Prinz music centre w/BSR P246-D2 cartridge advice please?

[Nickthedentist]

Probably made in the same era then.

[Beobloke]

Apologies for the poor quality picture shamelessly stolen from the internet, but this is the variant I was thinking of.

[ekjdm14]

Ah, I see what you mean. Looks the same except for the head shell (and the fact that it's playing with the transport screws out)!

Mid 1981 from the date codes on IC's.

[ekjdm14]

Aha, I have a possible manufacturer. (or at least the Mfr. that was copied perhaps)...

This is a photo of a National Panasonic with an SG prefix model number that looks rather on the similar side... Would NP have been an OEM for others or more likely they got cloned?

[paulsherwin]

That sort of styling was very popular towards the end of the music centre era and most manufacturers used it, so that may not mean very much. I'd be surprised if Panasonic OEMd for house brands but I can't say it didn't happen.

A close look at the component manufacturers may give you a hint.

[ekjdm14]

Lost my reply as the power went off! anyway, I don't see any Panasonic branded components & I would have expected them to use their own brand electrolytics at least so I think that's them ruled out as they're mainly Delcon. I have to admit, I've not seen anything quite as similar in design as the Prinz and that above but it's a nice look all the same.

I got a couple of photos of the interior, the only other branding I've dug up appears to be GoldStar and I think the unit was made in Korea so maybe they were the OEM for Dixons.

By the way, the last photo attached shows an obliterated ceramic disc cap in the diode bridge area. Any thoughts on what could have caused this? bearing in mind, the unit powers on and appears to work just fine. Could a fault previously repaired by replacing the "out of place looking" TO220 transistors and the "bodge resistor" (previous photos) have been a cause and the disc cap was just missed?

[paulsherwin]

Something has obviously gone bang in the past, and the output transistors have been replaced. It seems unlikely that the failed ceramic was part of this fault though. It looks as if there are 4 ceramics to suppress RFI from the rectifiers, and one of them has failed short circuit and self destructed.

One of the TO220s is a TIP31 and as you say is almost certainly not original. It's exactly the type that a UK repairer would use to replace an obscure far eastern part.

[ekjdm14]

Originally Posted by paulsherwin

Something has obviously gone bang in the past, and the output transistors have been replaced. It seems unlikely that the failed ceramic was part of this fault though. It looks as if there are 4 ceramics to suppress RFI from the rectifiers, and one of them has failed short circuit and self destructed.

One of the TO220s is a TIP31 and as you say is almost certainly not original. It's exactly the type that a UK repairer would use to replace an obscure far eastern part.

Ah that would make sense, the outputs are also TIPxx as well and I did note that there was plenty of thermal compound that seemed pretty "green" for 30+ years old. So it's had plenty of issues in the past then but been deemed worth fixing. I'm surprised at the ceramic cap having failed short, I don't recall seeing that before! At least it blew clear though, worth replacing all 4 if one has gone that way.

Out of interest, how did you deduce that the outputs were replaced? I didn't think they were really visible from the photos & don't recall mentioning their numbers! (you must have better eyes than me if you could see them in photo 2!)

[paulsherwin]

I assumed the TO220s were the outputs, but I now see that they are probably the drivers. I thought it was a bit odd that they weren't heatsinked. This points to a cascaded transistor failure, which is not an uncommon fault in DC coupled power amps. The original drivers will have been medium power types but the repairer may not have had direct equivalents to hand so just used TIP3xs.

Have the transistors been changed on both channels? It's unusual for both to fail like this, but maybe a PSU glitch took everything out, or maybe the repairer changed both channels to keep them balanced.

[ms660]

Those sharp edged disc ceramics can fail in other circuit positions too.

Lawrence.

[ekjdm14]

Both channels yes, I can't recall the exact numbers now but maybe TIP28 and TIP29?

It might have been that both channels were changed for balance, whatever the score it's interesting to note that it was felt worth repairing whenever it was done.

There is a couple of permanent marker scrawls under the chassis, one of which is "85 4" so perhaps it was only 4 years old when it went. I have to pop out now but will have to investigate further when back as this has piqued my interest now! Thanks for the help

[paulsherwin]

It may have been repaired by Dixons if it was only 4 years old at the time, especially if the owner had bought an extended warranty. I'm surprised that a major service centre didn't have a better stock of spares though. Maybe this was a stock fault and this was the official way to fix it.

[ekjdm14]

Could have been, whatever the situation it's working ok currently although I count around 19 of those knife-edged ceramics of the same value (437Z). Am I right in thinking that's 47(000) so .047uF at +80% tolerance? So not hugely important to get the exact replacements but as they're likely cheap I might get a bag of replacements.

The bodge resistor seems a strange value too, given that the still in-circuit original is red red red (2.2k yes?) it would appear to be much higher (either 8300 or 9300k 5% if I'm reading it right, white/grey? orange green gold).

I guess as it's working for now I'll leave it at changing some caps, but if I find a schematic for it any time it'd be nice to find out what's going on there.

[paulsherwin]

I would leave the ceramics alone if not faulty. It's unusual for them to fail, so you're just making work for yourself. 473 is 47000pF or 0.047uF.

The resistor may be something to do with modifying the bias or other operating conditions of the new transistors. It's difficult to know without a circuit diagram.

[ekjdm14]

That's true, I guess I'm just itching to get the soldering iron out again haha. May just leave it at replacing the 4 around the rectifier, lest I fix it 'til it's broke!