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« Last post by Roundtone on July 12, 2019, 01:35:00 PM »
In review of my comment I realized that the connector shown on the right is actually the preamp output to the power amp (the tank return is out of the photo). There are only two wires from the reverb circuit send to the tank. Also there are only two wires from the tank out to the preamp reverb recovery. They all share common ground. What range of voltage should I expect to see on the signal send to the reverb tank? Guessing... my reading is low and the tank is absorbing all the signal. This might point to a faulty drive transistor?
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« Last post by Jazz P Bass on July 12, 2019, 12:51:42 PM »
Nice find on the L3 schematic, PE.
It appears that R9 is a variable resistor for idle current adjustment.
Measuring the voltage drop across R16, 0.22 ohm ballast resistor, will give you your bias voltage.
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« Last post by phatt on July 12, 2019, 08:29:02 AM »
Thanks Phil - I appreciate the explanation very much. I need/want to comprehend the theory of operation of everything I work on. I have much to learn. You are welcome.  This guy has a ton of well written pages and I highly recommend it; Main page; http://sound.whsites.net/index2.htmlClick on Articles,, Projects,, and Site Map to find a ton of in depth info  I've found it easier to use the site map to find stuff of interest. The Reverb circuit (preamp) schematic is posted earlier in this thread.
Durh  silly me yep I should pay more attention,,  I can't see the driver plug clearly but the return plug has 3 wires. This makes me think there might be a ground missing for the drive. Darn hard to explain this with words but always remember you have to check the ground path of circuits as well as the hot wires. If driver output is grounded via the return plug it may not be connecting back through the tank. look at the schematic might help. Phil.
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« Last post by phatt on July 12, 2019, 08:13:39 AM »
Sounds like you are mixing up test points;
CO is the Poweramp output which sits at Half the supply voltage.
The highest voltage in the amp is at +B (next to the 3 Amp fuse,, which supply the collectors of TR20 & TR22,, as well as TR24 & TR19.
Or pin 6 on the board.
PIN5 should read half voltage which is the *CO* voltage,
So if you have 100VDC on +B supply then *CO* will read 50VDC.
IF point *CO* also reads 100VDC then likely the power transistors are dead.
The fact that you have altered that trim pot means the amp is now overheating and pulling a LOT MORE Current dragging the supply lower likely to kill even more parts.
As I said before check those voltages which are all over the schematic,, the ones in little boxes,, they are the DC voltages you should be seeing if they are way off it gives a clue as to what might be stuffed.
If the Amp is working close to spec then you should see some where close to 137VDC at the Collectors of TR24,19,20,22. with all controls at zero.
if you only get 90~100 then something is likely blown and dragging the supply down.
Phil.
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« Last post by ryanckelso on July 12, 2019, 03:56:42 AM »
Phatt,
I am going to try that test later today.
I thought I would just update everyone.
I took some extra readings. The +CO Voltage is supposed to read +45Vdc according to the service manual.
The reading was +115V, which is way off.
There is a pot in the MA board (VR2) which is a B470k trimmer. I turned this anti-clock wise to the max and now the +CO Voltage = +90Vdc
This has resulted in a louder sound than before, and stopped the surging when powering off. The volume is still too low, and when maxed is a reasonable room level sound.
So I feel that the MA circuit is the culprit.
Any thoughts on what might have failed on the MA board?
16
« Last post by Roundtone on July 11, 2019, 11:27:53 PM »
Here are some voltages and a layout shot of the reverb output section to the tank input. The dual molex connector on the left edge of the photo is the send to the tank. The one on the right is the return from the tank output.
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« Last post by edvard on July 11, 2019, 09:33:15 PM »
Try this: https://mega.nz/#!1ph3CSrY!44oxSuZl6oZOZnnXJ12ikMNFncLJ2-AqZqEFzqUrnKgInteresting schematic, that L3; it uses a CD4007 strung up to use the top half of it's 3 inverters as P-MOSFETs running the tone controls and another for the reverb driver/master out. I don't see anywhere that counts as adjustable bias, but I could be wrong...
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« Last post by Jazz P Bass on July 11, 2019, 11:18:09 AM »
I can't seem to put my finger on the L3 schematic. If it is anything like the L5/L7, then the bias is fixed. 3mv across any output ballast resistor. http://www.rru.com/~meo/Guitar/Amps/Lab/schem.html
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« Last post by Roundtone on July 11, 2019, 10:07:31 AM »
Thanks Phil - I appreciate the explanation very much. I need/want to comprehend the theory of operation of everything I work on. I have much to learn.
The Reverb circuit (preamp) schematic is posted earlier in this thread. I will post voltages again tonight for that part of the circuit (at the correct magnitude to ground) if you or anyone else wants to take a look. I have successfully tested the wires/jacks/tank and all are ok. Here is a close up - the +/- 50 Vdc input voltage is good.
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« Last post by phatt on July 11, 2019, 08:34:46 AM »
I may not be reading it right but the best I can make out is that it all seems ok.
Easiest way to grasp this is maybe I try and explain the basic concept.
It's a split supply and the output sits at Zero VDC.
It's never perfect so always a little DC offset. So 160mV of DC on the output (Pos or Neg) is normal, as long as it's close to Zero.
So the Emitters of the top 3 Transistors are going to be very close to Zero VDC.(which is what i think I'm reading) While the Emitters of the bottom 3 Transistors are going to be within a bees dick of the Neg rail hence you noted -52Volts on those resistors.
The basic plan of these circuits is to keep the base's of the two input transistors which form a LTP (long tail pair) as well as the output as close to Zero as possible, Feedback (that 100k) helps to keep this in check.
The diode string (2 in that dotted box and the one below) bias the output Tr's. The 2 Tr's beside the bias are just for safety,, they limit the output current if the output is shorted out.
The Tr down the bottom left is the voltage gain and then sent to the output TR's which turn that into current to drive the low Z speaker.
Hope that helps.
without a full schematic No way to tell what might be wrong with Reverb.
Phil.
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