It is strange that this console do not feature a tube amp section, like this :
http://www.diyaudio.com/forums/tubes-valves/197525-gramps-zenith-mlt-15-a.html
with the same MLT 15 model name ...
http://www.diyaudio.com/forums/tubes-valves/197525-gramps-zenith-mlt-15-a.html
with the same MLT 15 model name ...
so no other chassis tucked away inside the cabinet? it could well be solid state output section like i thought.
any chance we can get a look at the cabinet?
any chance we can get a look at the cabinet?
if the output is solid state the fading and cruddy sound is most likely leaky output caps check for the presence of dc at the speakers.
I agree with Streten on this. You are in over your head, as is obvious from the kinds of questions you're asking. This set has an early generation solid state amplifier, using germanium type transistors. These are poorly performing parts and notoriously unreliable. Replacements are also getting hard to find. The fade out of sound after turn on can be due to all kinds of problems, none of which can be solved for you over an internet discussion forum. If you have no experience in troubleshooting solid state amplifiers, then the best I can do is suggest that you have someone with experience help you to get it fixed.
Solid state audio power amp, yes. Quasi, no. It wasn't popular yet. This amp is from 1961. At that time nearly every solid state audio amp used the same design. It is the classic transformer driven totem pole output stage using PNP germanium output transistors, probably 2N2147's.
Google that type number to find a typical schematic. The driver transformers are visible in the under chassis picture of the power amp. The wires on the transformers are even the right colors! The "274" in the center of the transistors denote that they were made by RCA.
Some designs used bipolar power supplies and directly fed the speakers, others used a single supply and a speaker coupling cap. Most designs used a diode to set the idle current.
Operate the amp until the sound fades. UNPLUG it and check to see if any of the output transistors are hot. Germanium transistors are prone to runaway. If this is the case the transistors or the bias diodes are probably bad. Replacements may be hard to find. If the transistors are warm, they are probably OK and the problem lies elsewhere. DO NOT touch the transistors with the unit powered. The cases are the collector, and at least 2 of them are connected to B+.
could be interesting (if the output stage is toast) to run a couple of decent chip amps to drive the speakers or maybe convert the output to a tube stage...
Hi there - owner of "Gramps MLT15" here. I've done a lot of research on my console and it appears to be one of the last tube tuner/amp combos that Zenith produced (circa 1964/early '65), so it wouldn't surprise me if they started replacing their tube chassis with SS components around this time.
I will tell you this, bastafari... this was my first foray into tube audio repair and restoration, and it was a big undertaking that involved a LOT of homework as well as a lot of elbow grease. After replacing all paper/wax caps, re-wiring some components, and refurbishing a replacement VM/Zenith turntable, it has given me years of (mostly) trouble-free service. I use it almost every day.
I have the schematics for my tuner (chassis 9L20) and amp (4L21) if you need either. Good luck, and remember to spend plenty of time reading and understanding before you start swapping out components.
-Mike
Hi Mike, is the 4L21 amp some kind of SE unit, with series heater arrangement? Or it is a push pull one?
Hi - I've attached the schematic to this post - sorry for the medium-ish resolution. It's a 6BQ5 output stereo amp.
It was a great amp to cut my teeth on.
Here's a pic of the cabinet and some of the equipment. Thanks allot Mike. I appreciate the sympathy and definitely intend on reading as much as possible before I start. Where did you get your replacement components? I've always dealt with Angela.com and mojo tone for my capacitors. Mouser would probably be more cost effective.
Attachments
One possible line of action would be to use the SS amp chassis to build from scrap a replica of the 4L21 amp. Whoa! 355 volts on the plate, and 320 on G2, the guys liked hi voltage!
There is a strange derivation in the OPT primary, hum cancellation maybe?
There is a strange derivation in the OPT primary, hum cancellation maybe?
That's essentially the same VM changer I have. If you're up for it, here is my rebuild thread:
Antique Radio Forums • View topic - V-M 1297 Dis-assembly/Re-Lube thread
For projects like this, I've used http://www.justradios.com/ for caps and resistors.
In case that schematic didn't post before:
Antique Radio Forums • View topic - V-M 1297 Dis-assembly/Re-Lube thread
For projects like this, I've used http://www.justradios.com/ for caps and resistors.
In case that schematic didn't post before:
Hey thanks! Encouraging to know someone else had already done it. Will read through the thread. Thanks again.
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Hi,
You are of course right, I just saw the same output devices,
and made a premature jump, I couldn't see the transformer.
rgds, sreten.
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