| Kyllswytch |
$U?"¯ª4igned up and do not have electronic repair skills. I thought by searching and reviewing some posts I might be able to figure out what's wrong with my amp. What I found is just way over my head.
Here is what I have and what is wrong:
Phase Linear Model 700B Series II-
The right side channel gets very low sound, but does produce some and goes up and down when I turn the sensitivity knob. The sound is extremely low (light bar never moves). The left side seams to work ok except that while turning up the sensitivity knob the sound cracks and goes in and out. Playing with it I can get great sound out of the left side.
I don't think I have the skills to fix this on my own and was hoping it is worth fixing.
I live just outside of Philadelphia, Pa maybe someone here lives close by that knows how to fix this or knows a good shop that won't rape me for repairs.
Thanks for any help. |
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| Netlist |
Before you take heavy measures, you will want to look at your interconnects and the input sockets of the amp. The latter are prone to fail, at least in the units I serviced. Next, tighten all output transistor screws mildly and check if your speaker cables are well attached.
/Hugo |
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| bear |
First check the B+ fuses (rail fuses) that are on the back rear top of the amp... If one or more is blown, that indicates a blown fuse, and possibly a blown output transistor(s).
If you replace the fuse - take the fuse from the other channel's fuse holder. It needs to be the proper TYPE (physical size) which is slightly off the beaten track for fuses and rating (amperage).
So, once you have replaced the fried fuse and turn it on, and it turns the good fuse into vapor, you know the outputs are blown - at least in part.
PM me if you want to unload the beast.
The repair is on the order of $200 from a tech who is being nice - since it takes time and the power transistors need to be tested and replaced as a "unit" per rail, per channel. So if ONE was bad, unless you had the matching OLD devices, you'd have to swap in an entire row of new devices on that side (preferably on both sides...)
_-_-bear |
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| Kyllswytch |
Guys, thanks for quick responses. I checked the fuses they are not blown. I will check the input sockets next. I've tried switching the cables and I get the same issue.
I'm pretty good with an ohm meter, but I don't have a good schematic or testing instructions to go much further.
Again thanks for your input. |
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| Kyllswytch |
Netlist, you are the man. I played with the input connections and the channel came up. What great feeling hearing this thing again. There are still two things I wish I could get fixed.
1. The right channel light bar does not move at all. Just stays all the way to the letf.
2. The sensitivity knobs crack while being adjusted.
Thanks again! |
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| Netlist |
Don't forget to tighten the big transistors on the back of the amp. There are 24 of them. Make sure you unplug the mains and leave the amp for at least half an hour before proceeding.
As for the schematic, it's here
You could use a tiny bit of contact cleaner on the sensitivity knobs - inside the amp of course - and see if the light bar comes up when playing with the meter range switch. If it does a bit of cleaner might fix that as well.
/Hugo |
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| Kyllswytch |
Hugo, I'll try that tonight. Do you think I should replace the input sockets? If so where could I find a replacement?
Thanks |
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| Netlist |
I like the Neutrik gear a lot and something like this is cheap and widely available. Of course any other good quality socket will do.
Together with these plugs I make nice interconnects.
Note that I'm not affiliated with Neutrik in any way.
/Hugo |
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| Kyllswytch |
I used a little contact cleaner on the sensitivity knobs and meter switch. Wow what a difference that made. Absolutely clean sound while turning the knobs. Very sweet! Thanks Hugo you have probably saved me a few hundred bucks here if just dropped this off at a local shop.
Although the right side meter still doesn't get any movement, would you have any suggestions?
Paul |
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| Netlist |
Paul,
No idea about the meter. As you can see on the schematic the meter PCB is located on the top right.
Both left and right outputs are connected via a 240k resistor and the meter range switch to both pin fives of the PL37. I don't have a more detailed schematic of the LED meters but with a simple multimeter it should be possible to trace the signal.
However, suppose the good working meter has a LED lit even without music playing and the other channel hasn't, it could be a power supply missing. Does gentle tapping on the meter solve the problem? Look for a loose wire around the range switch or PL37.
In any case - and I don't want to sound lame - be careful when you poke around in the working amp.
/Hugo |
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| d3imlay |
What you have is a 700II. The 700B is an earlier, all discrete version. The 700II uses an opamp on the front end.
1. The input jacks are standard switchcraft devices. I almost always replace them during a repair as they are usually loose.
2. In the last year every series II I've worked on has had bargraph troubles. There is a coupling cap that fails. C304, I think.
3. Be very careful when tightening the screws. Use a nut driver with thumb and index finger only. Hold the nut driver loosely. When it starts to spin in your hand the screws are tight. If you go any further you'll strip out the socket or puncture the insulator. The sockets are not widely available.
4. There are a number of updates that were published in the tech manual. I have them at my web site. Here is a link.
http://home.earthlink.net/~dimlay/
5. There are some unpublished updates. For the most part it involves drifting power resistors. |
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| Kyllswytch |
Just to be clear the right side LED does light up, but does not meter or move along when playing music as does the left side meter. Does this info help?
Also on the back of the amp it says Model 700B S/N 7408 and on the front it says Model 700 Series Two.
I hope this helps and thanks for all your input.
Paul |
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| Kyllswytch |
By the way, there are red trim pots on each of the display boards. Should I try adjusting the one on the right side (non working side)? What do these do?
Paul |
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| Netlist |
This is your amp.
All previous models didn't have the LED bar display.
I wouldn't touch the pots for now. Also, since we have no schematic of the boards, it could be interesting to post a picture of the component side if you feel like disassembling the front. That would make it easier to find C304 or other faulty parts.
/Hugo |
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| Kyllswytch |
Yes, that's the one.
I'll take some pics front and back of the display board. I have no problem taking it apart.
Paul |
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| d3imlay |
| The trim pots are for calibration of the level controls. C304, as I recall is near the center of the board and is mounted horizontally on the board. I think it's a 4.7 mfd. |
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| Netlist |
Nice to know. Any change you might have a schematic of the PL37?
/Hugo |
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| d3imlay |
| Here is the display info. C304 is in the upper left hand corner near the xistor. I'm not responsible for any problems associated with the use of this information. |
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| Kyllswytch |
I’ve been called away on business and will be back on the 2nd. I will take some pics when I get back.
Thanks again for all the help. |
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| Netlist |
Thanks for the PDF's. :)
/Hugo |
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| d3imlay |
| If anyone finds the PDF's I've posted to be of value please show your support to DIY audio on behalf of me. |
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