I have a 700B that came in the shop that I want to plug into a dim bulb tester before I turn it on. The customer states that it screeches and the panel lamps dim when a signal is applied. YIKES! He said it had been recapped, a watts relay board was installed and new gold inputs jacks installed. I did pop the cover and read the instructions for installing the watts relay. The wiring is professional looking and is wired according to watts. Also he said all the caps came from Mouser including the large ones. I will post more when I get the correct wattage. Thanks.
What wattage rating for a 700B dim bulb test?
- Thread starter DSMtech
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https://www.youtube.com/shorts/y3uaSBfGMFM
Hooked up through a dim bulb test unit. Apply a signal to either input and both channels buzz. Weird. Gonna be a tough one. Never had any amp do this. Won't be back in the shop till Monday. A lot of thinking and reading to do in the meantime.
Hooked up through a dim bulb test unit. Apply a signal to either input and both channels buzz. Weird. Gonna be a tough one. Never had any amp do this. Won't be back in the shop till Monday. A lot of thinking and reading to do in the meantime.
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I have read that thread and have some questions about it I'll post in that thread. I intend to polish all the connections to the caps including the buss bar and get some star washers this weekend for all connections. It could be the person installing the new caps didn't reuse any original hardware. Thanks.
I checked the fuses early on for continuity but not whether the fuses were the correct length and type. I also brought it up on a variac using the dim bulb tester. No excessive current on my Sencore isolated variac. I am in the habit of looking closely at units for burned components and will notice an odor right away. No odor this time around. I also tested the outputs for CE shorts and found none. This is a weird one.
1. Read the attached manual, especially troubleshooting/testing transistors.
2. Read the attached manual, especially troubleshooting/testing transistors.
3. Read the attached manual, especially troubleshooting/testing transistors.
4. If you refuse to read the manual, don't ask for more help.
5. Do not pull the outputs/drivers.
6. Bulb is only used to start the amp. You don't have any input/output connections on the amp with the bulb.
7. Check the AC ripple on the caps.
8. If bulb dims, that's mostly good, then check DC offset. Should be less than 50 mv.
2. Read the attached manual, especially troubleshooting/testing transistors.
3. Read the attached manual, especially troubleshooting/testing transistors.
4. If you refuse to read the manual, don't ask for more help.
5. Do not pull the outputs/drivers.
6. Bulb is only used to start the amp. You don't have any input/output connections on the amp with the bulb.
7. Check the AC ripple on the caps.
8. If bulb dims, that's mostly good, then check DC offset. Should be less than 50 mv.
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Also check cap polarity. The upper cap is B+, it's
negative post attaches to the buss bar. The lower cap is B-, it's positive post attaches to the buss bar.
You can check the rectifier by disconnecting it's output wires from the caps.
It's a center tap transformer, so the transformer wires to the buss bar are relevant, as it'll still be your ground. See what value VDC the rectifier is outputting to those ring terminals when they are disconnected from the caps.
Also look for cold or broken solder joints where the center tap and ground wires attach to the buss bar.
The other guys who rebuild the original control boards can fill you in on the bad caps, open resistors, and corroded/noisy transistors.
And, each channel has three wires going to a bias transistor mounted on the chassis. DO NOT stress that wiring/bias transistor connection.
And, read Dons' "wattsabundant" post again.
negative post attaches to the buss bar. The lower cap is B-, it's positive post attaches to the buss bar.
You can check the rectifier by disconnecting it's output wires from the caps.
It's a center tap transformer, so the transformer wires to the buss bar are relevant, as it'll still be your ground. See what value VDC the rectifier is outputting to those ring terminals when they are disconnected from the caps.
Also look for cold or broken solder joints where the center tap and ground wires attach to the buss bar.
The other guys who rebuild the original control boards can fill you in on the bad caps, open resistors, and corroded/noisy transistors.
And, each channel has three wires going to a bias transistor mounted on the chassis. DO NOT stress that wiring/bias transistor connection.
And, read Dons' "wattsabundant" post again.
The PL700B is a very simple amplifier. You approached the amp correctly. This amplifier has three physical 'blocks'.
Block 1 is the Power Supply. Nothing good happens with a defective Power Supply, of course. As others have said (like resident expert George S.) , disconnect the DC "Rail" wires on each of the main Power Supply capacitors first. Check for the DC voltage between the Positive Rail and the STAR ground, which is the buss bar between the Power Supply capacitors. Verify +100V and -100V. Then switch your meter to AC and measure the AC ripple voltage just like you did the DC voltage. If it is below 50mV, you are good. I always replace the Bridge Rectifier, they are inexpensive ($7.00 on the website below - with snubber capacitors). A Power Supply capacitor installed incorrectly will cause the DBT to get bright very quickly. Both Power Supply capacitors installed incorrectly? Hmm, never saw that one...
Block 2 is the Control Board. No voltage checks here, too many possible issues and if you fix one there will be another; the original control board is not worth troubleshooting. Purchase a new White Oak Audio Rev G1 Dual Mono Control Board kit with the Phoenix connectors here: https://www.whiteoakaudio.com/estore.aspx Instructions included. Huge improvement in sound quality, your customer will like it.
Cut the wires close to the original PL Control Board. Then tie wrap the three leads going to the Bias Resistors located on the back chassis wall to something immovable or the leads will break. Wire connections are ID'd by color code but they were not always consistent, best to verify the origin of each wire (in the Instructions).
Block 3 is the Backplane and Output Transistors. Look for obvious issues on the Backplane, typically burnt/cracked resistors, cold solder joints, broken wires, etc. You can pull all the fuses except the 120V main power fuse and isolate Left and Right channels individual B+ and B- Rail voltages to identify a bad Output Transistor bank. However your symptoms do not include blown fuses or a bright DBT indication, so the Output Transistors are probably OK. You CAN bring the amp up and play music with only the bottom row of Driver Transistors installed, likely RCA 410 or RCA 66546 (they are the same). Just keep the input very low or you will fry them. Installing one row at a time and then checking for shorts, smoke, and DC voltage at the Left and Right outputs of less than 10mVDC, plus a good output sound will help identify any faulty Output Transistors.
Lots of good answers here on this forum and some critics too (aren't we all). Good Luck
Block 1 is the Power Supply. Nothing good happens with a defective Power Supply, of course. As others have said (like resident expert George S.) , disconnect the DC "Rail" wires on each of the main Power Supply capacitors first. Check for the DC voltage between the Positive Rail and the STAR ground, which is the buss bar between the Power Supply capacitors. Verify +100V and -100V. Then switch your meter to AC and measure the AC ripple voltage just like you did the DC voltage. If it is below 50mV, you are good. I always replace the Bridge Rectifier, they are inexpensive ($7.00 on the website below - with snubber capacitors). A Power Supply capacitor installed incorrectly will cause the DBT to get bright very quickly. Both Power Supply capacitors installed incorrectly? Hmm, never saw that one...
Block 2 is the Control Board. No voltage checks here, too many possible issues and if you fix one there will be another; the original control board is not worth troubleshooting. Purchase a new White Oak Audio Rev G1 Dual Mono Control Board kit with the Phoenix connectors here: https://www.whiteoakaudio.com/estore.aspx Instructions included. Huge improvement in sound quality, your customer will like it.
Cut the wires close to the original PL Control Board. Then tie wrap the three leads going to the Bias Resistors located on the back chassis wall to something immovable or the leads will break. Wire connections are ID'd by color code but they were not always consistent, best to verify the origin of each wire (in the Instructions).
Block 3 is the Backplane and Output Transistors. Look for obvious issues on the Backplane, typically burnt/cracked resistors, cold solder joints, broken wires, etc. You can pull all the fuses except the 120V main power fuse and isolate Left and Right channels individual B+ and B- Rail voltages to identify a bad Output Transistor bank. However your symptoms do not include blown fuses or a bright DBT indication, so the Output Transistors are probably OK. You CAN bring the amp up and play music with only the bottom row of Driver Transistors installed, likely RCA 410 or RCA 66546 (they are the same). Just keep the input very low or you will fry them. Installing one row at a time and then checking for shorts, smoke, and DC voltage at the Left and Right outputs of less than 10mVDC, plus a good output sound will help identify any faulty Output Transistors.
Lots of good answers here on this forum and some critics too (aren't we all). Good Luck
Yes I read the manual last week and tested the outputs according to instructions. In my reply I should have said check for defects rather than CE shorts. I know it would be a bitch and waste of time at least at this point to pull all outputs. I was getting frustrated as I have been spinning my wheels on this one. On a scope the AC ripple is very very low so I don't expect defective caps. When powering up the bulb dims very low and the DC offset on both channels is very low. I was kind of surprised at the readings compared to other brands of 70's amps and receivers I've serviced over the years. I;m going for the power supply. I have a feeling it is unstable for some reason. Thanks to everyone here for the suggestions. I'm not expecting you guys to fix it for me. I'm not one to give up on anything. I'll get it going if I can.
Naturally, the Power Supply is common to both channels. However, if the amplifier is quiet with no input signal (idle) then the problem is not with the Power Supply. The Power Supply provides the Control Board and the Output Transistors with full rail voltage whether there is an input or not.
On the original Phase Linear Control Board there are many shared power connections common between the Right and Left channels. On the other hand, the White Oak Audio Rev G1 Control Board is a true Dual Mono circuit board. You could break it in half and it would work just fine.
Save yourself some time and just order a new WOA Control Board.
On the original Phase Linear Control Board there are many shared power connections common between the Right and Left channels. On the other hand, the White Oak Audio Rev G1 Control Board is a true Dual Mono circuit board. You could break it in half and it would work just fine.
Save yourself some time and just order a new WOA Control Board.