Showbud D15 steel SS amp hiss

[LarBal]

I've got one of these here, it works but has a fair amount of hiss.
Hiss is on both channels, and increases as the volume(s) are turned up.
Tone controls have no effect on hiss.
There's no hum in the amp except when I turn one of the volumes 10 ten, and that channel's Treble past 5. The VU meter freaks out, goes in the red, and the amp immediately hums, almost like a LF oscillation I assume.
I replaced the 4000UF main caps with some new units from Mouser, and also replaced every other electolytic cap in the driver section.
The amp is super loud, and otherwise seems to be working normally.

My understanding of SS devices is limited, so I'm not sure where to even look next.
What are the usual culprits of this kind of hiss? It's the equivalent of an ultra-high gain amp turned up loud.
It's likely usable as-is, but I'd like to get to the bottom of it.

This schematic isn't the exact amp I have, but there are at least many many simiarities to it and the amp in front of me.
Sorry for the awful resolution, apparently it's the only one on the web

thanks!

[J M Fahey]

>>The VU meter freaks out, goes in the red, and the amp immediately hums, <<

Sounds like HF oscillation to me

A scope is the only sure fire method to catch it.
Regular meters do not, frequency is too high.
Also too high for our ears..

[loudthud]

Important point: How is the hiss when both Volume controls are all the way down ? If no hiss, that would indicate Q1 and Q4 are the source. There is no protection on the inputs, one or both parts probably got zapped by static or other Voltage gremlins. They still may pass some signal with attenuation. My info says J305 is a good sub but pin-out is different. TIS58 Gate is middle pin, J305 Gate is what would normally be the Collector of an American TO-92 transistor. Drain and Source pins can be swapped. TIS58: Idss: 2.5-8mA Gate breakdown: 25V. Those specs should help you find something that will work.

To add protection to input connect a signal diode (1N914 or 1N4148 or 1N4152) between Gate and ground. Cathode (end with a band) goes to the Gate, other end to ground.

Edit: See post #9 in this thread:
https://music-electronics-forum.com/...tremolo-issues
For how to test JFETs with an Ohm meter.

[The Dude]

Have you checked bias? If the amp is overbiased, any slight noise would be made worse by output over-conduction.

[LarBal]

Have you checked bias? If the amp is overbiased, any slight noise would be made worse by output over-conduction.

I have not, how would I go about that, and which components need to be biased? I assume the output transistors

Loudthud: The hiss definitely goes away when volumes are on zero. Turning either up increases the hiss as you travel up the pot, and each channel's hiss has a slight different tonal quality to it.

[The Dude]

There's a note on the schematic near the bias pot. I THINK it says to set for 30mA idle current, although it's difficult to read on your upload. Maybe you can read it better on your copy? Measure mV across output emitter resistors and calculate current using ohms law.

[LarBal]

There's a note on the schematic near the bias pot. I THINK it says to set for 30mA idle current, although it's difficult to read on your upload. Maybe you can read it better on your copy? Measure mV across output emitter resistors and calculate current using ohms law.

Ok yes I see it it, it does say 30ma. So I'm reading mV across the .33r 5W? (after actually measuring them)

[g1]

Yes, mV DC across each of the 0R33's. No signal, all volumes turned all the way down. mV/.33 gives mA current.

[LarBal]

Yes, mV DC across each of the 0R33's. No signal, all volumes turned all the way down. mV/.33 gives mA current.

Ok , Measuring across the 0R33's (that are reading 0R4 on my meter), I get 4.2mV across one and 1.6mV across the other .

On closer inspection on the blurry schematic it says set to 20mA, so I'm at roughly 10mA and 4mA. I can't locate the trimpot but I'll keep looking. I would think that something else is going on though, with readings that far apart?
Also, the two 0R33 resistors are of different makes. One is brown one is white, which seems odd

[mozz]

12.7ma and 4.8ma is what i am figuring. That's assuming the resistors are actually .33 ohms. They may have been changed is they are different colors, look at the solder connections.

[LarBal]

12.7ma and 4.8ma is what i am figuring. That's assuming the resistors are actually .33 ohms. They may have been changed is they are different colors, look at the solder connections.

Yes they are 0R4 ohms. Nonethless I'm way off from where they need to be.
I traced the board and realized that someone put in a 56R resistor where the 100R trimpot should be .

I don't understand why the current is so differnt across each output 0R33s. Why could that be?

I found a clearer schematic, it's a the single channel showbud amp(mines dual), but the power section is the same

sho-bud-singlechannel-amplifier-schematic.pdf

[Helmholtz]

Don't expect a DMM to correctly measure resistance below 1R.
To test the meter short the leads. This will show how much the leads and contact resistances add to the reading.

I don't understand why the current is so differnt across each output 0R33s. Why could that be?

Repeat measurements without a load connected to output.
Also check for DCV without load.

[LarBal]

Ok I didn't have a 100R trimpot so I paralleled resistors in until I got 8.6mV across one 0R33 . With no load, those resistors are reading 0R5, and my meter's continuity reads 0R1. so I'm calling them 0R4.
The other has 6mV across it, so one side's flow is 21.5mV and the other is 15mV. Somehow that seemed to help the hiss, by bringing the current up, but it's still there.

So I'm looking at replacing Q1 and Q4. I found a NOS TIS58 on ebay https://www.ebay.com/itm/403060160512

[loudthud]

So I'm looking at replacing Q1 and Q4. I found a NOS TIS58 on ebay https://www.ebay.com/itm/403060160512

Price is very high. Keep looking for a substitute from company's like Toshiba.

[LarBal]

Price is very high. Keep looking for a substitute from company's like Toshiba.

Mouser's got the J305 for $3, just ordered a couple https://www.mouser.com/ProductDetail/106-J305