Recapped Technics SE-A70 new issues now!

[banebadboo]

So finished recapping my SE-A70 and as soon as I powered it on both fuses blew.(used a DBT after that to prevent more damage) It was working prior to the recap just not well. After rechecking all my work I found that I was missing an insulator pad for one of the power amp transistors. Once it was install no more fuses issues everything powers on correctly and seems to work. I rechecked all the caps and found no bridging or incorrect polarity.

So now my new issue... when I connected the speakers I got a loud buzz through both speakers with the volume off. All the indicator light are good the meters don't move due to the buzz. I turned off the speaker selector and the buzzing stopped. I tried music with the speakers off and the meters seem to move normally and if a select the main speakers the buzz is there and you can hear music mixed in with it. I was told be someone that "Sounds like DC present at the output\ - I would check for blown outputs now." Ended up replacing
all the power transistors, the speaker relays, I also found a bad thermistor and a resistor that was burned out. Replaced those and re rechecked all the caps, wires and found no bridging. Hook up the DBT and turned on the amp and DBT light went bright and the resistor I replaced burned out again. I need help figuring this out... the resistor is R364 on the diagram its a Panasonic ERD25FJ271.

Any thoughts on what to do next?

 

[TLS Guy]

So finished recapping my SE-A70 and as soon as I powered it on both fuses blew.(used a DBT after that to prevent more damage) It was working prior to the recap just not well. After rechecking all my work I found that I was missing an insulator pad for one of the power amp transistors. Once it was install no more fuses issues everything powers on correctly and seems to work. I rechecked all the caps and found no bridging or incorrect polarity.

So now my new issue... when I connected the speakers I got a loud buzz through both speakers with the volume off. All the indicator light are good the meters don't move due to the buzz. I turned off the speaker selector and the buzzing stopped. I tried music with the speakers off and the meters seem to move normally and if a select the main speakers the buzz is there and you can hear music mixed in with it. I was told be someone that "Sounds like DC present at the output\ - I would check for blown outputs now." Ended up replacing
all the power transistors, the speaker relays, I also found a bad thermistor and a resistor that was burned out. Replaced those and re rechecked all the caps, wires and found no bridging. Hook up the DBT and turned on the amp and DBT light went bright and the resistor I replaced burned out again. I need help figuring this out... the resistor is R364 on the diagram its a Panasonic ERD25FJ271.

Any thoughts on what to do next?

View attachment 33749View attachment 33750

I have down loaded the service manual from Hi-Fi engine. I can not locate the resistor in question. There are just far to many parts and the manual is hard to read.

What I can say is that omitting that mica washer was a terrible mistake and has probably caused widespread damage.

My other observation is that that amp is absurdly complicated and typical of gear of that era from that part of the world. No power amp needs to have that high a part count. That thing is s real thrown together mess. I think it will be an uphill fight to get that sorted out now. I personally would not worry with such a contraption and move on.

 

[banebadboo]

Here's a photo I have circled in green the resistor, if that helps. The pic was taken pre repair/restoration. I bought this new amp new in Germany back in 88. It never had any issue, until the main caps started leaking. My JBL L100T's sound much better with this amp than my Sumo Andromeda which is filling in for it until I get it up an running again. I love the looks of the Technics and I have all the match Technics components to go with it, so I if I have to replace every part on it I will. I just need to figure where to begin. So any help will be greatly appreciated

 

[TLS Guy]

Here's a photo I have circled in green the resistor, if that helps. The pic was taken pre repair/restoration. I bought this new amp new in Germany back in 88. It never had any issue, until the main caps started leaking. My JBL L100T's sound much better with this amp than my Sumo Andromeda which is filling in for it until I get it up an running again. I love the looks of the Technics and I have all the match Technics components to go with it, so I if I have to replace every part on it I will. I just need to figure where to begin. So any help will be greatly appreciated

View attachment 33751

All I can tell for sure is that Q328 is broken down and shorted internally. But that begs the question why. I suspect that its partner Q 326 is probably blown also, in any case they will be a matched pair I'm sure. Whether any of the diodes are bad and the driver transistors I have no idea.

I don't know what test equipment you have available and your level of expertise. You may need professional help here. So I would check the diodes, and then replace Q 326 and 328 with a new matched pair. Restart the amp carefully on a Variac, and watch the voltages carefully with an amplified high impedance volt meter as you cautiously increase voltage. A high impedance amplified meter is an absolute essential. You can do an awful lot of damage with a standard multi meter.

I think you are going to need a signal generator and scope also.

This may well not be easy, as you won't know how far the damage has spread. The other issue is these kind of cases you can keep blowing the same components until you can get a clear idea of the extent of the damage.

When you left out that insulator you will have shorted a collector or emitter to ground depending on whether is was PNP or NPN. That has the potential to cause widespread trouble.

You are not going to rebuild that amp and expect it to work. That is a pipe dream.

If you don't have the test equipment I mentioned, then you need professional help if you ever expect that unit to be functional again. That amp seems to be some type of feed forward design, and those have serious trouble in service as it is hard to make them stable again.

Which transistor did you miss install? It might be helpful to know that.

 

[banebadboo]

The following parts were all replace just prior to the R364 resistor that just failed:

TH302
TH301

R364
R363

Q501
Q503
Q338
Q502
Q337

Q328-Q326(paired)
Q325-Q327(paired)

The following were all paired:
Q333
Q329
Q335
Q331
Q334
Q336
Q330
Q332

I not aware of a "miss install" transistor, the board is not only labeled but has icons for which way parts are to be installed I also took tons of photos prior to and during disassembly.

 

[TLS Guy]

The following parts were all replace just prior to the R364 resistor that just failed:

TH302
TH301

R364
R363

Q501
Q503
Q338
Q502
Q337

Q328-Q326(paired)
Q325-Q327(paired)

The following were all paired:
Q333
Q329
Q335
Q331
Q334
Q336
Q330
Q332

I not aware of a "miss install" transistor, the board is not only labeled but has icons for which way parts are to be installed I also took tons of photos prior to and during disassembly.

I mean the one you failed to install the mica washer. This is in your first post. [ I found that I was missing an insulator pad for one of the power amp transistors.]

That was a huge mistake and I'm pretty sure what is responsible for your troubles. So what transistor did you install without the insulating washer?

I'm now pretty sure you are not going to solve this without professional help, and even then this unit may now be a write off.

 

[TLS Guy]

One more question for you. Did you use clip in heat sinks on the terminals of these transistors when you soldered them in?

 

[banebadboo]

I mean the one you failed to install the mica washer. This is in your first post. [ I found that I was missing an insulator pad for one of the power amp transistors.]

That was a huge mistake and I'm pretty sure what is responsible for your troubles. So what transistor did you install without the insulating washer?

I'm now pretty sure you are not going to solve this without professional help, and even then this unit may now be a write off.

OIC Q334

 

[banebadboo]

One more question for you. Did you use clip in heat sinks on the terminals of these transistors when you soldered them in?

Yes and my dwell time was extremely short

 

[TLS Guy]

Well that Q334 would have been shorted. That transistor is on the ICQ bias control board. That is a complex board. It is not one that I personally would be comfortable servicing. It does require a scope to set it up, which I have. One would be required to do the waveform checks and adjustments on page 19 of the service manual.

The trouble is that this board has vital connections to other boards so your troubles have a good chance to have spread far and wide.

There is nothing further I can do to help you on a forum like this. In fact no forum will be able to help you. A very experienced tech indeed now needs to have that unit on his test bench if you are to stand any chance at all getting that unit up and running.

I have to be brutally honest with you that your attempts at repair have caused more serious problems than you had to start with.

You now need to find a good tech to send it to and not attempt any more repairs yourself.

 

[banebadboo]

Thank for the information and your input, it is greatly appreciated. I have all the needed gear, I am lacking in experience but I learn best buy doing and learning from my mistakes. I seek out people that have experience to learn from them. I will fix this amp it may take me some time and a part or 2

Thanks again

 

[TLS Guy]

Thank for the information and your input, it is greatly appreciated. I have all the needed gear, I am lacking in experience but I learn best buy doing and learning from my mistakes. I seek out people that have experience to learn from them. I will fix this amp it may take me some time and a part or 2

Thanks again

That is well and good. But I have gone as far as I can with this at long range. I would caution you though that, that unit is unusually complex for a power amp. It has all the indications it was designed by committee and not one or two designers. I have been doing this for 60 years plus now. I firmly believe that when it comes to power amps, sound quality is very much in inverse proportion to part count.

I really don't see anything about that power amp that really excites me, in fact I find it quite a depressing item, lacking in elegance of design. What I see is layer on layer of circuitry to solve problems that lacked an elegant solution in the first place.

I know the great Peter Walker, founder of Quad, who I knew personally and cherish everyone of my visits to Huntingdon, would have some choice words for that design to say the least. His designs where elegance personified and sound marvelous. They also are known for longevity. Those two facts are strongly linked.

 

[banebadboo]

I really do appreciate you taking the time to offer your help and insights. Please keep in mind that because you don't like something, see the beauty of its design or the quality of its sound doesn't mean other people don't. These amps are hard to find and when they do pop up on ebay or some other site for sale they go quickly and normally for a significant amount of money. I have not met or read of any one that owns a Technics amp in this series complain about in fact it's just the opposite.

If I may offer some advise... It makes for a much more pleasant exchange if you don't insult and demean what someone else cares about.

Once more Thank for your advise and knowledge.