Panasonic SU-2700 Repair (Otherwise known as the Technics SU Z1) - diyAudio
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Old 5th April 2013, 08:38 PM   #1
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Default Panasonic SU-2700 Repair (Otherwise known as the Technics SU Z1)

I hope i've put this in the right thread, I'm new here!

So i've picked up a Panasonic SU-2700 for cheap, mainly to 'restore' and sell on, however when starting the amp there was a loud 'thud' (deep and flexes the drivers a lot) so i replaced the main caps in the hope that they were bad. The amp sounded good better for the whole of 2 hours.

After that, the left channel locks completely, pulling the driver in and outputting about 10V between the speaker terminals. Emitting a hum that gets louder until the speaker locks in the goes silent. I'll add that the amp is putting out 15v across the RCA + and - on all terminals, and now puts out 10V across all speaker terminals.

All transistors are good, all diodes appear to be good (I'm not entirely sure how to test zenners). On all terminals of the IC 101 that should register 0 it is registering 15V (even when it's been removed), and on most of the STK2028 terminals they are off, registering 33-35V where they are not supposed too. Removed the STK2028, speakers still lock, UV meters still lock. Also replaced C117 and C118 as was getting funny readings from them. Transformer is outputting the right voltages at the terminals. Also at Q301 and Q302 where it should measure 1.2v it's closer to 34V.

Can someone point me in the right direction? I can't see anything else that would be causing this!

(Sorry for an external link, the PDF file-size restriction means I cannot upload it here)

Schematic: http://msc-media.co.uk/wp-content/up...3/04/su-z1.pdf

Last edited by MChandler; 7th April 2013 at 08:26 PM.
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Old 5th April 2013, 10:08 PM   #2
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Welcome to the forum. There is usually similar experience to your own among the thousands of members, so it pays to do a search for other threads by just typing in the model number in the address bar, top of page. Here's the one other experience I turned up - it may shed some light.

Tecnics SU-Z1 to refurbish
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Old 5th April 2013, 10:10 PM   #3
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Thanks for the reply, however after searching that thread already, I cannot find anything related.

If it helps anyone, I can upload pictures of where I'm at and label a schematic of the voltages I am measuring.

Last edited by MChandler; 5th April 2013 at 10:14 PM.
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Old 6th April 2013, 01:30 AM   #4
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Well, the relevant comments in the thread are that first, the STKs are just twin darlington output stages and if you measure incorrect voltages on the pins whilst the supply voltages are correct, you do have a similar problem. From your description of the sound you likely started with a partly blown hybrid I.C. and that means DC on the output terminals because there is no protection relay of any kind and shorted speaker leads or excessive output often means "phutt!"

However, you say you have 10V on the RCA's. RCA's here are signal input sockets so I'm puzzled about how you are measuring such voltages through coupling capacitors and also when most will be isolated by the input selector switches. Are the outputs via RCA sockets too? The obvious check of these voltages is to perform the same measurement on the working channel. Perhaps you had better post that schematic with voltages marked and describe where you place the ground or reference probe of your multimeter when measuring 10V at all inputs and outputs and 15V on all IC terminals. Methinks you have a supply rail reference point there or the unthinkable and probably unrepairable alternative of a completely blown channel of the amplifier.

Are you actually referring to just the output connectors?

You can't determine whether all components are good in-circuit without proper test equipment. You can check transistor Vbe which is usually sufficient but that's not the whole story. It's a pity, but it seems more damage has occurred since you first powered up the amplifier.

The VU meters monitor output voltage so that's why they max. out and will likely blow their diodes or meter movements too if you leave them connected whilst powered up. It may be worth removing power from the relevant side of the STK2028 (pins 2,4 or 13,15) to identify the source of the voltage that has "flattened" the amplifier. If this is too difficult, you'll need to remove it completely with the aid of fluxed solder braid and then identify collateral damage and repair that first. Either way, it will be down to sourcing a working replacement STK2028 or similar.

Consider the likely cost first.
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Old 6th April 2013, 06:45 AM   #5
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Thanks Ian, All my voltage measurements are taken from the ground referenced on the schematic (negative from the transformer) and compared to the values that they should be. After removing the STK the voltages are still bad.

As for measuring the transistors I was checking them with the hfe function on my multimeter.

I'll try to put everything back in today and i'll mark up a new schematic with the voltages.
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Old 6th April 2013, 05:21 PM   #6
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You haven't yet confirmed whether both channels are equally affected by this odd 15V DC on the inputs and 10V on the outputs. If, when you operate the input selector switch S1, you still have 1V on Phono, Aux and tuner sockets, you have a measurement problem. Verify by reading directly across the socket connections, otherwise the whole input stage must be floating 15V above ground for that to be the case. You may realise when you look at the schematic, that the selector disconnects the inputs that are not selected so you'll see the logical problem there, if you still measure 15V regardless of which input is selected.

Before refitting the STK (and I suggest you don't whilst ever the voltages are haywire), what are the voltages measured on the output terminals now with the speaker switch to on? If you measure so much as a few mV across the terminals, you can't be using the meter correctly. At least the VU meters will not be showing an output and that should be a reality check.
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Old 6th April 2013, 05:52 PM   #7
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Well, I am the starter of that thread, and I believe that Jan is correct, the problem is related.
More than all, over some years I had it more than once, always the same, a blown STK chip. In the past I could even find an original Panasonic spare chip, through friends at a big Hi-Fi dealer, but after a couple of years and a some partys at full volume, I had it again. That´s why I have decided to replace it with a P3A, and from that time, I am again a happy owner of a good (better than original) amp.
I believe it is worth the effert, the SU-Z1 is a great looking amp, and once fixed properly a great sounding as well!

Last edited by effebi; 6th April 2013 at 05:55 PM.
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Old 6th April 2013, 08:56 PM   #8
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Thanks effebi,
I'd like to just fix the amp and sell it on I think, do you have any tips on checking the STK 2028? I can't seem to find any schematics on the internet for the chip.

Sorry about the delay in the marked up schematics, I've been waiting for a new multimeter to arrive and it still hasn't - I suspect mine is acting up!

Last edited by MChandler; 6th April 2013 at 09:16 PM.
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Old 7th April 2013, 08:34 AM   #9
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Quote:
Originally Posted by MChandler View Post
Thanks effebi,
I'd like to just fix the amp and sell it on I think, do you have any tips on checking the STK 2028?
Just going from Sunday morning memory, (so do not trust me 100%), I remeber the STK chip is on only the final stage of the amplifier. In practice a double channel darlington EF. I am less sure of this, but the chip should include also the emitter resistors of the OP .
Before (outside the STK) it there is a transistor VAS (bootstrapped with a capacitor) and an imput stage using a SIL IC (one for each channel). The imput stage acts a preamplifier and is involved in the tone controls as well. I had in some of mine the VAS collector resistor burnt as well.
So in theory you could fix it building a simple EF darlington from discrete BJTs for test (and maybe keep it at the end!). Do not forget to add conspicous heat sinking, (see my thread pictures) the original,that relies only on the bracket, is ridiculus and may well be the source of all the problems.
About your specific problem, I am suspicious that you have such DC output also after having removed the IC. It might well be a multimeter/measuring problem.
You might try loading the meter inputs with a 10 K in parallel to the probes.
Hope this helps, good luck
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Old 7th April 2013, 10:12 AM   #10
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Default Maybe Still Available...

Wagner Electronic Services in Sydney, Australia list STK 2028 in their catalog...I get trade price and don't know the price that you would pay.
What is the driver SIL chip, STK3042 ?....these usually fail first and take out the output module.

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