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/uploads/2013/04/su-z1.pdf
 
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Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
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. :(
 
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.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
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.
 

effebi

Member
2010-11-10 11:08 pm
Milan
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!
 
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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!
 
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effebi

Member
2010-11-10 11:08 pm
Milan
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
 

effebi

Member
2010-11-10 11:08 pm
Milan
I do not have the schematics with me, I am abroad. But Max can be right about the SIL chip.
BTW the thread opener I believe let us understand that has the original schematics. I have only some old copies of a similar model, courtesy of one of our most eminent members. If Mr Chandler has them, I would like a copy too (per mail or whatever). Thanks in advance
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Hi Dan
The front end SIL ICs are AN7060s as described in effebi's thread and as per the OP schematic. I have heard of repairs where STK2028, 2038 only were replaced and the amplifier restored simply that way until the next time someone turned the volume up to party level or shorted the speaker leads. After a couple of such repairs, the SIL chips eventually died but that could be related to non-OEM replacements too :(.
 
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Finally got my new meter, recorded the new readings. All of them were taken with the IC101 removed and the STK 2028 in place.

The volatges that are ranges ie. 0-36 are positive unless the range is 0--36, for some reason at varying times the voltage would fluctuate. All measurements are to within 1v.

I have no idea whats going on with this circuit, someone enlighten me else I'm going to start removing everything and testing it all individually.

http://msc-media.co.uk/wp-content/uploads/2013/04/export.pdf

For some reason I can't preview it on my computer in the browser, right click and download works for me.
 
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Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Thanks, I received your marked PDF OK, so what was the drama with your original DMM?

I did see that the voltages were different between the the driver stages of the STK so again I'll ask, was the fault apparent in in both channels? Do you still measure 15 and 10V across the in/output terminals? Will you check the other highlighted voltages, those rectangled in the regulated supply (lower right side, bottom of diagram) and those on both the AN7060 ICs and Q301,2? Marking voltages near rather than over the top of the schematic ones will be helpful, if you can manage it.

IC101 is only a phono preamplifier and with the supply voltages correct at +/- 15V, it will have no bearing on circuit operation when other inputs are selected, however, as there are caps C301,2 isolating the preamp input. Check them if you like by measuring any DC across them.

There is no way that you can remove parts like the ICs and simply test them individually. Nor will you get a good result if any replacements of failed parts you find pop as soon as you power up again because they were not the basic problem. Other transistors may have failed as collateral damage but the source of your problem will most likely be one channel of the STK and probably only one AN7060, as previously suggested.

These are DC coupled circuits here so voltage errors at one point do not mean the problem lies there. It could be elsewhere in the voltage or signal path to that point or in a feedback loop.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
You'll have to match a switch design like that one by measurements or guesstimation from generic or close match components in Mouser, Digi-key or possibly Farnell/CPC catalogues. Spares aren't maintained for 30+ year old budget amplifiers - particularly when the brand no longer exists either. The extension arm (if I understand you correctly) is likely a custom addition by Technics or the subcontractor who actually built the amp. Reuse it if you can.

I assume you will have gathered from effebi's thread that this is not going to be a simple or economical repair. If you plan to sell the amplifier on, think again about getting any return for your effort.

The STK and 7060 are both available in the UK from SW Electronics' Ebay store for about 20 & 7 UKP ea. It would be sensible to check the other voltages present, as I asked, so that you have some idea of whether other diodes and transistors need replacing before fitting the ICs.

There is little worse than seeing expensive replacement parts promptly fail for no apparent reason. If you are not sure of what you are doing and why, stop and think about where this is going if you feel a need to adopt a shotgun parts replacement method.
 
I've managed to source all of the IC's on the board for £15, and as for the switch I'll have to look into it. There was one Transistor that had 'gone-bad' since it was last tested which has been ordered. The C301,2 caps were tested this morning, they are still holding 3uf.
I'm certain I haven't missed anything now, the new IC's and transistor should sort this amplifier out as from the start I suspected that at least one of the AN7060's had died.
Oh well, If it doesn't sell I might end up modding it! It is a good looking amplifier after all, and I wouldn't mind a project..
 
I wonder ...in the flea market of Athens this will not cost more than 30-40 euro only if in mint condition ( looks ) other than that faulty .

DO potential buyers exist in Uk for such an amp ?. If so i may be able to set you up with a few more ...hundreds ...:D

I find very optimistic that you expect to make some profit from it also
you never know .....

Kind regards
Sakis