Hiwatt Maxwatt G100R repairing distorting amplifier

[fai1984]

Hi, i'm having same issue that Leandro has. The amp is a Hiwatt Maxwatt G100R with the same power amplifier PCB, with the same circuit. It came to me with a low level and very distorted sound.

When I put the oscilloscope in the speaker output with a 8 ohms dummy load and 1khz sine wave in the fx loop return, I see only a semicicle of the sine wave signal.

I found the B+ supply has an arcing with ground. B+ has two wire bridges over the B- and ground tracks and this bridge's lugs are soldered more near to ground and B- tracks. I've replaced this two bridges with a bigger one on the bottom of the pcb.

In the emitter of Q8 i found erratics voltage readings, but when i put the multimeter positive probe in any diode or resistor around Q8, the amp start to work perfect.

I did power on and off the amp several times with the oscilloscope and audio generator, and the amp worked perfect. I returned the amp to my friend and he has playing with it about 2 hours and the same issue came back.
Wednesday i take the pcb off and take some measurements in the transistors and all was OK. I power ON the amp with the dummy load and osciloscope in the speaker output but witouth the input signal cable, then R18 and Q15 blow off. I replaced those and power On again without the dummy load and don't blow anything. I measured power transistors, drivers and diodes 1N4148 and all are good.
I need some advices please, i have to much experience with vaccum tube amplifiers, but solid state amplifiers don't like me and I don't like they jajajjajaja

Thank you.

 

[Ylli]

On the Randall RH100; If I am looking at the right schematic (says RH2-100-B in the lower right corner) then pin 1 of the input connector is ground, and pins 2 & 3 are differential inputs. If you short those together you are just feeding the amp a common mode signal and it will have very little or no gain. Unshort them and feed your test audio source into *either* pin 2 or pin 3, not both. [In addition, looking at the schematic, I believe there should be a connection 'dot' at the junction of the low end of R10 and the right end of R9]

 

[Ylli]

fail1984 - all the references I can find on the Hiwatt Maxwatt G100R show it as a tube amp. Are you sure it has the same power amp board as the RH100?

 

[fai1984]

Hi Yllim thanks for your help. Maxwatt G100R is a solid state amp, there is no schematics for this amp in the web.

This is the issue. I'm feeding 1Khz / 100mv in the Fx loop return. The preamplifier is fine.

 

[fai1984]

R9 and R10 are joint like you say.

 

[fai1984]

I found 50V in the emiter of Q14 and -10V in the emiter of Q15 when the issue apears, measuring with the amp off.

 

[Jacounet]

Hello.

Here Jacounet from France .

Where do you plug your 100mV 1 kHz signal ....?

Make this trie :
- make a short cut beetween pin 1 and 2 in the enter mark CON1 ==> that's became ground , and plug the ground of your generator at this pin/ground /1/2 , and plug the hot point of the generator at the input 3.

If it's OK on the output , without distorsion , .... , give a return .

Jac .

 

[Ylli]

I found 50V in the emiter of Q14 and -10V in the emiter of Q15 when the issue apears, measuring with the amp off.

With the amp *off*?? Assuming you measured those voltages with the amp powered on..

What is the AC voltage on the transformer secondary (across C21)? First thing we have to do is determine why B+ and B- are not symmetrical. Could be an issue with the rectifier bridge, the filter caps, or the load on the two outputs.

 

[fai1984]

Ylli, this measure i told you about was with the amp off. Every time i have to solder any component, I discharge the power supply caps.

In the secondary of the transformer I have 74,5VCA. In the power supply I have -49,5V and +49,9V

I test the amp for about 2 hours with 200hz / 120mv and all was good. I try a while with about 530mv (70VPP at the speaker output). I plug the guitar and play for about 15 minutes and the amp return to the same issue.

 

[fai1984]

Hello.

Here Jacounet from France .

Where do you plug your 100mV 1 kHz signal ....?

Make this trie :
- make a short cut beetween pin 1 and 2 in the enter mark CON1 ==> that's became ground , and plug the ground of your generator at this pin/ground /1/2 , and plug the hot point of the generator at the input 3.

If it's OK on the output , without distorsion , .... , give a return .

Jac .

Hi Jacounet, I plug my audio generator to the return of the FX loop.

If i put the generator like this: - make a short cut beetween pin 1 and 2 in the enter mark CON1 ==> that's became ground , and plug the ground of your generator at this pin/ground /1/2 , and plug the hot point of the generator at the input 3.

I get the same results.

 

[Ylli]

Fail - with no signal input, (unplug the cable on Con1) and the amp powered on, what is the DC voltage at the output (either side of L1)? Then measure the DC voltages across R24 & R25 - these should both be very small. Still with no input signal, measure the DC voltage at the base of Q12 and Q13.

 

[Jacounet]

I found 50V in the emiter of Q14 and -10V in the emiter of Q15 when the issue apears, measuring with the amp off.

Hello Fail.

As said highter in this subject , the fact you found + 50V in the emiter of Q14 and ( the + power supply) ,...and -10V in the emiter of Q15 ( the - power supply) , when the power amplificator fall in failure, is not normal .
Here is certainly the problem .
As said by Ylli , you have so , to verify the voltage of B+ and B- ...normaly indentic , when good fonctioning , at 0.1 Volt .
Certainly a problem of bad soldering , or bad contact beetween wires, somewhere , ... or the bridge with 4 diodes IN5402 , or the capacitor C20 that's in end of live and block the voltage...but in this case you have probably ,D12 and D13 very hot ...if not destroy .
If I resum , you have certainly a power-supply problem .

But you have an intermittent failure ...and this kind of failure is not easy to repaired, bad contacts or bad soldering resolved .
With time ...we can repair.
So if you found no problem in power supply , (I would be surprise) make a test at each end part of the amplicator on transistors , the differential , the driver and the power , in good fonctionning , and in bad .
And track the way , where there is a problem .
But I'm pratically sure , that you have a problem of power supply.

Can you make a photo of your ampli. , open .


Good work .
Have a good day .

Jac

 

[unclejed613]

Hi Yllim thanks for your help. Maxwatt G100R is a solid state amp, there is no schematics for this amp in the web.

This is the issue. I'm feeding 1Khz / 100mv in the Fx loop return. The preamplifier is fine.

View attachment 114939

looks as if you are only driving the negative half of the amp. if Q10 were shorted or leaky, it could cause the problem shown. assuming the schematic you posted is correct, there are two signal paths in this amp, with the signal path for the negative side beginning with D2 and Q6 acting as a current mirror to drive Q9. the positive half (which is where your signal is disappearing) is Q7, 8, 12, and Q14. check these transistors, and also, check the resistors that go to the +Vcc rail R15, R18. check R17 also. you are likely to find your problem is caused by one of the + side transistors, and/or one of the resistors. check the resistor from Q14 to the speaker output (R24)

 

[unclejed613]

I found 50V in the emiter of Q14 and -10V in the emiter of Q15 when the issue apears, measuring with the amp off.

you can't trust any voltages you read after powering the amp off. however, with the amp on you should be able to measure and compare the voltages across certain resistors (again, assuming the schematic is somewhat accurate for the amplifier you have) the voltage drop across R18 should be the same as it's "opposite" R19. the same for R15 and R16. R13 and R14 should also match, as well as R11 with R13 (R11 and 13 are part of a current mirror). R17 should have almost the same current through it as R15, so should have approximately 1/2 the voltage drop of R15.

the voltage drop comparisons will most likely indicate where the trouble is. you will likely find one resistor on the positive side that either has full rail voltage across it (meaning it's open circuit), no voltage across it (meaning the transistor it's associated with is open circuit), or vastly different than it's negative-side twin (meaning the associated transistor is leaky, or something else in the nearby circuits are awry) some of the symptoms you mentioned also seem to point to some bad solder connections around Q8, so you might wnt to poke around near Q8 and see if there's any breaks in the solder or in the PCB traces.

it seems there are/were multiple companies using the Hiwatt brand name, the most recent, some chop shop in China that likes using an op amp and 2 current booster transistors as the "power amp". this is probably one of the reasons a schematic is so hard to find, it may have been a knock-off of another amp, and probably so inexpensive that they expected people to replace it rather than get it fixed (which means they don't know much about musicians).

 

[fai1984]

Well fellas, thanks very much to all of you.

I changed Q7 for an BC556, (in collector-emitter are 45V, I think it will works fine). The amp is working perfectly. This 2SA970 is intermitent malfunctioning and caused this random issue.

It was the same solution that Leandro Dias tooked in his Randall RH100.

Francisco.

 

[unclejed613]

I changed Q7 for an BC556, (in collector-emitter are 45V, I think it will works fine). The amp is working perfectly. This 2SA970 is intermitent malfunctioning and caused this random issue.

keep in mind that Vceo of the BC556 is 65V, and the Vceo of the 2SA970 is 120V and the rail-to-rail voltage in your amplifier is at least 100V. Q7 is in a part of the circuit where it's possible to get more than 65V across it. so, if Q7 ends up failing, get the right part. 2SA970 transistors are dirt cheap, about 0.10 USD.