• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

SAE Two model A7 - need technical info for rebuild (500+ mV per channel...)

david winter

david winter

Classical music lover.
Hi,

I'd like to restore an old SAE Two type A7 amplifier.
I'm not used to redo SAE amps hence my request.
I already recaped the output section but still have over 500mV on both channels. Three output transistors are not original.
The relay clicks after two seconds once powered on despite the DC at the speaker outputs.
I tried to search the web for a service documentation but didn't find any so far.


Thanks !

David
 
Register to hide this ad
I have a schematic, if that will help. The scan quality is good, but the document that was scanned was in poor condition. If you want a copy, start a PM (conversation) with me, and let me know an email address where you can receive a PDF file of about 7MB.

The DC offset is not adjustable, according to the schematic. It is controlled by the STK3042 voltage amplifier. It seems unusual that both channels of the STK3042 would degrade in the same way by about the same amount. I wonder if there could be a poor ground connection that is causing the offset? Or perhaps the coupling capacitors for the input to the power amp section are leaky? (I don't know if you have already replaced C401 and C402.)

Bonne chance,

chazix
 
I've repaired a few P10s and they too use the STK3042 IC but there's more circuitry ahead of it. There might be a dual transistor used for the input diff. They are a common cause of DC offset.

Craig
 
Just got the schematic from "Chazix". The input goes straight to the STK, IC401, first check the power supply voltages on the STK, +44VDC on pin 7 and -44VDC on pin 9. Then we'll go from there.

Craig
 
I just remembered the A14 I rebuilt, the power supply regulators run very warm and was the cause of the problems I had. These units use oddball zener part numbers.
 
I changed all electrolytics. Checked the voltages across the zeners which seem to be ok.
Changed two 150 ohm resistors on a channel (drifted to 175), changed one driver pair on a channel (had one sub and one original, so took the usual KSA1013/KSC2383 subs we see on the forum).
Also checked all parts after the voltage regulator: all resistors ok, tried a new transistor for bias setting, nothing improved.
Voltages are all ok at the STK3042 except for the two outputs which are not -1.5/1.5 but something like -0.5/1.8 hence the DC offset.

I took another 3042 from an Akai amp I have stocked for years but this didn't solve the problem had generated the same DC as with the other 3042.
I was quite surprised but can't know for sure if both 3042s were bad with the same failure or if something else in the SAE is causing the problem.
I checked most parts around the 3042 and they seem to test ok (all resistors in value).

The question is what is a serious source for the STK ? There are plenty on ebay at various prices andI wonder how good they are and if they are not counterfeits as lots come from Asia. Moreover, anybody knows the difference with the STK3042, STK 3042 II, and STK 3042 III ?

What I need to know is the voltages of all zeners. I did a crude test by checking the voltages across them on both positive/negative parts and they are almost same.

What I also found is something like 150mV at both inputs of the 3042 despite the new Elna C401/402 (I have 0V before them so the voltage must come from the STK).

What I must say is that three of the output transistors were replaced with Sanken subs (can't recall the values now as I closed the amp and put it on hold).

The relay clicks after two seconds so I presume that the small TA73xx IC is bad, most probably due to the damage caused by the channels when they got short (and not repaired in the clean manner).

Sooo..... I keep wondering what can happen. I would be really surprised that both STK3042 ICs show the same trouble AND with the same resulting voltages.

Thanks for your hints !
 
The time delay for the protector IC should be from the R468/C454 combo, since there is a delay AND the relay energizes I would say the IC is OK. 500mv is just not enough of an offset to trigger the DC protection, +/-1.1 VDC is typical per the data sheet.

The different STK caused the exact same offset? How about Q401, can you adjust the bias?

I cringe when I get an amp in for repair that has any type of STK in it, you just can't trust them anymore. You get a new one, install it, and it still doesn't work. Is the new STK bad or do you have other problems. If it does work how long will it last?

Craig
 
Yes I can set the bias. The DC offset decreases if I increase the bias but I don't know what bias should be used here. I'm in the 10-12mV range across one of the 0.47 ohms resistors.
I used the 3042 found in another amp but have no clue if it's good or not. I just made the test knowing that it may work or not.
Of course I know those parts can work & fail but what else can you do ? Is there another solution using modern parts ? There are not many parts in the 3042 so a talented technician with good CAD knowledge could do a small SMD equivalent, IF (and really IF) there is enough demand.
I see numerous 3042 on ebay for example, but have no idea which seller can be trusted. What would be interesting is to determine a date code and search for late productions which may last longer than old ones. Just an idea....
 
Hello, I've exactly THE SAME PROBLEM, I wonder if David or anybody else found a solution. I tryed two original (one old and one NEW old stock) stk 3042 and one fake chinese (but working) stk 3042 and had the same unbalanced DC offset (-1,6 V and 0,7 V circa on 5 and 6 pin of STK3042) Thanks for answering.
 
Last edited:
Just took on the same amp for a refurb and have the same issue as well. This amp is reading ~160mV at both left and right speaker terminals. Has a solution been found for the dc offset issue yet? Thanks!
 
Look at this mess. Conductive glue on the STK3042 voltage amplifier. Started the recap process today and discovered this. Think that could cause a dc offset issue, among other things?

20190331_173010.jpg
 
Did you ever figure it out? I have about -135mv as well - schematic I found on elektrotanya is very hard to read and follow. If anyone has a good readable schematic and board view that would be nice too.
Other channel only ~35mv but there is definitely a loss of volume in one channel and there are new drivers and outputs being used that were fairly reasonably matched.
 
Sorry, never did figure this one out. Replaced transistors, STK modules, etc. The owner was fine with it being this way because it wasn't tripping the protect circuit. The owner was also not interested in putting anymore time or money into it.
 
Bummer, thanks for the reply and information! Same boat here, owner already did not want to do much and it plays fine just has that bit of DC stuck on it. Hate STK's.
 
You must log in or register to reply here.
Back
Top Bottom