AKG solidtube power supply help

[georgerarnold]

Hello,

I hope someone will be able to help me here.

A friend of mine has an AKG solid tube microphone and the power supply is blowing fuses.
I opened it up and see two varistors have completely blown up.
I’d like to replace them and see if that solves the problem.
I have the schematic but honestly I can’t make head nor tail of it.

I was wondering if somebody might be able to tell me what the parts are that I’m looking for.

I will attach a photo of where they are on the board and the schematic.

Apologies in advance if this is a silly idea.

Thanks.

 

[rock soderstrom]

I was wondering if somebody might be able to tell me what the parts are that I’m looking for.

Hi, I have highlighted the varistors for you.
I would definitely test the mains transformer for function, if I remember correctly then this power supply is notorious for problems with the transformer (due to overload?). I'll have to check this again in my documents.

 

[rock soderstrom]

I would definitely test the mains transformer for function, if I remember correctly then this power supply is notorious for problems with the transformer (due to overload?).

Yep, check this:

https://groupdiy.com/threads/akg-solid-tube-psu-failure.28382/

 

[Accelerator]

@georgerarnold

Hi georgerarnold and greetings from Greece.

As you can see by the attached AKG Solidtube 2800z0001 Service Manual they were 2 versions (old – new) for the AKG Solidtube P.S.U. so, it will be smart for you to check it out which one P.S.U. version is the P.S.U. version that your friend’s AKG Solidtube microphone have…

Mine AKG Solidtube microphone with the old version P.S.U. still goes strong after all these years of usage, so I have only good words to say about it, but I understand that the newer version of the AKG Solidtube P.S.U. maybe tends to be more notorious for problems with the power transformer, so it will be a good point of view to check the P.T. if they are any issues there…

 

[Rob Flinn]

I had a SolidTube in for repair as few years back with a similar sort of issue. The problem was the mains transformer was blown. If you do a bit of Google research this is a common issue with these mics.

You should definitely check the (blue I think) varistors, but in the case I had they were ok. The varistors normally crack open when they're fried.

 

[georgerarnold]

Thank you everyone for the replies.
Yes the varistors have completely exploded.
On the schematic there are no values of the varistors. Any ideas what they should be.

It seems it’s likely that the power transformer is bad in that case.
Does anyone know if there are replacements available?

Again thanks for being so helpful.

 

[Rob Flinn]

Thank you everyone for the replies.
Yes the varistors have completely exploded.
On the schematic there are no values of the varistors. Any ideas what they should be.

It seems it’s likely that the power transformer is bad in that case.
Does anyone know if there are replacements available?

Again thanks for being so helpful.

You may find the transformer is ok. The varistors are spike suppressors. I think I replaced the ones on the unit I was repairing but it was still blowing fuses. I would probably replace them first.

 

[Walrus]

According to the data Accelerator posted they are 130 volts RMS rated.

 

[Walrus]

Are most these failures in countries that use 220-240 Volt mains.
As that would explain why the varistors die, a short in one half of the primary would lead to one of the varistors being subject to much more than 130 V.
It goes short circuit, transformer burns open and takes out the other varistor.

 

[georgerarnold]

You may find the transformer is ok. The varistors are spike suppressors. I think I replaced the ones on the unit I was repairing but it was still blowing fuses. I would probably replace them first.

What did you do in the end?
In the link that someone kindly posted of an old thread, all the links for the replacement transformer don’t seem to work anymore so if someone has the specifications or even better, an idea where I would find one in the Uk, that would be great.

I’ll try the varistors first though.

 

[georgerarnold]

Are most these failures in countries that use 220-240 Volt mains.
As that would explain why the varistors die, a short in one half of the primary would lead to one of the varistors being subject to much more than 130 V.
It goes short circuit, transformer burns open and takes out the other varistor.

Does this mean that the root of all this is the transformer is bad or that the varistors are not in spec for European voltage?

 

[georgerarnold]

Sadly AKG don’t sell the part anymore. Not sure where I might find one.

 

[Rob Flinn]

What did you do in the end?
In the link that someone kindly posted of an old thread, all the links for the replacement transformer don’t seem to work anymore so if someone has the specifications or even better, an idea where I would find one in the Uk, that would be great.

I’ll try the varistors first though.

I got a replacement transformer from AKG or whoever owned them at that time.

 

[HerbertR]

May I ask what is the function of the T3 ZTX857 transistor which is connected from the B+ voltage rail to the heater voltage rail via the D40 12V zener diode?

 

[rock soderstrom]

May I ask what is the function of the T3 ZTX857 transistor which is connected from the B+ voltage rail to the heater voltage rail via the D40 12V zener diode?

Softstart for B+ or fancy bleeder? What value is R7? Is this 100R??

 

[Walrus]

Does this mean that the root of all this is the transformer is bad or that the varistors are not in spec for European voltage?

260 Volt total for the varistors on a 240 Volt ‘nominal’ supply, in the UK, is a bit close for my liking. I’ve often seen the mains at 250 V, which doesn’t leave a lot of room for spikes…..

 

[georgerarnold]

260 Volt total for the varistors on a 240 Volt ‘nominal’ supply, in the UK, is a bit close for my liking. I’ve often seen the mains at 250 V, which doesn’t leave a lot of room for spikes…..

Do you think I should get ones with higher values? What would be your suggestion?

 

[Walrus]

In the UK, I would probably pick a 275 V varistor across the mains, so in this case
maybe two times 140 V ones would be a better replacement.

 

[georgerarnold]

In the UK, I would probably pick a 275 V varistor across the mains, so in this case
maybe two times 140 V ones would be a better replacement.

Thank you.

 

[HerbertR]

Softstart for B+ or fancy bleeder? What value is R7? Is this 100R??

View attachment 140113

According to the AKG P820 tube microphone schematic, which is pretty much the same as the Solitube power supply without the pattern voltage part, the R7 on the emitter to ground is a 100 ohm resistor.

I think you are right, the transistor ZTX857 might be to "bleed" the HT/B+ rail to ground when no load is connected to the power supply. Then the "unloaded" heater rail will show 15VDC instead of 12VDC for the 12AX7 and drain 3VDC through the 12V zener diode, which then triggers the transistor ZTX857 through the base connection to let the connected HT/B+ rail at the collector pass through the emitter to ground. Maybe a safety feature to cut the HT rail? What do you think?

I bought a "non" working P820 for a few bucks and will check this out.