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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 11th Feb 2010, 6:20 pm   #1
tonewheelkev
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Default Cyrus 1 Amp Dead....Help!!?

Hi All
I have a Cyrus ! amp....issue 07....which just decided to die on me!
At the time of 'death', it was idling (connected to my PC...as it had been for some months!)
Now it just blows Main fuse, and won't come on at all.

It has been a great amp, and I'd like it back

I have a decent digital multimeter, and lots of patience,
....but I'm stuck

Any advice gratefully received

Kev
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Old 12th Feb 2010, 10:00 am   #2
valvekits
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Default Re: Cyrus 1 Amp Dead....Help!!?

Hi Kev
A good starting point might be to have a look round the net for schematics.
You might want to check out www.audio-circuit.dk where DIYers share their audio info.

Cheers
Eddie
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Old 12th Feb 2010, 10:19 am   #3
ajs_derby
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Default Re: Cyrus 1 Amp Dead....Help!!?

This situation -- amplifier blows fuse immediately on power up -- is almost always due to an output transistor failing short-circuit.

There should be 4 hefty transistors on the heat sink; they may be four the same, or two pairs of two the same. There are two for each speaker: one connected to the positive supply rail to push a current into it for the crests of the waves, and one connected to the negative supply rail to draw a current out of it for the troughs. If either of them fails short-circuit for any reason, the other one usually dies in the same way very soon afterward. This leaves a path between the supply rails which admits much more current than it's supposed to, and the power fuse blows.

Check the motherboard for any obviously-overheated components. If anything seems amiss, post back here with a picture. Otherwise, draw a diagram or take a photograph, so you can put everything back in the right place when you are done, and disconnect the output transistors from the motherboard. Now fit a fresh fuse and try switching on. If the fuse holds, that means the fault is indeed due to the power transistors; but don't leave the amplifier in this state for long, in case you damage something else.

Assuming it is a faulty transistor, look up the type number on the Internet (if it's just A, B, C or D and a number, try prefixing it with "2S") and find a data sheet. Now you know whether it's NPN or PNP; and which lead is the collector, which is the base and which is the emitter. Test each transistor with your multimeter.

If your multimeter doesn't have a transistor test function, then you can still do a simple test. It won't tell you much, but it will show whether a transistor is NPN or PNP and whether it is definitely faulty or possibly working. And if it is working, then you can also identify which terminal is the base. The collector is usually the one in the middle of a plastic package, or the actual body of a metal package.

Any transistor looks like two diodes joined together, as follows.
NPN: E ----|<|---- B ---- |>| ---- C (base = common anodes)
PNP: E ----|>|---- B ---- |<| ---- C (base = common cathodes)

Test between each pair of terminals on diode range (if your meter has one) or a low resistance range (choose one by testing a known good diode). If either of the "diodes" seems to conduct in both directions or not at all, or you see continuity from collector to emitter, the transistor is dead.

What you'll probably find is that 2 of the transistors are short-circuit collector to emitter and the other 2 are fine. Re-connect the two "workers", referring to your picture from before, and try the amp; you will only get sound in one speaker, of course, but it shouldn't blow the fuse this time. Again, don't run the amplifier without a full set of transistors for longer than necessary to test it.

Now, re-connect the two working transistors where the two dead ones came from, and apply power. If you are very lucky, the amplifier will work (in just that channel, obviously). Order four new transistors of the correct types (they're all the same age, so you might as well replace the two "good" ones and save yourself a job later), fit them and carry on enjoying the music. And treat yourself to a (If you can't get the exact replacements, ask on here for advice. Someone may well know of a suitable substitute.)

If the fuse blows again, that means there is another fault which caused the output transistors to fail in the first place. If you get bad distortion, this probably means that some alignment is required after replacing the output transistors. In either case, post back here and we'll take it from there.

Best of luck to you!
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Old 12th Feb 2010, 11:33 am   #4
tonewheelkev
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Default Re: Cyrus 1 Amp Dead....Help!!?

Thanks for your reply AJS....this is exactly what I need!

I downloaded a manual from the HiFi Engine site....
The 4 'hefty' transistors, according to component list on page 38, are:Style TO220, PT7, NPN Power transistors.

No obvious evidence of overheating..

The PT7 seems to be discontinued.....do I go to Mission for 'em........?

Will test when I get them off the board
Thanks again
Kev
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Old 12th Feb 2010, 11:53 am   #5
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Default Re: Cyrus 1 Amp Dead....Help!!?

Kev,
Download the circuits here
http://www.tcaas.btinternet.co.uk/index-4.htm
It shows a couple of fuses in the LT lines, are these OK?
If they have not blown, I would suggest taking a look around the powers supply, particularly the rectifiers and smoothing capacitors
Hope this helps

Mike
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Old 12th Feb 2010, 11:57 am   #6
John123
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Default Re: Cyrus 1 Amp Dead....Help!!?

I agree, make sure it's not a power supply fault. If you can isolate the PSU and see if it powers up OK without a load, then that's one thing out of the way.
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Old 12th Feb 2010, 3:01 pm   #7
ajs_derby
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Default Re: Cyrus 1 Amp Dead....Help!!?

Quote:
Originally Posted by tonewheelkev
The PT7 seems to be discontinued.....do I go to Mission for 'em........?

Will test when I get them off the board
Thanks again
Kev
"PT7" might well be Mission's internal code for a standard device. Even if you can't get actual PT7s but you can get a specification for the transistors (polarity we know, package style we know, bandwidth we can take a good stab at as it's an audio amp; voltage and current ratings and hFE {gain} we need) then it might be possible to locate a suitable equivalent.

I haven't got a Mission Cyrus myself; but I have fixed a few amps with S/C power trannies, and I'm sure at least one other forum member has one.
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Old 12th Feb 2010, 3:39 pm   #8
tonewheelkev
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Default Re: Cyrus 1 Amp Dead....Help!!?

Ta AJS

it seems that PT7 is equiv to BUV58
...and RS have them!!

still haven't tested them !
Kev
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Old 12th Feb 2010, 5:19 pm   #9
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Default Re: Cyrus 1 Amp Dead....Help!!?

http://www.tcaas.btinternet.co.uk/cyrus1pwr.gif
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Old 13th Feb 2010, 3:12 pm   #10
tonewheelkev
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Default Re: Cyrus 1 Amp Dead....Help!!?

Transistors all appear fine...
...one of the 3A rectifiers has gone 'continuous' (wrong term I know) and is behaving as a conductor in both directions. I'm told I can replace all 4 with 1N5404's from Maplin....23p each I think!
Yippeeeee
Will report back
Kev
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Old 13th Feb 2010, 7:05 pm   #11
Michael Maurice
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Default Re: Cyrus 1 Amp Dead....Help!!?

At least its not the output transistors, because you'll end up having to replace all of them in the affected channel. I would use 1N5408's as the PIV is much higher.
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Old 13th Feb 2010, 7:26 pm   #12
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Default Re: Cyrus 1 Amp Dead....Help!!?

Kev,
Make sure to fit them the right way round!!

Mike
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Old 14th Feb 2010, 12:52 am   #13
tonewheelkev
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Smile Re: Cyrus 1 Amp Dead....Help!!?

Rectifiers replaced....the right way round!!...and everything works

Had to visit 2 local Maplin stores to get the 1N5404's.....may well uprate later with 1N5408's...IF I can get 'em.

Thanks for all you help and advice......

.....Hhhmmnn......now what shall I fix next

Kev
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Old 14th Feb 2010, 10:58 am   #14
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Default Re: Cyrus 1 Amp Dead....Help!!?

Excellent. It's always good when a thread comes to a successful conclusion.
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