Distortion/Low Signal/Noise from Ampex 620 after being on for 5 minutes?

[templemark]

I'm having an issue with an old Ampex 620 speaker unit.

When i first turn it on I can get a really nice sound, tone and volume. However about 5 minutes into warming up the sound quickly degrades and gets very low and distorted.

Any guesses? Obviously must be some thermal related issue?
(Possibly just some connection expanding? Should i do a basic solder re-flow?)

 

[templemark]

Schematic here:

 

[radardoug]

This unit has approx 40 parts in it. Any of those parts could be faulty.
Your question is equivalent to saying to your doctor 'I have a pain in my abdomen. Whats wrong with me?'
The simple answer is we dont know! You need to do faultfinding! If you dont feel confident to do this, consult a professional!
And be carefull, this unit uses high voltages!

 

[templemark]

Ok. I understand it is a pretty general question, but I figured I would ask first in case someone had a similar problem in the past.

Just like at the doctor's you gotta start somewhere. I'm asking for your first guesses here......(before i open her up)


I have all the tools and have serviced other vintage electronics (some high voltage tube circuits) so no worries there.

 

[templemark]

In the schematic what is 'compensation network' mystery box??

Is this just an RC Filter network or something?  Was this special proprietary info for  Ampex they didn't want to get out?

 

[radardoug]

You have the box in front of you, open it up and have a look! How hard can that be?

 

[ruffrecords]

The most common cause of this type of problem is leaky capacitors. Coupling capacitors often go leaky after many years.  I would try swapping out the two caps feeding the grids of the 6V6s. The HT smoothing caps might also be leaky.

Before changing anything it might be worthwhile making a few simple tests. First, monitor the HT voltage from turn on until the onset of distortion. If the HT stays up then it is probably not a n HT smoothing cap problem.

Next I would monitor the bias on each tube (by measuring the cathode volts) and see if any of these change over time. This will give you clues as to the part of the circuit that has the problem.

Cheers

Ian

 

[templemark]

Ok finally got around to checking this out!

I monitored voltage across
C304
and C308A (i think)

C304 reads 21.9V and may go up slightly to 22.1V but nothing too significant

C308A reads 364.6V and only varies within a few volts.

No smoking gun so far? Any other suggestions?

I would go ahead and replace all Electrolytic Caps, but if possible I would like to only replace bad ones and keep it original as possible.

 

[emrr]

Ive seen this many times.  Leaky old coupling caps, not filters.  They are fine for awhile, then leakage goes up and the amp biases itself off or down.  I bet tou can turn it off for 5 minutes and it works again, goes down. 

 

[templemark]

Ive seen this many times.  Leaky old coupling caps, not filters.  They are fine for awhile, then leakage goes up and the amp biases itself off or down.  I bet tou can turn it off for 5 minutes and it works again, goes down.

Yes that is correct.

So I should start with coupling caps C302, C303, C304?

It seems there are 3 main types of capacitors in this circuit? There are the paper looking ones C304, C306 and C307 and then the rest are of the harder type except C308 which is mounted next to the tubes in a metal can.

While I am familiar with modern capacitors I don't really know these older ones. I believe the soft ones are electrolytic and the one in the can is oil filled but what are the harder ones? Some seem to have polarity while some do not.

(note: the attached picture isn't an exact one from the amp but a similar-ish one i found on google)

 

[emrr]

values tell you the type.  Anything from that era is likely to be bad if one is bad, it's usually humidity based so  I'd strip it out and start over.  Especially if you want to actually use the unit without future failure interruptions. 

 

[templemark]

Wasn't exactly the answer I was looking for but I see your point....Although isn't there some crossover.

This explains it pretty well: (except doesn't show vintage parts)
http://www.mojotone.com/amp-parts/capacitor-type

Based upon this I guess the 'hard-ones' Im referring to in my previous post are film and foil?

 

[emrr]

the coupling are probably all paper/foil, regardless of external package. 

 

[PRR]

> after being on for 5 minutes

So *what* changes after 5 minutes?? Do any good voltages drift bad?

> C304 reads 21.9V
> C308A reads 364.6V

The screens are high but the cathode is low.

You may have a dead 6V6.

Try again with one, then the other, 6V6 in.

*Do it quickly!!* With just one 6V6 on a cathode resistor sized for two 6V6, the one wants to go to double current. It won't quite do that, but will go over rated dissipation. This is safe if less than a minute. So just long enough for the cathode-voltage meter to almost stabilize, read, power-off, then try other side.

Agree the coupling caps are paper-caps, and even the Black Beauty Spragues are past their best performance.