Hacker Centurion GA550 - UK Vintage Radio Repair and Restoration Discussion Forum

Herald1360 · 23rd Mar 2017, 1:40 pm

Posting here since it's the radio section that appears to be misbehaving.

Initial turn on not promising- lots of pops and crackles and a dominant 100Hz hum. Opening up revealed a very sad looking 4700uF main smoother that had been sick on the floor. Disconnected the PSU section and fed in 30 odd volts from a bench PSU- no hum and apart from a few popping noises it appeared to be working. The volume control was quietened down (but not completely so) by a squirt of Deoxit and some exercising.

Jumping to the obvious conclusion saw a nice new C in the PSU, unfortunately the hum was still there, though only when radio selected. Humph!

So far a quick probe at the dc supply shows 36V or so with about 1V p-p ripple- maybe a bit high? I remembered that the bench supply was showing a current of about 600mA and a quick simulate suggested that this is consistent with the ripple level seen, but..... is it a reasonable quiescent current for the set, I wonder? I suspect not.

That's where I am for now, bright ideas welcome- I'll be looking to see where the mA are going this evening.

Another question- there's a mysterious big green rectangular component on the stereo decoder board (a CA758 PLL incarnation of some sort that Hacker were unforthcoming about in the service data). Anyone know off the cuff what it is?

mhennessy · 23rd Mar 2017, 4:12 pm

The green thing is an MPX filter... It's mentioned in the GA500 mk2 manual.

The Iq is stupidly high in these things. 150mA per channel IIRC. It can be turned down a bit to cool things down somewhat - just keep an eye on the THD.

The supply voltage should be 42V with a GAR550.

I'm not aware of any "stock faults" that could cause hum on the radio only, but I'm sure you'll soon get there. Any number of bad connectors or capacitors could be the cause. To add to the "fun", there appears to be some errors on the drawings in the area of the decoder. Enjoy!

Herald1360 · 24th Mar 2017, 2:25 am

After a bit of midnight oil I've concluded that the hum is coming from the stereo decoder. Voltage on the CA758 supply pin is around 40V

. Looks like that's dead, then.

I don't think much of the decoder supply arrangement- it seems to rely on the chip current dropping the supply volts via a series 330R resistor which seems to be a flaky way of doing it.

I've traced out the decoder PCB circuit- it bears a reasonable resemblance to the RCA application circuit plus that MPX filter on the audio outputs.

Unfortunately the chip seems to cost anything from £10 upwards from anywhere that doesn't look dodgy. Any kind soul got a CA758E/LM1800/MC1311P decoder going spare?

23rd Mar 2017, 1:40 pm	#1
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Hacker Centurion GA550 Posting here since it's the radio section that appears to be misbehaving. Initial turn on not promising- lots of pops and crackles and a dominant 100Hz hum. Opening up revealed a very sad looking 4700uF main smoother that had been sick on the floor. Disconnected the PSU section and fed in 30 odd volts from a bench PSU- no hum and apart from a few popping noises it appeared to be working. The volume control was quietened down (but not completely so) by a squirt of Deoxit and some exercising. Jumping to the obvious conclusion saw a nice new C in the PSU, unfortunately the hum was still there, though only when radio selected. Humph! So far a quick probe at the dc supply shows 36V or so with about 1V p-p ripple- maybe a bit high? I remembered that the bench supply was showing a current of about 600mA and a quick simulate suggested that this is consistent with the ripple level seen, but..... is it a reasonable quiescent current for the set, I wonder? I suspect not. That's where I am for now, bright ideas welcome- I'll be looking to see where the mA are going this evening. Another question- there's a mysterious big green rectangular component on the stereo decoder board (a CA758 PLL incarnation of some sort that Hacker were unforthcoming about in the service data). Anyone know off the cuff what it is? __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

23rd Mar 2017, 4:12 pm	#2
mhennessy Dekatron Join Date: Jul 2007 Location: Evesham, Worcestershire, UK. Posts: 4,244	Re: Hacker Centurion GA550 The green thing is an MPX filter... It's mentioned in the GA500 mk2 manual. The Iq is stupidly high in these things. 150mA per channel IIRC. It can be turned down a bit to cool things down somewhat - just keep an eye on the THD. The supply voltage should be 42V with a GAR550. I'm not aware of any "stock faults" that could cause hum on the radio only, but I'm sure you'll soon get there. Any number of bad connectors or capacitors could be the cause. To add to the "fun", there appears to be some errors on the drawings in the area of the decoder. Enjoy!

24th Mar 2017, 2:25 am	#3
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Hacker Centurion GA550 After a bit of midnight oil I've concluded that the hum is coming from the stereo decoder. Voltage on the CA758 supply pin is around 40V . Looks like that's dead, then. I don't think much of the decoder supply arrangement- it seems to rely on the chip current dropping the supply volts via a series 330R resistor which seems to be a flaky way of doing it. I've traced out the decoder PCB circuit- it bears a reasonable resemblance to the RCA application circuit plus that MPX filter on the audio outputs. Unfortunately the chip seems to cost anything from £10 upwards from anywhere that doesn't look dodgy. Any kind soul got a CA758E/LM1800/MC1311P decoder going spare? __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O