DeArmond Bassist 2

[dpm309]

I have a DeArmond Bassist 2 bass guitar combo with a blown power transformer. There is very limited information out there on the web and was wondering if anyone had a schematic or knows where to find a PT?
I have attached a picture of the inside of the chassis.

[sluckey]

How many wires coming outta the PT? What are the numbers on the power transistors? Draw a minimal schematic showing how the PT connects to the diodes. The replacement PT probably only needs to be somewhat close to the voltage of the original.

[dpm309]

Sluckey, the picture above is of a similar Bassist 2 from an ebay post. Not a stock PT and the power transistors look different. Also looks like there were several mods made to this. I have attache 2 photos of the inside of the amp I am working on. The numbers on the PT:   7-62, 220-1, 549-7444. The power transistors (2) numbers are: 8440 (TI symbol), TIP41C. Will try to pull the board to see where the PT connects to the diodes.

[shooter]

Will try to pull the board to see where the PT connects to the diodes.

try and get a good shot of the bottom side, the circuit looks simple enough to schematic up from the foil traces, and top view

[dpm309]

As it turns out, the amp is working but has some 60Hz hum that the owner wants fixed. He read somewhere that a new PT would do the trick. Have only worked on a couple of SS amps in the past and that involved board swaps or pot replacements. I am thinking that there has to be another less expensive way to take care of the hum. Would the same techniques for determining the source of hum in a tube amp also apply to a SS amp?

Measured 315VAC on the secondary of the PT.

[PRR]

> has some 60Hz hum that the owner wants fixed. He read somewhere that a new PT would do the trick.

Very very unlikely.

A PT is all about hum; that is what it eats. *Other* parts filter the big raw hum into steady DC.

Hum can be many things. I'd start with the Main Cap(s), because it/they are the first line of hum reduction and because this thing is old enough for cap-rot.

Hmmmmm. The 2200uF 75V is the main cap and the 1000uF 50V is the speaker cap. The speaker cap won't make hum. Go 3300u or 4700u 75V for main cap, mount it solid, observe polarity.

(An alternate topology has two equal main caps and no speaker cap. Modern fashion swings this way. But the main+speaker design works well, can't be "modded" to the other way without major re-design.)

There's surely another level of hum filtering. Without insight (a schematic), I'd try to figure what medium size e-cap comes right from the main cap through a resistor. (You know this from tube amps: raw B+ to 6L6, then 10K+40u for driver, and another 22K+20u for preamp. Sand-state will use lower resistances and fatter uFd to do the same thing.)

[sluckey]

Measured 315VAC on the secondary of the PT.

Really! Everything on that board should be fried. I suspect an error in measuring that secondary voltage. I would expect more like 50VAC.

[dpm309]

I have not pulled board yet to see where the secondaries go. I measured 45VDC on the 2 blue diodes closest to the PT, 15VDC in the middle and 0 on the far end. I will re-measure the secondary voltage (tabs 9 and 10).  Will have to wait until tomorrow before I can get back at this. Can I jumper a new e-cap instead of pulling and replacing the caps? Looks like I can do it for the 2200uf cap without too much trouble.
Thanks.

[shooter]

Can I jumper a new e-cap

You can, but it may not eliminate much if the old ones are crusty enough.  clip your meter across the 2200, measure DC, then switch to AC and measure.  Then piggy back a new Ecap, repeat, DC should get a small bump up and hopefully AC gets a good bump down.

[dpm309]

Remeasured the secondary and I am getting 34VAC. Must have misread the first reading. Clipped a 2200uf, 63V ecap across the main filter cap and it did not reduce the hum. DC and AC readings were the same before and after. Pulled the board and checked the underside for cold joints etc and did not see any. See picture. The secondaries are connected to the 2 diodes closest to the PT. I have also posted a picture of an old PT from a Yamaha SS bass amp that I have and was going to see if this would provide the same voltage. Not sure which wires are the primary or secondary side and want to make sure before I apply power to it. I am assuming the grey and brown are the primaries.

[PRR]

> Would the same techniques for determining the source of hum in a tube amp also apply to a SS amp?

Primary info: which knobs and jacks do what to the hum?

[dpm309]

The hum increases with the volume control. Does not matter if anything is plugged into either jack (I cleaned both input and aux jacks). I also used a dummy grounding input plug with no change in hum.

[shooter]

Did you try a test cap on the 2200uF?

can you measure the POS+ side of the 2200, DC AND AC

here's a start on schematic, ya, ya, we could guess this much
can your eyes get a read on the 3 little transistor guys closest to the power transistors

[dpm309]

I am getting 45VDC on the + side. AC voltage varies between 2 and 16 volts. Got the same reading with the 2200uf test cap clipped to the one on the board.

[shooter]

AC voltage varies between 2 and 16 volts

that's a big swing on 45vdc

can you "lift the + of the 2200 n re-measure?  I'm thinking a diode isn't dioding well

here's some more schemo, NOT correct, but still workin at it

[PRR]

> The hum increases with the volume control.

So it is before the volume control. (Those earlier stages likely have their own power filtering, so the main cap would not be the problem.)

I see the 2 has a Gain and a Master (but you have a 1?). Which does what to hum? Any tone controls? What do they do to hum? Do you have gut-pictures of the actual amp now on your bench?

[dpm309]

Shooter - I lifted one end of the cap and installed the new 2200uf cap and am still getting the same AC and DC readings with no reduction in hum. I tested the diodes with 2 different meters and they seem to test out fine.
PRR- I have a couple of gut shots earlier in this thread. Ignore the one in the first post, this was from an e-bay ad for a Bassist 2. This is actually a Bassist 1 with gain, low, mid and high controls. There is still hum with all controls at 1 and increases as you turn up the gain, and to a lesser extent the tone controls.

[PRR]

Replace the caps I have circled.

[dpm309]

Replaced the 2 e-caps, 330uf/50V and 100uf/35V and am getting less hum with all the tone controls on 1. When I turn up the tone controls, I am getting a worse hum and buzzing (120 cycle?). Double checked the polarity etc.

[PRR]

With clear square images of top and bottom of PCB, and excess time, I could maybe work-out the circuit and diagnose intelligently. But time is precious.

The amp is old enough (also simple enough) that wholesale replacement of ALL e-caps may be the best next path. Also a bright light and magnifier inspection of all traces and joints.

[dpm309]

I missed soldering one of the leads of the 2 new caps I installed, re-soldered it and am now back to where I was before but with no buzzing. I have magnifier light and went over the whole underside of the board and could not find any problems except with the lead I missed. I will go ahead and replace rest of the e-caps. I appreciate your help and I will report back when I get this resolved.

Thanks

[shooter]

But time is precious.

yup, I was trying but time....
410 shotgun I would agree

I did get a little more worked up, I think it's starting to look a lot like the valco 67 SS schematically, excluding values, non split rail supply, Darlington type driver final