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23v (not mV!) of DC offset on Amstrad IC2000

MunkeyQ

MunkeyQ

Super Member
Today I scored a little Amstrad IC2000 amplifier Mark III from a bin, so I got it home, fired her up and checked the DC offset before connecting it to speakers. There's a very unhealthy 23v on both channels. :jawdrop:

The transformer is a single-winding non centre tapped thing which puts out 49v after smoothing and rectification. Since 23v is roughly half of 49v, I'm guessing the output transistors have shorted.

Before I pull them out to test, does anyone have any other suggestions? Any advice would be much appreciated - thanks.
 
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This is why god gave us fuses on the eighth day. Are there none present or did somebody put bigger ones in? Your assessment seems correct so far. Does this amp pre-date self-protection era? No relays, no current limiting?
--Tom in Tucson
 
Ah, good point about the fuses. I just checked them and they're intact and of the correct rating.

I forgot to add that there's no self-protection relays, current limiting of anything of the sort.

Thanks for the input.
 
Ok, I've hit another hurdle here - the transistors are marked (from top to bottom) "17300" "RCA" "RHG 536"

I can't find any information on these via Google to find an equivalent one to replace these. Anyone know what I can use to replace these?

Thanks.
 
Are all the outputs identical? If so, you might assume this is a quasi complimentary design where the outputs are all NPN and the drivers are mixed PNP and NPN. Further dangerous assumption is that a person could start with the common-as-dirt 2N3055 standard issue NPN output device. But of course the drivers possibly fried too so don't waste money replacing outputs and then blowing them in the first millisecond with bad old drivers. I should have clarified: are these fuses in the output path or more toward the power supply end of things? If in PS area, they are in the wrong place at the wrong time to do any good.
I mean no offense with this question but how much electronic circuit knowledge do you have? Willy-nilly parts changing is a costly and non-productive approach unless you have a clear hypothesis and are using test equipment. --Tom
 
You may try hooking a junk pair of speakers to them with no volume and then remeasure offset.
If it's anything like cap coupled Marantz units they will show high voltage until they have a load.
Could be the case as they both measure the same.
Not familiar with that brand myself.
 
No need...connect a 100 ohm 1/4W resistor. If it burns up, the unit is NOT cap-coupled, and connecting any speaker will be the end of said speaker.
 
I put a 100 ohm resistor across the output as Echo said, and it got very got within a few seconds of being switched on. There's a lot of current here...turns out it's not cap-coupled.

The output transistors are paralleled pairs of RHG 536's and the drivers are indeed mixed pairs of NPN and PNP - a BD138 and BD137. Board layout is dual-mono, which makes troubleshooting a tad simpler.

As for my experience, I'm nowhere near as good as most of the older members on this board but I've seen and repaired quite a few amplifiers.

Amstrad were a big British personal computer manfacturer back in the late 70s, and this was their attempt at hi-fi. They only made the IC2000 and two models of speakers - all pretty budget gear but fairly well made. From the same bin as the amp I also fished out a pair of Amstrad speakers which despite being in need of a refoam, are surprisingly good.

Thanks for all the help.
 
I tried to find data sheets on these transistors; only the drivers came up. The 7 is NPN and the 8 is PNP. Do you have a variac dial transformer? If not, beg borrow or steal one. It is indispensable for this sort of repair. If it were me faced with so few good options, I would buy a pair of 2n3055 and put them in one channel making sure pinout matches the originals. Data sheet for them is widely available. Remove all PS connections from the side with blown parts and ramp up the variac slowly with a resistor and voltmeter on the output. You might get lucky. I am happy to talk off-forum with you if it's easier. thomash85715@yahoo.com --Tom
 
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