The amp was noisy and the reverb worked, so I recapped and changed tubs. Re-assembled the amp sounded good but the reverb did not work. I opened it up and checked all my solder joints. Replaced both 3.9 ohm (R39 & R40); both 390 ohm 3w (R42 & R43); and both 150k ohm 2w (R49 & R50) resistors. I re-assembeled the amp and I got a bias read of 24mv across one of the 3.9 ohm resistor and 0 volts across the other 3.9 ohm resistor. With both 6CA7 tubes pulled I energized the amp and with an Infrared Thermometer I got 150 F on R32 (3.3k ohm 2w resistor); 130 F on C22 ( 20uf/25vdc cap); 150 F on R75 (3.3k ohm 1w resistor) and 130F on D8 (4003). I guessing if I located R75 & D8 on the schematic correctly. Chassis # 2165RD & circuit board #GD-2. Any ideas on why I am getting the high temp read on those components?
Sun, 03/05/2017 - 18:50
#1
Things to try
Hi Craig, If you know how to do a simple forward bias test with DMM, check both power transistors to make sure one has not gone open circuit on you. https://www.google.com/search?q=dmm+transistor+test&ie=utf-8&oe=utf-8 After that, verify that both 3.9 ohm resistors are connected to ground as shown in the schematic. Verify that the other end of each 3.9 ohm resistor is connected to its respective power transistor emitter. If one of the resistors is not grounded or not connected to the transistor as described, you won't measure a 25mV voltage drop across the 3.9 ohm resistor. Make sure the tubes are properly seated in their sockets. The power transistor collectors need to connect to the tube cathodes for current to flow. You can measure the voltage at the transistor collector to see if the tube is behaving properly. You should see about +60 to +80 VDC at the collectors if things are working properly. The tubes need to be installed to make these measurements. For the bias control circuit, make the following checks. Verify R32 resistance is around 3.3kOhm. Verfify +40VDC at the top of R32. Verify +1.2 to +1.4 VDC on the other side of R32 where it connects to the two series Diodes D7 and D8. With power off, check C22 20uF cap to ensure it is not shorted. Finally, verify that the 4ea 470 ohm resistors (R34-37) in the bias network are not cracked, have the correct value and are properly connected to the circuit. Long story short, you need to ohm out the circuit, verify componant values as best you can and ensure all proper connections. Hope this is not too confusing. We'll get you through this troubleshooting exercise. -mgriffin
JE 1692 transistors replacement
thank you for your help. I removed the power transistors from the board and the one in question tested bad. No read from the collector to the base. The transistors are JE1692. I see that someone mentioned in one of the threads that 511-2n6488 from Mouser is a good replacement, however in a MM bulletin the replacement transistor is NPN 2N6488 15amp, 80v. I did not have any luck finding this transistor listed in Mouser's on line site. Can you advise me on the transistors to oder, also can you think of any other components to oder? thx craig
2N6488G
Hi Craig, Try this...http://www.mouser.com/ProductDetail/ON-Semiconductor/2N6488G/?qs=sGAEpiM... Good job finding the bad transistor. -mgriffin
I had the bass, collector &
I had the bass, collector & emitter legs incorrect when I tested the transistors. I retested once I figured out the correct leads and the transistors tested good but when I removed them from the board I broke one of the leads on the transistor. So I ordered new ones from Mouser. R34-37 around 390 ohms; R32 3.3k ohm tested ok; C22 tested ok. I will install the new transistors when I receive them in the mail, insert the tubes and apply voltage to check the voltage values that you mentioned earlier in this thread. Do I need to mount the transistors to the classes before energizing the amp? I don't want to burn-up the new transistors.
You're on a roll.
Hi Craig, Was the broken transistor leg the one that showed 0mV across the 3.9 Ohm resistor? That would make sense. The hiccups you've been running into are common on an old amp that has not been serviced in a long time. Since you're getting new transistors, I would advise you to also change out the 20uF bias control storage cap C22. Your call. Also, while it's apart and powered off, it might be a good time to hunt for cracked resistors. It happens on those old Carbon type resistors. Connect your dmm to each resistor and tap the resistor with the end of a pencil while watching the reading on your DMM. If the reading gets bouncy, consider replacing it. Repeat for each resistor in the power amp circuit. There are like 6 470 Ohm resistors, the 2 3.9's and the 3.3K . Finally, yes you should mount the transistor tabs to the case heat sink before powering up. Don't forget the goop and insulator. The 2N6488G is a much beefier transistor than the OEM JE's, but that's no reason to bypass good assembly and test technique. Be safe and Good luck. -mgriffin
still not working after installing new parts
I installed the 2 new transistors; 6ea. 470 ohm 1/2w; 2ea. 3.3k ohm 2w; 1ea. 1.5k ohm 1w; and 5ea. 0.1uf 500v resistors. I get a read of 18vac from each orange lead to ground. What next?
What's not working exactly?
Hi Craig,
are you saying you have no sound at all?
Sincerely,
Lars Verholt
I found today that D16 was
I found today that D16 was lifted off the board. I will dig back into it tomorrow and report back. thanks craig
amp is working, reverb is working
I had help rebuilding and fixing the problems (thanks to Mike he told me where to check the voltages and posable parts to check). I did some sloppy soldering on the circuit board which caused some diodes and caps to fry. I replaced the fried components and rechecked all my soldering joints making sure that I did not solder the wrong leads together again. This type of circuit board is very delicate and is easy to break the foil tracers. In the future I would probably cut the old parts out leaving the leads soldered to the circuit board and solder the new parts to the old lead.
Congrats on getting your amp
Congrats on getting your amp back in service!
Although I don't find MM boards particularly delicate, they are 30-40+ years old and so I try to "disturb" them as little as possible. I was taught to cut the old components off close to their bodies, leaving as much original lead soldered to the board as possible. Once gently straightened and cleaned, the old leads become the "posts" around which I wrap the looped ends of the new components leads. With the correct temperature soldering iron and a quick, light touch, you can get a solid physical and electrical connection without issue almost every time.
if I work on another MM amp I
if I work on another MM amp I will snip the old parts out and use their leads as you did.