Blown-up 7812 Regulator - UK Vintage Radio Repair and Restoration Discussion Forum

jjl · 15th Aug 2020, 1:34 pm

I've been asked to look at a non-functioning Phil Jones Bass Superflightcase combo bass amplifier. It's quite a nice little thing with a 250 watt class D amplifier.
I've found what I think is the cause of the problem - a L7812CV +12V regulator IC with a hole in the body and signs of heat stress around it, see the photos below.

I'm asking for thoughts on this as I had the impression that it was difficult to kill these devices due to their short circuit and thermal protection. Could excessive input voltage or the regulator oscillating for some reason cause this kind of damage? There doesn't appear to be a short on the 12V line that the regulator supplies.

John

Mooly · 15th Aug 2020, 1:53 pm

I've had many 78 series regulators fail in commercial gear over the years but don't recall any self destructing. It need a lot of energy to crack the package like that.

Thoughts are:

1/ It may well just be a failure but the input side must be of high current ability to do that.

2/ Has the failure allowed an overvolts situation to occur on the output with possible collateral damage to other parts.

3/ Is the damage 'man made'. i.e. has someone been poking and prodding in there in the past and perhaps shorted one rail to another.

All you can do is change it and take it from there.

jjl · 15th Aug 2020, 3:01 pm

Hmm, I measured the DC voltage between the regulator's IN and GND terminals and I see 60V with my Fluke 77. The 7812 has a specified maximum input of 35V, so that might be the cause of its demise.
It could be that my Fluke is not telling the truth as the 7812 regulator appears to be fed from a rail provided by the amp's switch mode PSU, but I'm getting a bad feeling about this one. Having no schematic doesn't help.

John

Mooly · 15th Aug 2020, 5:56 pm

Hmm indeed...

If you have no circuit then I would begin by looking at any other rails off the SMPS tranny and see if they seem high as well. If so you might have a simple issue with a feedback network which would be used for monitoring one of the rails as a means of stabilising the voltages.

Look at marked cap voltages. What cap voltage is the 7812 input using? Is it open circuit? That would give odd results on a DVM.

A scope is your instrument of choice for this kind of thing really.

greg_simons · 15th Aug 2020, 6:21 pm

Might be worth seeing if the input terminal has a large amount of hf ripple on it, smoothing capacitors are a common failure mode in smps designs.
Greg.

Craig Sawyers · 15th Aug 2020, 6:36 pm

Why not contact Phil Jones. I knew him years ago when he was running Acoustic Energy in the UK before he decamped to the USA. He's a nice guy.

https://www.pjbworld.com/cms/index.php/about/

I can't imagine that they would not send service information. Although this looks like a current product https://philjonesbass.net/cms/index.php/product_bg-300/

jjl · 16th Aug 2020, 10:00 am

Craig

Thanks, yes, I've contacted Phil Jones; hopefully some service info will be forthcoming.

John

jjl · 18th Aug 2020, 9:44 am

An update on this one.

I contacted Phil Jones Bass on Saturday asking for a schematic for the amp and mentioned the rather spectacularly killed 7812. Yesterday (Monday) I received the requested schematic along with a statement that regulator IC failure is common with this amp and some advice on other parts to replace to effect a reliable repair.
I'm very impressed with this service; Phil Jones must really be a nice bloke

John

Mooly · 18th Aug 2020, 11:34 am

That's good to hear. Service like that goes a long way...

Craig Sawyers · 18th Aug 2020, 11:41 am

He's a great bloke with tons of pedigree. Back in the day, post Acoustic Energy, we were discussing him joining Wharfedale to head up a professional sound reinforcement section.

He didn't, which was a good job because Wharfedale imploded not long after.

In the States he has been going great guns and his company is a really serious pro-audio supplier.

I'm delighted at the response to the problem you're trying to sort out.

Craig

jjl · 5th Sep 2020, 5:26 pm

An update and a happy end result with this.

The 7812 regulator had suffered so much that its IN lead had mostly vapourised as had part of the PCB track that it was soldered to.

The reason is as follows :-

The amp's switch mode power supply provides +48V and -48V rails for the class AB power amp (not class D as I originally thought). The +48V and -48V rails are reduced to +12V and -12V for the small signal stages of the amp. This is done using the combination of a series 5W 18V zener diode and 7812 regulator for the +12V supply and a similar zener and 7912 combination for the -12V supply.
In this amp, the zener in the positive supply had gone short thus applying 48V or more to the poor old 7812.

Phil Jones Bass advised me that regulator failure is common with this particular revision of the amp and that it was recommended to replace both the 7812 and 7912 and change both the the 18V zeners to 3 off 8.2V 5W zeners in series in each case thus sharing the dissipation between the 3 zeners in each supply and reducing the input voltage to the regulators too. I did this, but chose to use 3 off 9.1V zeners in each case.

Luckily, no other damage was caused and the amp is now working properly.

I've attached a photo of the offending items.

John

Electronpusher0 · 5th Sep 2020, 6:59 pm

Glad you got it sorted! Sounds like you got good service from Phil.
I have reservations about the way the 12V rails are derived in the design, I am sure there is a more elegant way to derive this, LM317HV has a maximum input voltage of 60V for example.

Peter

Mooly · 5th Sep 2020, 7:48 pm

Thanks for the update. Pleased you have been able to fix it... and that failure mode sounds like it was just a matter of time.

Series Zeners are a valid approach for losing some voltage for low current rails but when you start talking 5 watt Zeners then things are getting a bit hairy.

turretslug · 5th Sep 2020, 8:29 pm

Linearly regulating from 48V to 12V is a little lumpen and wasteful- apart from the highlighted hazard to the 78/79 devices. A couple of advantages of HF PSUs are low turns-per-volt and small reservoir requirements- two more low-volume windings on the bobbins (small gauge, not many turns), a couple of HF diodes (I'm assuming flyback-type topology) and a couple of c. 100uF 25V providing 18-20V to the reg inputs would be more efficient, plus keeping the low-noise advantage of linear regs. About 50 OEM pence, and less of a sword dangling over the regulators.

15th Aug 2020, 1:34 pm	#1
jjl Octode Join Date: Jan 2003 Location: Ware, Herts. UK. Posts: 1,082	Blown-up 7812 Regulator I've been asked to look at a non-functioning Phil Jones Bass Superflightcase combo bass amplifier. It's quite a nice little thing with a 250 watt class D amplifier. I've found what I think is the cause of the problem - a L7812CV +12V regulator IC with a hole in the body and signs of heat stress around it, see the photos below. I'm asking for thoughts on this as I had the impression that it was difficult to kill these devices due to their short circuit and thermal protection. Could excessive input voltage or the regulator oscillating for some reason cause this kind of damage? There doesn't appear to be a short on the 12V line that the regulator supplies. John Attached Thumbnails

15th Aug 2020, 1:53 pm	#2
Mooly Octode Join Date: May 2011 Location: Lancashire, UK. Posts: 1,351	Re: Blown-up 7812 Regulator I've had many 78 series regulators fail in commercial gear over the years but don't recall any self destructing. It need a lot of energy to crack the package like that. Thoughts are: 1/ It may well just be a failure but the input side must be of high current ability to do that. 2/ Has the failure allowed an overvolts situation to occur on the output with possible collateral damage to other parts. 3/ Is the damage 'man made'. i.e. has someone been poking and prodding in there in the past and perhaps shorted one rail to another. All you can do is change it and take it from there.

15th Aug 2020, 3:01 pm	#3
jjl Octode Join Date: Jan 2003 Location: Ware, Herts. UK. Posts: 1,082	Re: Blown-up 7812 Regulator Hmm, I measured the DC voltage between the regulator's IN and GND terminals and I see 60V with my Fluke 77. The 7812 has a specified maximum input of 35V, so that might be the cause of its demise. It could be that my Fluke is not telling the truth as the 7812 regulator appears to be fed from a rail provided by the amp's switch mode PSU, but I'm getting a bad feeling about this one. Having no schematic doesn't help. John

15th Aug 2020, 5:56 pm	#4
Mooly Octode Join Date: May 2011 Location: Lancashire, UK. Posts: 1,351	Re: Blown-up 7812 Regulator Hmm indeed... If you have no circuit then I would begin by looking at any other rails off the SMPS tranny and see if they seem high as well. If so you might have a simple issue with a feedback network which would be used for monitoring one of the rails as a means of stabilising the voltages. Look at marked cap voltages. What cap voltage is the 7812 input using? Is it open circuit? That would give odd results on a DVM. A scope is your instrument of choice for this kind of thing really.

15th Aug 2020, 6:21 pm	#5
greg_simons Octode Join Date: Jul 2004 Location: Evesham, Worcestershire, UK. Posts: 1,422	Re: Blown-up 7812 Regulator Might be worth seeing if the input terminal has a large amount of hf ripple on it, smoothing capacitors are a common failure mode in smps designs. Greg. __________________ Picture, sound?, DOOR.

15th Aug 2020, 6:36 pm	#6
Craig Sawyers Dekatron Join Date: Nov 2010 Location: Oxford, UK. Posts: 4,984	Re: Blown-up 7812 Regulator Why not contact Phil Jones. I knew him years ago when he was running Acoustic Energy in the UK before he decamped to the USA. He's a nice guy. https://www.pjbworld.com/cms/index.php/about/ I can't imagine that they would not send service information. Although this looks like a current product https://philjonesbass.net/cms/index.php/product_bg-300/

16th Aug 2020, 10:00 am	#7
jjl Octode Join Date: Jan 2003 Location: Ware, Herts. UK. Posts: 1,082	Re: Blown-up 7812 Regulator Craig Thanks, yes, I've contacted Phil Jones; hopefully some service info will be forthcoming. John

18th Aug 2020, 9:44 am	#8
jjl Octode Join Date: Jan 2003 Location: Ware, Herts. UK. Posts: 1,082	Re: Blown-up 7812 Regulator An update on this one. I contacted Phil Jones Bass on Saturday asking for a schematic for the amp and mentioned the rather spectacularly killed 7812. Yesterday (Monday) I received the requested schematic along with a statement that regulator IC failure is common with this amp and some advice on other parts to replace to effect a reliable repair. I'm very impressed with this service; Phil Jones must really be a nice bloke John

18th Aug 2020, 11:34 am	#9
Mooly Octode Join Date: May 2011 Location: Lancashire, UK. Posts: 1,351	Re: Blown-up 7812 Regulator That's good to hear. Service like that goes a long way...

18th Aug 2020, 11:41 am	#10
Craig Sawyers Dekatron Join Date: Nov 2010 Location: Oxford, UK. Posts: 4,984	Re: Blown-up 7812 Regulator He's a great bloke with tons of pedigree. Back in the day, post Acoustic Energy, we were discussing him joining Wharfedale to head up a professional sound reinforcement section. He didn't, which was a good job because Wharfedale imploded not long after. In the States he has been going great guns and his company is a really serious pro-audio supplier. I'm delighted at the response to the problem you're trying to sort out. Craig

5th Sep 2020, 5:26 pm	#11
jjl Octode Join Date: Jan 2003 Location: Ware, Herts. UK. Posts: 1,082	Re: Blown-up 7812 Regulator An update and a happy end result with this. The 7812 regulator had suffered so much that its IN lead had mostly vapourised as had part of the PCB track that it was soldered to. The reason is as follows :- The amp's switch mode power supply provides +48V and -48V rails for the class AB power amp (not class D as I originally thought). The +48V and -48V rails are reduced to +12V and -12V for the small signal stages of the amp. This is done using the combination of a series 5W 18V zener diode and 7812 regulator for the +12V supply and a similar zener and 7912 combination for the -12V supply. In this amp, the zener in the positive supply had gone short thus applying 48V or more to the poor old 7812. Phil Jones Bass advised me that regulator failure is common with this particular revision of the amp and that it was recommended to replace both the 7812 and 7912 and change both the the 18V zeners to 3 off 8.2V 5W zeners in series in each case thus sharing the dissipation between the 3 zeners in each supply and reducing the input voltage to the regulators too. I did this, but chose to use 3 off 9.1V zeners in each case. Luckily, no other damage was caused and the amp is now working properly. I've attached a photo of the offending items. John Attached Thumbnails

5th Sep 2020, 6:59 pm	#12
Electronpusher0 Nonode Join Date: Jun 2016 Location: Bognor Regis, West Sussex, UK. Posts: 2,296	Re: Blown-up 7812 Regulator Glad you got it sorted! Sounds like you got good service from Phil. I have reservations about the way the 12V rails are derived in the design, I am sure there is a more elegant way to derive this, LM317HV has a maximum input voltage of 60V for example. Peter

5th Sep 2020, 7:48 pm	#13
Mooly Octode Join Date: May 2011 Location: Lancashire, UK. Posts: 1,351	Re: Blown-up 7812 Regulator Thanks for the update. Pleased you have been able to fix it... and that failure mode sounds like it was just a matter of time. Series Zeners are a valid approach for losing some voltage for low current rails but when you start talking 5 watt Zeners then things are getting a bit hairy.

5th Sep 2020, 8:29 pm	#14
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,396	Re: Blown-up 7812 Regulator Linearly regulating from 48V to 12V is a little lumpen and wasteful- apart from the highlighted hazard to the 78/79 devices. A couple of advantages of HF PSUs are low turns-per-volt and small reservoir requirements- two more low-volume windings on the bobbins (small gauge, not many turns), a couple of HF diodes (I'm assuming flyback-type topology) and a couple of c. 100uF 25V providing 18-20V to the reg inputs would be more efficient, plus keeping the low-noise advantage of linear regs. About 50 OEM pence, and less of a sword dangling over the regulators.