HH MA150 NO SOUND?

[J M Fahey]

Houston, we have a problem  :grr

The same transistor designation was repeated twice in the same schematic and PCB  :grr   :trouble
Q1 and Q2 are used at the beginning of the power amp and also as power transistor designations.
To differentiate them I'll use Q1 and Q1(p) (power)  , same with Q2 and Q2(p) so please follow the same system.

Modern equivalents first best, then acceptable if best is impossible.
Q1: TIP29C = available
Q2: SM2177 = MJE15031 or TIP32C
Q3: BC204 = available
Q4: BC207 = available
Q5: BC204 = available
Q6: TIP47 = available but MJE15030 is better
Q7: SM2177 = MJE15031 or TIP32C

Q1(p)/Q2(p) TO3 case: HH9080 = MJ15024  BUY ONLY FROM AN ON DISTRIBUTOR OR MOUSER OR (maybe) FARNELL .

[Enzo]

When you see a house number like SM2177, look at the circuit.  Q6 is a TIP47, which means that Q7 will just be its PNP complement.

I also think you might as well use more modern transistors.  TIP47 could be replaced with MJE15032, while the PNP would fit a MJE15033.   Your power amp has 120v of power supply, so the transistors need to be at least 150v types, I chose 250v types to be safe.

[g1]

  For the SM2177 you can use a PNP complement to the Tip47.
Or some use MJE15033 in the HH with no problems (and you can use MJE15032 for the Tip47).

Edit:  sorry Enzo, we were thinking the same thing at the same time. 

[bazmusicman]

JM Fahey,

Thank you for beating me up, I deserve it! 
I'm sorry if I have driven you to 'tear your hair out' with frustration but I did say I was a little green working with Solid State. You have helped me  tremendously for which I am very grateful.
Now you have pointed me in the right direction I can move along. Thank you once again.

Also a big thank you to Enzo and g1 for their help.

Best regard,
Baz.

[bazmusicman]

Latest update.
I made a 'light bulb limiter' fitted the new transistors, used a hundred watt bulb switched on and the bulb flashed and went too dull to see any glow, but the amp was on.  I switched everything off and fitted a 40 watt bulb, switched everything back on, the bulb flashed and went to a very dim red - you can just see it in a very dark room - at least the Amp wasn't cooking - so I disconnected the limiter and tried the amp, but still no sound.
I took a couple of measurements from the Driver PSB that I saw on the other thread that I thought may be relevant.

+C  =  53.4v.} these two I believe are in spec?
- E  =  53.3v.}    "      "          "        "        "
  O  =  minus 51.0v.}  ( I know that this one is way too high) should be about 200ma?

Any help with the next step would be greatly appreciated.

Regards,
Baz.

[Enzo]

I think you mean 200mv - millivolts not milliamps.   But yes, 200 of them is a reasonable maximum.  You have 50v there, which means the whole thing is slammed over to the -50 rail.   If you had a speaker there the thing would blow a fuse and the speaker burn out.  No speaker until we have the output near zero.

The whole power amp is one big loop, so lets look at the 741 op amp IC.  Pins 4 and 7 are the power supply pins, +15 on 7 and -15 on 4.  Are they there?  Note that for each 15v supply, there is a zener diode and a dropping resistor from 50v.  If one voltage is missing, look at those.

Output of the 741 is pin 6, what voltage is there?  That is what tells the output where to be.  If that is over to one side, so will be the output.  See D1 in the middle?  What voltage is sitting on its ends?  That should be roughly the voltage at the bases of the outputs, they follow it.

[bazmusicman]

Hi Enzo,
Yes I did mean 200mv, not ma.  I did connect the speakers when I tried the amp but the speakers still work and no fuses blown!  Before that I noticed
Resistor R13 was very burnt, but it still measured 1K!  Neadless to say I replaced it anyway.

With your suggestion's this is what I found.

Pin 4   minus  15.03v
Pin 7   minus    0.68v
Pin 6   minus    1.15v
D1      minus   52.2v  anode,    52.8. cathode
ZD1    minus     0.69v
ZD2    minus    15.10v  anode end only?  would this be reversed biased?

These measurements were taken with the light bulb limiter in circuit.

Regards,
Baz.

[Enzo]

So you are missing +15v.  Go find it.  You have two zeners.  Note one has its cathode to ground the other has its anode to ground.  Zeners work by applying a reverse voltage to them, then their reverse voltage "breaks down" in a designed way to leave a steady voltage across the part.

Without me looking at the schematic at the moment, either your zener diode is shorted, or perhaps the resistor from the higher voltage source is open, R7.  If R7 gets hot, then ZD1 or the IC are likely shorted.  If R7 stays cold, it may be open.  However also check the upper end of R7 to make sure the +50 has reached it.

Have we verified that +50 is coming from the power supply and getting to the Q6, Q3, R7, etc.

[bazmusicman]

Hi Enzo,
Resistor R7 is open, I have a boxful of resistors
but not a 3K3!

I have + 53.2v  power feed to Q6, Q3, and to R7. I will replace the faulty R7 and take some more readings and let you know how I get on.

Regards,
Baz.

[Enzo]

You got a 3.9k? 3.6k? Just get close, the resistor limits current, the voltage is established by the zener.  Or do you have two 6.8k?  Put two in parallel for 3.4k, certainly close enough.

[bazmusicman]

No I don't have anything near or I would have used it. the nearest I have is 4.6k. 
That's a good idea with two 6.8k in parallel,  but I have some 3.3k's coming tomorrow.

Thank you for your help Enzo, I really appreciate it.   

[bazmusicman]

I fitted the resistor and remeasured:

Pin 4  minus  14.9v
pin 7  plus     14.5v
pin 6  plus     00.6v
ZD1   plus     14.6v
ZD2   minus  15.0v
speaker out   02.2mv

As the speaker out level is now right I tried it. The amp now works but very low on power and distorts.  Is there something I've missed,  Any ideas please?

Regards,
Baz.

[Enzo]

Well, maybe not something you missed so much as something you haven't got to yet.  You needed to get your power supply right before anything else mattered.  Now that we have that, we continue on to find out what else the amp might need.

Now start checking out the amp.  Plug the guitar into the AUX IN or SLAVE LINK jacks.  Does it sound OK?  That is the power amp input, so you are testing the power amp alone.

You can test the preamps too.  Set up some other amp, with a guitar cord to that amp's input.  Now plug that cord into the SLAVE jack.  Does that preamp sound OK coming out the other amp?  Now connect the other amp to the ECHO SEND instead.  Turn on the FX switch on the channel ins, now how does that ECHO send sound?  All input channels sound alike?  Or just one or two sound bad?

Remember how that fouled up +/-15v situation made the power amp not work?  Well the preamp has similar supplies.  Look just left of the SLAVE jack, see the +/-15v supplies there?  From the V+ and V- supplies there are 1k 3w resistors dropping down to 15v zeners.  So are both 15v supplies present?  Note these are totally separate from the ones in the power amp.  And do you have good 15v supplies right at the ICs on the preamp?

We need to determine which section of the amp has the problem, THEN we can think about what to do about it.

[bazmusicman]

I plugged the guitar into  the AUX IN and SLAVE jacks, both sound fine.
I set up another amp as advised to test the preamp, slave jack fine, the echo send is fine and the same on all input channels.  The two 1K3w   R24/R25 resistors both measure fine.

R15 should be 3K3 but is 1K70?  also R17 should be 3K9 but is 1K53?  would this cause the low output/distortion?
A couple of others are a little out of speck.

Measured supplies to preamp;

supply 75  minus  14.9v
supply 76  plus     14.8v
Z1            plus     14.8v
Z2            minus  14.9v

IC3
pin 4        minus  14.75v
pin 7        plus     14.79v
pin 6        plus     00.10v

IC4
pin 4       minus   14.95v
pin 7       plus      14.81v
pin 6                     0.00v

Regards,
Baz.

[Enzo]

1k7 you measure for R15?  Likely just parallel circuit paths lowering the readings.  Look at R17 3k9.  I bet that 1k5 reading changes if you turn the presence control up and down.  Look at the schematic.  In paralle with R17 is the path through R18, R19, and the presence control.  That path adds up to something 4k4 to 8k9 depending on the control, and those parallel 3k9 cuts the total roughly in half, as you measured.

You have good 15v supplies at the ICs, all that looks fine.

If playing into the slave jack sounds fine out the speaker, and the preamp out the slave jack sounds OK elsewhere, I am hard pressed to think why it wouldn't sound fine going straight through.