Peavey Backstage - Constant Loud Hum

[PorteX]

Hello all,

I am trying to fix the Peavey Backstage (I think it is a model from 2003) and I need some help as I don't have much experience with fixing guitar amp.

The problem is that when I am switching on the guitar amp there is a constant loud hum from the speaker. Adjusting the potentiometers does not change anything.
The DC voltage on the speaker is 10V and as far I know this is much to high.

So far I have checked:
IC2 - RC4558p (outside the circuit) - looks OK
Q10 - PNP B1366 (outside the circuit)
Positive lead to base and negative to emitter: OL
Positive lead to base and negative to collector: OL
Negative lead to base and positive to collector: O.638
Negative lead to base and positive to emitter: 0.640
Negative emitter, positive collector: OL
Negative collector, positive emiter: OL
Q9 - NPN D2058 (outside the circuit)
Positive lead to base and negative to emitter: O.606
Positive lead to base and negative to collector: O.606
Negative lead to base and positive to collector: OL
Negative lead to base and positive to emitter: OL
Negative emitter, positive collector: OL
Negative collector, positive emiter: OL
All other transistors - (all of them are NPN C1740)
Positive lead to base and negative to emitter: O.716
Positive lead to base and negative to collector: O.710
Negative lead to base and positive to collector: OL (except: Q2 -> 2.6V, D8 -> 2.2V -> Q6 2.7V , Q4 -> 1.8V but outside the circuit all of them have OL)
Negative lead to base and positive to emitter: OL
Negative emitter, positive collector: OL (except: Q1 ->2.6V, Q7 -> 2.2V, Q5 -> 2.7V Q3 -> 1.9V but outside the circuit all of them have OL)
Negative collector, positive emiter: OL (Q1 -> 0.7V, Q7 -> 0.7V, Q5 -> 0.7V, Q3 -> 0.7V but outside the circuit all of them have OL)

All diodes have around 0.615 and in other direction they have OL (except: D5 D4 D2 they have 2.6) checked hen connected to circuit.

On RC4558p the +VCC is 24V and -VCC is -24V

I have attached the schematic that I have found in the internet.

Do you have any suggestion what I can check to find the issue?

[Jon Snell]

"On RC4558p the +VCC is 24V and -VCC is -24V"
That is far too high. +-15volts is the norm.

Definately no more than 30 volts between pins 4 and 8.

There must be zero volts DC across the loudspeaker.

[glebert]

PorteX, are you running this on 120V mains? I am not sure how you could get the +/- 24VDC unless you are running with much higher mains or someone replaced the power transformer with the wrong part.

[PorteX]

Hello,

I am running on 230V mains, the power transformer is EI-02-11-10, Primary 230V/60Hz, Secondary 32V (CT). ( I have found one picture on the internet showing the inside of the Backstage amp and it had the same transformer)
Directly on the transformer output there is 2x18V AC.
When I am checking on the capacitors C30 and C29 there is -23V and +23V DC (please correct my if I wrong but capacitors can increase Voltage?)

On RC4558p between pin 4 and 8 i have around 46V, but despite higher voltage it seems RC4558p is not damaged. (I have compared it with another rc4558p)

Unfortunately I don't have any other transfer to test.

[PorteX]

Sorry I didn't mention that I have measured the voltage without any load, When the load is connected I have 15V on RC4558p PIN 8 and -21V on PIN 4

[glebert]

That makes a bit more sense. Seems like the load on the negative voltage rail is less than expected. You could check the voltages across R51 and compare to R49 to confirm. If R51 doesn't have any current through it I would look close at Q10 and that area of the circuit to see if it is conducting.

[PorteX]

Under the load:

R51 -> -21.3V
R49 -> 14,20V

Q10 - B1366
Emiter -> +9V and slowly increases,
Collector -> -21V
Base -> 10V and slowly increases,

R46 -> +9V and slowly increases,
R47 -> +9V and slowly increases,

I checked all resistors and capacitors around and they look ok (but I checked them only in circuit)

[glebert]

Since Q10 tested OK with the diode tests I would probably replace the 4558.

[g1]

Even unloaded, there is no reason there would be that high of supply voltage on the IC. Have you checked your meter on a DC voltage like a battery? Have you checked your meter's battery?

[The Dude]

I agree voltage well exceeds the op amps max rated supply voltage. However, if you look at the supply, there's only 10 ohms separating op amp supply from the main rails and no regulation, so I think readings are likely correct. It looks to me like it's just a crappy design. With those op amp supply voltages, the 4558 is a good candidate for failure. I'd probably look at some op amps with a higher max supply voltage or raise the values of R49 & R51.

[g1]

if you look at the supply, there's only 10 ohms separating op amp supply from the main rails and no regulation, so I think readings are likely correct. It looks to me like it's just a crappy design.

I was thinking more like all the voltages are high and it should be more like +/-15V at the main supply. Not sure which exact amp this is, picture would help. If it's the 10W Backstage, +/-15V main rails would get you around there. If reported voltages are correct, this would be a 25 to 30W Backstage.
Nothing on this schematic to indicate output power or main supply voltages.

edit: I do now see some points marked 20V, and that the main caps are 35V. Still confused how those IC's would survive that voltage.

[The Dude]

I can't imagine why you'd use 25W/60V/3A output transistors for a 10W amp, so I'm betting that the main rails are higher than 20V. That said, it's speculation without actually checking.

[The Dude]

...... Still confused how those IC's would survive that voltage.

I don't think it did!

[g1]

I wonder if there is a typo on those 10R, and someone installed them because the schematic showed. Look at point A, it has a 1K dropper. Then elsewhere (pg.1) it goes through a 2k2 (R28) and seems to become the 20V supply.
If those 10R were 10K, 1mA of IC current would drop 10V across them.

[The Dude]

To sum up, IMO: Either there are schematic errors, or it's a really crappy design.

Edit: The 10K instead of 10R makes more sense.