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Messages - ilyaa

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1
Amplifier Discussion / Re: Heathkit TA-16 Tripping Breaker
« on: January 05, 2022, 07:05:16 PM »
switched breakers - no dice

it trips the other breaker, as well -

2
Amplifier Discussion / Re: Heathkit TA-16 Tripping Breaker
« on: January 05, 2022, 02:37:13 AM »
i was passing a 1khz sine through it into a dummy load, with a scope on the load.

no distortion - i crank it up slowly, it gets nice and square, achieves full power for about 3 to 4 seconds, and then *poof* turns off.

3
Amplifier Discussion / Re: Heathkit TA-16 Tripping Breaker
« on: January 05, 2022, 01:35:16 AM »
i tried a 1A slo blow - will try 2A slow blow and see what happens!

reason i thought 1A slow blow would be okay as a first go was that my ammeter was showing 0.5 amp when the breaker was tripping - was showing 0.5A when the 1A slow blow cooked.

i might try switching the breakers first and then try a 2A slow blow fuse depending on the results -

4
Amplifier Discussion / Re: Heathkit TA-16 Tripping Breaker
« on: January 04, 2022, 08:49:02 PM »
i did try that by replacing the breaker with a 1 amp fuse. that blew, too

and its the second breaker, on the secondary side of the PT, that is being tripped.

5
Amplifier Discussion / Heathkit TA-16 Tripping Breaker
« on: January 04, 2022, 08:13:22 PM »
hey happy new year everyone!

wouldnt be posting if i wasnt stumped....

got a Heathkit combo on the bench. kind of a weird one, but sounds rad! works alright, until you turn it up. after 3-4 seconds of full power, it trips the breaker (this amp has two 2 amp breakers rather than fuses).

i hooked an ammeter to it and its only drawing 0.5 amps or so right before it trips the breaker, so the high current draw must happen fairly quickly.

here is what ive tried so far:

replaced c104. thats the oldest electrolytic in there - the other ones are fairly new. replaced c34, c21, c17, c22. no diff.
outputs looked fine but i replaced em anyway (Q10 and Q9).

before i start swapping the rest of the transistors out, wanted to see if any of you brilliant minds had some thoughts as to what might be happening. the strangest thing is it works totally fine and if i reset the breaker it comes right back on, no problem. its only when it reaches near full power that something seemingly starts to cascade and draw too much current. the output stage is a bit tricky for my brain (mixed germanium and silicon, for starters...) - please enlighten me!!


6
Amplifier Discussion / Re: Fender M-80 Chorus Bias/Avalanche
« on: March 05, 2021, 01:32:01 AM »
ebay.....am i bad?

7
Amplifier Discussion / Re: Fender M-80 Chorus Bias/Avalanche
« on: March 04, 2021, 04:29:51 PM »
wow great thinking everyone -

the heatsink compound WAS old and dried out -

i gooped some more on there and we are back in business! both amps work BUT they are not quite matched. the good amp that never had any issues idles at a lower current - about 24 mA - and is able to deliver more power. the fixed amp idles at a higher current - about 30 mA - but seems to have a bit less gain/does not get up to full power - there is still some headroom but compared to the other amp there is a significant difference, for sure.

i wonder: is this likely a transistor problem? i got what looked to be legit transistors to fix the bad amp but i dont think they were a matched pair....everything else about the amps is identical. any thoughts?

8
Amplifier Discussion / Re: Fender M-80 Chorus Bias/Avalanche
« on: March 02, 2021, 11:40:00 AM »
thanks, phatt!

ill give that a shot and see what happens!

9
Amplifier Discussion / Re: Fender M-80 Chorus Bias/Avalanche
« on: March 02, 2021, 12:22:19 AM »
this bias mod is in a fender service bulletin -

its in the schematic i attached!

and i have reason to believe the original issue (a blown output) was due to the overheating/runaway problems associated with the unmodded bias circuit

10
Amplifier Discussion / Re: Fender M-80 Chorus Bias/Avalanche
« on: March 01, 2021, 09:25:55 PM »
anyone have any thoughts??

11
Amplifier Discussion / Re: Fender M-80 Chorus Bias/Avalanche
« on: February 23, 2021, 12:35:48 PM »
yeah the power resistors are all good -

all voltages read pretty much exactly the same both sides until the bias starts to crawl up

12
Amplifier Discussion / Fender M-80 Chorus Bias/Avalanche
« on: February 23, 2021, 02:10:40 AM »
hey!

got one of these fuzzy carpet amps on the bench -

R155 and Q12 were blown - had -40 VDC on the left output (yowza!)

i replaced R155, Q12 and Q13, and then did the bias mod for good measure (swapped out R127, 129,154 and 156 with 2K resistors) -

the left side had some funky looking electrolytics so i replaced those, too, as well as Q10 and Q11 (just being thorough!)

now: amp turns on without blowing up, BUT if i measure bias across R165 or R166 i see it starts out okay (around 25 mV) and then starts to creep up, with no sign of stopping. i turn the amp off when it gets above 60 mV because i can tell itll keep going until kaboom.

any thoughts what might be the issue? the so-called avalanche diodes (the BYV 26Ds) measure OK but i have a feeling they are not, in fact, OK. however, i am not that familiar with this bias circuit so i am not sure best way to proceed -

thoughts/advice? thank you!

ilya

14
Amplifier Discussion / Re: Randall RM50 Channel Switching
« on: December 18, 2020, 12:14:35 AM »
dang that my only choice?

i dont want to wait till march!!

15
Amplifier Discussion / Re: Randall RM50 Channel Switching
« on: December 16, 2020, 06:47:43 PM »
hmmm

dang just not getting through -

im suspecting the relays - i can hear/feel one of them clicking but not the other (on the breakout/switching board)

but im having a damn hard time finding any of these:

SYS-S-112D -

any suggestions for suitable replace?? thank you!


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