Hughes & Kettner

[Valvehead]

Just picked up a cool amp--its a H&K CF 200 , solid state , 1 rack space power amp.

Has some problems :
no input to and no output on R channel .

left channel does work , but seems like it puts out 20 watts instead of 100

the amp has input signal lights and clip lights.

Whats weird is when Im in the right input--the Left input and clip lights come on when i touch the cable end.

When im in the left input , the input lights up normal

I replaced 2 , 5w resistors , 22 ohm , and put in 2 new input jacks--the old ones were corroded . This didnt change anything .

Where to start ? I dont own a scope so Im stuck with just my multimeter...


EDIT ----- the manual says both inputs are activated if going into the R input.

[mozwell]

Hi Valvehead
Lots of Hughes & Kettner schematics here http://music-electronics-forum.com/t25740/
Lots of manufacturers use teh same power amp circuit on different amp models.
For 100W power amp into 4 ohm load +/- 42V is the pwoer supply level, for a 100W 8 ohm load, arounf +/-55V is about right.
Check out the ATS120, bassforce 100, clubreverb, matrix 100, tour reverb 100 as one of these may have a similar pwoer amp circuit to the one you have.

Are the power transistors mosfet (2SK... 2SJ...) , or bipolar (2N... MJ... TIP...)

You mention replacing 5W 22 ohm, are you sure these shoudl not be 5W 0.22 ohm ? are these close to the power transistors ?

[Valvehead]

the outputs are mj4032 / mj 4035
the 5w say 22R on them---the good ones measured 21.3 ohms

when you say 55V , where and how do i measure that ?

[J M Fahey]

Power supply level means power supply rails, the main voltages coming out of the PSU and feeding the output transistors.

[Valvehead]

ok, I have 38.5v out of the rectifier , and 38.5v feeding the output transisitors

[mozwell]

Ok, this looks about right for a 100W power amp.
Do you have + & - 39.5V from the rectifier ?

you say you have low output on one amplifier & no output on the other.
first check, do you have + & -38.5V to both power amps.
do you have any DC offset on the speaker outputs ?
Are there any op amps near the input circuit, for an 8 pin op amp, pin 4 shoudl be around -15V & pin 8 should be around +15V, for a 14 pin its pin 4 & pin 11, do you have these voltages ?
With power turned off & the unit unplugged, measure the power transistors, on ohms, between all 3 pins, are any short, also there will be some driver transistors, are any of those short ?

Have you tried contacting HK to get a service manual. Tell them you have one brought in by a customer, and you are after the service manual as he has a gig next weekend. It helps sometimes to let them think you are a repair shop (even if you are not)

EDIT, the manual says there are two speaker fuses, are these blown ?

[Valvehead]

Thanks moz-----i appreciate your time.

Here we go--

i have + and - 38v at the output transistors

the dcv at the speaker jacks is about 300mv dc

measured the ohms on the out transistors.
i get about 3k between the 2 pins on all.

Now , on just 2 of those, i get 0 ohms from pins to case (shorted? )

the other 2 have about 3k to 5k from pins to case.

the 3 op amps do have the + 15 and - 15

[Valvehead]

I just got the schematic !! i will now try to post it----

[J M Fahey]

Fine.
A classic design, won't be hard to repair, but output transistors may be hard to find and expensive, obsolete for many years.
Remeasure your transistors using the "Diode test" scale on yout meter.
Kohms readings do not make much sense; diode test shows voltage across junctions which is more useful.
You will typically measure "open" one way and around 0.6V or 1.3V the other side (because these are Darlingtons); what you do not want to find is zero or a very low value (say, less than 100mV).
All transistors have 3 terminals ; in this case the 2 legs and the case.
Download the datasheet to know which is which:
MJ4032 datasheet pdf datenblatt - STMicroelectronics - COMPLEMENTARY SILICON POWER DARLINGTON TRANSISTORS ::: ALLDATASHEET :::
Also measure the emitter ballast resistors: 0.15 ohms 5W.
And, as usual, good luck

[mozwell]

OK, lets find out where the problem is.
Do you have an oscilloscope, or just a multi meter ?
Can you inject a signal (1kHz test tone) into the inputs ? even a CD player signal will do if you have nothing else.
Dont connect any speakers yet.
Make sure the bridge mode switch is NOT set to bridge mode.

First check the power supplies (already done) and check for DC on the speaker outputs to check for a blown output transistor.
Then, as per standard fault finding, start at one end & work through, checking at each step to see if we get the correct signal.

1. There are fuses on the transformer output, 4A, check these are ok.
2. There are fuses on the speaker outputs 3.15A, check these are ok.
3. When you measure the DC output, measure also from both sides of the speaker fuse (3.15A) to 0V. If the fuse is blown, by checking on both sides, we can make sure there is no DC output before we go further.

If the transistors check ok on the diode check, we can go on.
If you have replaced the 22 ohm 5W resistors, i wouldl also replace the 47n capacitor also, C41 & C40. These are the zobel network on the amplifier output & these shouldl be working correctly.

We can check the input op amp circuit.
4. We can inject a signal, & check the outputs of IC2, pin 1 & pin 7, Pin 1 is for left channel, pin 1 is for right channel.
If you use the right input only, it goes to both channels, so that makes it easier.
We expect no DC on the output and we should see the same signal as on the input. Make sure each volume control is turned up.
Got correct signal here, if so, we move on.

The unit has a mute circuit, so this can be checked, as general checks seem to indicate we are ok.
5. What voltage do you get at TP7 (on C6)

6. We could temporarily disable the mute circuit, carefully desolder the drain pin of T6 for the right channel & T3 for the left channel, does this fix it ?

[Valvehead]

Ok, ive got some info...No i dont have a scope.

Measured the output trans, in diode mode ,

from pin to pin , all were the same = .5v and 1.2v

from pins to case =
T1 = .4v / 0v , .5v / 1v

T2 = .4v / 0v , .5v / 1.2v

T3= 1v/.4v , .5v/ 1.2v

T4 = .4v/ 1v , .5v / 1.7v

the 5w 0.15 resistors all measure .3 ohms


i used a mp3 player 4 signal ,

at the speaker jacks i got about -300mv

at IC2 i got the same -300mv pin 1 /7

at TP7 measured the 3 legs , i got -0.4v / -300mv / -300mv

when you say disconnect the drain on t3 / t6 , which leg is that ?


all fuses in the amp are ok

[Valvehead]

Do those numbers help ?

[Jazz P Bass]

,

from pin to pin , all were the same = .5v and 1.2v

from pins to case =
T1 = .4v / 0v , .5v / 1v

T2 = .4v / 0v , .5v / 1.2v

T3= 1v/.4v , .5v/ 1.2v

T4 = .4v/ 1v , .5v / 1.7v

I cannot seem to wrap my head around exactly what you are measuring.

What does T1 = .4v/0v mean?
0 volt what?

[Valvehead]

ok, T1 /2 /3 /4 are the 4 output transistors .

i measured each one in diode test mode. each trans has 2 pins ,

so T1 (transistor 1 ) form pin1 to case measured .4 volts , then with leads reversed i got 0 volts

on the same transistor on pin 2 to case , i get .5 volts , then reverse leads i got 1 volt

[Jazz P Bass]

The schematic shows the output transistors as (L Ch T24 & T25) (R Ch T21 & T22)
Are these the ones that you are testing?
If so the ones that read 1 volt when reversed are bad.
The diode check function must only read one way (hense 'diode' check)