B120 component values: tech help?

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B120 component values: tech help?

Postby johnc1 on Sat Aug 30, 2014 11:39 am

Got my latest B120.

Needs work & to minimise chargeable time with the tech, I'd like to replace as many of the old caps & resistors as I can myself.

got the schematic, got my soldering iron, got my meter ... but one thing I haven't got ... knowledge of the power rating of the existing components to match what I'm replacing.

No use to substitute a resistor with a new one, having the correct resistance value, if it blows due to insufficient power capacity as soon as I switch on ...
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Re: B120 component values: tech help?

Postby Mr. Foxen on Sat Aug 30, 2014 2:04 pm

Need to get kind of specific to the actual resistors, since various positions have different ratings. If the existing ones are there, can pretty much read off them. Also happy to go higher rated everywhere, only thing to avoid is going silly with wirewounds in some places due to their inductive nature, it matters in some positions.
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Re: B120 component values: tech help?

Postby Papa Dog on Sat Aug 30, 2014 8:04 pm

why on earth would you want to replace all the resistors and capacitors??? WHY????
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Re: B120 component values: tech help?

Postby johnc1 on Sat Aug 30, 2014 8:57 pm

Sorry, not ALL ... just those which may have wandered out of spec, or if any are obviously burned.

Haven't even took the chassis out of the casing so far, so don't know yet just how bad it may be.

did try an alternative pre into the reverb loop though ... and aside from a few pops & complaints, it soon settled down & was sounding pretty damn good. Guess the power & output stage are ok then.

no sound from the normal channel though & sound through the brilliant channel is a tad intermittent. Nothing at all from bass & mid pots.

wonder if there's nothing more sinister going on than faulty v1 & v2 preamp tubes?

I'll try swapping them out tomorrow. :lol:
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Re: B120 component values: tech help?

Postby Papa Dog on Sun Aug 31, 2014 1:12 am

that would be my guess...bad #1 and #2 preamp tubes.
lots of times the issue with a Mk4 pre-amp is that there are issues with the input jacks and the resistors there.
If a grounding wire has come loose all manner of noise gets in.
if the "grid leak" resistors (which are located there) are bad or not soldered well you can also get hiss and noise.
the Cliff jacks on the input do more than similar jacks on other amps and if the switches get dirty and are not making contact all manner of badness creaps in.
Sadly many folks just do not bother starting there and cleaning and fixing before they go ripping out a lot of other components or trying to make it into something which they might better understand.

Of course in it's stock for it is never going to have the same kind of front stage crunch that other amps have. It is the nature of the way the input stage is designed...it is meant to be clear, very clean and articulate.
changing one value on V1 will change the overall character of the amp, and more than one change can give very unpredictable results.
But...I like to bring the grid leak up to a higher value, and ground it directly instead of through the slave input.
results may vary, void where prohibited by law.
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Re: B120 component values: tech help?

Postby johnc1 on Sun Aug 31, 2014 3:44 pm

Thanks PD.

Replaced all the preamp tubes. Now have bass & mids, but still no normal channel volume.

What sound I DO have from the brilliant channel is clear, bright and strong though ... and with minimal hum & hiss, so I guess it's relatively healthy, but needs some attention to the inputs at least.

Turn the chassis over ... and I find spiderwebs all over the inputs and generally dull solder joints & even corroded wire legs on several resistors & capacitors ... plus white rust on the heads of the large filter caps (despite the clear sound). Bears out the several years of storage it had in cold & damp conditions before I scored it.

Need to decide whether to begin my own overhaul of the old beastie (as far as I can ... and also whether my marriage is strong enough to allow it!)), or just give it to the tech' to handle. Confident of my soldering ability (do lots of re-wiring & upgrades on my guitars) ... but guitars don't carry lethal voltages

:?
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Re: B120 component values: tech help?

Postby Papa Dog on Sun Aug 31, 2014 4:20 pm

I would, fer sure, change the electrolytic filter caps.
those are the only ones to worry about re: deadly voltages wise.
however, you gotta be sure you frind them all.
Always make sure when you are going to work on that thing that you dis-charge those big caps before you do anything else.
Once they are grounded out they should pose no further problems. Just make sure that every time you plug that beast in and turn it on that before you go to work on it again you ground and drain the caps. Ideally, once you have a good 3 line power calbe installed they will self drain, but a good rule o'thumb is to drain them to ground anyway...just to be on the safe sid. Ampheads like us, we sometimes learn from our mistakes, but in matter of high voltage we might well die from them...seldom any second chances with leathal voltages.
Anyways, a good shot of air (like from a vacuum cleaner turned around to blow) thru a fine nozell to blow all the crap out of it, followed by a fine brush, more air, and then maybe some de-oxit kind thangon the crusty joints and jacks.
Might wanna take some pics of the jack assembly, pull it out to clean and then make sure you put it back correctly (if it is indeed correct at present).
And, last but not least, got shots would be helpful in troubleshooting this thing long distance.
Good luck
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Re: B120 component values: tech help?

Postby tracynorton on Mon Sep 01, 2014 2:37 am

DON'T #UCK with anything until you get the power supply all sorted and recapped. Don't make me come over there.......I'm just sayin'.....
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Re: B120 component values: tech help?

Postby johnc1 on Mon Sep 01, 2014 11:32 am

Legacy of a mis-spent youth eh, TraceyNorton?? ... do tell :wink:
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Re: B120 component values: tech help?

Postby tracynorton on Tue Sep 02, 2014 4:46 am

johnc1 wrote:Legacy of a mis-spent youth eh, TraceyNorton?? ... do tell :wink:

hahaha! I'm just raising heII.....seriously, start with the power supply.
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Re: B120 component values: tech help?

Postby Papa Dog on Tue Sep 02, 2014 6:30 am

good point, Tracy.

sometimes I miss the obvious.
one should always make sure that there is a grounded power cable, and that the filter caps are not shorted or leaking.
In some cases one needs to be sure that they actually are still installed.
and,do please remove the old school switched grounding and "death cap".
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Re: B120 component values: tech help?

Postby johnc1 on Tue Sep 02, 2014 10:43 am

Check you both on that. Ground switch was already removed & blanked anyway. Shame, I could have re-used it to replace the shoddy aftermarket & non-matching power switch!
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Re: B120 component values: tech help?

Postby Papa Dog on Tue Sep 02, 2014 3:16 pm

johnc1 wrote:Check you both on that. Ground switch was already removed & blanked anyway. Shame, I could have re-used it to replace the shoddy aftermarket & non-matching power switch!

the grounding switch usually is a SPDT and the power switch is a double pole. Good idea but most likely would not have worked.
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Re: B120 component values: tech help?

Postby johnc1 on Tue Sep 02, 2014 4:00 pm

Papa Dog wrote:
johnc1 wrote:Check you both on that. Ground switch was already removed & blanked anyway. Shame, I could have re-used it to replace the shoddy aftermarket & non-matching power switch!

the grounding switch usually is a SPDT and the power switch is a double pole. Good idea but most likely would not have worked.


Ok ... moot point anyway ... BUT ... should the sensitivity switch be a DPDT also? If so, I could maybe swap those two over. The current (sic) power switch works ok, it just doesn't match the correct standby/on switch (where the sensitivity switch DOES, visually at least).

edit ... Ok ... scratch that ... just re-checked the schematic & that one's a single pole too.

Guess I'll have to buy a replacement then ... if I can.
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Re: B120 component values: tech help?

Postby Papa Dog on Tue Sep 02, 2014 9:14 pm

the sensitivity control is one place where I honestly take issue with the designers.
it seems like a good idea, on paper, but I think that there is a better way.
with the sense switch it is either all or nuthin. Well, not exactly, but close enough.
I like to take that switch out and replace it with a 22k 1 watt linear pot, wired as a rheostat (variable resistor) so that it affords a more "granular" control. Gotta remember to leave the 12k resistor in place right at the cathode. And a switching pot would give one the ability to go "all in" or to "turn it down" as you please.
It isn't exactly a master volume (which could be wired in that spot instead) and it changes the tone, feel and character of the amp, but is kinda fun to mess with.
Another way to get a master volume control without doing any soldering is to put a 500k or 1 meg audio (log) in the "reverb" loop. Those two things together afford some very interesting ways to control volume.
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