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Transcendent 2000 Powertran PCB diagram

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Re: Transcendent 2000 Powertran PCB diagram

Postby Hugh Robjohns » Fri Jan 16, 2015 2:13 pm

midierror wrote:Are there any components you would advise me buying as replacements?


It might be wise to identify the fault first, however...

Clearly there is something very wrong with the power supply! It would appear that IC1 isn't working properly to control Q1. The negative rail is derived from the positive rail voltage, so if you can fix V+, V- should sort itself out.

It would be worthwhile looking very carefully at all the solder joints under the board to make sure none are 'dry', and if IC1 is in a socket check that it is seated firmly and all its pins are in the right holes (it's not unknown for a pin to miss its socket and get folded underneath).

I've not come across a failed regulator of this type before, but it's obviously possible, so if all the other visual checks look good, and IC1 is in a socket, I'd be tempted to substitute a new uA723 regulator. If it's soldered directly into the board, some further measurements might be wise to try to confirm the 723 has failed, since removing it will risk board damage.

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Re: Transcendent 2000 Powertran PCB diagram

Postby Wonks » Fri Jan 16, 2015 3:12 pm

Midierror, whereabouts are you? I'm in Reading. If you're not too far away I could bring mine over and compare. Unless we both have exactly the same fault, it would probably benefit checking one against the other.
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Re: Transcendent 2000 Powertran PCB diagram

Postby Folderol » Fri Jan 16, 2015 5:58 pm

As Hugh says, there is no point thinking about changing things until you're pretty sure they are faulty.

It seems to me you have at least two faults, and I think we should concentrate on the PSU one first.

The voltages you get on pin 4 of IC1 do indeed prove the pot and resistor chain are OK. Doing a bit of maths gives almost exactly the same figures.

What I'd like to find out now is what all the other relevant voltages are on IC1. Going on all the actual chip pins listed below, what are their voltages (with the meter - on 0V)?

3, 5, 6, 7, 11, 12

Hey Wonks, are you telling me that Reading did not close down at the end of the 1960s :tongue:

P.S.
The reason I want measurements on the actual pins is that I want to eliminate bad connections etc. and see what the chip 'thinks' it's got.
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Re: Transcendent 2000 Powertran PCB diagram

Postby Wonks » Fri Jan 16, 2015 7:20 pm

Still there, Folderol. Still a PITA to get through.
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Re: Transcendent 2000 Powertran PCB diagram

Postby midierror » Sat Jan 17, 2015 10:21 am

Yeah, perhaps I`m jumping the gun a little! Thanks for your continued help.

Wonks - I`m about 4 hours from Reading. We could exchange some pictures of boards and speak over the phone, maybe?

Folderol, numbers for IC1 are:

3 17.9
5 7.20
6 7.20
7 0
11 20.2
12 20.3

Can you explain some of the maths you`re doing? I`d love to learn more about it.
Thanks all
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Re: Transcendent 2000 Powertran PCB diagram

Postby Wonks » Sat Jan 17, 2015 10:23 am

I've still to take the plunge and gear myself up to take all the knobs off and unscrew the beast again.
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Re: Transcendent 2000 Powertran PCB diagram

Postby Hugh Robjohns » Sat Jan 17, 2015 1:20 pm

midierror wrote:...numbers for IC1 are:

11 20.2
12 20.3


So the rectified and smoothed DC is around 20V, which seems appropriate.

3 17.9


Therefore the series regulator transistor Q1 is dropping about 2V -- so it's doing something... but it should be dropping more like 7 or 8V to get the output down to 12V.

5 7.20
6 7.20


This is the internal reference voltage generated inside the regulator chip. I think it should be about 7.5V, so this is plausible, but maybe a bit low.

7 0


Yes, that is connected to the ground rail.

So it seems that Q1 is simply not getting sufficient drive from IC1 to do its job properly, so we need to know that Q1 is working okay. For that we need to know its base-emitter voltage (measured between IC1 pins 2 and 10. Folderol asked for this earlier, but I don't think you gave it.

I should think either Q1 is knackered, although that is unlikely since it is apparently applying some level of regulation, or IC1 is knackered -- and that's more likely as it doesn't seem to respond to voltage changes received on pin 4.

But dry joints or corroded IC sockets are still possibilities.

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Re: Transcendent 2000 Powertran PCB diagram

Postby Folderol » Sat Jan 17, 2015 7:09 pm

Ok, I'm now 99% certain it's IC1 itself. My reasoning is as follows (and is why I specifically asked you to measure directly on the chip pins).

Pin 7 at zero - This proves there is a good connection to ground, a dry joint or bad socket would have left this high and could have accounted for the problem.

Pin 3 at 17.9V - This proves it is actually connected to the output. It is the current control feedback pin so could give strange results if it was floating.

Pins 5/6 at 7.2V - Proves they are actually connected together and the reference voltage is about right.

Pins 11/12 at 20V (ish) - Proves both parts of the chip are getting supply and the 'raw' DC is about right. 1.414 x AC voltage - a volt for rectifier losses.

According to the write-up, this IC should be socketed, and for me the final proof would be to carefully remove it. Like that, you should get zero on the +12V line unless either the transistor has a very strange leakage problem or something else on the board is tracking across.

If you don't have a proper IC extraction tool (I can never get them to work reliably) the best way to remove it is with two watchmakers screwdrivers. Ease the blades in under opposite ends of the chip then slowly rock it out, only lifting each end a little each time so you don't bend or break the pins.

If it's been soldered in directly, then for test purposes, it would be easier and safer to desolder the base of Q1. Just heat it with an iron and a smidgen of solder, then lift it up with a pair of thin pliers.

The good news, is that this chip is readily available and quite cheap.
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Re: Transcendent 2000 Powertran PCB diagram

Postby midierror » Tue Jan 20, 2015 10:20 am

Wonks, how can you let this beast lie dormant!?

Thanks for all the advice and assistance - I will go through all the joints today and see if this makes any difference (though even with a big magnifying glass I can`t see much wrong. Would you recommend re-soldering EVERY joint on the off chance?

Have ordered a new IC1 - luckily its in a chip socket, so removal will be (careful, but)easy!

Thanks guys

;)
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Re: Transcendent 2000 Powertran PCB diagram

Postby Wonks » Tue Jan 20, 2015 11:19 am

I've got as far as moving if from against the wall and putting it on the bed in my studio room so that it now keeps calling out 'fix me'.

But I've been setting up the bass and have just loaded Cubase 8 and some other new software and I've got the world to save on various forums. And I can't find any of my numerous small flat bladed screwdrivers to take the knobs off with. :headbang:
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Re: Transcendent 2000 Powertran PCB diagram

Postby Hugh Robjohns » Tue Jan 20, 2015 11:30 am

midierror wrote:Would you recommend re-soldering EVERY joint on the off chance?

If your soldering skills are good then yes, personally I would run an iron over and re-flow all the joints just to be sure. However, if your soldering skills are not good you run the risk of doing more harm than good.

Have ordered a new IC1 - luckily its in a chip socket, so removal will be (careful, but)easy!

It's not unknown for cheap sockets to tarnish and corrode, and go high-resistance or open-contact. So it might be worth getting hold of a replacement socket too, just in case.

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Re: Transcendent 2000 Powertran PCB diagram

Postby Folderol » Tue Jan 20, 2015 5:36 pm

On this occasion I disagree with you Hugh.

I never resolder joints unless I've a good reason to suspect they need them (such as the ends of very hot resistors). Mostly you don't improve things, and even with years of practice you stand a good change of making things worse.

If the socket had been dodgy the OP wouldn't have got the readings he did, so again I would not be inclined to change it and risk lifting old and fragile trackwork.
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Re: Transcendent 2000 Powertran PCB diagram

Postby Hugh Robjohns » Tue Jan 20, 2015 6:56 pm

Folderol wrote:On this occasion I disagree with you Hugh.

There's no law against it ;)

I never resolder joints unless I've a good reason to suspect they need them (such as the ends of very hot resistors). Mostly you don't improve things, and even with years of practice you stand a good change of making things worse.

With a DIY synth of unknown history, I think I would be a reasonable precaution... But I take your point.

If the socket had been dodgy the OP wouldn't have got the readings he did...

Fair point... And to further the fairness, ;) I suggested getting hold of one just in case, not to replace it on spec.

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Re: Transcendent 2000 Powertran PCB diagram

Postby Wonks » Tue Jan 20, 2015 7:12 pm

You have to remember that these synths were put together and soldered by a lot of amateurs. Midierror's might even be the one that my friend made (with my help) when we were 17 or 18. Some of the soldering could be decidedly dodgy.
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Re: Transcendent 2000 Powertran PCB diagram

Postby midierror » Tue Jan 20, 2015 7:51 pm

Looking at the PCB, its probably the best soldering I`ve ever seen! I`ve been using a soldering iron for about 10 years; even then I can see the blobs I`ve made quite clearly.

If I post a picture I may have to censor the really good soldering for being `too raunchy`.

Thanks for the advice about the chip chassis - I`d never have thought of that.
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Re: Transcendent 2000 Powertran PCB diagram

Postby midierror » Thu Jan 22, 2015 6:42 pm

Good news folks, IC1 has been replaced and is now showing much more stable numbers. I have set it to 12V and it is steady as a rock!

Still no sound out of the oscillator though.
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Re: Transcendent 2000 Powertran PCB diagram

Postby Wonks » Thu Jan 22, 2015 7:15 pm

One step at a time. Follow through the instructions about setting up the voltages.
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Re: Transcendent 2000 Powertran PCB diagram

Postby Hugh Robjohns » Thu Jan 22, 2015 7:22 pm

midierror wrote:Good news folks, IC1 has been replaced and is now showing much more stable numbers. I have set it to 12V and it is steady as a rock!

Fab! Don't forget to make sure that the negative rail is tracking properly at -12V as well.

Still no sound out of the oscillator though.

Not entirely surprised. With the power rails so far out of wack, it's entirely possible that some other parts of the circuitry have been taken out. The process is first to check taht the right rail voltages are reaching the right places on all the other circuit boards, and then to work through the synths signal path checking that all is as it should be along the way.

So... once you've confirmed the -12V rail is present, and that these +/-12V rails are reaching the other boards properly, we can now turn our attention to the oscillator, with more measurements to be made... standby!

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Re: Transcendent 2000 Powertran PCB diagram

Postby midierror » Thu Jan 22, 2015 7:35 pm

Hi Hugh
Yeah both rails working as they should -12 and +12. They are arriving at the boards like this too.
It`s funny how unstable these numbers were before the chip change. Where next??!
Thanks
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Re: Transcendent 2000 Powertran PCB diagram

Postby Folderol » Thu Jan 22, 2015 8:31 pm

I see I'm late to the party again :)
I was busy dealing with my cellar door, the previous owner had run out of screws and fixed it in place with cut flooring nails :protest:

N-E-way

You need to go back to those strange voltages around IC10 and Q9.

Q9 emitter/IC10 pin 3 should be approximately -0.6V and I believe you said it was 0V. My first suspicion would be R39 (or associated track work) open circuit as this supplies the current for Q9 to operate. Is the 'bottom' of the resistor actually seeing -12V?

The value of this resistor isn't terribly important, and to prove the point you could just tack a similar value one across it temporarily.

Alternatively, it is possible, but extremely unlikely that Q9 has gone short circuit.

Another fairly remote possibility is C19 short circuit.
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