Problem with Crack - Audio cut out, driver tube cracked. [resolved]

[jt23]

Hello,

First time post. I'm using a Crack with Speedball upgrade, built in 2015 by another member. Other changes are that it's using a Tungsram E80CC alongside a Sylvania 7236 (replaced 237's with 470 Ohm Resistors on the A and B boards).

I had an issue occur a while back, but have only managed to look into it now. It happened when I was listening to someone's home recording; the source music was noisy, however over the duration of listening a background buzz steadily grew louder. I took this to be a part of the recording.

I left the amp on, came back and tried listening to another recording - nothing audible. I turned off the amp. A little while afterwards, I noticed that the silver lining at top of the driver tube appeared to have "burned off". I also noticed a crack in the tube. I gathered that this was not a good thing.

I checked the underside of the amp: the Black wire at Terminal 12L had come loose, possibly due to an inadequate solder connection. I re-soldered it. I then took resistance readings, and they didn't match the recommended ones in the manual.

Here they are:

Terminal   Resistance
1   0L
2   6.4M - Fall downwards
3   1.5
4   2.2M - Fall Downards
5   3.286M Steady
6   0L - Fluctuates
7   0L
8   0L Flututates
9   0L
10   0.4
12   0.3
13   0
14   0L Fluctuates
20   0L Fluctuates
22   0
B3   0
B6   0 - Fluctuates
RCA Jacks - Ground Lug   1.1
RCA Jacks - Center Pin   81.4K (L); 90.4K (R)

Any advice on what to check next - or any more information that I can provide?

Thanks for any help in advance.

[Paul Birkeland]

You have a Speedball, so your resistances won't match those listed in the manual for the amp built without it.

I would perform voltage measurements.

[cddc]

I think the easiest solution is to try a good driver tube to see if the amp is still good.

When the chrome inside tube (getter flash) turns from silver to white, it means the tube has lost its vacuum and is dead. So need to replace the tube for sure.

[jt23]

Hello,

Just made voltage checks. I had a few anomalous readings:

Terminal   Voltage
A1   77
B1   170
B3   170
B6   Fluctuates 90 or 0
0 on A Board   180
0 on Large PC Board   170

I also did the check on the tip of the headphone jack after 5 mins, and it checked out fine.

[Paul Birkeland]

A1   77

How is that anomalous?

B1   170

B1 connects to T1 and A6, but you didn't report problematic readings there.

B6   Fluctuates 90 or 0

That would tend to suggest a loose or broken wire somewhere.

0 on A Board   180

So this is what I would go after first.  If that voltage isn't pulling down, the 6080 will never operate properly.  Do both halves of the 9 pin tube still glow?  Do both LEDs on the socket still glow? 

[jt23]

How is that anomalous?

Sorry, you're right: I must have misread - A1 is fine.

That would tend to suggest a loose or broken wire somewhere.

I reheated the terminal at B6, and removed some excess solder - it seems to be reading at 90V steadily.

Do both halves of the 9 pin tube still glow?  Do both LEDs on the socket still glow? 

Both halves of the 9-pin tube are still glowing, but the LED near A7 is not glowing. The other LED near A3 is glowing.

Thanks,

[Paul Birkeland]

Swap the small PC boards side to side and see if the voltage issues move with the board.

If you have the new Speedball with one small board up front, you can just swap the wires on the outside corners of the board.

[jt23]

Swap the small PC boards side to side and see if the voltage issues move with the board.

Just swapped the boards, and re-took voltages. The problem didn't move with the board, it repeated in the same location (next to headphone jack) on the swapped board.

I also realized that in my first voltage check, I missed one of the '0' position for the large PC board. Here are both:

Terminal   Voltage
0 on Large PC Board Left Side (black to 7L)   170
0 on Large PC Board Right Side (black to 9L)   90

The LED attached to A8 was still not lighting up.

[Tom-s]

Do you have another E80CC to try?

Are all connections on the 9 pin socket firm? I'd double check or reheat all points coming from and to pin 6 - 7 - 8.

[Paul Birkeland]

You'll either have the situation where one LED on the 9 pin socket isn't lighting and that would likely coincide with half of the 9 pin tube not glowing inside, and this boils down to the solder joint at A4/A5 not being sound.  You could also find that the LED isn't lighting and the DC voltage on the 9 pin socket where the LED connects will be about 12V, in which case the LED has somehow been damaged. 

A tube with a dead half will also mimic these issues potentially.

[jt23]

Same result - I reheated A4/5, and checked A6, 7, and their connecting wires. A8 with with unlit LED seemed fine, bit couldn't reach to reheat due to the 2 PC boards being in the way.

I also swapped out the E80CC with a spare and this didn't change.

However, I took a voltage reading at A8 where the unlit LED connects, and it read at 7.4V - might that indicate the LED has been damaged?

A3 with the glowing LED measured at 1.55V.

[Paul Birkeland]

Yes, you can run a wire between A3 and A8 or replace the LED.

[jt23]

Thanks - I ran the wire, all voltages now match the manual. Plugged in headphones, and music is only coming through the right channel. I switched L + R cables, and swapped out a new cable with the same result. I did notice slight bleed through of the working right channel, when plugging a cable into the left channel in isolation.

Any tips of where to look to sort the problem? - I'll double check solder joints connecting to the left channel later on today.

Thanks again,

[Paul Birkeland]

That is usually an issue either with the wiring from the RCA jacks to the pot to the socket, or an issue with headphone jack wiring.

[jt23]

It seems to just be a physical issue with the jack - turned a certain way, both channels are loud and clear - time for a new cable, I guess!

I really appreciate all the help above, it's great to be able to listen to music again through the amp.

Many many thanks.