Need help fixing an odd Phase Linear 400
I recently bought a relatively inexpensive "As Is" Phase Linear 400 series II off of E-Bay. The seller was selling off a lot of lab equipment (oscilloscopes and whatnot), and it looked like this PL400 had been re purposed for lab use in some manner.
When it arrived, the first odd thing I noticed was that the RCA input jacks had been replaced with BNC connectors. I hooked up some cheap satellite speakers I didn't care about and powered the unit on. The LEDs come up on the meters, but I got buzzing from the right channel and nothing from the left channel.
Upon cracking the case open, I saw that all of the output amplifiers and driver transistors for the left channel had been removed! And this is what the main board looked like:
Several components had been removed. After comparing against a schematic I found on the internet of the PL36 board, it looks like the following was removed:
* two 3.9k Resistors (R212 and R112) The PCB is dark brown where these resistors used to be, so I assume they burned up.
* One FPN 3569 transistor (Q102)
* The leftmost 40327 (Q101)
* Both LF356h's (Z101 and Z201)
* A diode (1N4744)
I bought 12 MJ21196's for the amplifiers and 4 MJ15024's for the drivers off of DigiKey. I also bought replacement LF356's, a 2N3440 to replace the 40327, and the appropriate diodes and resistors to replace the other missing ones. I've also tested all the resistors in the unit with a multimeter and the appear to be fine.
My question is, what's the best order to replace these parts to minimize damage to the circuit? Should I populate the PL36 board with the replacement parts before I replace the output transistors so I don't risk blowing them?
As far as test equipment goes, this is what I have:
a digital multimeter
a nearly 50 year old Tektronics 585 oscilloscope.
Several cheapo Insignia satellite speakers I don't mind destroying
What I don't have:
Experience fixing amps
Any help would be appreciated. I'd like to avoid ordering more replacement parts than I have to.
When I repair my amps I usually disconnect the output transistors and feed the output of the VAS back into the LTP to test the driver.
The VAS stage should output a pair of voltages that track each other by a few volts.
If the driver works OK I would then turn the bias right down and connect the output transistors but only one pair.
If your driver isnt working then I would remove all transistors and check every component on the board.
While the transistors are out I would check the HFE on them.
One fault I had was a transistor with an HFE of one !
Its all slow and hard work but you get there in the end doing it logically.
Sometimes its more than one component that has died.
PL's by design will sort-of "function' without output transistors. It should have normal offset and drive a 1k ohm or so load with the outputs and/or rail fuses removed.
Start with no outputs or drivers then if all goes well add them first and rough-check the bias. Make sure the Vbe of the combined positive-side driver/output is under a volt - which will keep them from turning on hard the instant collector voltage is applied. If that's in the ballpark, put in rail fuses and check bias.
Variacs won't help PL Series 2's anyway. At around 30-60VAC, they act "stupid". Horrific DC offsets, even audible oscillations. Sometimes they "squeal" a bit during power-down. Use a "light bulb limiter" when servicing these. It still goes through the oscillation phase as it comes up, but it will drive right through it.
The light bulb limiter is just a bulb in series with the power supply, right? Will that limit the amperage so I don't burn out any additional components?
60 watt bulb in series with the AC mains. While it still may be possible to damage something low-power using this, it will almost always save expensive high power components - including speakers.
Thanks, I'll swing by Home Depot and get a bulb socket.
Which way do the LF356h's go in? My board takes the 8 pin DIP chips, and the schematic I downloaded shows them in the "can" packaging. The two PL36 schematics I found online don't exactly match the board in my PL400. The layout is slightly different, and there are two 12k resistors on my board that aren't on the schematic.
Also, the schematic has two large 1.8k 5 watt resistors where my board has two 3 watt 7.5k resistors. I don't know if those are the right values for this board or if it's something the previous owner put in for some reason...
There are quite a few minor variations on the schematics. I doubt I've even seen them all - and I've been playing with PLs since 1980.
On the op amp ... pin 8 to the +15, pin 4 to the -15. The rest of the pins should line up. It's easy enough to trace the main +/-75V supply through the shunt regulators.
The big sand-box resistors are supposed to be 1.8k, 5 watt. They run hotter than the blazes of Hell and damnation - and one of the main failure modes is thermal cycling and oxidation where they solder to the board. I always touch this area up at a bare minimum. The previous owner may have replaced them with a higher value hoping to reduce the heat. I replaced each of these resistors with three 5.6k 3 watt MOX (flameproof) resistors in parallel and used the leads to beef up the PCB traces on the PL400 I'm using.
There are 7.5k resistors, but they are 1 watt and are the collector loads of the VAS. They run a little warm, and it never hurts to check their solder connections. They used a higher value (18k or something) in early versions.
I hooked the PL400 to a light bulb in series with the power supply and powered the unit on without making any changes. The bulb glows brightly for a moment, then dims. The LEDs on the PL400 power up as normal.
Then, I replaced all the missing components on the PL36 board (but leaving the old output drivers and whatnot) and powered the unit up. Bulb glows brightly, LEDs on the unit do not power up.
I removed all the output transistors and drivers and powered the unit up. Bulb glows dimly, LED powers up.
I put 4 new drivers in and power the unit up. Bulb glows dimly, LED powers up.
I put one bank of 3 output transistors in and power the unit up. Bulb glows brightly, LEDs do not power up.
I'm assuming this means I still have a ways to go. I'm going to reheat all the solder joints on the board and recheck all the resistors and whatnot. Is there something more specific I should be looking for?
I've fixed a few Phase 400 Series II's and have to warn you they can have some serious stability issues due to their rather odd design using an op-amp in place of the usual input stage. Bob Carver, as has often been the case, seemed bent on being "different" with his designs and the 400-II was no exception.
The relatively high open loop gain of an op-amp was probably why he thought it might be superior (especially in terms of damping factor which was a popular spec in those days). But it also presents some rather unique stability/compensation issues. There's a reason few commercial designs have done anything similar.
In my experience the 400-II is quite fussy as to driver and output transistors even when they're the correct part number. I also believe there were several revisions to the design during production to correct various problems that cropped up--some stability related. The early ones were kind of half baked.
And, running the amp on low AC line voltage (as created by your light bulb for example) may cause the power supply to the op-amp to drop out of regulation. And that, in turn, exposes the front end to ripple on the power rails which the op-amp should be able to reject but doesn't seem to like in practice. So it's possible your circuit is either oscillating due to low voltage and/or unable to reach a happy DC operating point due to the light bulb.
So the light bulb *may* be full on because the amp is oscillating or just can't reach the correct DC operating point. It could also be a wiring error or dead/incorrect component somewhere. A scope will tell you if it's oscillating as it tries to power up.
Some 400-II's had some significant turn on "thumps" so that might also contribute to a DC operating point issue. It's possible the circuit is trying to power up normally and in the process the light bulb is interfering--i.e. it *MIGHT* be OK if you took the bulb out. But you could blow a lot of transistors finding out the hard way. It's much better to figure out if there's a wiring error, a dead component somewhere, or if it's oscillating. But with no load connected most amps don't have a problem with low voltage causing massive current draw so something else may well be wrong.
Also, the dropping resistors for the op-amp power supply, as pointed out, ran WAY too hot and cooked everything nearby on the PCB including the solder connections. So that's probably why you're seeing different values. The factory resistors were too low in resistance and in their power rating. And be aware of any and all nearby components and solder connections. Also check to make sure the op-amp is getting a properly regulated supply even when the rails dip under load (when and if you get it working).
To figure out the op-amp pinout, just trace the power pins back to the big power resistors and see which rail they go to.
Overall the only thing I really liked about the 400-II were the cool (for their day) LED meters. And, of course, it was cheap on a dollar per watt basis. Frat houses everywhere loved them. Until they blew up (and sometimes took speakers with them). I've heard it's very rare to find any these days that haven't been repaired at least once and some (like yours) have been hacked to death because the original design had so many issues.
Personally, I'd be tempted to give up on Carver's 1980 "better idea" and use a 2007 solution like the LME49810 which should drive the 400-II output stage (or otherwise make use of all those transistors you bought). And you can dump that toaster of an op-amp power supply too. The stability, distortion, noise and slew performance should all be significantly better than the Carver design plus you get the Baker Clamp clip circuit. There are some LME49810 boards out there including the driver board from http://pansonaudio.com (panson_hk here in diyAudio) that should make it a relatively easy upgrade if you can sort out the output stage, Vbe multiplier (bias tracking), etc.
And, finally, you may want to replace and upgrade the main power supply caps if you can find something better that will fit. They were seriously undersized and, if they're the originals, are now nearly 30 years old and electrolytics don't age very well.
If the lamp is continuous lighting then there is a serious short circuit somewhere.
In my experience it is usually down to an output transistor gone or a capacitor gone short.
If there is a problem with the bias stage then just putting in new output transistors wont fix the problem.
What I do is remove all transistors and check them for hfe on a multimeter. While the transistors are out I also check resistors for value and caps for shorts.
I connect the VAS output back into the LTP to check teh driver is working OK.
Then I would turn the bias down to zero and reconnect the output transistors.
By this point you should have found the problem or have a working amp.
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