Dolby A301A repair

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living sounds

Well-known member
Joined
Jul 26, 2006
Messages
3,930
Location
Cologne, Germany
I always wanted one of these for the cool high end sizzle and low end punch. A few years ago these could supposedly be picked up for 50 bucks, but mine cost me a lot more. After it finally arrived from the States it took me a few more weeks to get hold of the proper power cord, which, were it not for a semi-closed specialty electronics store, is pretty much unobtanium in Europe.

Anyway, I finally tried the unit out yesterday, and found it to be non-working. There is intermittent crackling, sometimes hum, one channel distorts badly and there isn't much of the actual compression action going on no matter how you set the switches. I traced back the distortion to one of the amp cards, which luckily uses only easily optainable transistors. Only one of the meters shows a reaction.

Does anyone here have experience working on these units?

The input voltage selector can be set to a wide variety of voltages, I chose 230V (though in reality it is somewhat lower here) to be on the safe side. I doubt 220V would make much of a difference, but I thought would probably stress components unnecessarily.

The caps are mostly WIMA electrolytics from the 60s. Usually it is a safe bet that a lot of old caps have dried up, and replacing them would fix most if not all of the problems. Has anyone here already done that and found to be the case? Can you suggest a type/brand?

Thanks!
 
living sounds said:
The caps are mostly WIMA electrolytics from the 60s. Usually it is a safe bet that a lot of old caps have dried up, and replacing them would fix most if not all of the problems. Has anyone here already done that and found to be the case? Can you suggest a type/brand?
A301's were famous for their bad capacitors, 40 years ago.
this is a case where replacing all the aluminum  electrolytics is in order.
brand?  whatever you like.  the unit does not get all that warm, 85 degree caps might be fine.
check the Coutant psu, they are reliable but since you are hearing hum, it is suspect.
do you have the necessary extender card required for alignment?  if not, you may need to fabricate one.
 
Thank you! It's a really high-quality build otherwise.

I don't have an extender card. But right now I haven't even got the manual with the alignment procedure either. With the top removed and only one channel inserted alignment might be possible without an extender, don't you think?
 
living sounds said:
It's a really high-quality build otherwise.
indeed.  with a touch of British "class".
living sounds said:
But right now I haven't even got the manual with the alignment procedure either. With the top removed and only one channel inserted alignment might be possible without an extender, don't you think?
David Kulka (Studio Electronics) has the manual.

I do not think it possible to align without an extender.

You can check overall performance by looping encoded channel 1 into decoded channel 2.

Since you are unlikely to record the Budapest Strings with your Scully 280 and A310, just replace the capacitors and don't tweak the controls.  A bit "off" won't ruin it as an effects unit.
 
gridcurrent said:
Since you are unlikely to record the Budapest Strings with your Scully 280 and A310, just replace the capacitors and don't tweak the controls.  A bit "off" won't ruin it as an effects unit.

I could use it with my Telefunken M15... But I really bought it for off-label use as an effects unit.  :p

Thanks again, I'll report back after the recap.
 
Is there a trick to access that Coutant PSU PCB without completely taking it apart? The caps are the same as in the rest of the unit, so I will have to replace those, too...
 
living sounds said:
Is there a trick to access that Coutant PSU PCB without completely taking it apart?
assuming you mean the entire unit.
4 screws on the bottom, loosen the screws holding the tag strip,
and the "brick" can be removed for servicing.
 
gridcurrent said:
assuming you mean the entire unit.
4 screws on the bottom, loosen the screws holding the tag strip,
and the "brick" can be removed for servicing.

No, I mean the brick. It looks like one of those things that are hard to take apart and even harder to get back together again.
 
living sounds said:
No, I mean the brick. It looks like one of those things that are hard to take apart and even harder to get back together again.
remove the 4 shiny button head screws holding the aluminum frame.
see attached.
 

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Thanks,  but it's how the PSU (as well as the other pcbs are built into the brick that's bothering me. Since it has to be done I'll carefully remove one screw after the other, take lot's of photos and try my best.  8)
 
So, I replaced every single electrolytic cap. The PSU can be taken apart a lot easier than I thought by removing the screws at the bottom.

I powered it up and got a rather distorted signal. Luckily the PSU can be used and measured by itself. Turns out, there is no negative voltage present. So far, I haven't been able to find a fault. I do not understand the discrete active circuit though (schematic attached). The three transformer secondary pairs deliver the correct voltage, and the diodes in the diode bridges measure fine. But there is no voltage present at the junction of MR7 and MR8 and very little at the junction of  MR14 and MR17. There appears to be a loose connection somewhere, since the voltage at the latter junction chances a little (it's mostly + 0.3 V DC though). Please see the voltages in the attached schematic. I couldn't find any broken traces so far.

I will eventually resolder everything, but with all the wires I hope to be able to put it back together again correctly.

Is there an obvious component / circuit part to check?

Thanks!
 

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living sounds said:
So, I replaced every single electrolytic cap. The PSU can be taken apart a lot easier than I thought by removing the screws at the bottom.

I powered it up and got a rather distorted signal. Luckily the PSU can be used and measured by itself. Turns out, there is no negative voltage present. So far, I haven't been able to find a fault. I do not understand the discrete active circuit though (schematic attached). The three transformer secondary pairs deliver the correct voltage, and the diodes in the diode bridges measure fine. But there is no voltage present at the junction of MR7 and MR8
it appears MR7/8 should be a rough mirror image of MR6/9 so -16V(?)  The 29V total from the bridge may be a little low but reasonable.
and very little at the junction of  MR14 and MR17. There appears to be a loose connection somewhere, since the voltage at the latter junction chances a little (it's mostly + 0.3 V DC though). Please see the voltages in the attached schematic. I couldn't find any broken traces so far.
mr14/17  looks like a floating negative supply referenced blow the +16V(?) rail.  The 18V from that bridge may be reasonable too.  If the +18 was dropped down to +16V or less, the negative bias rail would drop down the similar voltage difference.
I will eventually resolder everything, but with all the wires I hope to be able to put it back together again correctly.

Is there an obvious component / circuit part to check?

Thanks!
The problem could be anywhere in the discrete regulator (all those discrete transistors.

Time to start measuring every base, emitter, collector voltage and trying to understand what devices are working correctly and what aren't.

It could be something simple, it almost always is after we find it.  8)

JR
 
moamps said:
IIRC, it's single power supply, +18V.

Well, it does say +18V and -18V. I was wondering where the ground is supposed to be though... BTW, for lack of a PSU ground I did the measurements against chassis ground.

Has there even been a convention to denote ground with the negative voltage? But why are there three pairs of secondaries to generate a single voltage? Or even to generate two voltages?
 
So the -18V from the PSU is connected to "earth" on the distribution bar. None of the modules seem to use -18V either. This PSU might be working after all...

Distortion on the two channels seems to be the same though. No hum above the (reasonable for such an old unit) noisefloor. Hm... anyone else ever had this problem?
 
living sounds said:
....I was wondering where the ground is supposed to be though...
They call it earth.  -18V output is connected to the "earth".
Has there even been a convention to denote ground with the negative voltage?
No. When germanium PNP transistors were used, + output of PS  was usually common reference (ground).
But why are there three pairs of secondaries to generate a single voltage?
Two are used for generating floating voltage references.
 
Thanks!

I experimented with selectively taking out modules. It appears one amplifier module is to blame, the unit works fine with it not fitted. It uses only easily attainable transistors, so this will probably be an easy fix.
 
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