laney LX120....120watts....Really.

[teemuk]

I don't know why get hooked on details of the power amp when this all could be just due to something like dirty FX loop contacts or even simpler stuff like "quieter" voicing of the Laney amp overall.

Yes, apparently the other power amp has less output. That doesn't mean it has a fault; could be that it just receives a much lower input signal to begin with.

[RODNEY]

I've already looked into FX loop issue by connecting a guitar cable from FX send to the FX return on the LX, made no difference. Also.... I have used the FX send of the LX to the FX return of the HCM and the result was that the HCM sounded "normal" which tells me the LX pre-amp is outputting fine, the other way around using the pre-amp of the HCM and the power-amp of the LX, reletivly low output again. Ok the LX might have a "quieter" voicing but surely it should louder than the HCM @ 65W if only slightly but i promise you the HCM65R is noticeably louder LX120. I'll do some voltage test tomorrow as suggested here.

If everything check out ok, the only thing that can explain it is the the HCM is putting 65W in to one speaker causing the speaker to distortion making is just sound louder and that the LX (using 412) just has more headroom. Would that make sense?

[J M Fahey]

All I can repeat is: donīt touch the bias trimmer (nor the offset one), they are absolutely unrelated to your volume problem, you will repair nothing and may cause a lot of trouble.
Thanks for the LX120 schematic, now we need the HCM65 one so we can compare the closed loop gain of each power amp.
Thanks.

EDIT: two possible culprits for your lowered volume/sensitivity:
1) The LX120 has a mute/delayed turn on circuit, where the attenuator is Q1, a 2N3819 FET.
*If* its gate does not get negative enough, a few seconds after turn on, it pads input signal down.
Check that on the track that joins Q1īs gate/C5's negative leg/D4īs anode/TS8's emitter.
You should have over -5V there, preferably closer to -10V.
If TO92 cased, lift Q1's drain to check, if SMT it will be impossible, unless you cut the track that joins it to R3/C1
2) the other suspect(s) are D1/D2 Zeners.
If somebody plugged a speaker out in the Power amp in, he might have burnt them ... and now that I think about it, also R3.

[g1]

Mr. Fahey is (once again) bang on in his assessment, it could well be either the muting Fet or the clamping zeners. Removal of Q1 and either of the zeners will tell you right away if they are causing signal loss.

[teemuk]

The closed loop gains of LX120's and HCM65's power amps are kinda difficult subject since they vary throughout the effective bandwidth due to use of current feedback. But all in all they are about the same, with differences at mid-range frequencies being about 2dB (higher gain on HCM65) and about 3-4dB at resonant and at higher frequencies (higher gain on LX120). The HCM65's power amp overall has much more linear response to speaker loads.

Anyway, with such minor differences in closed loop gains the power amps should be somewhat comparable loudness-wise but naturally the input sensitivities of the two designs are very different since HCM65 uses +-28V power supply while the LX120 uses +-45V. The input sensitivity of the HCM65 is about 0.4V RMS while the input sensitivity of the LX120 is about 0.7V RMS. Given same input signal source the power amps of HCM65 and LX120 should thus be about as loud but the HCM65's power amp will overdrive much faster, which then could then explain an impression of greater loudness.

Laney schematics usually turn pretty well up in google search. At least elektrotanya.com will host the HCM65R.

[RODNEY]

okaayyyyyyyy. now my head is starting to hurt, on the circuit diagram, what/where is Gmu and Gi? I've hooked the unit back up for now so I can check the voltages on the power rails and darlington so I cant trace where Gmu/Gi is sorry.
The -5V/-10V is measured relative to what, Ground, Gmu or Gi or Q1 drain, i'm very much an amateur?
So i'm to lift of Q1 drain so as to not give the input signal a path to ground (or Gmu)?

Ok test done. power supply check while still connected to amp +- 32.6 V but got 87.6v across the collector of both Darlington so power supply is fine.
Base to collector on both darlington was +-1.1V

Power now off so....D1 and D2 show a forward volt drop of 0.730V so i assume they haven't failed in the other direction either. R3 @ 1K was ok

Please make note, All check done on the component side rather than the solder side. Yet to check the -5v/-10V on Q1 gate as I need advice on where these voltages are in respect to.

[J M Fahey]

okaayyyyyyyy. now my head is starting to hurt, on the circuit diagram, what/where is Gmu and Gi? I've hooked the unit back up for now so I can check the voltages on the power rails and darlington so I cant trace where Gmu/Gi is sorry.
The -5V/-10V is measured relative to what, Ground, Gmu or Gi or Q1 drain, i'm very much an amateur?

Donīt worry .
Iīm quite sure that G(something) means ground, plain and simple.
The (i) or (mu) probably mean different traces, in the PCB, to which some part is grounded to in preference to others, ground loop problems and all that, but they all eventually get connected to actual ground (the union between the big filter capacitors) so for DC measurement purposes (what you need) itīs all the same.
So just to check Iīm right, touch the black multimeter probe to metallic chassis or the C9 negative/C12 positive or the wire link (it might look like a white resistor, no color bands or a single center black band) Ln7 (it *should* be labelled so at the PCB) and the red probe to C9 positive or C12 negative and you should read +/-45V respectively.
Now that you checked you can trust your ground connection, leave black there and measure the voltage at Q1īs gate.

So i'm to lift of Q1 drain so as to not give the input signal a path to ground (or Gmu)?

Thatīs the idea, but you still didnīt answer whether Q1 was a TO92 case or SMT
Good luck.

[RODNEY]

Donīt worry .
Iīm quite sure that G(something) means ground, plain and simple.
The (i) or (mu) probably mean different traces, in the PCB, to which some part is grounded to in preference to others, ground loop problems and all that, but they all eventually get connected to actual ground (the union between the big filter capacitors) so for DC measurement purposes (what you need) itīs all the same.
So just to check Iīm right, touch the black multimeter probe to metallic chassis or the C9 negative/C12 positive or the wire link (it might look like a white resistor, no color bands or a single center black band) Ln7 (it *should* be labelled so at the PCB) and the red probe to C9 positive or C12 negative and you should read +/-45V respectively.
Now that you checked you can trust your ground connection, leave black there and measure the voltage at Q1īs gate.


Thatīs the idea, but you still didnīt answer whether Q1 was a TO92 case or SMT
Good luck.

Sorry... it is a TO92 case.....phewwwww....Ok thanks....I just need to find the data sheet for Q1 so I can be sure which pin is the gate

[RODNEY]

ohhhh....Getting some where now, 0v @ the gate of Q1 and 0v drop across the drain and source so my input signal is shorting to ground. I'll remove circuit-board and check the soldering again and the other components linked to Q1, if nothing obvious i'll lift off the drain. I assume I'm ok to hook up the amp back and switch on with the drain not connected?

[J M Fahey]

Yes, maybe your amp now thumps a little at turn on/off like a million others, no big deal. Thatīs what the mute is for.
Just unsolder the drain leg and lift it out of its hole, carefully.
Anyway for a full repair we need to know why you donīt have the negative bias there.
You have a small negative supply there: AC from the transformer charges C6 negative; after a couple seconds that negative voltage charges C5 negative through R13 and D4 ; when amp off Ts8 discharges C5 quickly, as to deal *now* with turn off thump.
Some part is bad (shorted cap, open resistor or track, bad Ts8) which kills proper voltage.
Even the Fet might be shorted!
Oh, the joys of servicing!
Thatīs why usually (unless itīs a power transformer or another very expensive part) I simply donīt charge the parts, but my work.
The bad part may be a 10 cent resistor;but if it took an hour to find, it costs U$50/60.

[g1]

Maybe things are different now and newer (muting) Fets are more reliable, but I have replaced tons of them and it is almost always the Fet rather than something causing loss of gate voltage. So normally I just pull them out and see if the signal comes back, if so, replace with new ones.
Sometimes the meter has a hard time reading the gate voltage without loading down the circuit so it's usually easier to measure it with the Fet removed.

[RODNEY]

Disconnected the drain on Q1 and I'm sure that's done the trick, much much better now.
Checking TS8 and found there is a short between it's collector and emitter (tested using a diode test then a continuity test). I can only think that TS8 is faulty as the only other path would have to go through D4 > R12 which would of shown up on my meter. If I replace this TS8 (i have some on hand) would I still be able to get the negative voltage on the Q1 gate with it's drain floating? I'll worry about changing Q1 later as i'm not sure how to really check FETs.

[J M Fahey]

I always test parts "functionally", meaning than rather than using a specialized piece of test gear (which either I donīt have or left somewhere else ) I just check: "whatīs this supposed to do?" ... "*is* it doing it?".
This FET is supposed to mute with 0V on its gate, unmute with -5 to -15 ... which so far we donīt have.
Pull the entire FET and recheck: if now you have the negative voltage at the gate pad, on the PCB itself, the Fet was shorted as g-one suggests; if not, check the "mini negative supply".
Or pull TS8. It will unmute as normal, only it will re-mute (on power off) taking a longer time.
Sometimes you can pull a suspect part and the rest of the circuit works, although not in a perfect way, but it helps you diagnose.
Good luck.

[g1]

Checking TS8 and found there is a short between it's collector and emitter (tested using a diode test then a continuity test). I can only think that TS8 is faulty as the only other path would have to go through D4 > R12 which would of shown up on my meter.

Either a shorted Q1 (gate to source) or a shorted C5 would give you your low reading from C to E of TS8. I would test C5 and TS8 out of circuit, if they are fine replace Q1.

[RODNEY]

yep.....C5 was shorting so took it of the board and tested again..shorting . Removed Q1, lifted one side of D4 removed TS8 (MPSA42) . Junction-tested a new MPSA42 (6 ways) soldered it in, tested again, all was fine but i then had a junction volt-drop cross the emitter/collector one way (cant remember which) removed the little bugger again, re-checked it and it was fine...agghhhhhhhh..how is that possible as the emitter was isolated !!!!!!. lol....and now one of the solder pads on TS8 come away (ok...I can get round that)...aghhhhh.lol
I'm gona leave it now until tomorrow.