correct also…

 
Posted : 16/06/2022 6:23 pm
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