Trying to fix guitar amp Randall RH100 G2 head - power amp RG75B

[giokat]

Hi everybody! I'm new to the forum, I hope that someone will be able to help me!
I'm trying to fix a guitar power amp, schematics attached.
It is the power amp from a 100 W guitar amp head by Randall.
I think the fault was caused by the amp loaded with 2 Ohms loudspeaker(!)
Q11 .. Q15 where blown as well as Q8 and some diodes.
After having replaced them the amp came back to operation but it sounds low and very distorted. I noticed that:
VCE of Q8 is about 0,2 V (Q8 is in saturation);
Q7 and Q9 are heating a lot;
Voltage across R24 is about 0,4 V;
voltage across R25 is .. zero!
Both R24 and R25 are ok.
I replaced Q7 and Q9 too, with no results.
I suspect that Q10 may be faulty, despite it is ok if I check it with a multimeter.
I also miss some theory in the sense that the function of Q6 is not clear to me,
as well as I don't understand if Q7 or Q9 is the VAS (or both???)
Any idea enyone?
Thank you so much!!!
Best regards!!
Giorgio

 

[unclejed613]

Q6 and D2 are a current mirror. the current through D2 is mirrored by Q6 to drive Q9 symmetrically to Q7 (i.e. whatever Q7 is doing, Q9 is doing the opposite.). Q8 and D5 set the bias for the output transistors (Q14, 15, and their drivers Q12,13). Q10,11 are protection transistors and their purpose is to shut down the bias and drive signal to the outputs. have you replaced R24,25? if they're open, Q10,11 could be saturated constantly because the slightest amount of bias to the drivers and outputs has nowhere to go except to drive Q10,11 into saturation instead of flowing through the low resistance of R24,25 and the speaker. that would also account for the lack of bias and Q7,9 running so hot. R24,25 are the "emitter resistors" for the outputs (which are arranged as "composite" or Sziklai Pairs, Q12,13 act as a high beta NPN and Q13,15 act as a high beta PNP), and if they're open the voltage sampled by Q10,11 will be too high, and activate Q10,11. normally Q10,11 are in the "off" state, and essentially out of circuit. R20,23 sample the voltage across R24,25, and activate Q10,11 if the voltage is too high, shutting down the bias and drive to the output devices.

 

[Mr RB]

I would remove all 6 transistors right of Q8, the entire power stage.

Then run a sine signal in and check everything looks good at the collector of Q8. If so, (which it should) then just replace all 6 transistors in the power stage with new ones. You should also check R24 and R25 to see if they are damaged or high resistance.

It's an easy amp it should only be a 1 hour fix.

 

[unclejed613]

replace Q10 and 11, D6,7. check Q8 out of circuit.

Q7 and Q9 are a symmetrical VAS to answer one of your questions...

 

[giokat]

Thank you very much!
I'll try your suggestions during the weekend.
Don't want to bore you with questions but I'm pretty curious about the symmetrical VAS.
Q7 clearly takes the input signal from the differential input stage, Q1's collector, but what about Q9? It seems to take the input signal through Q6...
with the same phase of Q7 or opposite? Big question for me!
BTW thank you for the help you both gave me so far!
All the best!
Giorgio

 

[unclejed613]

like i said, D2,Q6 act as a current mirror, which drives Q9 opposite Q7 (but because it taps from Q2 instead of Q1, it's acting push-pull with Q7). i think "push-pull" is a better description of how it acts than saying "in phase". R11 and R13 have equal currents flowing through them. D2 drops 0.7V and Q6 B-E drops 0.7V and so whatever changes in the current through D2+R11 are imitated by Q6+R13, which drives Q9 with the same current. since Q7 and Q9 are complementary devices, in push-pull, they are a symmetrical VAS.

 

[giokat]

Hi ! I'm here again.
I replaced both R24 and R25, R25 was open.
But the problem still persists :-(
Q7 and Q9 are hot. I measured 48 V across D3 and D4 which is the power supply voltage.
I think this is symptom but I can't understand what's the problem.
Any suggestion please?
thank you so much, as usual!!!!
Best regards,
Giorgio

 

[unclejed613]

D3 and D4 should each have almost 1/2 the total power supply voltage across them (actually 0.7V less than that if Q7 and Q9 are ok), so from the base of Q7 to the base of Q9 should be about 1.4V less than the total supply voltage. it actually should be a bit less than that because of the voltage drops across R15 and R16. if Q8 and D5 are ok, there should be about 1.4V between the collector of Q7 and the collector of Q9. the voltage drops across both R18 and R19 should also be about 0.7V each, and the voltage drops across R24 and R25 should not be much more than 10mV (0.01V) but realistically should be more like 5mV (0.005V). have you checked the speaker? DC resistance should be about 6 ohms. you can also eliminate the protection transistors Q10 and Q11 as potential causes by lifting one end of D6 and one end of D7, which will take Q10 and Q11 out of circuit (just don't overdrive the amp, put just enough signal in for about 5 watts output, which would be 6.3Vrms out).

the attached drawing should be nearly correct for voltages. it won't be exactly right, but you can see approximately what should be there

 

[giokat]

Hi Unclejed! You are very kind, patient and competent! Thank you soo much !!!!!
I'll let you know about eventual progresses...
ALL the best!!!!!!!!!! Giorgio

 

[dmcastillor]

Hi, I need the RG75-B randall schematic or R29 and R30 Values, Thanks

 

[unclejed613]

two posts above.... Randall RG-75B schematic......

 

[dmcastillor]

I saw it but it does not content the R29 and R30. in fact i have the randall the schematic from randall but still missing R29 and R30. if you could give those values I will apreciate it. thanks

 

[dmcastillor]

I found this picture on the web, in the right corner near the C5198 are the R29 and R30, in the picture is seen the R30

 

[dmcastillor]

**broken link removed**

 

[giokat]

here it is:

 

[unclejed613]

that's an owner's manual

 

[giokat]

Yes, the last one is an owner manual, but the first two are schematics

 

[dobz3]

damn here is a copied version of that.
https://i262.photobucket.com/albums/ii81/dobz3/RandallRG75BPowerAmpsketch_zps6fa10afc.gif

 

[giokat]

Hello everybody,
three years have passed since I started this thread.
Finally, I fixed this amp!
It sounds ridicolous that it took me three years, well what can I say? many many personal problems...
I could not localize the fault, I replaced all components on the power amp board except the inductor and capacitors and resistors on the signal input.
Thanks everyone for the support.
Regards
Giorgio

 

[Alman]

Hi Giorgio, I see you repaired an amp with similar components to the amp I am currently working on. It is a Randall RG200. I am on the search for the transistors located in the output section. Namely these C5198, A1941, 2SC4793, 2SA1837. Where did you purchase your components? I am still searching for a supplier.

Thanks,
Allan