ROLAND BOLT 30- 60 cycle hum...

[czech-one-2]

Just got the amp a month ago, havn't used it at a gig yet but plan to next month for a fall tour. So when I say its my gigging amp I mean I'm hoping it can replace the JC-120 that I've been lugging everywhere [and save my spine in the process]
It had hum when I got it, got worse [more noticable] when I put the Century in. I dont mind some hum, but this is real annoying hum.
The stock speaker was 35w with a tiny magnet and probably 96db sensitivity.The Century is 100w with a 102db rating.
Are my AC readings even possible?

[czech-one-2]

OK, little confusing,
Now I'm Getting
R58 98v / 89v AC and R59 98v / 105v AC

Triple/ quadruple checked. These are the AC readings

[czech-one-2]

Well its ridiculous... actually I'm curious if the 13 and 6 volt readings were the same measuring this way with terminal 5
new test.. measure from that ground point with power off to the ground point you used before resistance reading on 200 ohms scale.

.3 ohms from pad#5 to the chassis ground point, power off

[czech-one-2]

Just tried it with a neighbors meter, r58 = 98/89v AC and R59 = 98/105v AC

Do these AC readings indicate a big problem?

[czech-one-2]

I presume the switching function works ? as it uses a cmos ic TC 4016 which contains 4 switches .
If there was ac ripple in the supply to this it would infest most of the preamp.
One footswitch controls 4 "electronic" switches at various points in the schematic which reconfigures the pre-amp for overdrive!

Yes, the switching jacks work.
Here's a high contrast board shot:

[g1]

Some meters can't measure AC properly if there is DC present. If so they need a cap in series to block the DC.
To check your meter (and your neighbours) set the meter for AC volts and measure a 9V battery.

[czech-one-2]

Some meters can't measure AC properly if there is DC present. If so they need a cap in series to block the DC.
To check your meter (and your neighbours) set the meter for AC volts and measure a 9V battery.

Ok, both meters measured the 9v battery at 6.3v dc / 13.4v AC

Do I need to put a cap on the + probe? If so, what value would you recommend?

[g1]

.047 or .1uf or something like that, rated for at least 600V.
Definitely should read 0VAC for a battery.
But why does it read 6.3VDC, is the battery weak, or is the meter also that inaccurate?

[Enzo]

Foo. I have a couple 9v batteries on my bench that measure about 6v. They are perfectly useful for testing speaker voice coils if nothing else.

[oc disorder]

Yeah well I choose to ignore these ac readings for the moment but maybe come back to them later.
Thanks for posting those pictures.
It seems to me the amp has been murdered sorry I meant modded ..(yes an intended pun) .. so at this point it seems we have two options
1.Restore it to original
2.Trace out and try to understand the mod using various posted schematics for modding this amp as a guide !
Then attempt to make it work properly.
In my mind you have the basis for a good amp- good speaker and good power amp.
It seems by comments on the web Roland/Boss didn't quite nail the overdrive sound back then .
So I'm not surprised an attempt has been made to improve it.
You could either get/build some kind of preamp (stand alone) and ignore the internal one i.e. disconnect it!
From where I'm looking I suggest we restore it first then think about some fancy changes .......
although as a clean amp it should be fine with pedals.
See if you can spot all the changes on the board then we'll think about it.

[J M Fahey]

Those cheap metars can NOT rear real AC.
The telltale indicator is that they have only 2 AC scales, usually 200V and something around 700VAC.

And I'm not so sure a series capacitor will work, for the very good reason tat it will become an RF probe (real oldtimers will rememberv that), a cap and diode network which yes, stopped DC ... and charged the cap to peak AC, not what we want here.



This one gives you roughly the RMS value if your VTVM/Digital multimeter has 10/11M input impedance.

Read all about it in:
N5ESE's Classic RF Probe

Of course, you set your meter to some DC scale.

And besides the multimeter "AC scale" is just the DC one, multiplied by 2.5 (or thereabouts), the conversion factor for the DC component in a half wave rectified AC so the end result of two mismatched measurements is anybody's guess.

That said, I haven't seen (yet) anything pointing to bad supply caps or excessive ripple.

First of all: does hum respond to any controls?

[czech-one-2]

oc, thats interesting, especially those 4 big resistors,missing diodes and jumper on the left? I did do a clipping diode change over on the right from germanium to silicone, and there were 2 small caps [c33 was one] over there that the mod suggested to remove to improve the brightness, but other than that I havn't touched anything.
I assumed that the other missing parts were for the Bolt 60 that was built on the same pcb??

[czech-one-2]

But why does it read 6.3VDC, is the battery weak, or is the meter also that inaccurate?

Weak battery

[czech-one-2]

From where I'm looking I suggest we restore it first then think about some fancy changes .......
although as a clean amp it should be fine with pedals.
See if you can spot all the changes on the board then we'll think about it.

So..I should print the bolt 30 schematic an replace all missing parts/jumpers?

I would be fine not using the overdrive channel, just the clean channel as thats how I've been using my JC120 forever. Just need that blasted hum gone!

[oc disorder]

Before I forget T.P. = Test Point and Juans question is pertinent !
At present I would think the preamp would be the same for both the 30 and the 60.
Will check the parts list and possibly some of the advertising blurb which is on the
site with the schematics.As the front panels are identical I doubt there is any difference.
Occasionaly they make a boo boo (mistake) when they do the art work for the PCB
and then correct it after production ie change the odd component or leave the odd one off!
However as the Japanese are so careful and take pride in their work I doubt it.
Normally they just fit lower powered transformers and speakers to make the amp more affordable thus the pre amp would be the same.
So if the switching works I presume the "two channels" both work and again Juans question........ I would presume
it hummed more on the super dooper overdrive.............