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Carver PM 1.5 Power Factor

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ceharden View Drop Down
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    Posted: 26 January 2009 at 12:09am
Just had an interesting experience on the bench.  Been working on a pair of old Carver PM1.5's for a long standing forum member.  The one on the bench at the moment had been attacked by someone rather ham-fisted with a soldering iron and started working once the solder bridges and unnecessary wire links had been removed.  Have had to replace the low voltage supplies smoothing caps as they'd gone dry and stopped actually storing charge.  That was a highly entertaining job for starters because of their location and the fact that whoever built them had used a whole roll of solder on the board!  I could go on for ages about how strange a design they are: Lighting dimmer for PSU regulation, no less than three sets of supply rails, 10 output devices per channel but only two connected to the output!

Anyway, I started running the amp up into a 4R load as a kind of final check before screwing the case on and deeming it finished but after only a few seconds of running at full power (slightly clipping) the amp cut out.  Very quietly, no bangs or anything.  Turned out the mains fuse had blown (6.3A slow blow).  So I replaced it with the one from the other amp, same value.  Started the testing again and it blew the fuse again.  Now at this point I've run out of 6.3A 35mm fuses so it's now got a 20A one temporarily but have downrated the plug fuse to 5A for testing and I decide perhaps measuring the input current would be a good idea.

Very quickly the reason for the fuse blowing was obvious, the amp has an appalling power factor.  At low powers it's about 0.3, rising to 0.5 at high power.  To give you an idea, with the amp delivering 300WRMS from one channel into a 4Ohm load, it's drawing over 5A or 1300VA from the mains!!!  Just before it took out the 5A fuse in the plug I clocked it at over 3000VA and this is still only with one channel running!  Maximum output power is about 800WRMS/4R for short bursts (100 ms or so)


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Wrighty Quote  Post ReplyReply Direct Link To This Post Posted: 26 January 2009 at 12:58am
I had one of these on the bench a while ago, which a customer said wasn't working. I took the lid off to have a look inside before I did anything and I thought the general design and construction was very strange indeed.

For starters, there was what resembled a model aircraft motor and propellor for a fan, which varied in speed according to the input and made a noise varying from quite annoying, all the way up to screaming for its life!

I ran the usual tests and came to the conclusion that there was nothing wrong with it, apart from the power LED for one channel not working. The level meter PCB was glued to the inside of the front panel, so I decided to leave it be!

I didn't actually do a flat out sine wave test, as I didn't want to stress an amp which was working but I had no service information for, knowing from the start that it was an unconvential design...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ceharden Quote  Post ReplyReply Direct Link To This Post Posted: 26 January 2009 at 1:25am
Luckily I was supplied with the service manual for these.  Without it the job would have been very difficult.

The amp has +/- 35V, 75V and 125V rails, all for an amp rated at something like 500W/4R.  8 output devices per side do rail switching/tracking and two actually drive the output.

The fan scared the sh1t out of me the first time the thermal switch cut in to take it to full speed.  I found myself grabbing for the power switch thinking the whole thing was going to blow up!  You probably didn't get to experience it at full whack.

The power supply is controlled by a triac in series with the mains transformer and the conduction angle is varied according to demand from the output stages.  The fan speed control is actually just a useful side effect!

Anyway, I've left that for now, got to work out where my hot glue gun is to mount the new caps in whatever space I can find!  Now on the bench is a Studiomaster 900E in which all the smoothing caps have failed at the positive terminal (as in corroded, you desolder the negative terminal and the cap comes off the board leaving the positive one still attached to the PCB!).

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Wrighty Quote  Post ReplyReply Direct Link To This Post Posted: 26 January 2009 at 1:41am
Yeah, I was dreading there being a fault on this one, as a quick search for some service info just revealed a load of dead ends...

I thought the fan speed switch on the front was an interesting inclusion for a pro audio amp until I fired it up and decided it was definitely necessary unless the amp is off stage somewhere! The fan speed seemed to be modulated by the input, so you could probably get a pretty good idea of the tune even when driving into a dummy load!

Apparently this is/was all amazing and innovative technology...

The Studiomaster sounds like fun - good luck finding some new caps that fit. If it's anything like most other amps, they'll be a funny size and I doubt the current incarnation of Studiomaster will have much to offer in the way of spares.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ceharden Quote  Post ReplyReply Direct Link To This Post Posted: 26 January 2009 at 1:54am
New caps weren't so much of a problem, RS had some of the right shape.  Did a 2000E last week with different value caps but the same problem, they must have bought a whole load of cheap sh1t caps.

However this one is still behaving strangely by turning the audio output on and off at 100Hz.  It was doing it much worse before I changed the main smoothing caps but hasn't cured it.  I don't have any service info for these and don't really want to pay for them....


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Wrighty Quote  Post ReplyReply Direct Link To This Post Posted: 26 January 2009 at 2:01am
When you say it's turning the audio output on and off at 100Hz, do you mean it's turning it off 100 times a second or it's turning it on and off randomly when you put 100Hz through it?

I'll be seeing a colleague of mine tomorrow who used to deal with Studiomaster a lot, back in the day and supplied a few of that series of amps over the years, so it's possible he's got some service info on them. I'll let you know how I get on...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ceharden Quote  Post ReplyReply Direct Link To This Post Posted: 26 January 2009 at 2:12am
If you could find out if he's got a schematic that would be fantastic please.

It's turning the audio on at 100Hz.  It's something like I get 8ms of on, then 2ms of off.

When the supply rails were sagging massively between mains peaks I could see why it was doing it but now that fault is cured I'm wondering what's going on.

Apparently the amps have some protection circuitry called 'gated output stage' or something wierd.  That would explain what I'm seeing but not why.  The second channel isn't doing anything at the moment but that's probably something simple....

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Wrighty Quote  Post ReplyReply Direct Link To This Post Posted: 26 January 2009 at 2:23am
A few ideas off the top of my head which you may well have already thought of and checked:

Have you checked the low voltage supplies (if there are any) on the input stages? I'd expect to see a +/-12V or similar supply, with its own rectifier, smoothing caps and regulators.

DC protection circuit gone mental? If there's no relay, it could be something similar to the LAB.1000 which (from memory) has some circuit involving a triac for DC protection...

Blimey, just noticed the time - better go to bed!!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 26 January 2009 at 2:39am




The Carver PM 1.5 is from the mid to late 80's. So, it is not surprising it pulls a lot of current.

Last year someone from PSW was searching for the Carver PT 2400 & 1800 manual. I sent it to him for he bought a few at a great price. The PT 2400 used two separate power cords and required 20 amps (240 volts) to operate @ 4 ohms stereo but was rated 2 ohms per channel. The amplifier only delivered 1200 watts per channel @ 4 ohms and, the THD was very low. So, what you are experiencing seems to co-inside with Carver Amplifiers.

Carver is a Home Audio company and dabbled in Pro Audio for a short period. While there amplifiers worked fine for Home Audio they failed constantly for Pro Audio. If I remember correctly the PM 1.5 was rated 375 watts per channel @ 8ohms. Seeing that the PM 1.5 is a Home Audio amplifier, the fuse rating reflex Home Audio use under an 8-ohm load.

Here is the Schematic

http://www.schematicsforfree.com/archive/file/_newly_uploaded/carver%20pm%201.5%20-input%20board.jpg

http://www.schematicsforfree.com/archive/file/_newly_uploaded/carver%20pm%201.5%20-power%20supply.jpg

http://www.schematicsforfree.com/archive/file/_newly_uploaded/carver%20pm%201.5%20-display.jpg

Best Regards,

Edited by Elliot Thompson - 26 January 2009 at 2:46am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 26 January 2009 at 9:07am
Chris,while you have it on your test bench,would it be possible take a photo of the inside and show us?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Deadbeat Quote  Post ReplyReply Direct Link To This Post Posted: 26 January 2009 at 3:03pm
The 'P' denoted pro models.

Just repackaged home ones, you can find their HT equivalents.

Even the 't' is carried over, if one has a PM1.5t, you've got one a Carver Special (transfer function-ized amp) - that emulates one of the $$$$ hifi amps.

When Phoenix gold bought them over, they kept quite a few of the schems -> the PT2400 and quite a few of them (non class D/lightweight amps) were like the older ones, rebadged hifi/ht amps.

Sound very good (I kept a Carver M1.5t in my hifi for a very long time...brill piece of kit), but what elliot said...

/uneeded info
Away on extended leave.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 26 January 2009 at 3:53pm
If I remember correctly, the PM 1.5 is 600 watts per channel @ 4 ohms and, 375 watts per channel like I mentioned previously @ 8 ohms.

Whatever you do, don't use it @ 2 ohms per channel.

Best Regards,
Elliot Thompson
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