Hi Folks
Re-capping and replacing all the obvious on my 39 year old 112RP - 65. Nearly finished but there's a ceramic disc cap across one of the valve bases that is marked N1500 1KV. On the schematic for the GP2 board it's the only one not written down, does anyone know it's value please? is it 1500pF?
There are 2 x 220 ohm resistors across the valve bases too, one was open circuit, so I've replaced those with metal film 2W. The amp worked fine so not sure about the knock on effect?
I'm just a hobbyist so any engineers on here don't get frustrated please.
Thank you all for your love and support for these wonderful amps. I've had mine from new, had Boogies too, but the 112 was always my work horse and has never let me down. If I were to buy another amp for gigging it maybe a Fuchs ODS, as my 112 is getting on a bit now.
I've also replaced all the smoothing and HT diodes as they were looking a bit tired, any other tips greatly appreciated
Thank you
Graham
500pF is the correct value
Hi Graham,
the HV transient caps in the RP/RD series are 500pF (noted as 470pF in later units) 3kV. They tend to be very stable, so unless they appear shorted can be safely left alone. N1500 is the temperature gradient marking for this type of cap.
The screen resistors (220ohm) are originally rated at ½ watt and as long as the tubes are working normally you are good. If a tube has an internal short and starts acting up drawing excessive screen current, the resistor will burn before the rest of the amp, sort of a fuse. It doesn't hurt to put 2W resistors in there but you lose part of the protection provided by the original design.
Glad to hear you are willing to put effort into keeping an MM amp on the road.
Cheers,
Lars Verholt
Lars,
Lars,
Thank you so much for taking the time to reply to my query. Sadly I know about the 470pf caps (you can see one in the enclosed picture) it's the one at the top left I can't find a value for, is that 470pF too? Didn't know that about the screen resistors being there as a safety feature, I'll take out the ones I've put in and replace them for 1/2W carbon film
Once again thank you
Graham
Oh! Sorry...
Hi Graham,
ok sorry for the misunderstanding. The capacitor in question is 50/47pf @ 1kv and is there to deal with RF interference.
Cheers,
Lars Verholt
You're a star Lars
Thank you
Graham
Re-capped but a fuzz box for now
Hi all
So I re-capped my 37 year old 112RP 65, along with all power supply diodes and resistors, plus new JJ 6CA7's. Good news is everything works, reverb, phasor, tone controls. But the output is low and distorted. Early days yet so I need to go back to the detailed photos I took before dismantling. everything looks right but thinking about it now there's a 2uF 50V electrolytic that may be the wrong way round. I did put a polarity dot on the boards before removing but may have got that one wrong. Failing my ineptitude is there anything else I need to check? Cheers G
No everything is right
Hoping someone can help. I've triple checked and all components are in their correct orientation, the bias readings for one of the 6CA7's is 23mv and the other 21mv, solder joints all good. Still low output and distorted?
Speaker?
Have you checked if the speaker is in good working condition? The OEM one in my 112RD died on the operating table while recapping. The amp will sound weak, thin and distorted with a speaker that is ready to die. Try an extension speaker plugged into the main speaker jack. -mgriffin
Not the speaker
Hi Steve
Definitely not the speaker, it's a JBL that's been in there for years and works really well. I've checked it and it's fine thanks.
Cheers
Graham
Where does it distort?
Hi there,
this amp being an RP you can pull and push signal into the unit via the loop/insert jacks. A super useful (if a bit exorbitant) feature. What happens if you feed signal of some sort into the hi and/or lo insert points? Do you have a way to test what comes out of the same jacks? T is input-to-next-stage, R is output from preceding stage if memory serves.
Cheers,
Lars Verholt
Goos idea
Hi Lars
Thanks for that, good idea. I've a 1khz tone generator that can output at several levels so will give that a go
Cheers
Graham
She's alive
Well it's 17 months since I posted about the distortion problems and in truth I just got disheartened that although I'm pretty good with a soldering iron, I'm not at the level of some of you guys (Lars) so the amp was just put on a shelf for a rainy day project.
In the meantime I visited relatives in New Jersey and was lucky enough to go to Fuchs Audio Engineering and meet Andy and try his formidable amps. I ended up buying an ODS II 1 x 12 combo. What really sold it to me was that I got my Music Man clean sound straight away so anything else would've been a bonus. Years ago I bought a Boogie, and got rid of it as it could do clean or dirty but not both in the way my MM with a pedal board could
So much as I love my new amplifier I felt guilty that I hadn't tried to get my MM amp up to spec. I've had her since I was 19 and it's never let me down. So I read a few of the bulletin boards and decided to replace the Driver Transistors with a matched pair of 2N6488s, and as they were cheap all the op amps for the original LM1458 types. I also read that I should check the solder joints on the board (GP 2) as they can age.
Well interestingly there were no dry joints or cracked traces, BUT one of the electrolytics that I replaced had an unsoldered leg DOH!!!
There was also an RF filter cap (0.5uf?) that went from ground to the speaker jack that when tapped gave a level change so I replaced that.
After a thorough check that everything was where it should be I turned it on... I got the clean volume back but when flat out there seemed to be a high pitched squeal. However checking the bias (44.8mv!!) I quickly re-biased the amp and got 23.6mv and 26.3mv respectively. That seemed to do the trick with the squealing.
To cut a long story short "She's alive" and as good as the day I got her albeit with a few battle scars. I'd also forgotten how loud she was.
Can I just say a big thank you to everyone who tried to help me and for everyone keeping these great beasts going
All the best
Graham
Weak phasor and reverb
Hi Folks,
Me again I need some advice....
Since I re-capped my MM 112RP65 she's working well except that the reverb and phasor are weak. I've tried changing op-amps with varying degrees of success, some put DC splats on the speaker output when the footswitch is operated on the phasor so I've stayed with the original RCA 1458's.
LDR voltages read -15 and -16v. I just wondered if anyone knows a fix as it's a shame that after all my work it's not 100%
Thanks in advance
Graham