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Old 22nd Sep 2021, 8:43 pm   #1
poppydog
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Default Pye Q7 high current.

I have a pye q7 here that has a high current issue that I’m struggling a bit with.
When switched on it would draw a high current (nearly 100ma) for a couple of seconds, then slowly come back down, then start rising again til I decided it was time to switch off which was around 50ma.
The radio sounds very good and has bags of volume with no distortion.
I lifted the ends of a couple of elec caps and tested on Chinese tester and they were not good. I have now replaced all of the elec caps and a few out of spec resistors that I managed to meter up.

I am using the Pye manufacturers sheet so resistors I have replaced are R26,R24 and R20, according to the sheet R26 was supposed to be a 470r but there was a 1k in its place but was out of spec so a 470r is in its place.
Current on switch on after the replacement parts is still very high but rapidly drops to 23ma at idle and is stable, so I have made some progress, the sheet states it should be around 15ma at idle.
All the transistors are cool.

Voltages on all the transistors are almost as per sheet but there is around 0.5v on the emitters of the output pair. V4, the 1st AF amplifier has around 0.5v on its emitter and base and its collector has around 4v. It has been replaced with an AC128 but not by me.

How can I measure the current that the output pair are drawing without too much disturbance? They are the delicate oblong NKT types and I don't want to unnecessarily unsolder them.
There is a fair bit of chirping and whistling up one end of mw but it does pick up stations on mw and lw, wether this has any relevance to what else is going on, I don't know.

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Old 23rd Sep 2021, 10:35 am   #2
Herald1360
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Default Re: Pye q7 high current

To measure the output pair combined current, measure the value of their emitter resistor and use the voltage across that to calculate the current. Be wary of the lead resistance of your meter when looking at low ohms! Maybe add a 10R or so series resistance (precise value doesn't matter), note reading for that alone then for that plus emitter R and subtract first reading to get emitter R.

To reduce idling current you could drop the 39R or increase the 2k2. There's no idle current stabilisation other than a bit of dc NFB from the emitter R but if it's worked for 60 odd years it'll probably go on managing OK. You could set the idle current as low as it will go without adding noticeable crossover distortion.
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Old 23rd Sep 2021, 11:11 pm   #3
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Default Re: Pye Q7 high current.

Thanks for the reply Chris, I will do some measuring as you suggested and let you know how I get on.

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Old 27th Sep 2021, 7:32 pm   #4
poppydog
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Default Re: Pye Q7 high current.

I have switched on this evening and the current was very high again and it will not settle below 50ma and the voltage on the emitter of the output pair is now only 250mv, not sure whats going on.

Chris mentioned a series resistor to minimise the chance of getting an inaccurate reading due to the low ohm emitter resistor, this meant lifting one end of the resistor, so I just stuck the ampmeter in on the end of the lifted resistor rather than adding the series resistor and it read around 45.5ma.
I clearly have a problem with the output pair so I ended up removing them for testing, the pics below show they are not good.

I don’t have any nkt251 here so will have to see what I have here, NKT transistor s are fetching silly money.

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Old 27th Sep 2021, 10:26 pm   #5
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Default Re: Pye Q7 high current.

I have managed to find a fairly evenly matched pair of ac154 and have temporarily tacked them on the solderside of the board just to make sure everything is ok, and they are fine.

No large current on switch on, it draws 11ma at idle, it sounds good and the voltages on them are good. The biggest headache is going to be installing them in the heatsink and trying to get the leads in tidily etc, it is a bit crammed up that end of the board.

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