shfa-2 amp problems

HI all: this amp has been recapped. it was working fine then just lost volume. when totally turned up is barely audible on both channels. we have checked the high and low voltage power supply and get close on all voltages. then went to the output from the 6973's and the primary 12ax7's, ok as well. the only voltage that is wrong(I think) is the grid voltage on the 6bj6 pin 6 is 30 volts not the 50 that it looks like in the best schematic I could get. there is also no difference if the mute/squelch plug is removed, or when the 6bj6 tubes are removed. any thoughts? thanks in advance. Mark

Just a WAG--did you replace the bias rectifier ??
Ron Rich

Well, so much for that good ole famous "recapping", eh?
The internet-wide endlessly touted "fix all" that fixes everything under the sun.

mb9513 wrote: the only voltage that is wrong(I think) is the grid voltage on the 6bj6 pin 6 is 30 volts not the 50 that it looks like in the best schematic I could get. there is also no difference if the mute/squelch plug is removed, or when the 6bj6 tubes are removed. any thoughts? thanks in advance. Mark

"MB" This voltage is the control on the 6BJ6 reactance tube that controls the gain. It will be high on high input and low if record level is low. It is dependent on the output of the preamp and record which may indicate that the power supply to the preamp has failed.

What voltages are you seeing at the transistor collectors or at pin 2 of the test socket (it call for a minus 22VDC )?

Another area to check is the plug for the volume control. If the ground opens or develops high resistance the two channels will now be out of phase and cancelling a lot of signal.

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Well, so much for that good ole famous "recapping", eh?
The internet-wide endlessly touted "fix all" that fixes everything under the sun.

Matt, this sort of comment is not helpful in a forum like this. It can lead to people not doing the necessary job of replacing capacitors in these old rigs. Capacitors are among the most likely failure points and also the most likely to do serious damage when leaky or shorted.

Even professional and common-carrier equipment must be recapped at intervals. A 60 year old jukebox amp -must- be done.

It is also common for multiple failures to appear when resurrecting old electronics. It should be expected that testing will reveal additional problems.

Rob

Thanks guys, Ron I replaced the low voltage diode CR102 but not CR101 if that is the rectifier you are referring to. what would be a good replacement, either by an "all in one case " item or just tying two diodes together? I will check the other suggestions Rob noted. For me recapping is a safety option, like looking in your rearview mirror before you put the car in reverse, even if it is your own driveway. This amp was done before we even plugged it in. I will get back as soon as I can get to it again. Thanks Mark

Hi Mark, and all,
A 1N4002 or better will work fine --
As to the re-cap suggestion--true, it most likely needs to be done, but the sellers of the re-cap kits, make it sound as if NOTHING else needs to be done--in other words, a "one button fix-all"-which it ain't --and could be dangerous ! Ron Rich

Ron Rich wrote:Hi Mark, and all,
A 1N4002 or better will work fine --
As to the re-cap suggestion--true, it most likely needs to be done, but the sellers of the re-cap kits, make it sound as if NOTHING else needs to be done--in other words, a "one button fix-all"-which it ain't --and could be dangerous ! Ron Rich

Exactly the reason for my previous post.
And I recently read posts from another "DIY" site where no one had a clue as to stubborn problems with a stereo receiver - seems ALL the putzers on there tossed in "it's a cap" suggestions.
"Could be C132"
"Nah donnie, it's C486"
"Oh just RECAP the whole thing!"
(These putzers ran over 14 pages of bickering over the unit and never once had a clue about the AMP distortion problem actually originating in the FM discriminator.)

To Rob:
I'm not saying old capacitors shouldn't be replaced, because they do outlive their usefulness.
But in this world of electronics, it's much more complicated of an affair, requiring technical knowledge, equipment, and experience that 14 year old Bobby just doesn't have.
And no matter what anyone insists, the internet and all it's "worldly" information cannot ever replace that.
No, capacitors are *not* the most likely failure in products, dispite what people tout.
It's a combined and complex set of factors all due to aging, design, previous "repairs" and even actual design flaws, among other things.

Hi

I agree with Rob, the comment made by Matt is not appropriate IF this board is intended to assist people. IF the intent of this board is to lord it over people and not offer any idea how to actually make a repair then Matt's comment is appropriate. It is our board and only by our actions will we show which kind of board this is. In my humble opinion, I doubt if we will have a lot of members/visitors if all we do is offer criticism with no advice.

I for one am a putzer, sorry but that is just life.. This board has been most helpful to me and it is greatly appreciated and I think I speak for a lot of other people. If we all knew all the answers there really would be no need for this board, other than to just yak at each other.

Just my observations. It is my desire than this board remain a board that offers assistance to like minded individuals and not a board where a person has to walk on egg shells because they may show they do not know everything. From my further observations there are people with a wide variety and depth of knowledge regarding the subjects discussed here. Surely, there is enough room for everyone without the need to to belittle anyone that may have less knowledge than we have. Many time sit is the tone of what is said, not the message itself that makes it unacceptable,

Best
Darrell

Hi Darrell,
I welcome "Putzers" like you who want to learn, and are willing to at least help themselves by reading the available information, including the "Service Manuals". I also learn from others, on this and other forums. It is very frustrating when a "newbie" just post something like,-- "it don't work--what should I do?" --no info, no ability to help-- Ron Rich

Ron

Thanks Ron. I agree and feel your pain when the only info they give you is "jukebox" and "won't work" or "Seeburg will not work". Of course usun xperts know that there is more than one kind of jukebox

I also understand Matt's frustration, I just do not think some comments do anything to promote this board or to help others.

Best
Darrell

ds100h wrote:Ron

Thanks Ron. I agree and feel your pain when the only info they give you is "jukebox" and "won't work" or "Seeburg will not work". Of course usun xperts know that there is more than one kind of jukebox

I also understand Matt's frustration, I just do not think some comments do anything to promote this board or to help others.

Best
Darrell

I'm not "frustrated", I merely attempted to clarify the reality, the fact, that "recapping" is not the "Golden Answer" to successfully repair or restore a piece of electronic equipment.
But alas, across the internet, it's all I seem to read from people, it's almost like the insane craze of holding onto a cellphone throughout the day and night.
Everybody's doing it, because others are, for whatever reasons.

There is nothing wrong with pointing out to others the limitations, hazards, and potential failure rate that comes along with something the internet has apparently made famous.
That is also "helpful" to others, if you think about it.

Hi Matt

Appreciate your thoughts. As I read the OPs post he was merely telling the board what thing(s) had already been done to the amp so the reader would have an idea what has or has not been done in order to offer an opinion and it appears that a component failed at some point in time after it was recapped. Could this happen to a component that checked fine during recapping, of course it could. I did not see where the OP ever suggested that recapping was a cure all. I think most of us, even a putz like me, understands that a cap kit only does not mean there are no other problems to deal with.

I for one find it amazing how with certain bits of knowledge that individuals can determine remotely what is or may be wrong with a unit. I further find it amazing that these individuals are willing to freely give of their time and talent to assist a stranger to arrive at a solution to a problem.

This is a wonderful board for those interested in these topics and I hope it continues to be one of friendship and trends towards offering help when possible.

Best
Darrell

HI all again:
Ok have replaced the Cr101 it was definitely bad. (thanks Ron) However still the same problem. as the problem exists whether I have the mute/squelch plug in or not. I am trying to understand those circuits. we have Ron's mech book but it does not have a schematic of the q160 mechanism and the copy we have is not the clearest. So please educate me. As I understand, the mute circuit is grounded during scan and is open during play. the squelch I do not understand and cannot follow it where it goes in the mech. is the squelch grounded at any time? does it loop to another line coming back to the amp or TSU? Oh and by the way I also checked the ground at the volume control and it was ok.

Mark, if the mech mute-squelch can be unplugged and the amp should play w/out AGC gain control.

Touching pins 2 or 3 at the 3-pin pickup plug should induce a loud hum when the pickup is unplugged. If neither pin results in a hum, it indicates a common fault such as B+.

Have you measured DC voltages in the preamp?

With pickup plugged-in and record playing some low voltage AC varying at an audio rate on pins 1 & 5 of the mute-squelch plug (plug removed).

Have you checked:

Volume control plug.

Speaker plug.

Speaker wattage selector.

Rob

Thanks for purchasing the Mech Guide !--but as it says in every "promo", and in the foreword for it--it is designed to be used as a "supplement", along with (a good --"legitimate") copy, of the service manual for which ever model you are working on. If you have the last printing of the Guide, look at the 245 mech schematic--the "squelch/MB" circuit is identical to that used on your model. The mute, was doubled when a stereo mechanism was installed. Ron Rich