Need Help w/ Ampeg BA210SP - blows the fuse, the output is bad

[Jiji]

@JM Fahey
The video is prior to the incident... that's what I was trying to fix.
As stated before, the sound cleared quite a bit when I increased the resistance of the Bias pot.
The new components haven't arrived yet.

* I might scope it myself when the time comes... I do have a telescope.. ;}

[Jiji]

silly questions:

Is it possible that the schematics(dated 2000-2001) have changed in 5 years for this amp? [I saw a '2006' year on the speakers and serial#(21)DYODS03061)
I am asking because last night I was testing components in the wake of putting the new transistors and I have located a few Rs and Qs that do not appear on the schematics. For example (as seen in the pic on the right) R25 is now in series w/ a 'new' R91, each measuring 4.7K, and on the schematics R25=10k.

I am attaching the schematics for both pre and power amp, and the 'search' function is working in the document (ctrl+F).

Also, how do I test OC1? (Vactrol VTL-5C7)


*the new components are arriving tomorrow. thank you very much for your help and time so far!



Ampeg BA210 bass amp SCHEMATIC.pdf

[J M Fahey]

Is it possible that the schematics(dated 2000-2001) have changed in 5 years for this amp? [I saw a '2006' year on the speakers and serial#(21)DYODS03061)

Yes, itŽs possible, although hopefully changes are minor.

I am asking because last night I was testing components in the wake of putting the new transistors and I have located a few Rs and Qs that do not appear on the schematics. For example (as seen in the pic on the right) R25 is now in series w/ a 'new' R91, each measuring 4.7K, and on the schematics R25=10k.

Maybe, but no big deal; total resistance is still the same, I guess youŽll find a small cap to ground from the midpoint between those resistors, to decrease RF interference.

Anyway the general purpose repair technique would be:
1) disconnect speaker and plug amp in lamp current limiter
2) turn it on, check that main rails are reasonable, say 25 to 40V each (because of series lamp); +/-15V should be there or very close because they are regulated.
3) check that you donŽt have DC at the speaker output
4) check that you donŽt have DC at U3b output.
5) check that you donŽt have DC at U3a output.
Obviously, the amp is not working, so it will fail one or more of these checks.
Please post results.
And for now forget OC1.
Working or not it will not prevent the amp from working.

[Jiji]

thank you J M Fahey.

*there is a cap to ground, just as you said above...


After replacing the bad components the amp powered up.
W/ a lamp limiter everything checked out ok. Set the Bias at 16 mV DC, according to the schematics' instructions.

W/ no lamp limiter, it reads: (cold amp, no signal, no load)
50V = 43.6V
16V = 16.3V
Bias = 16mV DC (very similar across both resistors)
Speaker out = 110mV DC

questions:
1. does the Bias Voltage go down after the amp warms up? I set the Bias to 16mV DC, and after 2-3 min of idle (transistors get warm) the voltage reads ~ 8mV DC.
2. R 6&7 get hot in the first 20 sec. Should I be concerned?
3. is it necessary to use the (SL22 10008) THERMISTOR? -it's mounted before the main fuse and as far as I can tell it is blown, but still works...

[J M Fahey]

After replacing the bad components the amp powered up.

Cool

W/ a lamp limiter everything checked out ok. Set the Bias at 16 mV DC, according to the schematics' instructions.

Good

W/ no lamp limiter, it reads: (cold amp, no signal, no load)
50V = 43.6V

Reasonable

16V = 16.3V
Bias = 16mV DC (very similar across both resistors)

Good

Speaker out = 110mV DC

Very slightly high but nothing to worry about.
Hope you measured it without speaker.
Remeasure with speaker. It might lower something but if not, not big deal.

questions:
1. does the Bias Voltage go down after the amp warms up? I set the Bias to 16mV DC, and after 2-3 min of idle (transistors get warm) the voltage reads ~ 8mV DC.

To be more precise, we hope it stabilizes at some nearby value; it might go slightly up or down, personally I feel better if it goes down.
Anyway those 8mV show there is still some idle current passing, so itŽs above the knee, good.

2. R 6&7 get hot in the first 20 sec. Should I be concerned?

No, itŽs normal, they are dissipating exactly 1/2W, their official rating.
Personally IŽd have used a somewhat larger (1W?) resistor there but donŽt rush to replace them; as is they will last a lot.
FWIW they are less abused than dropping resistors in Fenders, go figure.
[QUOTE]

3. is it necessary to use the (SL22 10008) THERMISTOR? -it's mounted before the main fuse and as far as I can tell it is blown, but still works...

Then itŽs not that blown
IŽd get another, just in case.

Back to the amp, play 5 minutes, check temperature and bias, then 10 minutes, repeat, etc.

If you can play 1 hour at rehearsal or Club levels without overheating but more important, keeping reasonable bias current (no more than the 20mV suggested) . pronounce it healthy.

Sorry about the very cautious testing but better safe than sorry

[g1]

I'm surprised at the low supply rails. If they are really supposed to be 50V, 43 seems kind of low. It is much more common for supply rails to read high than low.
Are the R6&7 you guys are talking about the ones near OC1 ?

[Jiji]

@g-one: that's correct; R6,7 are close to OC1. see page 2, upper left on previously attached pdf (post #17)

@JM Fahey
thank you, I will report back...

[Jiji]

I replaced the Bias pot, I was able to find a 1k/0.75W (1.25W max) 15 turns.
It is easier to adjust compared to the old one, so it is worth the trouble to make it fit...
I do not know what the wattage is on the original. It is a 500 Ohms, that much I know.
--------------------------------------------------------------------------------------------
@JM Fahey
*My multimeters are not calibrated, but they read within 0.02mV of each other(3 of them).


I readjusted the BIAS (about R72=17.5mV / R7=18.3mV DC) with the new multi-turn pot. cold start, 70F(21C)

Here are some reading, as suggested by you:

Cold amp:
no speaker connected~~~~~~~~~~~~~~~speaker connected
R72 17.5mV~~~~~~~~~~~~~~~~~~~~~R72 17mV
R75 18.3mV~~~~~~~~~~~~~~~~~~~~~R75 13mV
spkr 113mV~~~~~~~~~~~~~~~~~~~~~spkr 108mV

With warm amp (5 min idle)
no speaker connected~~~~~~~~~~~~~~~ speaker connected
R72 10.5mV~~~~~~~~~~~~~~~~~~~~~ R72 12.2mV
R75 12.1mV~~~~~~~~~~~~~~~~~~~~~ R75 9.8mV
spkr 98.2mV~~~~~~~~~~~~~~~~~~~~~ spkr 96.1mV
- output transistors temp 118F(48C)


+after 10 min idle (no spkr connected):
no speaker connected~~~~~~~~~~~~~~~ speaker connected
R72 9mV~~~~~~~~~~~~~~~~~~~~~ R72 11mV
R75 11mV~~~~~~~~~~~~~~~~~~~~~ R75 9mV
spkr 96mV~~~~~~~~~~~~~~~~~~~~~ spkr 94mV


+ another idle, 45min- 1hr (stable readings)
no speaker connected~~~~~~~~~~~~~~~ speaker connected
R72 11.4mV~~~~~~~~~~~~~~~~~~~~~ R72 12.8mV
R75 14.0mV ~~~~~~~~~~~~~~~~~~~~~ R75 11.2mV
spkr 91.0mV ~~~~~~~~~~~~~~~~~~~~~ spkr 89.7mV


+ 1hr loud music+bass guitar - 80% volum (reading after 1 min idle):
no speaker connected ~~~~~~~~~~~~~~~ speaker connected
R72 3mV ~~~~~~~~~~~~~~~~~~~~~ R72 5mV
R75 5mV ~~~~~~~~~~~~~~~~~~~~~ R75 3mV
spkr 93mV ~~~~~~~~~~~~~~~~~~~~~ spkr 92mV

+1 hr idle:
no speaker connected~~~~~~~~~~~~~~~speaker connected
R72 6.2mV~~~~~~~~~~~~~~~~~~~~~R72 8.2mV
R75 8.3mV~~~~~~~~~~~~~~~~~~~~~R75 6.4mV
spkr 90mV~~~~~~~~~~~~~~~~~~~~~spkr 88mV

+1hr music 90% volume (reading at no idle time, see video for 'loud' static humming noise/observation 1)
no speaker connected~~~~~~~~~~~~~~~speaker connected
R72 2.7mV~~~~~~~~~~~~~~~~~~~~~R72 0.2mV
R75 4.4mV~~~~~~~~~~~~~~~~~~~~~R75 0.4mV
spkr 90mV~~~~~~~~~~~~~~~~~~~~~spkr 88mV

+20 min music 20% volume
no speaker connected~~~~~~~~~~~~~~~speaker connected
R72 3.6mV~~~~~~~~~~~~~~~~~~~~~R72 5.8mV
R75 5.6mV~~~~~~~~~~~~~~~~~~~~~R75 3.8mV
spkr 92.8mV~~~~~~~~~~~~~~~~~~~~~spkr 91.4mV
- output transistors temp 160F(71C)

+30 min power off
no speaker connected~~~~~~~~~~~~~~~speaker connected
R72 16.4mV~~~~~~~~~~~~~~~~~~~~~R72
R75 17.5mV~~~~~~~~~~~~~~~~~~~~~R75
spkr 112mV~~~~~~~~~~~~~~~~~~~~~spkr




--------------------------------------------------------------------------------------------

How does it look?

Should the BIAS be set to be within the specified values (15mV-20mV) when warm/idling or that value should be set just for a cold amp?

Thank you!



observations:

1.as heard in the video, there could be a louder static humming after max volume for more than 30min. It is volume and tone independent. it went away as soon as I powered off the amp and restarted it.
the only other time this happened, the amp was idling and I unplugged it from the AC outlet and then replugged it back in. This time the noise went away after I turned the volume above 80% and than back to zero.
so it is safe to say that the 'loud' noise goes away with either a 'restart' or a high volume kick...

2.when speaker is connected, R75 reads the higher values between the two...

3.the polarity of the speaker output is reversed when measuring the DC.

4.a very low volume static 'humm' is present (as heard in the video @30sec), with everything turned down (zero). it is the amp, not the preamp.
w/ everything turned up (10) there is the usual "full power hiss"; low volume, nothing major ;]
The overall sound of the amp is satisfactory I would say..

[J M Fahey]

Looks *very* good, specially because as it gets hotter, idle current not only stays the same but *lowers* .

In theory not the best but in practice ***safe*** .
Personally IŽd set cold bias to schematic indications (around 20 mA) , let it warm somewhat (not necessarily boiling heatsinks, rather just "not cold to the touch" (30/35 Deg C? , ) , check that it goes down somewhat and raise it again to 15/20mA .

The idea is to avoid it distorting at berdroom levels on a cold day, yet still staying well inside the safe area.

Just a suggestion, huh? , nothing set in stone .

[Jiji]

Thank you to everyone for your help!
@JM Fahey: I sent you a private message.


The amp seems to be in good working order.


The last things that were creating problems, as heard in the previous video, I believe had to do with the "Grounding" between the preamp and the amp. J28 & 29(big white w/ white wires) carry the ground on pin4 and as seen in the picture that type of connector is anything but 'audio'. I managed to redo the connectors, but I would prefer if there were a way to solder everything... Has anyone ever attempted something similar?

I also made 2 aluminum sinks for the Q1 and Q4, it keeps the temps lower.
The ceramic resistors (R66&68) are the hottest components in the amp (>150C easy), it is a shame they heat everything up inside the box with no exhaust fan... I was thinking of adding a fan sometime in the future -on one rainy day...
The whole for the speakers' wire is the only thing that moves air inside the amp. I find it funny and sad...

The insulation inside the cabinet was a joke. I hope this is not the 'standard' for Ampegs... Anyways, I put in some nice and thick insulation (5inch) all around. It does make a difference, but nothing major. It might have a greater impact on higher volume...

The tweeter is useless; it's nicer w/ it disconnected - I think.

These are a fun bass combo, sounds decent and it is in no way worth the time to 'fix' everything with them... ;]] but it's fun.

Thanks again!

[Ligsy1]

Hi,

I know this thread hasn't been used in long time but this is the only bit of the Internet that seems to be helping me out.

I have had the exact same problem as Jiji. I have replaced all the mofsets and searched high and low for a short within the circuit (with no trace) and yet I'm still blowing the DC fuse on my amp and having some strange smoke appearing when I power up.

It is the exact same amp and same problem as Jiji.

[Enzo]

Hi, welcome.

Please start a new thread for your amp repair. More than one repair in a thread gets confusing.