1971 Randall RG-90 head, smoking speaker!

[czech-one-2]

Man if it was me, I’d also build me a dim bulb tester. This will help protect those new parts if you end up still having a short somewhere.

Yeah, its kind of a crap shoot...
I'll look into those [and what to do with them]

[tdlunsfo]

There are not that complicated. I built mine from parts I bought from a home improvement store. Hardest part these days in the USA is finding an incandescent bulb as we are quickly replacing them all with LED’s. Not sure if you have that problem in CZ or not.

[czech-one-2]

Oh yeah, probably harder to find over here!

[g1]

That should be TIP29 & 30, not TIS. They are not fet's. Pretty much the same as the 30 and 31 shown on the RG100 schematic.

[dmeek]

Remember to pay attention to the last letter in the TIP series:
ex TIP29C / TIP30C

No letter Vce = 40V
A Vce = 60V
B Vce = 80V
C Vce = 100V

This one uses C as would most amps

[olddawg]

Maybe it’s different over there but that amp is worth about $50 here in working condition. Just a thought.

[czech-one-2]

That should be TIP29 & 30, not TIS. They are not fet's. Pretty much the same as the 30 and 31 shown on the RG100 schematic.

Yes, TIP, I was going from memory at 2a.m, never a good thing.

[czech-one-2]

Thanks for the wisdom

[czech-one-2]

"You mean the big metal RCA's or one of the small black ones?"

No offense, but if you don't know where and which are the power transistors, you have no business trying to fix this amp.

RANDALL, your obviously a tech with all kinds of tools at your disposal, and yet even you with all your infinite knowledge still start threads in order to repair your Randall amp.
https://music-electronics-forum.com/...54050#poststop

Now, imagine you are a guitar player with a limited set of tools and only hands on knowledge, just coming here for some possible causes and a little direction. I really didnt start the thread to hear that its 'not that hard' and I can buy a schematic for 20 bucks or that my amp isnt worth what I paid for it.

[DrGonz78]

I am thankful every time someone initiates a thread that gets the group here troubleshooting. This website represents a wealth of information detailing so many different faults and resolutions to amplifier failures. Not to mention this site has great information on instruments, pick ups, and many many other topics of interest. The rules are simple: there are no dumb questions except the ones you failed to ask. I remember picking up a broken Carvin solid state amp from a shop I had worked for a while back. The owner of the shop pointed at the output transistors on the back of the amp and said they thought that these diodes might be faulty. I just acted polite and said I will check those output transistors to see if they are a problem. They do have diodes in them I guess so not too far off the mark really. When I first came to MEF I had to learn the terminology and the names of parts that were in an amplifier really quick as to not discourage the help that I could receive. In between questions I asked on the forum involved lots of google searches and reading up to be able to converse better on the site. Before too long my knowledge base on the basics of amplifier parts got better and my ability to read schematics improved. It took lots of time for sure but with the help of MEF members it set me on the path to learn these things better.

[czech-one-2]

Thanks Drgonz78! Thats refreshing to hear. I know most folks here are always extremely helpful [and extremely patient with non tech types]. I wish I had retained all the stuff I've learned here in the past but really, I end up here only when I feel I've bitten off more than I can chew, and I do do this often enough to retain all the info. I can get around a tube amp quite well, but these old solid state amps baffle me.
I'll report back when I get the transistors in. But I might just go ahead and change out all the small electrolytics cause my meter doesn't test capacitors.
I dont want to fry the new power transistors cause I bought genuine RCA replacements that werent exactly cheap. I'll double check everything that I can like resistors before I power it up again.

[olddawg]

RANDALL, your obviously a tech with all kinds of tools at your disposal, and yet even you with all your infinite knowledge still start threads in order to repair your Randall amp.
https://music-electronics-forum.com/...54050#poststop

Now, imagine you are a guitar player with a limited set of tools and only hands on knowledge, just coming here for some possible causes and a little direction. I really didnt start the thread to hear that its 'not that hard' and I can buy a schematic for 20 bucks or that my amp isnt worth what I paid for it.

There is no disrespect is intended I’m sure. We get all kinds of people posting with wildly varying levels of skill and competency. There seems to be an all too common belief lately that anything old is desirable/better. It simply isn’t true. Old solid state equipment can be especially problematic since parts are frequently obsolete and unobtainable. At that point you need the skill set to actually redesign/modify the circuits to use modern/existing components. Yes.. there are a lot of very experienced, competent techs on this site. Most are wiling to help you within reason. It’s like this.. if I tell you it’s probably not worth repairing.. it probably isn’t. If another tech looks at the conversation and says that if you really want it fixed, you should probably take it to a shop.. you probably should. But.. it’s just our educated opinions. Honestly we are just trying to save you time, grief, and money. There will not be a “quick” fix. If you want to turn the amp into a “learning experience”. That’s another story. In that case it’s time to stop burning it down and start reading some electronics theory.

[czech-one-2]

Again, I'm open to suggestions if somebody knows a tech in my neck of the planet, that would be great.
As far as the 'old is better' thing goes, I let my ears guide me there. These old solid state amps sound worlds better than most modeling or digital amps. I own a few old Japanese PEARL solid state amps that have beautiful, fendery clean tones. They are LOUD with unlimited clean headroom, half the weight of a Fender twin and a perfect clean platform for pedals.

[Jazz P Bass]

Unfortunately, solid state amp repair can be a real 'bear'.

Tube amps are relatively easy as the high voltage stages are coupled to the next stage by Vdc blocking capacitors.
Which makes things unlikely to blow up.

SS amps on the other hand are 'direct coupled'. No caps to stop the voltage.
So when things go bad , they go bad.

A tried and true tool is a 'lamp limiter'.
If the amp is in a failed state and tries to pull huge amounts of current, the filament in the lamp will help absorb it.
Along with a visual interpretation of what is going on. (bright is bad)

Another useful tool is an ammeter on the mains.
I use a device called 'Kill A Watt' along with an ammeter.

If you want to learn I would suggest fellow member Teemuck's online book.
http://www.thatraymond.com/downloads...ttala_v1.0.pdf
(Download Caution: it is 419 pages)

[czech-one-2]

While I'm waiting for new transistors and a bridge rectifier, I'm gonna recap with new electrolytics. One question, what are the yellow .22uf/200v caps? [pic] They look polarized so are they like 'speaker divider' type capacitors? There's 4 of them in this amp.
Also, these white/red stripe diodes, 1N4007 types?