Ampex AM-10 help

[FFF]

I have this nice Ampex AM-10 that I bought because it has plenty of gain for a pair of handcrafted ribbon microphones that I designed and made.

Problem is, one of the line amp cards is not working. Channel A output was dead while channel B was working fine. I pulled both line amp cards and swapped them. As expected, now channel A works fine and channel B is dead.

I pulled out the bad card again. Here it is:

Let me start by saying I am mostly a beginner when it comes to this kind of repair work. I like doing it, but I'm still learning. I learn best by doing. Forgive me if I have to make you explain something like I'm 5.

I'm planning on replacing all the tantalum caps hoping that that is all it will take to get my line amp up and running again. The question is, should I replace them with ceramic caps, polypropylene or something else? Would electrolytics be suitable replacements in this application?

I'd like to use high-quality audio grade components where appropriate. Nothing too big though, because space inside the chassis is pretty tight.

Any advice or recommendations would be appreciated. Thanks!

To make things easier I'm providing the schematic and parts list:

 

[elnaldo]

I'd start doing passive measurements on both boards, specially at the transistors Q6, 7, 8 and 9. Measure resistance across pins. Wherever you find a difference between boards, there is a problem to check.

I see most of the tantalums are coupling caps from the mixer, it would be a coincidence that all the channels went bad at the same time. I'd suspect from the amplifier section, Q6, 7, 8 and 9.

Anyway, if you measure resistance across the tantalums pins, you can find if some of them is shorted. Compare with the good board. Measure resistance across some component, and measure in the good board. If the same, keep going to the next component.

Measuring resistance from pin 1 to 8 (mix B+ to Mix ground) and from pin 11 to 10 (L-A B+ to L-A ground) can reveal a fail too. Always comparing both boards.

 

[Lavane]

Maybe start with Q8 and Q9? Check for hairline cracks on the tantalum caps. I've found several bad tantalums in the past and they had cracks. It may take a magnifier to see them. If space is a problem replacing them all with modern tantalums is a good idea if any are found bad. You may find some Wima stack films that will fit also.

 

[FFF]

Hi,

Thanks for your guidance in this. With your help, I was able to figure out which components to replace. You made it really easy, and I.m learning a lot in the process.

Now I’m on the hunt for the correct replacements. Finding the right pieces is a bit confusing, especially the transistors.

I found this Ampex Transistor Cross-Reference List:
http://www.uneeda-audio.com/ampex/xistr.htm

Parts I have to replace:

Transistors

Q6 silicon, 014-652 (USE 580-098 2N3906)

Q7 silicon, low noise, npn 014-698 (HEP 55 or 2N3117 / 2N3565 / 2N5088)

Q8 silicon, npn; 014-585 (2N3706 / 2N3719)

Capacitors

C6 Tantalum 3.9mfd +/- 20% 15vdcw

C9 Tantalum 15mfd +/- 20% 15vdcw

(BTW, what does the ‘w’ in vdcw mean)?

I was looking at WIMA caps, which are great, but I wasn’t able to find any with the values I need.

Film caps would probably be preferred (?), but if I were to use tantalums, I found this possible source for replacements. I bet there is a better option somewhere else though.
https://www.ebay.com/itm/0-10UF-to-...hash=item2849442697:m:m-AziR5LqW3VGGZQ7ddi-FQ

After spending the last two days pulling my hair out looking for the right parts I am seeing there are a lot of variables and different specs in transistors with the same part number. How do I know which ones are the correct ones?

Any advice on these things would be greatly appreciated. Thanks again for all your help!