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Pioneer HF-20 mono tube power amp

petejk

petejk

AK Subscriber
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While I was searching for a Pioneer M-22 amplifier to buy from Japan from Yahoo auctions via Buyee last year, I happened upon a Pioneer/Fukuin electric HF-20 tube amplifier. This wasn't however one of the later tube receivers which seem to be a lot easier to find, but instead something that I hadn't seen before.
I put in a bid, and won it for less than I was anticipating, and tried to find some information about the amp. The only mention of it online is a scan of an undated Pioneer/Fukuin catalog with a picture complete with tubes installed, and what is possibly the preamplifier that may have been sold with it - the HF-10A.

Here are the auction pictures:

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And the catalog scan that I was able to find:

Fukuin Electric catalog conv 0.png
Fukuin Electric catalog conv 1.png

My plan is to restore this amp, so that I can safely use it. Not a PCB in sight when I removed the bottom cover:

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Part way through disconnecting transformers to allow for them to be removed. I am making note of connections, and starting to sketch out a circuit diagram.

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Transformers free and clear:
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Unfortunately, even with the four flat head screws removed, the bottom cover is not removable. I will need to figure out how to transformers can be released from their enclosures, as I would like to see if there is annotation within showing their specifications.

I have spent months seeing if I could acquire a 2nd HF-20, and at this point have almost resigned myself to one not being out there. If anyone has a lead however, please see my WTB posting. Thanks.
 
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You can become a AK Subscriber, which will allow you to access BarterTown. There you can create a WTB (Want to Buy) thread for the HF-20 amp.
 
I was able to remove the bottom covers, but I fear that further progress to identify these transformers will be difficult.
They are held in place using a hard (although brittle) potting material. It will likely be a difficult and messy job to remove enough to allow for transformer removal.

IMG_9799.jpeg
 
Keeping the ball rolling today with disassembly, and drawing the circuit out by hand. I may try to produce a clearer version using KiCad, if there are symbols available for tube sockets etc.

Some of the components are well labeled, and it will be easy to find replacements for, such as the film capacitors, and some of the resistors.
I am not familiar with the two large capacitors in the center of the amp. They have three terminals (two positive and negative), and are a can form factor.
Are they oil-filled caps?

Some of the resistors have a '1P' label, and are larger in size. Are these 1W resistors?
There is one polar capacitor, which looks like an axial electrolytic.
The larger resistors are hollow, which is not something I have seen before. What would the modern replacements be?

The smaller transformer with two connectors, that has one leg connected to the Power transformer, and the other leg to the VR-90GT (voltage regulator?) tube has not markings or annotation on it. I am hoping there is a way to measure/test it to determine its specifications.


Pioneer HF-20 schematic.jpg

Exterior terminal connections for input and output. 4 - 16ohm speakers supported

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Larger 3-terminal caps:
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Hollow resistors should be wirewound on a hollow core. Unless they test out of spec, they should last just about forever.
 
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