Pearl Phono 1 - diagnostic help please

[tnargs]

Hi everyone, I am stumped on fixing a problem with my self-built Pearl Phono v1.

Any comments would be great however cursory, please.

The right channel has stopped working.

Both channels were working but when trying to root out some hum problems the right channel died. It *might* have been related to my plugging in interconnects while the amp was powered up.

All voltages on Fig 5 “The Unsimplified Circuit” of the Pearl Phono Stage article check out (within a bit of variance) except around Q4 (ZXT450) and Q6 (2SK389), where I get the following voltages:

Location: between Q4 and C9
Fig 5: 19.3v
Left ch: 18.5v
Right ch: 1.126v

Location: base of Q4
Fig 5: 14v
Left ch: 14.4v
Right ch: 1.833v

Location: between Q4 and Q6
Fig 5: 13.4v
Left ch: 13.9v
Right ch: 1.112v

Location: base of Q6
Fig 5: 0.05v
Left ch: 0.11v
Right ch: 0.30v

The right channel is the dead one. I first replaced Q4 – it made no difference.

I then figured it must be Q6 and replaced Q6. It made no difference.

At this point I ran out of ideas.

Any suggestions would be much appreciated. (The voltages shown above are the current circuit values – after replacing Q4 and Q6).

 

[mjf]

hello.
can you have a look at r24 (100k),perhaps contact lost.......bad soldering point or so.

 

[tnargs]

hi mjf, thanks for your comments.

I just looked into your suggestion and can see no problems. All connections around R24 are good. R24 itself measures 100k.

 

[mjf]

i woud look at q4........the volt reading at the emitter - and collectorpins are very similar, looks like some kind of short? (built in reverted,soldering bridge or something else....)

 

[mjf]

......i think there is something built in wrong/reverted.......if q6 is fully conducting........could be a mistake.

 

[tnargs]

thanks mjf, I will look into it tomorrow and get back

 

[wayne]

The base of Q4 should be fixed at half supply. Heavy current due to a short can pull it low through the emitter. The gate of Q6 should have almost no voltage unless Q6 is bad or the coupling cap is leak and that is pretty rare for a film cap.

 

[6L6]

2SK389 are pretty rare anymore - are you sure that your replacement was genuine?

 

[tnargs]

Maybe it wasn't.... where DO you get a genuine 2SK389?

 

[6L6]

I haven't the foggiest... The last one I got was most of a decade ago. Sorry.

 

[tnargs]

Hi, just to update those who generously contributed to my thread:

I replaced the Q4 ZRX450 (again), and still the same voltage measurements, including the base of Q4 being at 1-2 V instead of 13-14 V.

So now I have ordered replacement Q6 and will replace Q4 and Q6 simultaneously. I will let you know how it goes.

cheers

 

[h_a]

Typically the jfet blows and takes the ZTX with it; this is the very most common failure for the pearl. In that case replace both as otherwise the remaining damaged jfet will again take down the ZTX.

where DO you get a genuine 2SK389?

The K389 is not needed in this position, you can as good use the K170BL or even selected K369GR. These also have twice the power dissipation of the single jfets in the K389, so reliability will no longer be a problem with these replacements.

Hannes

 

[tnargs]

Typically the jfet blows and takes the ZTX with it; this is the very most common failure for the pearl. ....

Ha! NOW I discover this! I have been alternately blowing each one when I replace the other....

Anyway, I have bought 25 K170BL and will replace Q4 (ZTX450) and Q6 (with a matched pair of 170's) together and report back.

Thanks again all.

 

[h_a]

Q6 (with a matched pair of 170's)

Just for your information - there is absolutely no matching necessary in this position. Of course you can do it if you simply feel like it.

However, use the highest IDSS BL-jfets that you have in this position and you'll get a bit lower distortion.

Hannes

 

[tnargs]

Thanks Hannes, will do.

 

[tnargs]

Fixed! Replacing both components together did the trick. Measured DC voltages are now as expected.

A quick listen gave the impression of less output on the repaired channel, but this needs to be confirmed.

Thanks again to those who helped!

 

[h_a]

Fixed!

Great!

A quick listen gave the impression of less output on the repaired channel,

Might happen if your jfets are on the other end of the IDSS scale compared to the ones in the other channel, but still the difference should be very small.

Hannes