[KCMike]

Main amp has each side fuse protected. When I hooked up the main amp. I tried each side seperately (one fuse pulled). It drew the heavy current either side. For the short time I had power to main amp. I could here a steady hum from speaker and if you touched the chassis or controls the hum got lower. Keep in mind this was at 50 volts on variac and I didn't want to leave it on long. As to main amp. parts I replaced all transistors except Q1 & Q2 these are 2sc458LG. I did remove them from circuit and they tested good. I also replaced all the Ecaps on main amp.
Tested the 2-STV diodes in circuit each test good. They are down in the heat sink and I heard very hard to obtain if bad.
I did not replace any ceramic or mylar capacitors or any resistors.
If there is a overload on B2. That is the control amp. unit. Should I replace the ecaps on it and test the transistors out of circuit?
I have the parts to do it but don't want to change too much for fear of creating a problem that is not there.

[morzh]

1. Oh....how about powering main amps without speakers!
2. See if C15 - C18 are not shorted.
3. Put VR3 and 4 to the shorted (minimum resistance) psoition.
4. Measure +B voltage and at the same time the voltages at C13 and C14 Positive. Those should be about half of B voltage.
5. Measure voltage between the Bases of Q5 and Q7, and same air in the other amp (cannot make the ref designators).

[KCMike]

I tried powering amp without speaker,I just have one 4ohm. bookshelf speaker to test with.
Checked C15-C18 with ohm setting of meter in circuit I get a reading of like 1.504M on all of them.
V3 & V4 you said to minimum setting. I turned them full ccw.
Powered up and still drawing over 1 amp at 50V. I did have volt meter hooked to C13 + and it was reading 18volts when the B+ of amp was at 50 volts.
I unhooked the B+ to main amp and started taking volt readings at power supply again with outlet volts set to 117V. With the main amp out of circuit I can power up with amps staying .1v
I found something that might be the problem,not for sure.
I can get a AC reading on the B1 post which feeds the main amp. It is 25.40v ac. also can get reading on DC setting it is 54Volts.
I don't get any ac readings on the other B+ posts.
On the power board I have replaced all the E-caps, transistors Q1, Q2 ,Q3, the diodes D-2,3,4,5.
and the zener diode D6. I did remove D1- SB-3P and it tests good. I have not changed any resistors. Do you think something on power board is letting ac into the B+ to main amp? Your help is greatly appreciated.

[morzh]

Well, I would think these caps had something to do with it but they are 1) small and 2) should cancel each other.
Are you sure all your diodes in your bridge rectifier are good?
Seems like they are (otherwise why the regulated voltages are close to what they should be), but have you made sure?


Oh...wait a sec....the sch does not show a bulk electrolytic cap after the bridge rectifier on B1.
Where is it?

[KCMike]

I rechecked the diodes on the fluke DMM diode setting testing good. C4--2200/80 I think is the cap that goes after B1 to the main ampl. I have unhooked and bypassed it with a new one same uf and voltage rating I think it is ok though, because it holds a charge after power is off.

[morzh]

Mike


Acc. to the sch C4 is 220uF/25V and is filtering B4....

OK, on the top level C4 is what you said....

Well, as I said, you do not have DC loads there,
so if when attaching either L or R amp with no load you draw large current you then should look for either a short or a bad part in the amp.
There are switches connecting headphones and such....

When amp is connected, is its output disconnected from everything?
You need to connect ONLY B1 power but nothing else should be connected to the power amp. Only Power, that is B1 and E.

And make sure E1 - E4 are all connected.

[morzh]

And VR1 and 2 set the DC point, that should be the half of B1 at the output when measured before the output cap.

Also make sure reistors R24 and R28 (10 Ohm) and R31 and R33 (1/2 Ohm) are Ok.

[KCMike]

Well I did have all the wires connected to the amp. I did not know it was safe to leave then out of circuit. I will retry power up with just amp B+ and E which is the grounds I believe. I will also check the VR1 &2 settings. The 10 ohm and 1/2 ohm resistors are checking R 24 R28(10) R31 &33 0.5 and 0.7.

[morzh]

Yes. Remember there are two Es for each amp (E1-E4 ebtween the two of them), they separatly connect input and output commons to GND, both should be connected.

[KCMike]

Well the problem seems to still be here. I hooked up main amp. with B+ and the Es for each side connected. Drawing over .7 amps at 65v. Tried each side of amp with one fuse removed also doing it on both sides. when I turned VR1 &2 to clockwise the amperage came down a little ccw caused it to climb. I did take a voltage reading of the two power transistors on one side of amp.The Q10 & Q12. this is the (out#2 side)
(Q10) BtoE +.743v) B to C neg. 1.5v) (Q12) B to E .630v) B to E 34v). They were getting very hot and I noticed the amperage on the meter had climbed to 1 amp so I turned off with out getting any more readings.
The power transistors I installed were from mouser (pn. 2n3442G)-10A 140V 117W NPN. The only thing I know to do now is wait for your advice on how to proceed.

[morzh]

What I don't quite get is this:

1. 0.7A at 65V, does it happen with both channels or just one connected? Does not even matter which one? Are you sure.
2. You still know for a fact all your transistors are good?
3. If your Q10 Vbe is almost zero and Q12 Vbe is 34V and your B+ is 65V something should be really smoking. That something would be your R32/R34.

Now, IF both your E2 and E4 are grounded like they should be and B+ is connected, here is your DC path for Q4:
R14 - R20 - D2 - VR4- Q4 C to E - R22. This sets up the DC output.
Now the feedback tht sets it is
R16-VR2-R18.

If you were to remove the output transistors (totally, disconnecting all wires) the VR2 will bring the output (C14plus) to the midpoint of your B+. And the Vc of Q4 should be close to it, maybe a volt or so lower.
If after that you add your power transistors and it gets screwed again, either they are bad or 1/2 Ohm resistors are. Or your C6 or C8 is leaky.

[KCMike]

Ok Its my fault for not being clear on the 65v 0.7amps. Those figures are65 ac line volts that I brought the variac up to. The B+ going to main amp was 35V DC .
The 0.7 amps is what the ammeter in series with the varic is reading. If I raise the volts higher with variac the amps head for 2A and would probably peg the ammeter its on the 3A scale. I believe if were to just plug into house line 120v it would blow the fuse or burn something up.
I will recheck everything you just mentioned and remove and recheck power and other transistors.

[morzh]

SO, one of your transistors is fully open and another is fully closed. Namely, Q10 is closed. Which means that Q4 is fully open.
This sch with all transistors being OK cannot work like this. Make sure output AND the ones before them (Q6 and Q8 ) transistors are intact.

Q: how do you check them transistors?

[KCMike]

When I check a transistor I remove from circuit.Based on PNP or NPN I put pos. on base or neg on base and do the BE BC reading on ohm setting then do E to C. and CtoE should get no reading. then pos. or neg. opposite of what it should be. example NPN I put the neg on the Pbase and should get no reading to E or C. If this is wrong correction is appreciated. ( The Q3,4,5,6 (NPN 2SC497) and Q7,8, 2sa497 PNP I installed new with NTE128 for the NPNs and NTE129 for the PNPs Q7&8).
I suppose one of them could be bad or burnt out when I first repowered unit up but the symptom was the same as before I did any work on the unit. I have ordered new resistors and the small value caps for the board and will recheck all the transistors out of circuit.

[morzh]

No, your checking methodology is correct.

Have you looked at possible shorts in heatsinks etc?

Try to do that test without output transistors too.