Matsushita diode MA27A2 (MA26-2)

marcosentino

New Member
Hi everybody, this is my first post and definitely a tricky one :)
I own a Technics SU-V3 amplifier which had a major fault some years ago.
Thanks to the servicing manual and my electornics skills I was able to figure out what components needed a replacement and I was able to get one of the two channels back working.
In the other channel I still lack the component mentioned in the title of the post: diode MA27A2 (MA26-2) which is mentioned as "1.2V Bias supply" or just "bias supply" in other versions of the manual.

I think this diode provides stability to the bias of the various amplification stages and, when replaced with a generic diode, drives the power amp stage into instability causing the finals (power transistors mounted on the radiator) to blow up when the output power is grater than a few tenths of a Watt.

It would really appreciated if you could help me in figuring out what I could use to replace this part.

Thanks :banana::banana::banana:
 
Sounds like synchro bias diodes. A lot of the amps in the series used MA27W-A in this role.

One has to wonder the difference here. Key is to make sure that both diodes are the same. I would be trepidacious about using NTE replacements. I will see if I can find out the diff between 27A and 27W-A
 
Here is data
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Hi SVI2004A,
thanks a lot for the info. I did some more research and I found the following datasheet
http://datasheet.octopart.com/MA26-Panasonic-datasheet-108588.pdf

The MA26 packaging looks really like the diode in the circuit (but the cathode is blue, not yellow) but I suspect that the "-2" extension means actually something of key importance.

The symbol on the service manual is also puzzling: looks like a "double-diode" because it has 2 triangles pretty much like this: -->>|---
This and the fact that they specify "1.2V" in the service manual (for example also in service manuals of SE-A5 which employs the same diodes) makes me think that version MA26-2 is like two MA26 "in series" within the same package (if this makes sense:sigh:).


There is also a similar thread (in German) here:
http://www.repdata.de/wbb2/index.php?page=Thread&threadID=19703

From that thread it pops out that the following could make it as a replacement:
MA26V02 or MA2C029

Any other thoughts?
 
if it has fwd voltage of 1.2V and is mentioned as a double diode, probably it is just 2 silicon diodes in a single package.

In push-pull amp output stages it is quite typical to have a voltage of about 1.2V between the bases of the two output transistors (usually a complimentary pair).

Sometimes this is done by using a current source and a low value trimmer resistor (50-100 ohms say), which then has to be manually adjusted to set the bias.

Sometimes it is done with a pair of fwd biassed diodes. In this case the designers have usually matched the current with the VI curve of the diodes chosen to make the bias voltage correct with no need of adjustment.

If this is the case, the important thing is just to get that 1.2V correct. Maybe two 1N4148's or similar would do it. You need to try that and measure the voltage to see if it is correct before loading the amp or driving it with signal.

Can you post the schematic so we can see what is happening?
 
Hi danmcb,
thanks for sharing your thoughts. Unfortunately I tried with 1N4148 in series and similar but no luck :(

Here the final stage of the amp. The guys are D318, D320. They can be found in different configurations through many stages of the amp.
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I just wanted to post an update after 7 years for people who come across this thread and want to know what a suitable replacement can be for the MA26-2 or MA27-A2.
I managed to find some surplus of an equivalent but more modern diode from Panasonic with part number MA2C029.

Those come in various variant which are basically combination of 2 letters:
- first letter can be (nothing), W, T, Q: This decides the forward voltage and (since power dissipation is fixed at 150mW) the forward current handling capability
- second letter can be A or B which is a finer variation in the forward parameters of the first letter
Datasheet is very clear and readable in comparison to the older MA26 series
In this case I used MA2C029WA and worked ok: I use the repaired amp on a daily basis and so far has worked fine.
Still I wasn't able to find a decent datasheet for the MA26-A2 so I cannot compare the performance according to the manufacturer...

In general I think for this application there aren't many parameters which have to match if you want to find a replacement:
- The forward voltage at operating current (obviously, because this parameters sets the operating point of the transistors)
- The thermal behaviour, because with the warming up of the amp, the bias point needs to move towards a safe operating area instead of moving towards thermal runaway.

Also you don't want to break the symmetry of the push-pull configuration so if you replace a diode in the push side, you want to replace the corresponding diode on the pull side so that the balance in the bias point is symmetric between the push and the pull side.
 
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