The Electric Piano Forum

Repairs, Maintenance & Upgrades => Parts, Service, Maintenance & Repairs => Topic started by: sopranojam85 on August 17, 2015, 09:25:27 PM

Title: Rhodes Janus I 50W Amplifier Crackling
Post by: sopranojam85 on August 17, 2015, 09:25:27 PM

I'm about to get cracking on a Rhodes Janus I 50W Amp. This is the vertical free-standing ones that Rhodes intended you to buy either one or two of them together. Anyway, there's only one, and it's developed a fairly horrendous crackling sound.

I'm not 1,000% certain, but am mostly certain that this is the proper schematic for this little beastie:

It's been interesting trying to differentiate between the suitcase built-in amps, and the stand-alone amps. If this is not the right schematic, I'd appreciate a gentle shove in the right direction.

This is coming back to the shop for a re-work. The original problem was a crackle, and I noticed that the Power transformer was dangling off the chassis by one nut. The hardware mounting it was all stripped out. I replaced all the hardware, and the crackle went away. While the amp was out, I put a dummy load on it, powered it on, and measured 38 V on the 30 V rail. So that's an area of concern. Something is probably not drawing enough current, or the voltage regulators are not doing their job, or both.

Does anyone know what the expected AC voltage is on the power transformer secondary windings for these things? (Power on, and secondary windings disconnected, freely measured?) I have a notoriously bad time with power transformers failing, and before I start chasing IC problems, I'd like to make sure the power transformer is doing its job.

Also a recap is in order, and replacing any off-spec resistors... Any advice is much appreciated. I'm more a tube guy.. this solid state stuff is easier to work on, but harder for me to understand.
Title: Re: Rhodes Janus I 50W Amplifier Crackling
Post by: vanceinatlance on August 17, 2015, 10:29:47 PM
I'll offer my 2 cents, but you might want to wait and see if someone with more experience can chime in and verify if I'm taking you down the right avenue.
I can't verify if what you have is indeed the same circuit as the suitcase amp, but I would not be suprised if it was. I don't know the unloaded ac voltage of the transformer's output, but your reading seems to indicate to me that it is doing it's job within reason. I would definately suspect the regulator board as the culprit or at least a part of your issue.
If it is basically the same as the janus suitcase amp, I personally like to rebuild the voltage regulator boards when I run across these amps especially if they appear to be original. A basic re-cap is usually long over due and I consider a few caps and possibly a regulator or two cheap insurance even if the voltages seem to be in spec. I have not personally run across a bad transformer in one of these yet, but I can't say I have worked on a large volume of them either.
To me, it seems to me like you are at least on the right track. Clean and proper power is a great place to start.
Best of luck with it.
Sorry I can't offer you more or better info, but maybe at least a start?
Title: Re: Rhodes Janus I 50W Amplifier Crackling
Post by: spacecho on May 19, 2019, 05:51:11 AM
Hello All,

Following this thread, I too have a pair of Janus that i recently biased and adjusted. As part of that service i checked all rails and noticed that the 30v rails both measure around 38v on both units (measured at the caps), much the same as is the case with vanceinatlance's original post.

I don't see any voltage regulators for the 30v rails, just a Bridge Rectifier (which check out okay on both units). My units were a little crackly (when plugging in and out) until i adjusted the DC offset (which was quite out) and the Bias, now they sound great. All of the connections are switching jacks, and all of mine were oxidised - switch deoxit really helped here, as did fader deoxit for the pot. Also as mentioned in other threads, reflow the molex connections to each of the amplifier and regulator boards as the solder joints can crack (causing random crackling and issues).

There is a video that outlines how to calibrate and adjust the bias and DC offset for these amps - but please, don't even think about going in there unless you know what you are doing!!!!!