Crate BT220 problem

[iceregent]

New here, not sure where to post or even if I can find answers. I have Crate BT220 bass combo, worked beautiful til I went to pick it up from where it was. Took it to rehearsal, and the sound started getting real low, the limiting ligt started coming on, and heard a 60 hz hum starting, which the volume control did not affect. Now, there is barely any sound coming out except for the 60 hz hum, doesnt matter where the volume settings are set to, same sound. And ideas?

Ice

[phatt]

At a guess, Main Filter Capacitors on the power supply.
Usually two in SS amps  and usually the biggest ones in the amp.
Sorry that's all I can do with the info. Cheers Phil.

[J M Fahey]

It *might* also be a thermal problem, touch the heatsink to see if it gets abnormally hot.
Good luck.
J M Fahey

[iceregent]

Thank you for the guidance, I will check those things out. I just this morning pulled the amp out of the cab, and am visually inspecting it. I will go over the suggestions, and let you know what my results are..

Ice

[iceregent]

Ok, so, I looked over the mainboard, and found that a lead on one of the main power supply electrolytics had apparently blown a solder hole, and the lead because void of solder. I removed the cap, and checked it with a meter, but the cap checked good, not blown. So, I fathomed the possibility of a solder void becoming overheated and the gasses blowing the solder away from the lead, so I placed the cap back in, correct polarity as it was marked on the board, and resoldered it, and then checked on the power amp section, checking a bunch of caps and the power tranny's on there, but did not find any shorted trannies or caps, so, I put the amp back together and screwed it back into place, and plugged it in.

When I turn on the power, now there is no 6o hz hum, however, the limiter light does a rapid pulsation of about 1-2 hz, and the sound out of the amp without anything plugged in, and with all volume knobs turned to zero, pulses along with it. Not sure where to go from here, since I do not have the schematic for it.

[phatt]

Oh dear,, Likely something else died when the filter went.
You still may have a power supply issue,,
the highest voltages are on those big Caps  but the preamp needs much smaller volts
from the main supply.

Without seeing the amp it's hard to know as there are many ways to derive the preamp supplies. Some just use resistors and zener diodes for regulation while some have dedicated chips to do it.

If the amp is out of box and nothing is melting/glowing/smoking while it's on then,,,
IF it has chips with 8 pins then you can check pins 4 and 8 on those chips.

usually these run on a split supply,,ie, up too 15/0/15 VDC
Pin 4 should read a *Negitive voltage* and pin 8 a *Positive reading*.

Normally these are very close to the same reading ie you should read something like this; Pin 8= 14.9 pos and Pin 4= 15.1 neg, with respect to ground.
If one or both of those voltages are way off then it's certainly a supply issue again.

Everthing may have been restored in the supply dept but the original fault might have killed one of the chips and that may not be as easy to track down.

If you're good at back engineering circuits then disconnect the preamp supply and see what reading you get.
Some circuits are easy but a lot of modern ones are just nightmares to work out.
Cheers, Phil.

[J M Fahey]

Hi Icregent.
I don´t have the B-220 amp schematic, but usually most amp makers have a given set of design ideas, a certain "style" if you wish, which they use not in only one specific amplifier but also in a very similar form to create its "brothers". So, often the schematic of a "brother" amp, although of higher or lower power, can be of great help. Please tell us the codes on the output transistors, how many of them, the codes on the power supply filter capacitors , maybe a closeup picture of two.
A couple Crate (and Ampeg, same factory, and often *very* similar) schematics have been posted, they might help.
Bye.
J M Fahey

[iceregent]

Ok, I have news to report...

I removed and re-installed one of the main filter caps, the large electrolytic, on the pcb. The resultant operation appeared to be due to the fact that I neglected to re-install the faceplate on the opposite side of the pcb from the heat sink on the power amp pcb. I noticed the metal plate on the table beside the amp after I got that initial cycling result I mentioned in the previous post. I replaced the metal plate, and put the amp back together, and it appeared to work just like it was new. No buzz or anything else, just a clean clear signal. I didnt play the amp at any sort of power or timeframe until a few days later. I played the amp for about 5 minutes, and then suddenly it popped and the 60hz buzz came back and the sound went dead except for the 60 hz buzz. I immediately turned it off, and now have the amp apart again, but, havent proceeded any further. I still do not have any schematic, so I am unsure of how the circuit is laid out. There are ref des on the pcb, but that doesnt help without the schematic. I used to troubleshoot and repair dc-ac power inverters at a company for a living, but, that was with schematics and o-scope etc. Now, with this, I dont have schematic yet and I only have digital voltometer. And , I am only a junior tech, never had any schooling beyond usaf back in 1982. So, I have knowledge of how solid state parts should be but not sure how to troubleshoot right now without knowing what signals should be where. Only thing I can do is probe in circuit for parts that appear to be bad, since there is no apparent visual damage. ANy help? I cannot afford to take the amp to a shop, as I am un-employed at the moment.

Ice

[J M Fahey]

Hi Ice.
At least it´s not blown, only some thermal or stability or similar problem.
Amps don´t repair themselves, so if at random they work properly, they're not dead yet.
For lack of better information, take some clean, readable pictures from both sides of the PCB and any other ones you find useful.
If too much for this forum, post them in Flickr or similar.
The multimeter will do, if nothing else is available.
Download some 1kHz MP3 to have some repeatable stable signal to test.
Also post whatever codes are on the transistors, supply voltages, what´s written on the power transformer, etc.
Good luck.
J M Fahey
I forgot:build a series lamp outlet. It has been explained by Teemuk elsewhere in the Forum.

[iceregent]

Ok, not sure what kinda pics you are talking of, but I took pictures of the main input/power supply pcb so far, and they are at: http://amp.iceregent.com. I am working on getting the 60 watt bulb setup, simple to wire in a lamp with a 60 watt bulb in series with a plug. If I need clearer pictures or different pictures please let me know.

Ice

[phatt]

Ok, not sure what kinda pics you are talking of, but I took pictures of the main input/power supply pcb so far, and they are at: http://amp.iceregent.com. I am working on getting the 60 watt bulb setup, simple to wire in a lamp with a 60 watt bulb in series with a plug. If I need clearer pictures or different pictures please let me know.

Ice

Again,,, most likely a dud Electro Cap, the big black ones.
FIRST,, Establish you have both AC (Before any Rectification)
and then,,,,,,, DC (without Caps)

Your transformer has 3 AC wires. (Or/Blueygreen/Or)
The greeny blue one is the centre TAP (zero volts from there onwards)

Pull all 4 terminals from Bridge Rect. (Crist sakes mark them first!)

Now set meter for AC Max reading,
Neg probe to ZERO blueygreen center tap,,now read the AC volts of BOTH Orange wires.
Should both read the same AC voltage if one side is way off the power transfomer is stuffed,,unlikely.

Next set Meter to DC reading.
Re-install AC (orange wires) from transfomer onto the bridge rect.
(Don't connect the DC wires that go to Caps yet)

Now power up again and read the DC on the Bridge terminals.
One is a postive voltage the other is negitive.
(again Neg probe to ZERO blueygreen center tap)
These should be within a volt of each other, don't expect to be exact.

Now reconnect the Caps and see what readings you get,,one side will likely be down a fair bit.       
But I would just replace both CAPS First and be safe.

I've added some hints on this pic for you..
Now One word of warning those black and white wires are close to where you're working and they look to be mains wires so be carefull

Remember the DC reading goes up x 1.414 after you add the Capacitors.
Hope you fix it.  Phil.