GK 800RB low volume

[pontiacpete]

This amp came into my possession in really bad shape. It had a lot of work done to it in attempts to get it going by another tech. It has the toroidal style transformer that GK supplies when these original power trannies go.
I changed out the output transistors throughout the amp. Many of the bolts that hold down the cases of the 15025s and 15024s had broken from the circuit. I think I repair all of them....
One thing that's bothering me is the high voltage that's expected +/-85, here I'm only getting +/-70vdc. The +/-60v is present. Across the winding off of the power transformer is about 105vac. The lower voltage winding has about 90vac.
I'm not expecting this slightly lower voltage on the HV winding would be the reason for the low output but could this be a problem going forward?
I have a good clean signal coming out of both outputs of U1. But plugging into the effects return I'm getting a good strong sound.
When I link the pre to another amp the sound is very weak and distorted. So the problem seems to now be in the pre amp section.
I will post more as I trace through the pre amp.

[g1]

Maybe it's some other toroid and not the one GK supplies for retrofit?
105VAC sounds like it would give around what you are getting, unless something is pulling it down.
Did you measure that with the winding disconnected from the amp circuit?

[nevetslab]

I just opened the GK 800RB that's in the shop waiting for parts. Here's a couple images of its' Toroidal Power Xstr. Tomorrow, when the parts arrive, I have to remove the bottom panel, and will get GK"s part number on the xfmr, since it will be unmounted in the procedure. I did just check the AC voltage feeding the LF amps' power supply bridge: 108.3VAC C/T



Power supply voltages at the LF supply is +/- 73.3VDC. So much for believing the +/- 85VDC values on the schematic!

[pontiacpete]

Maybe it's some other toroid and not the one GK supplies for retrofit?
105VAC sounds like it would give around what you are getting, unless something is pulling it down.
Did you measure that with the winding disconnected from the amp circuit?

Yes I measured with each of the windings disconnected.

Perhaps I'll call them tomorrow and check the numbers on the transformer.

[pontiacpete]

I just opened the GK 800RB that's in the shop waiting for parts. Here's a couple images of its' Toroidal Power Xstr. Tomorrow, when the parts arrive, I have to remove the bottom panel, and will get GK"s part number on the xfmr, since it will be unmounted in the procedure. I did just check the AC voltage feeding the LF amps' power supply bridge: 108.3VAC C/T

108vac is really close to what I'm seeing here. What is the high voltage winding measuring on yours?



Edit: sorry you answered my question already.

[Enzo]

A few volts difference on V+ and V- won't cause much difference in loudness, and the preamp doesn't run on 85v anyway.

[g1]

The transformer should be part # 080-0004.
From reading elsewhere, the supply voltages listed on the schematic are very optimistic. Your numbers sound ok.
Also you did not mention your AC line voltage at the outlet?

[nevetslab]

The transformer should be part # 080-0004.
From reading elsewhere, the supply voltages listed on the schematic are very optimistic. Your numbers sound ok.
Also you did not mention your AC line voltage at the outlet?

In my example, I forgot to state the line voltage. 119.8VAC, Amp at idle, Mains current was around 180mA, I think...maybe as much as 240mA.

[pontiacpete]

Thanks G1. this is the correct transformer, same part #.
My line voltage is at 120.5vac and i'm also getting 180mA current draw at the mains.

[pontiacpete]

Tracing through the pre amp I found no signal on U3 pin 7 but has signal on pin6. Changed U3 LF353 and she's working!! Finally. This is one of those repairs that's been on the back burner for a couple years. yay

When biasing I found that with no signal I'm seeing a negative voltage, schematic is showing to set at a positive .6v on the wipers of the trim pots.
So I set it at -.6v for both and it's sounding very good.
Should I set this with a signal? Like the pre amp schematics(5/25/95) page says, 2mV 200hz sin at input. Probably should, I'll give that a go.

[nevetslab]

Tracing through the pre amp I found no signal on U3 pin 7 but has signal on pin6. Changed U3 LF353 and she's working!! Finally. This is one of those repairs that's been on the back burner for a couple years. yay

When biasing I found that with no signal I'm seeing a negative voltage, schematic is showing to set at a positive .6v on the wipers of the trim pots.
So I set it at -.6v for both and it's sounding very good.
Should I set this with a signal? Like the pre amp schematics(5/25/95) page says, 2mV 200hz sin at input. Probably should, I'll give that a go.

On biasing, I'd look at the voltage between any pair of output xstrs' emitter resistors such as R48 & R54, where they're calling out for 10mV typical. That equates to 15mA per stage. If you look at all three pairs of outputs, that voltage will vary. A trick I've heard Enzo state is watching the AC mains current (assuming you have the means of reading that), and set the bias to where that current just begins to rise, then back it off a touch. What you're doing is setting the output xstrs to be nearly turning on. What will happen in use, as the output stage warms up, the bias level will shift some, and the outputs will turn on sooner. That heat-sink-coupled bias transistor compensates for that temperature rise (the little TO-92 xstr held by a funny copper clamp on the GK800RB).

If I was setting bias under signal drive conditions, I'd be looking at the crossover distortion on my distortion analyzer. My scope is always coupled to the Signal Monitor ports of my Amber 3501a Audio Analyzer. It has a differential input, so I never have to be concerned (within reason) of connecting to single channel or bridge mono channels when I need to see the signal waveform. Adjusting bias with the distortion analyzer, you'll see the crossover notch vary as you adjust the bias level. Watching that, and watching the AC mains current can help optimize distortion. Though with guitar and bass amps, I normally don't nitpick so much in that area.

Nice find, spotting no signal thru U3B. Love it when a simple parts swap restores order.